Rice Seed Invigoration: A Review

Rice Seed Invigoration: A Review M. Farooq, S.M.A. Basra, A. Wahid, A. Khaliq and N. Kobayashi Abstract Rice (Oryza sativa L.) provides about 55–80% of the total calories for people in South Asia, Southeast Asia, and Latin America. Elsewhere, it represents a high-value commodity crop. Change in the method of crop establishment from traditional manual transplantation of seedlings to direct seeding has been adopted in many Asian countries in the last two decades, in view of rising production costs, especially for labor and water. Seed invigoration is ascribed to beneficial treatments, applied to the seeds after harvest but prior to sowing, that improve germination or seedling growth or facilitate the delivery of seeds and other materials required at the time of sowing. Many seed invigoration treatments are being employed in a number of field crops, including rice, to improve seedling establishment under normal and stressful conditions. The treatments used to invigorate rice seed include hydropriming, seed hardening, on-farm priming, osmopriming, osmohardening, humidification, matripriming, priming with plant growth regulators, polyamines, ascorbate, salicylicate, ethanol, osmolytes, coating technologies, and more recently presowing dry heat treatments. In the wake of the day-to-day increasing cost of labor and shortage of water, direct seeding approaches in rice cropping systems are the subject of intensive investigation throughout the world and offer an attractive alternative to traditional rice production systems. In this regard, seed invigoration techniques are pragmatic approaches to achieving proper stand establishment in the new rice culture. They help in breaking dormancy and improving seedling density per unit area under optimal and adverse soil conditions. Induction and de novo synthesis of hydrolases, such as amylases, lipases, proteases; and antioxidants such as catalases, superoxide dismutase and peroxidases are reported to be the basis of improved performance using these techniques. The rice seed priming can be performed by soaking simply in water, a solution of salts, hormones, osmoprotectants, matric strain-producing materials, and other nonconventional means. Despite certain limitations, such as water potential, oxygen and temperature, rice seed invigoration has M. Farooq (B) Department of Agronomy, University of Agriculture, Faisalabad-38040, Pakistan e-mail: farooqcp@gmail.com E. Lichtfouse (ed.), Organic Farming, Pest Control and Remediation of Soil Pollutants, Sustainable Agriculture Reviews 1, DOI 10.1007/978-1-4020-9654-9 9,  C Springer Science+Business Media B.V. 2009 137 138 M. Farooq et al. been worthwhile in improving rice yield and quality. Nevertheless, in-depth studies are imperative for understanding the physiological and molecular basis of rice seed priming. Keywords Direct seeding · Dormancy · Growth · Rice · Seed priming · Stand establishment · Stress tolerance · Vigor · Yield Contents 1 Introduction . . . . . . . . . . . . . 2 Seed Invigoration Strategies . . . . . . 2.1 Seed Hydration Treatments . . . . 2.2 Other Seed Invigoration Tools . . . 3 Factors Affecting Seed Priming . . . . . 3.1 Oxygen . . . . . . . . . . . . . 3.2 Temperature . . . . . . . . . . . 3.3 Water Potential . . . . . . . . . . 4 Mechanism of Rice Seed Priming . . . . 4.1 Physiological and Biochemical Basis 4.2 Molecular Basis . . . . . . . . . 5 Seed Priming and Dormancy Management 6 Rice Seed Priming and Stress Tolerance . 6.1 Drought . . . . . . . . . . . . . 6.2 Salinity . . . . . . . . . . . . . 6.3 Low Temperature . . . . . . . . . 6.4 Submergence and Water Logging . . 7 Conclusion . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138 139 140 157 159 159 159 159 160 160 165 165 166 166 167 167 168 168 169 1 Introduction Rice (Oryza sativa L.) is a staple food for more than half of the world population which provides about 55–80% of total calories for people in South Asia, Southeast Asia, and Latin America. In the rest of the world, it represents a high-value commodity crop. Global food security is confronted by escalating food demand and endangered by dwindling water availability. In this scenario, both farmers and researchers are devising strategies for water-wise crop production without compromising on yield (Gleick, 1993). Incessant and dedicated efforts of the researchers have lead to a new way of cultivating rice that requires lesser water than the conventional production system. In this newly devised method, rice is grown in aerobic soils using Rice Seed Invigoration 139 supplementary irrigation like other cereals and aims for high yields (Huaqi et al., 2002). Although aerobic rice is an attractive alternative to the traditional rice production system (Balasubramanian and Hill, 2002; Farooq et al., 2006d), poor stand establishment and high weed infestation are major constraints in its mass scale adoption (Farooq et al., 2006c, d, j). One major advantage of a traditional transplanting system is weed control, which would need special emphasis in aerobic rice culture (Farooq et al., 2006j). Many recent studies have dealt with improving germination and the subsequent growth in this crop. The age of nursery seedlings is one of the important determinants of seedling establishment in transplanted rice. Traditionally, seeds are used for growing nurseries in most parts of the world, which result in poor and erratic seedling growth. In a system of rice intensification, younger nursery seedlings are transplanted than the conventional transplanting system (Farooq et al., 2006i). The growth of rice nursery seedlings and, subsequently, their performance in transplanted culture can also be improved through seed priming (Farooq et al., 2006 h, 2007a, b). Seed priming is reported to increase the root proliferation that enhances nutrient and water uptake (Farooq et al. 2006 k). It improves the tolerance to low temperature (Naidu and Williams, 2004; Sasaki et al., 2005), salinity (Ruan et al., 2003; Kim et al., 2006) and drought (Harris and Jones, 1997; Du and Tuong, 2002) by enhancing the activities of antioxidants, including superoxide dismutase, catalase (Fashui, 2002; Deshpande et al., 2003), peroxidases, and glutathione reductase (Fashui, 2002). Moreover, priming reduced the levels of active oxygen species and plasma membrane permeability (Fashui, 2002). Although earlier reviews (Khan, 1992; Basu, 1994; Bray, 1995; Taylor et al., 1998; Welbaum et al., 1998; McDonald, 2000; Harris, 2006) dealt elegantly with seed invigoration in various crop species, no single review is available on rice. This review sums up the current work accomplished on rice seed invigoration. 2 Seed Invigoration Strategies Seed invigoration techniques are value-added treatments applied on a given seed lot to improve its field performance. This term is often used interchangeably with seed priming. However, it is an umbrella term, which comprises many presowing techniques. Seed invigoration or seed enhancements are “post-harvest treatments to improve germination and seedling growth or to facilitate the delivery of seeds and other materials required at the time of sowing” (Taylor et al., 1998). This definition includes four general methods (Fig. 1): presowing hydration treatments (Lee and Kim, 1999, 2000; Basra et al., 2003, 2004, 2005a, 2006b; Farooq et al., 2004a; Farooq and Basra, 2005; Farooq et al., 2006b, e, 2007a, b, c), low molecular weight osmoprotectants seed treatments (Taylor et al., 1998), coating technologies (Ross et al., 2000; Song et al., 2005) and, more recently, presowing dry heat treatment (Farooq et al., 2004b, 2005b, d). These treatments focus on shortening the seedling emergence time and protecting the seeds from biotic and abiotic 140 M. Farooq et al. Seed Invigoration Seed hydration treatment Thermal treatments Seed priming Pre-soaking Hydropriming Seed coating Hardening On-farm priming Osmopriming Matripriming Osmohardening Humidification Hormonal priming Fig. 1 Classification of seed invigoration techniques. Broadly, invigoration techniques can be divided into hydration, coating, and thermal treatments subdivided into Chilling treatment and Drought treatment. Seed hydration may be uncontrolled (presoaking) and controlled (seed priming). Depending on the nature of osmoticum used, seed priming may be osmopriming, osmohardening, humidification, matripriming, and hormonal priming factors during critical phases of seedling establishment. Such treatments synchronize emergence (Tables 1, 2) and lead to uniform and vigorous stands and improved yield (Tables 3, 4). 2.1 Seed Hydration Treatments Seed requires water, oxygen, and a suitable temperature for germination. Water uptake follows a triphasic pattern (Bewley, 1997). Phase I is imbibition, which commences with the physical uptake of water by the seeds, whether alive or dead. It is usually very rapid because the water potential difference between the dry seeds and water is usually great. In alive seeds, little metabolic activity occurs during this phase. In fact, dead seeds will imbibe water at the same rate as the viable ones. Phase II is the lag period. During this phase, there is little uptake of water, thus a little change in fresh weight, but considerable metabolic activity. The seed converts stored reserves (proteins, fats, and lipids) into compounds needed for germination. Phase III is radicle protrusion. This phase usually coincides with radicle emergence and is characterized by a period of rapid water uptake (a rapid increase in fresh weight). Seeds are desiccation-tolerant during Phases I and II, but frequently become intolerant during Phase III. Each phase of water uptake is controlled by water available to Seed priming treatment Rice type Variety/cultivar/genotype Improvement recorded over control Reference Final germination percentage Hydropriming 48 h Hydropriming 48 h Hardening 24 h Hardening 24 h Hardening 24 h Hardening 24 h Ascorbate priming 10 ppm Ascorbate priming 10 ppm Ascorbate priming 20 ppm Ascorbate priming 20 ppm Salicylicate priming 10 ppm Salicylicate priming 10 ppm Salicylicate priming 20 ppm Salicylicate priming 20 ppm Hardening 24 h Osmohardening CaCl2 Osmohardening NaCl Osmohardening KNO3 Osmohardening KCl Coarse Fine Coarse Fine Fine Coarse Fine Coarse Fine Coarse Fine Coarse Fine Coarse Fine Fine Fine Fine Fine KS-282 Super-Basmati KS-282 Basmati-385 Super-Basmati KS-282 Super-Basmati KS-282 Super-Basmati KS-282 Super-Basmati KS-282 Super-Basmati KS-282 Super-Basmati Super-Basmati Super-Basmati Super-Basmati Super-Basmati 36.67% 36.67% 16.70% 13.00% 36.67% 13.67% 16.33% 12.00% 3.25% 4.66% 3.25% 7.00% 4.66% 5.53% 17.66% 11.00% 00.00% –11.34% 11.00% Farooq et al. (2006g) Farooq et al. (2006g) Basra et al. (2006b) Basra et al. (2005b) Farooq et al. (2004a) Farooq et al. (2004a) Basra et al. (2006a) Basra et al. (2006a) Basra et al. (2006a) Basra et al. (2006a) Basra et al. (2006a) Basra et al. (2006a) Basra et al. (2006a) Basra et al. (2006a) Farooq et al. (2006a) Farooq et al. (2006a) Farooq et al. (2006a) Farooq et al. (2006a) Farooq et al. (2006a) Rice Seed Invigoration Table 1 Efficacy of various seed priming treatments on the germination of rice 141 142 Table 1 (continued) Seed priming treatment Rice type Variety/cultivar/genotype Improvement recorded over control Reference Mean germination time (MGT) Hydropriming 48 h Hydropriming 48 h Hardening 24 h Hardening 24 h Hardening 24 h Ascorbate priming 10 ppm Ascorbate priming 10 ppm Ascorbate priming 20 ppm Ascorbate priming 20 ppm Salicylicate priming 10 ppm Salicylicate priming 10 ppm Salicylicate priming 20 ppm Salicylicate priming 20 ppm Coarse Fine Fine Fine Coarse Fine Coarse Fine Coarse Fine Coarse Fine Coarse KS-282 Super-Basmati Basmati-385 Super-Basmati KS-282 Super-Basmati KS-282 Super-Basmati KS-282 Super-Basmati KS-282 Super-Basmati KS-282 1.06 days 0.79 day 0.47 day 0.76 day 0.79 day 2.22 days 2.25 days 2.03 days 1.88 days 1.62 days 0.85 day 1.82 days 0.88 day Farooq et al. (2006g) Farooq et al. (2006g) Basra et al. (2005b) Farooq et al. (2004a) Farooq et al. (2004a) Basra et al. (2006a) Basra et al. (2006a) Basra et al. (2006a) Basra et al. (2006a) Basra et al. (2006a) Basra et al. (2006a) Basra et al. (2006a) Basra et al. (2006a) M. Farooq et al. Seed priming treatment Rice type Variety/cultivar/genotype Improvement recorded over control (%/days/mg) Reference Final emergence percentage Hydropriming 48 h Hydropriming 48 h Hydropriming 48 h Ascorbate priming Osmohardening KCl Osmohardening CaCl2 Hardening 24 h Hardening 24 h Hardening 24 h Ascorbate priming 10 ppm Ascorbate priming 10 ppm Ascorbate priming 20 ppm Ascorbate priming 20 ppm Salicylicate priming 10 ppm Salicylicate priming 10 ppm Salicylicate priming 20 ppm Salicylicate priming 20 ppm Hardening 24 h Osmohardening CaCl2 Osmohardening NaCl Osmohardening KNO3 Osmohardening KCl Coarse Fine Coarse Coarse Coarse Coarse Coarse Fine Coarse Fine Coarse Fine Coarse Fine Coarse Fine Coarse Fine Fine Fine Fine Fine KS-282 Super-Basmati KS-282 KS-282 KS-282 KS-282 KS-282 Super-Basmati KS-282 Super-Basmati KS-282 Super-Basmati KS-282 Super-Basmati KS-282 Super-Basmati KS-282 Super-Basmati Super-Basmati Super-Basmati Super-Basmati Super-Basmati 44.18% 50.76% 23.34% 40.56% 42.78% 33.15% 32.78% 46.78% 46.78% 28.77% 21.99% 18.58% 21.99% 31.83% 30.23% 29.83% 19.87% 38.82% 41.78% 35.56% 25.34% 30.78% Farooq et al. (2006g) Farooq et al. (2006g) Farooq et al. (2006k) Farooq et al. (2006k) Farooq et al. (2006k) Farooq et al. (2006k) Farooq et al. (2006k) Farooq et al. (2004a) Farooq et al. (2004a) Basra et al. (2006a) Basra et al. (2006a) Basra et al. (2006a) Basra et al. (2006a) Basra et al. (2006a) Basra et al. (2006a) Basra et al. (2006a) Basra et al. (2006a) Basra et al. (2004) Basra et al. (2004) Basra et al. (2004) Basra et al. (2004) Basra et al. (2004) Mean emergence time (MET) Hydropriming 48 h Hydropriming 48 h Coarse Fine KS-282 Super-Basmati 2.87 days 2.97 days Rice Seed Invigoration Table 2 Efficacy of various seed priming treatments on the emergence of rice seedlings Farooq et al. (2006g) Farooq et al. (2006g) 143 144 Table 2 (continued) Rice type Variety/cultivar/genotype Improvement recorded over control (%/days/mg) Reference Hydropriming 48 h Ascorbate priming Osmohardening KCl Osmohardening CaCl2 Hardening 24 h Hardening 24 h Hardening 24 h Hardening 24 h Hardening 24 h Ascorbate priming 10 ppm Ascorbate priming 10 ppm Ascorbate priming 20 ppm Ascorbate priming 20 ppm Salicylicate priming 10 ppm Salicylicate priming 10 ppm Salicylicate priming 20 ppm Salicylicate priming 20 ppm Hydropriming 48 h Ascorbate priming Osmohardening KCl Coarse Coarse Coarse Coarse Coarse Coarse Fine Fine Coarse Fine Coarse Fine Coarse Fine Coarse Fine Coarse Coarse Coarse Coarse KS-282 KS-282 KS-282 KS-282 KS-282 KS-282 Basmati-385 Super-Basmati KS-282 Super-Basmati KS-282 Super-Basmati KS-282 Super-Basmati KS-282 Super-Basmati KS-282 KS-282 KS-282 KS-282 1.66 days 2.36 days 2.85 days 2.34 days 2.30 days 1.2 days 0.5 day 2.87 days 2.87 days 2.52 days 2.28 days 1.82 days 2.28 days 3.14 days 0.08 day 1.67 days 0.51 day 0.6 day 0.7 day 1.9 days Farooq et al. (2006k) Farooq et al. (2006k) Farooq et al. (2006k) Farooq et al. (2006k) Farooq et al. (2006k) Basra et al. (2006b) Basra et al. (2005b) Farooq et al. (2004a) Farooq et al. (2004a) Basra et al. (2006a) Basra et al. (2006a) Basra et al. (2006a) Basra et al. (2006a) Basra et al. (2006a) Basra et al. (2006a) Basra et al. (2006a) Basra et al. (2006a) Farooq et al. (2006c) Farooq et al. (2006c) Farooq et al. (2006c) M. Farooq et al. Seed priming treatment Rice Seed Invigoration Table 2 (continued) Seed priming treatment Rice type Variety/cultivar/genotype Improvement recorded over control (%/days/mg) Reference Osmohardening CaCl2 Hardening 24 h Hydropriming 48 h Ascorbate priming Osmohardening KCl Osmohardening CaCl2 Hardening 24 h Hydropriming 48 h Ascorbate priming Osmohardening KCl Osmohardening CaCl2 Hardening 24 h Hydropriming 48 h Ascorbate priming Osmohardening KCl Osmohardening CaCl2 Hardening 24 h Coarse Coarse Fine Fine Fine Fine Fine Coarse Coarse Coarse Coarse Coarse Fine Fine Fine Fine Fine KS-282 KS-282 Super-Basmati Super-Basmati Super-Basmati Super-Basmati Super-Basmati KS-282 KS-282 KS-282 KS-282 KS-282 Super-Basmati Super-Basmati Super-Basmati Super-Basmati Super-Basmati 1.4 days 1.7 days 0.32 day 0.36 day 0.89 day 1.75 days 1.55 days 0.78 day 2.05 days 2.02 days 2.33 days 2.16 days 0.74 day 1.58 days 1.58 days 1.98 days 1.89 days Farooq et al. (2006c) Farooq et al. (2006c) Farooq et al. (2006l) Farooq et al. (2006l) Farooq et al. (2006l) Farooq et al. (2006l) Farooq et al. (2006l) Farooq et al. (2007b) Farooq et al. (2007b) Farooq et al. (2007b) Farooq et al. (2007b) Farooq et al. (2007b) Farooq et al. (2007b) Farooq et al. (2007b) Farooq et al. (2007b) Farooq et al. (2007b) Farooq et al. (2007b) 145 146 Table 3 Efficacy of various seed priming treatments on the seedling dry weight of rice Rice type Variety/cultivar/genotype Improvement recorded over control Reference Seedling dry weight Hydropriming 48 h Ascorbate priming Osmohardening KCl Osmohardening CaCl2 Hardening 24 h Hardening 24 h Hardening 24 h Hardening 24 h Ascorbate priming 10 ppm Ascorbate priming 10 ppm Ascorbate priming 20 ppm Ascorbate priming 20 ppm Salicylicate priming 10 ppm Salicylicate priming 10 ppm Salicylicate priming 20 ppm Salicylicate priming 20 ppm Hydropriming 48 h Ascorbate priming Osmohardening KCl Osmohardening CaCl2 Hardening 24 h Hydropriming 48 h Ascorbate priming Osmohardening KCl Osmohardening CaCl2 Hardening 24 h Seed coating Coarse Coarse Coarse Coarse Coarse Coarse Fine Coarse Fine Coarse Fine Coarse Fine Coarse Fine Coarse Coarse Coarse Coarse Coarse Coarse Fine Fine Fine Fine Fine Coarse KS-282 KS-282 KS-282 KS-282 KS-282 KS-282 Super-Basmati KS-282 Super-Basmati KS-282 Super-Basmati KS-282 Super-Basmati KS-282 Super-Basmati KS-282 KS-282 KS-282 KS-282 KS-282 KS-282 Super-Basmati Super-Basmati Super-Basmati Super-Basmati Super-Basmati – 3.1 mg/seedling 2.77 mg/seedling 4.5 mg/seedling 3.27 mg/seedling 3.1 mg/seedling 2.7 mg/seedling 4.53 mg/seedling 4.35 mg/seedling 23.50 mg/seedling 7.25 mg/seedling 17.75 mg/seedling 7.58 mg/seedling 1.00 mg/seedling –6.00 mg/seedling 3.75 mg/seedling –4.75 mg/seedling 0.28 mg/seedling 0.91 mg/seedling 0.97 mg/seedling 1.30 mg/seedling 1.28 mg/seedling 1.45 mg/seedling 1.12 mg/seedling 2.03 mg/seedling 2.44 mg/seedling 2.26 mg/seedling 400–870% Farooq et al. (2006k) Farooq et al. (2006k) Farooq et al. (2006k) Farooq et al. (2006k) Farooq et al. (2006k) Basra et al. (2006b) Farooq et al. (2004a) Farooq et al. (2004a) Basra et al. (2006a) Basra et al. (2006a) Basra et al. (2006a) Basra et al. (2006a) Basra et al. (2006a) Basra et al. (2006a) Basra et al. (2006a) Basra et al. (2006a) Farooq et al. (2007b) Farooq et al. (2007b) Farooq et al. (2007b) Farooq et al. (2007b) Farooq et al. (2007b) Farooq et al. (2007b) Farooq et al. (2007b) Farooq et al. (2007b) Farooq et al. (2007b) Farooq et al. (2007b) Ross et al. (2000) M. Farooq et al. Seed priming treatment Seed priming treatment Rice type Variety/cultivar/genotype Improvement recorded over control Reference Emergence to heading days Hydropriming 48 h Ascorbate priming Osmohardening KCl Osmohardening CaCl2 Hardening 24 h Hydropriming 48 h Ascorbate priming Osmohardening KCl Osmohardening CaCl2 Hardening 24 h Coarse Coarse Coarse Coarse Coarse Fine Fine Fine Fine Fine KS-282 KS-282 KS-282 KS-282 KS-282 Super-Basmati Super-Basmati Super-Basmati Super-Basmati Super-Basmati 7.00 days 07.00 days 13.00 days 11.00 days 11.00 days 7.67 days 10.76 days 15.00 days 17.34 days 15.00 days Farooq et al. (2006c) Farooq et al. (2006c) Farooq et al. (2006c) Farooq et al. (2006c) Farooq et al. (2006c) Farooq et al. (2006l) Farooq et al. (2006l) Farooq et al. (2006l) Farooq et al. (2006l) Farooq et al. (2006l) Heading to maturity days Hydropriming 48 h Ascorbate priming Osmohardening KCl Osmohardening CaCl2 Hardening 24 h Hydropriming 48 h Ascorbate priming Osmohardening KCl Osmohardening CaCl2 Hardening 24 h Coarse Coarse Coarse Coarse Coarse Fine Fine Fine Fine Fine KS-282 KS-282 KS-282 KS-282 KS-282 Super-Basmati Super-Basmati Super-Basmati Super-Basmati Super-Basmati 5.00 days 07.00 days 10.00 days 08.00 days 09.00 days 4.00 days 4.33 days 1.33 days 8.66 days 7.66 days Farooq et al. (2006c) Farooq et al. (2006c) Farooq et al. (2006c) Farooq et al. (2006c) Farooq et al. (2006c) Farooq et al. (2006l) Farooq et al. (2006l) Farooq et al. (2006l) Farooq et al. (2006l) Farooq et al. (2006l) Rice Seed Invigoration Table 4 Efficacy of various seed priming treatments on some agronomic traits and grain yield of rice 147 148 Table 4 (continued) Rice type Variety/cultivar/genotype Improvement recorded over control Reference Grain yield Hydropriming 48 h Ascorbate priming Osmohardening KCl Osmohardening CaCl2 Hardening 24 h Hydropriming 48 h Ascorbate priming Osmohardening KCl Osmohardening CaCl2 Hardening 24 h Hydropriming 48 h Ascorbate priming Osmohardening KCl Osmohardening CaCl2 Hardening 24 h Hydropriming 48 h Ascorbate priming Osmohardening KCl Osmohardening CaCl2 Hardening 24 h Coarse Coarse Coarse Coarse Coarse Fine Fine Fine Fine Fine Coarse Coarse Coarse Coarse Coarse Fine Fine Fine Fine Fine KS-282 KS-282 KS-282 KS-282 KS-282 Super-Basmati Super-Basmati Super-Basmati Super-Basmati Super-Basmati KS-282 KS-282 KS-282 KS-282 KS-282 Super-Basmati Super-Basmati Super-Basmati Super-Basmati Super-Basmati 3.70% 11.11% 18.51% 14.81% 11.11% 28.43% 24.64% 30.80% 40.28% 30.33% 12.25% 14.52% 21.93% 15.95% 14.24% 11.22% 19.64% 25.26% 31.57% 25.61% Farooq et al. (2006c) Farooq et al. (2006c) Farooq et al. (2006c) Farooq et al. (2006c) Farooq et al. (2006c) Farooq et al. (2006l) Farooq et al. (2006l) Farooq et al. (2006l) Farooq et al. (2006l) Farooq et al. (2006l) Farooq et al. (2007b) Farooq et al. (2007b) Farooq et al. (2007b) Farooq et al. (2007b) Farooq et al. (2007b) Farooq et al. (2007b) Farooq et al. (2007b) Farooq et al. (2007b) Farooq et al. (2007b) Farooq et al. (2007b) M. Farooq et al. Seed priming treatment Rice Seed Invigoration 149 the seeds (Taylor et al., 1998). Pre-sowing hydration techniques can be grouped into two categories depending on whether water uptake is uncontrolled or controlled. 2.1.1 Pre-Soaking Presoaking includes the methods in which water is freely available to seeds, and its uptake is not restricted by the prevailing environment. The water uptake is governed by the affinity of the seed tissues to water. Common techniques include imbibing seeds on moistened blotters or soaking seeds in water. Important presoaking techniques that are employed to prime rice seed are detailed in the follow subsections. a. Hydropriming In hydropriming, seeds are soaked in water and dried before sowing (Soon et al., 2000). Soaking by submerging seeds in water can be performed with or without aeration (Thornton and Powell, 1992). Since an ample amount of water and oxygen and suitable temperatures are available, nondormant seeds would readily germinate. Because no chemicals are used during this process, it is an environmentally safe technique. A likely disadvantage of this technique is that the seed hydration is sometime uneven, which results in nonuniform germination (Pill and Necker, 2001). Hydropriming duration is of vital importance for seed invigoration. To our knowledge, only one study was conducted to hydroprime rice for seed invigoration (Farooq et al., 2006 g). Coarse and fine rice seeds subjected to hydropriming for 12, 24, 36, 48, and 60 h in aerated tap water manifested improved vigor in both rice types except seeds hydroprimed for 60 h. Of these, maximum vigor improvement was noted in seeds hydroprimed for 48 h, which was followed by that of 36 h in both rice types. Improved germination and seedling establishment finally contributes towards the grain yield, thereby substantiating that hydropriming has the potential to improve germination and early seedling growth in coarse and fine rice (Farooq et al., 2006 g). In a field study, hydropriming for 48 h improved the emergence, seedling establishment, growth, and yield in direct-seeded coarse and fine rice cultivars (Farooq et al., 2006e, k). In another study, hydropriming for 48 h improved the growth of nursery seedlings and subsequently the growth, yield, and quality of both coarse and fine rice in transplanted cultures (Farooq et al., 2007a, b). In nutshell, hydropriming can be employed to improve the performance of transplanted and direct-seeded rice. The best priming duration was 48 h for both rice types. b. Hardening Hardening, also called wetting and drying, or hydration-dehydration, refers to repeated soaking in water and drying (Pen Aloza and Eira, 1993). The hydrationdehydration cycle may be repeated twice, thrice, or for more times (Lee et al., 1998b; Lee and Kim, 2000). The beneficial effects of seed hardening are primarily related to pre-enlargement of the embryo (Austin et al., 1969), biochemical changes like enzyme activation (Lee et al., 1998a; Lee and Kim, 2000; Basra 150 M. Farooq et al. et al., 2005a), and improvement of germination rate, particularly in old seeds (Lee et al., 1998a). During seed hardening, the number of cycles and, above all, duration between the cycles are important for improving vigor. For rice seeds, two cycles of alternate wetting and drying are effective (Lee and Kim, 2000; Basra et al., 2003, 2004, 2005a). Hardening for 24 h proved best in vigor enhancement in both coarse (Basra et al., 2004) and fine rice (Farooq et al., 2004a). Seeds hardened for one and two cycles of 12 and 18 and 24 h enhanced vigor in both fine and coarse rice types, except the seeds hardened for 24 h (2 cycles) that behaved similarly to that of the control. Maximum vigor enhancement was noted in seeds hardened for 24 h (1 cycle), which was similar to that of seeds hardened for 12 h (Farooq et al., 2004a). Seed hardening treatments have been found to be very effective in improving the germination and seedling stand establishment in rice (Lee et al., 1998a, Lee and Kim, 2000; Basra et al., 2003, 2004, 2005a; Farooq et al., 2004a; Farooq and Basra, 2005; Farooq et al., 2005a, 2006c, d, 2007a). Seed hardening was more effective for invigoration of normal and naturally aged rice seed than osmoconditioned ones (Lee and Kim, 2000; Basra et al., 2003). Mathew et al. (2004), after a series of laboratory and field experiments, established the superiority of the seed hardening strategy in improving the various attributes such as speed of germination, germination percentage, and seedling vigor that facilitated crop establishment in the field under subdued soil moisture. Seedling mortality was minimal and seedling density was higher in treatments involving hardening. Seed hardening for 24 h (1 cycle) also improved the growth, yield, and quality in direct-seeded coarse and fine rice types (Farooq et al., 2006l). In a separate study, seed hardening for 24 h not only improved the growth of nursery seedlings but also the subsequent growth, yield, and quality of both coarse and fine rice in transplanted culture (Farooq et al., 2007a, b). This suggests that seed hardening is an important approach in improving seed germination, stand establishment, and ultimately seed yield, when used up to 1 cycle of 24 h each (see Tables 1, 2, 3, 4). c. On-farm seed priming It is evident from the recent research that in a range of crop species, faster germination, early emergence, and vigorous seedling growth may result in high-yield crops by soaking in water for some time followed by surface drying before sowing referred to as “on-farm priming” (Harris et al., 1999, 2000; Musa et al., 1999). On-farm seed priming is a simple, low-cost, and low risk method for promoting seedling establishment, as well as vigorous and faster seedling growth. The duration of soaking is critical and should always be less than the safe limit (time to prime seed) for each crop cultivar. If the priming time exceeds that, it may lead to seed or seedling damage by premature germination (Harris et al., 1999). The concept of a “safe limit” differentiates on-farm seed priming from pregermination. Primed seeds will not germinate unless placed on a moist substrate, or unless moisture becomes subsequently available (e.g., rain). In contrast, seeds that have been soaked for longer than the safe limit will continue to Rice Seed Invigoration 151 germinate even in the absence of an external moisture source. Use of pregerminated seed presents inherent risks, whereas primed seed behaves as dry seed if sowing is delayed or seedbed conditions are suboptimal. Soaking overnight is also successful for rice (Harris et al., 2002). It is highly cost-effective for farmers, produces better stand; the crop matures earlier and gives higher yields for little cost. Primed rice seed germinates and seedlings emerge faster (1–3 d), more uniformly, and vigorously, leading to a wide range of phenological and yield-related benefits (Table 4). On-farm seed priming results in better emergence (91% vs 61%), earlier flowering (71 days vs 74.7 days), taller plants (108 cm vs 94 cm), longer panicles (22.4 cm vs 20.3 cm), and more numbers of panicles per plant (5.7 vs 4.9) in direct-seeded rice (Harris et al., 2002). For instance, in their experiment, Harris and Jones (1997) tested the germination response to the seed priming of 11 varieties of upland rice, including traditional and improved O. sativa and O. glaberrima varieties and new inter-specific hybrids. Seed priming with water for 24 h did not affect the final germination percentage, but it reduced time to 50% germination in all varieties from 46 h down to 32 h, which agrees well with the actual time saved by priming (7–20 h). In summary, on-farm priming is a simple strategy for improving the phenology and yield of rice even under adverse soil conditions. Overnight soaking (before the actual radicle protrusion) is the best strategy in this regard. 2.1.2 Seed Priming (Controlled Hydration) Seed priming is a technique by which seeds are partially hydrated to a point where germination-related metabolic processes begin, but radicle emergence does not occur (Heydecker and Coolbear, 1977; Bradford, 1986). During this process, seeds are placed in solutions with a high osmotic potential. This prevents the seeds from taking in enough water to enter Phase III of hydration. This actually results in an extension of Phase II, essentially restricting the seed within the lag phase (Taylor et al., 1998). During this period, the seeds are metabolically active and convert stored reserves for use during germination, wherein membrane and genetic repair is better than normal imbibition. The seeds are then removed from the priming solution, rinsed with water, and dried. Such seeds when planted show faster germination than the unprimed ones. Primed seeds usually display increased germination rate, greater germination uniformity, and sometimes greater total germination percentage (Heydecker and Coolbear, 1977; Brocklehurst et al., 1984). These changes have been attributed to metabolic repair during imbibition (Burgass and Powell, 1984; Bray et al., 1989), a build up of germination-promoting metabolites (Farooq et al., 2006l), and osmotic adjustment (Bradford, 1986). However, for seeds that are not redried after treatment, a simple reduction in the lag time of imbibition takes place (Heydecker, 1977; Bewley and Black, 1982; Brocklehurst and Dearman, 1983; Bray, 1995; Taylor et al., 1998; Welbaum et al., 1998; McDonald, 2000). Seed priming can be accomplished by different means as detailed in the following subsections. 152 M. Farooq et al. a. Osmopriming The primary objective of employing seed osmopriming is to improve the germination and stand establishment. Osmoconditioning, osmopriming, or halopriming synonymous seed priming methods are used to describe the soaking of seeds in aerated low-water potential solutions to control water uptake and prevent radicle protrusion (Bray, 1995). Such treatments, followed by the dehydration of the seeds, have been demonstrated to improve the germination of numerous vegetable seeds, especially under suboptimal conditions (Heydecker et al., 1975; Brocklehurst and Dearman, 1983; Brocklehurst et al., 1984; Bradford, 1986; Bradford and Haigh, 1994; Karssen et al., 1989). In fact, osmoconditioned seeds are less sensitive to temperature and oxygen deprivation (Guedes and Cantliffe, 1980; Brocklehurst and Dearman, 1983; Corbineau et al., 1994). Of the different osmotica used as priming agents, polyethylene glycol, a variety of inorganic salts, proline, and mannitol are of special consideration. Osmopriming with calcium chloride, potassium nitrate, sodium chloride, and polyethylene glycol-8000 improved the energy of germination and lowered mean germination time in rice (Ruan et al., 2002a). Priming with polyethylene glycol8000 also accelerated the germination of coarse and fine rice (Basra et al., 2005a). In a greenhouse study, osmopriming with calcium chloride alone, and combined with sodium chloride, improved the seedling vigor index, and seedling and stand establishment of rice in flooded soil. The addition of gibberellic acid to a solution containing a mixture of calcium chloride and sodium chloride did not significantly promote either the speed of emergence or stand establishment as compared to the mixture of salts alone in solution (Ruan et al., 2002b). In a field experiment, rice seeds were primed with 4% potassium chloride before sowing, and 50 ppm paraquat sprayed at the tillering or booting stages. With seed priming, plant moisture content, leaf-area index, chlorophyll content, and nitrate reductase activity increased. Plant-moisture content and leaf-area index were the greatest when paraquat was applied at tillering, and chlorophyll content and nitrate reductase activity were greatest when paraquat was applied at booting (Sarma et al., 1993). Likewise, priming with lanthanum nitrate solutions can also accelerate the germination of the rice seeds, whilst significantly increasing seedling vigor in terms of root growth (Fashui et al., 2003; Zhang et al., 2005). Researchers are trying to use different fertilizers for seed priming to improve their efficiency. In this respect, nutripriming with fertilizers to improve the performance of direct-seeded rice is another area of great interest (Du and Tuong, 2002). Kalita et al. (2002) found that nutripriming in 4% monoammonium phosphate resulted in the highest number of effective tillers and greater grain yield of direct-sown summer rice (Table 4). However, contrary to the above, micronutrient priming in a series of field experiments did not improve grain yield or the grain micronutrient content of rice (Johnson et al., 2005). In a laboratory study, fine and coarse rice seeds primed with urea, nitrophos, di-ammonium phosphate, and potassium sulphate resulted in a complete failure of germination and emergence. This was due to increased membrane damage (as seen from increased conductivity of seed leachates) with these fertilizers (Farooq et al., 2005c). This Rice Seed Invigoration 153 suggests that a certain level of each fertilizer should be preoptimized before carrying out nutripriming. Priming duration and salt concentration are of key importance and inversely proportional to each other. Priming seeds in higher concentrations of salts for longer durations may result in reduced and uneven germination and stand establishment. The performance of primed seeds is also dependent on the temperature during priming. For example, Lee et al. (1998c) concluded that priming of rice seed in distilled water (0 MPa) for 4 days at 15◦ C and 1 day at 25◦ C performed in a similar way, whereas four days was optimum time in polyethylene glycol solution (–0.6 MPa) regardless of the priming temperature. In another time-course study, osmopriming for 48 h was the most effective in both fine and coarse rice when seeds were soaked in aerated solutions of polyethylene glycol (ψs –1.25 MPa), while seeds osmopriming for 72 h behaved similar or inferior to untreated seeds, possibly due to priming for longer durations (Basra et al., 2005a). Many researchers have reported improved germination and seedling stand establishment owing to a wide range of osmopriming protocols. Osmoconditioning (–1.1 MPa potassium nitrate solution) for 24 h improved germination and early seedling growth in fine (Basra et al. 2003, 2005b) and coarse rice (Basra et al., 2006a). Ruan et al. (2002a, b) reported a significantly enhanced energy of germination and declined mean germination time after osmopriming with calcium chloride singly and in combination with sodium chloride. Likewise, Lee et al. (1998c) found that osmopriming with –0.6 MPa polyethylene glycol solution at 25◦ C for four days took up to three days less time from planting to 50% germination, and improved the rate and final percentage of germination than those of untreated seeds. In addition to under optimal environmental conditions, the priming of rice seeds might be a useful way for better seedling establishment under adverse soil conditions (Lee et al., (1998a). Significantly greater and rapid germination of osmoprimed rice seeds under low temperature (5◦ C) and salt (0.58% sodium chloride) stresses were observed (He et al., 2002). The ultimate advantage of osmopriming is yield enhancement (Table 4). In an adoption study in five states of Nigeria, 83 farmers out of the 300, who participated in the upland rice seed priming technology transfer between years 2000, and 2002 accrued 33–84% yield advantage by primed over nonprimed seeds. In view of this yield benefit, most of these farmers (94%) diffused the technology to their fellow farmers. This showed a wide acceptance of rice seed priming technology in the areas of coverage (Bakare et al., 2005). In summary, a number of inorganic salts in appropriate concentrations can be used to improve germination in rice. Lowered osmotic potential of the solution appears to be instrumental in seed priming. The priming can be more beneficial if carried out up to 48 h; after that, it may lead to suboptimal germination and seedling stands. b. Osmohardening A new technique for rice seed invigoration has recently been introduced in which both seed hardening and osmoconditioning are successfully integrated—named 154 M. Farooq et al. osmohardening (Farooq et al., 2006a). In this technique (like hardening), both the number and duration of cycles are important for improving the seed vigor. Because this is a relatively new technique, extensive work is imperative to find the most effective salts to be used as priming agents for rice seed invigoration. Available data show that a variety of salts were used to osmoharden coarse and fine rice. In a laboratory trial, both coarse and fine rice seeds were hardened (with water) and osmohardened (chlorides of calcium, potassium, sodium, and potassium nitrate solution) in such way that osmotic potential of all the solutions was –1.25 MPa. For both the rice types, osmohardening for 48 h with calcium chloride was better than other treatments followed by hardening and osmohardening with potassium chloride (Farooq et al., 2006a). Osmohardening with calcium chloride was the most effective in improving the growth of rice nursery seedlings (Farooq et al., 2007b), and stand establishment in direct-seeded coarse and fine rice (Farooq et al., 2006a, c, d, k, 2007a, b, c). In fine rice, osmohardening with calcium chloride produced 2.96 t ha–1 (vs 2.11 t ha–1 from untreated control) kernel yield, 10.13 t ha–1 (vs 9.35 t ha–1 from untreated control) straw yield, and 22.61% (vs 18.91% from untreated control) harvest index (Farooq et al., 2006l). In coarse rice, on the other hand, osmohardening with potassium chloride produced greater kernel and straw yield, and harvest index, followed by that of calcium chloride hardening. Improved yield was attributed principally to the number of fertile tillers and 1,000 kernel weight (Farooq et al., 2006c). In another study, osmohardening with calcium chloride improved the initial seedling vigor and resulted in improved growth, yield, and quality of transplanted fine rice; the improvement in kernel yield was 3.75 t ha–1 (control: 2.87 t ha–1 ), straw yield 11.40 t ha–1 (control: 10.03 t ha–1 ), and harvest index 24.57% (control: 22.27%) (Table 4). The improved yield was attributed to the increase in the number of fertile tillers (Farooq et al., 2007b). In essence, osmohardening is quite practical for enhancing the emergence, seedling stand establishment, growth, yield, and quality in both transplanted and direct-seeded rice. Osmohardening with calcium chloride was more effective for fine rice, and potassium chloride for coarse rice, in both culture methods. Further studies will provide the basis of enhancements in seedling density and economic yield in rice. c. Matripriming Matripriming involves controlled seed hydration similar to the natural moisture absorption of the plant media. Seeds are mixed into moist solid carriers such as granulated clay particles or vermiculite (Gray et al., 1990; Hardegree and Emmerich, 1992a, b). The surface of these compounds creates matrix forces that hold water to facilitate slow absorption by the seed (Taylor et al., 1998; Khan, 1992; Beckman et al., 1993). After treatment, the seed is separated from the solid carrier and allowed to dry. Matripriming is a more effective vigor enhancement tool used in bold-seeded crops; the reason only very few studies have been conducted on small seeded crops such as rice. More recently, a matripriming method has been developed for rice using sand as a priming solid matrix (Hu et al., 2005). The seeds of four rice Rice Seed Invigoration 155 varieties were mixed with sands that contained 3.8% (v/w) water and sealed in plastic boxes at 18◦ C for 72 h. This improved the emergence and seedling density of direct-sown rice in the laboratory. Moreover, seedling height, root length, number, and the dry weight of the root were significantly greater than the nonprimed controls. Field trials showed that the seed establishment and yield in matriprimed seeds were increased by 20–23% and by 10–31%, respectively (Table 4), as compared to soaked seeds without priming (Hu et al., 2005). d. Priming with hormones and other organic sources Improved seed performance has been achieved by incorporating plant growth regulators, polyamines, and certain other organic sources during priming and other presowing treatments in many vegetable and field crops, including rice (Kim et al., 1993; Jeong et al., 1994; Lee et al., 1999). Of the phytohormones, gibberellic acid is well-known to activate β-amylase for the breakdown of starch stored in seeds to be utilized by growing embryos during germination (Taiz and Zeiger, 2006). Both gibberellic acid and ethylene stimulate the elongation of mesocotyle, coleoptile, and internodes of rice seedlings after germination. Also, abscisic acid promotes elongation of the mesocotyle of rice seedlings (Kim et al., 1989; Lee et al., 1999). In rice, gibberellic acid treatment without seed priming enhances the time of seedling emergence by 1–2 days, depending upon gibberellic acid concentrations (Kim et al., 1993). In a study on kinetin and gibberellins applied to the dehusked seeds of indica and japonica rice under aerobic conditions, both these hormones stimulated the germination of rice. However, under anaerobic conditions, the effect of kinetin was negative while that of gibberellins was positive (Miyoshi and Sato, 1997a). Studies of the individual or combined effects of gibberellic acid, urea, naphthaleneacetic acid, etc. on hybrid rice revealed that the application of 200 g of naphthaleneacetic acid ha–1 resulted in the improved percentage of emerged panicles while incurring the lowest cost. This treatment, along with 50 g gibberellic acid + 50 g naphthaleneacetic acid ha–1 and 100 g gibberellic acid ha–1 , recorded the maximum paddy yield. Based on the cost-effectiveness, naphthaleneacetic acid proved to be a viable alternative to gibberellic acid for hybrid rice seed production (Deshpande et al., 2003). Chen et al. (2005) soaked the seed of four rice cultivars in gibberellic acid and noted that seedling emergence and dry matter were increased significantly in some cultivars in response to seed gibberellic acid treatment. Polyamines are known to have profound effect on plant growth and development (Watson and Malmberg, 1998). Polyamines, being cations, can associate with anionic components of the membrane, such as phospholipids, thereby stabilizing the bilayer surface and retarding membrane deterioration under stressful conditions (Basra et al., 1994). Conclusive evidence is available about the involvement of polyamines accumulation in the protection of plants against various environmental stresses (Bouchereau et al., 1999). Fine rice seeds soaked in lower (10 and 20 ppm) concentrations of polyamines (spermidine, putrescine, and spermine) displayed earlier, synchronized, and enhanced germination. Improvement in shoot and root length, seedling fresh and dry weight, and root and leaf 156 M. Farooq et al. score was also observed. Seed treatment with 10 ppm putrescine solution was highly effective for most of the studied attributes (Farooq et al., 2008). Salicylicate is an endogenous growth regulator of phenolic nature, which participates in the regulation of physiological processes in plants (Raskin, 1992). These include effects on ion uptake, membrane permeability, etc. (Barkosky and Einhelling, 1993). In addition, salicylicate interacts with other signaling pathways including those regulated by jasmonic acid and ethylene (Szalai et al., 2000, Ding and Wang, 2003). It also induces an increase in the resistance of seedlings to osmotic stress (Borsani et al., 2001), low or high temperature by activation of glutathione reductase and guaiacol peroxidase (Kang and Saltveit, 2002). Studies on the coarse rice seed priming with salicylicate resulted in greater vigor enhancement as compared with the control group. However, a prompt and most uniform germination and emergence was observed in seeds primed with 10 ppm ascorbate solution (Basra et al., 2006a). In a study, presowing seed treatments with 10, 20, and 30 ppm salicylicate resulted in earlier, synchronized, and enhanced germination. Improvement in root length, leaf score, and seedling fresh and dry weight was also recorded with these treatments, although 30 ppm concentration was the most effective (Farooq et al., 2007c). Ascorbate is one of the most important antioxidants. Quite a few reports highlight the role of ascorbate in improving germination of cereals, including wheat, barley, and rice at lower concentrations (Naredo et al., 1998). In a laboratory study, it was revealed that priming with ascorbate at various (10–50 ppm) concentrations improved the germination and early seedling growth in both coarse and fine rice types, although priming with 10 ppm was the most effective (Basra et al., 2006a). Ascorbate priming also improved the growth, yield, and quality in direct-seeded coarse and fine (Farooq et al., 2006l) rice. In another study on transplanted rice, ascorbate priming not only improved the growth of nursery seedlings, but also the yield and quality of both coarse and fine rice types (Farooq et al., 2007a, b). Among other organic sources, butenolides are a class of lactones with a fourcarbon heterocyclic ring structure (Joule and Mills, 2000). The most common and important example of a butenolide is ascorbate. Butenolide derivatives are produced by some plants upon exposure to high temperatures, and these compounds can trigger seed germination in plants whose reproduction is fire-dependent (Flematti et al., 2004). In a recent study, low concentrations of butenolide greatly promoted seedling root and shoot length, and the number of lateral roots. The vigor index of smoke-water (1:500) and butenolide-treated rice seeds were significantly greater than that of untreated seeds (Kulkarni et al., 2006). Effectiveness of imidacloprid (an insecticide) as a priming agent to improve yield has also been explored (Mathew et al., 2004). In a study, priming with imidacloprid, sodium chloride, potassium chloride, and Azospirillum, the imidacloprid greatly improved the seedling density and yield performance of rice seeds over the rest of the treatments (Mohanasarida and Mathew, 2005). Ethanol has been reported to have stimulatory effects on the germination of seeds in many plant species (Taylorson and Hendricks, 1979; Bewley and Black, 1982). Farooq et al. (2006f) soaked fine rice seeds in 1, 5, 10 and 15% (v/v) Rice Seed Invigoration 157 aerated solutions of ethanol for 48 h. None of the seeds could germinate at 10 and 15% (v/v) ethanol concentration, while 1 and 5% concentrations prompted a more uniform seedling emergence followed by more number of leaves per plant at 1% concentration. In another study, the inhibition of germination caused by de-husking japonica rice was overcome by 0.5–5% ethanol (Miyoshi and Sato, 1997b). In short, priming with plant growth regulators and various other organic sources in relatively lower concentration has the potential to further enhance the uniformity in germination, stand establishment, growth, and harvestable yield. e. Priming with low molecular weight osmolytes Osmolytes help to maintain cyptoplasmic turgor pressure during water stress, stabilize the structure and functions of certain macromolecules, and ultimately promote the growth of plants under stressful conditions (Mickelbart et al., 2003). It is well-established that seed treatment and foliar application of these solutes might have some advantages, as they improve the tolerance ability of plants (Agboma et al., 1997). In several temperate rice-growing countries of the world, the prevalence of low temperature at sowing results in poor rice seed germination, seedling establishment and vigor. Seeds of four rice cultivars (Sasanishiki H433, HSC-55, and Doongara) were soaked in various combinations of glycinebetaine in Petri dishes placed in a low-temperature glasshouse (18/13◦ C; day/night) for two days. After this soaking period, seedling emergence was faster in cold tolerant cultivar HSC-55 as noted from mean emergence time, while the other three cultivars (noncold-tolerant) displayed reduced seedling emergence, implying that glycinebetaine was ineffective on the latter cultivars. This indicated significant differences in the responses of genotypes to seedling emergence and vigor towards applied gibberellic acid and glycinebetaine under low temperature (Chen et al., 2005). f. Humidification Humidification is a presowing, controlled hydration treatment in which seeds are equilibrated under conditions of high humidity (Perl and Feder, 1981; Finnerty et al., 1992). In this technique, seeds are in direct contact with water vapor (Khan, 1992). To our knowledge, only one study was conducted to investigate the possibility of rice seed invigoration by this means. Humidification of normally germinating rice seeds did not increase germination under favorable conditions, but was accelerated in unfavorable soil and suboptimal temperatures (Lee et al., 1998a). Aged seeds humidified at 60% relative humidity showed no effect on germination rate or time to 50% germination. However, 80% relative humidity reduced the germination percentage and enhanced the time to 50% germination (Lee et al., 1998a). 2.2 Other Seed Invigoration Tools Certain nonconventional means have been successfully used to invigorate rice seed to accomplish optimal seedling density and ultimate yield per unit area. 158 M. Farooq et al. 2.2.1 Thermal Treatments The dry-heat treatment of seeds is used for two purposes one is to control the external and internal seed-borne pathogens, including fungi, bacteria, viruses, and nematodes (Nakagawa and Yamaguchi, 1989; Fourest et al., 1990); and the other is to break the dormancy of seeds (Zhang, 1990; Dadlani and Seshu, 1990). In general, high temperature in dry heat treatment reduces seed viability and seedling vigor, but optimum temperature for breaking dormancy promotes rice seed germination and seedling emergence (Lee et al., 2002). In a study on coarse and fine rice seeds, dry-heat treatment at 40◦ C for 72 h shortened the time to 50% germination and improved germination index, radicle and plumule length, root length, root/shoot ratio, root fresh and dry weight, radicle and plumule growth rate, and shoot fresh weight in fine rice. In coarse rice, none of these treatments improved germination and seedling vigor (Farooq et al., 2004b). In a laboratory study, coarse and fine rice seeds were exposed to thermal hardening (heating followed by chilling followed by heating and vice versa; and heating followed by chilling and vice versa). In fine rice, the heating–chilling–heating cycle was the best, while in coarse rice, the chilling–heating–chilling cycle performed better than all other treatments (Farooq et al., 2005b). 2.2.2 Seed Coating Seeds vary greatly in their size, shape, and color. In many cases, seed size is small, making singularization and precision placement difficult. In addition, seeds should be protected from a range of pests that attack germinating seeds or seedlings. Seedcoating treatments can be employed in both situations; they can facilitate mechanical sowing to achieve uniformity of plant spacing, and can be applied in target zones with minimal disruption to the soil ecology and environment. Ross et al. (2000) found that rice seedling emergence was depressed by 40–60% when seeds were coated with a single super phosphate, mono ammonium phosphate, or potassium phosphate. By contrast, seed coating with rock phosphate did not affect final emergence, although it delayed seedling emergence by 2–3 days. Twenty days after sowing, coatings increased shoot dry weight, but decreased root dry weight of seedlings. The effect of coating treatments persisted up to 40 days after sowing, and at this stage, plant growth in terms of root length and dry weight and shoot dry weight increased by 400–870% (Table 3). Coating rice seeds with rock phosphate may be more promising in stimulating early rice growth on low P soils (Ross et al. 2000). Song et al. (2005) reported that film coating of rice seeds may improve the performance of direct-sown rice. In Japan, coating rice with a source of oxygen (as CaO) has been practiced for decades to promote seedling emergence of direct-seeded rice in flooded soil (Ota and Nakayama, 1970). If seeds are broadcast in standing water, they remain floating due to lower specific gravity and tend to be poorly anchored, leading to floating or lodged seedlings. Yamauchi (2002) reported that the specific gravity of rice seeds can be increased by iron coating, which increases seed germination and thus stand establishment. Rice Seed Invigoration 159 3 Factors Affecting Seed Priming Many environmental variables in the priming protocols have different effects on the physiology of seed performance (McDonald, 2000). However, Corbineau and Côme (2006) opined that among the factors affecting seed priming, oxygen, temperature, and water potential of priming medium are the most important ones. 3.1 Oxygen Oxygen has been identified as one of the most important variables modulating the effectiveness of seed priming. To the best of our knowledge, little information is available on the effects of aeration on rice seed priming and its subsequent performance. In a study, osmopriming in an aerated solution of polyethylene glycol solution with osmotic potential of –1.25 MPa improved germination and early seedling growth (Basra et al., 2005a). Osmohardening in aerated solutions of calcium chloride and potassium chloride, each with osmotic potential of –1.25 MPa, improved the germination, stand establishment, growth, and yield in transplanted (Farooq et al., 2007a, b) and direct-sown rice (Farooq et al., 2006c, k). 3.2 Temperature Low temperatures during priming can change the seed performance (Lee et al., 1998c). This may delay the physiological processes of germination, even though the seed absorbs water in optimal amounts. Lower temperatures also reduce the possibility of microbial contamination during priming. Lee et al. (1998c) primed rice seeds at 15◦ C and 25◦ C, and the seeds were germinated at 17◦ C, 20◦ C, or 25◦ C. Considering germination rate, the optimum priming duration in water was four days at 15◦ C and one day at 25◦ C. However, priming in –0.6 MPa polyethylene glycol solution at low temperature did not affect the effectiveness of seed priming. Further studies are necessary to establish whether low, high, or optimum temperatures have any specific roles for enhanced germination in response to seed priming. 3.3 Water Potential Seeds germinate when water potential reaches a critical level in the seed. This varies within and between plant species, but generally occurs when the seed environment is between 0 and –2 MPa (McDonald, 2000; Corbineau and Côme, 2006). Exceptions occur when seeds have impenetrable seed coats, or contain dormancy-causing chemicals that must be removed before germination occurs. Seeds having permeable seed coats usually go through three distinctive phases of germination: (1) imbibition; ψw of the seed environment is higher than that in the seed, causing water molecules to flow through the seed epidermis into the embryo, leading to (2) the activation phase; in which stored seed hormones and enzymes stimulate physiological 160 M. Farooq et al. development leading to (3) growth of the radical; ending the germination phase (Taylor et al., 1998). Dormant (dry) seeds are usually at very low water potential, in the range of –350 to –50 MPa. Some metabolism occurs even at these low water potentials. Water movement into dry seed during the imbibition phase is rapid at first, but slows as the water potential of the seeds approaches that of the environment. If imbibition is too rapid (from an environment where water potential is very high), damage to hydrating cells often occurs (Soon et al., 2000; Pill and Necker, 2001). Many researchers have reported improved germination and seedling stand establishment due to wide-range water potentials. Osmoconditioning with KNO3 and water potential at –1.1 MPa improved germination and early seedling growth in coarse (Basra et al., 2006a) and fine rice (Basra et al., 2003, 2005b). Likewise, Lee et al. (1998c) found that osmopriming with –0.6 MPa polyethylene glycol improved the rate and final percentage of germination, and that rice seeds primed in this solution at 25◦ C for four days took lesser time from planting to 50% germination than that of untreated seeds (Lee et al., 1998c). 4 Mechanism of Rice Seed Priming 4.1 Physiological and Biochemical Basis Seed vigor enhancement initiates the very early stages of germination, but not those associated with radicle growth. Still, fundamental understanding of the physiological and biochemical mechanisms of priming as to how they affect seed germination is elusive. For priming to be generally successful, seed moisture content should be maintained at 40–45% on a fresh weight basis, or about 90–95% of the seed moisture content necessary for germination (Gray et al., 1990). Priming of seeds triggers changes in the activities of enzymes, leading to changes in the levels of germination substrates (Fig. 2). These are discussed for rice in the following subsections. 4.1.1 Enzymes Seeds represent a well-defined system as a sink, where resources are utilized for the production of seedlings. Rice seeds store starch, storage proteins, and a small amount of oils in the endosperm. Hydrolytic enzymes are mainly responsible for the hydrolysis of these reserves into the useable and readily available source of energy for embryo growth. Seed priming is reported to modulate enzymes of carbohydrate metabolism (Kaur et al., 2000, 2002), thereby increasing the available food for the growing embryo. Studies on rice showed that priming increases the activity of hydrolytic enzymes (Table 5) and counteracts the effects of lipid peroxidation. During priming, de novo synthesis of α-amylase has been documented (Lee and Kim 2000). The α-amylase activity is directly related to the metabolic activity, leading to higher vigor of the rice Rice Seed Invigoration 161 Seed priming Physiological and biochemical basis Breakdown of reserved food Activation of hydrolases and other enzymes Molecular Basis Metabolites biosynthesis Nucleic acid biosynthesis Protein biosynthesis Improved germination and stand establishment and economic yield Fig. 2 Mechanism of rice seed priming. Improvement in germination, stand establishment, and economic yield owing to seed priming can be explained on a physiological, biochemical, and molecular basis. Seed-priming techniques increase the activity of hydrolases and some other enzymes (including antioxidants under stress conditions), which enhance the breakdown of reserve food. Meanwhile, some other metabolites are also synthesized. Nucleic acids and protein biosynthesis are also enhanced by seed priming techniques seeds (Basra et al., 2005b; Farooq et al., 2006 k). Significantly higher and more rapid germination of osmoprimed rice seeds under low temperature (5◦ C) and salt (0.58% NaCl) stresses were observed. However, no significant changes in the activities of seed α-amylase and root system dehydrogenase were observed, while activities of seed β-amylase and shoot catalase were enhanced in low temperatures (He et al., 2002). Under salt stress, a significant increase in the activity of seed α-amylase, β-amylase, and root system dehydrogenase, and a moderate rise in the activity of shoot catalase occurred (He et al., 2002). Lee and Kim (2000), while investigating the effects of osmoconditioning and hardening on the germination of normal and naturally aged seeds, showed that the α-amylase activity of normal seeds was greater than the aged ones; the latter being more effective than the former. The α-amylase activity was positively correlated with the total sugars and germination rate. The increase in gibberellic acid concentration and the duration of exposure increased the α-amylase activity and seed germination (Vieira et al., 2002). Lanthanum ion has been found to be effective in modulating the activities of seed enzymes in rice. The influence of soaking in lanthanum salt on the germination and seedling growth of rice indicated that the range of 1–20 mg L–1 increased 162 Table 5 Effect of various seed priming treatments on some metabolic changes in rice seeds Rice type Variety/cultivar/genotype Improvement recorded over control Reference Soluble Sugars Hardening 24 h Hardening 24 h Hydropriming 48 h Ascorbate priming Osmohardening KCl Osmohardening CaCl2 Hardening 24 h Hydropriming 48 h Ascorbate priming Osmohardening KCl Osmohardening CaCl2 Hardening 24 h Coarse Fine Coarse Coarse Coarse Coarse Coarse Fine Fine Fine Fine Fine KS-282 Basmati-385 KS-282 KS-282 KS-282 KS-282 KS-282 Super-Basmati Super-Basmati Super-Basmati Super-Basmati Super-Basmati 9.18 mg g–1 fresh weight 7.88 mg g–1 fresh weight 2.11 mg g–1 fresh weight 4.53 mg g–1 fresh weight 9.21 mg g–1 fresh weight 6.94 mg g–1 fresh weight 6.25 mg g–1 fresh weight 3.23 mg g–1 fresh weight 2.11 mg g–1 fresh weight 4.0 mg g–1 fresh weight 6.56 mg g–1 fresh weight 4.97 mg g–1 fresh weight Basra et al. (2006b) Basra et al. (2005b) Farooq et al. (2006c) Farooq et al. (2006c) Farooq et al. (2006c) Farooq et al. (2006c) Farooq et al. (2006c) Farooq et al. (2006l) Farooq et al. (2006l) Farooq et al. (2006l) Farooq et al. (2006l) Farooq et al. (2006l) Reducing sugars Hydropriming 48 h Ascorbate priming Osmohardening KCl Osmohardening CaCl2 Hardening 24 h Coarse Coarse Coarse Coarse Coarse KS-282 KS-282 KS-282 KS-282 KS-282 0.26 mg g–1 0.36 mg g–1 0.44 mg g–1 0.40 mg g–1 0.30 mg g–1 Farooq et al. (2006k) Farooq et al. (2006k) Farooq et al. (2006k) Farooq et al. (2006k) Farooq et al. (2006k) α-amylase activity Hydropriming 48 h Ascorbate priming Coarse Coarse KS-282 KS-282 5.31 units∗ 5.78 units fresh weight fresh weight fresh weight fresh weight fresh weight Farooq et al. (2006k) Farooq et al. (2006k) M. Farooq et al. Seed priming treatment Rice Seed Invigoration Table 5 (continued) Seed priming treatment Rice type Variety/cultivar/genotype Improvement recorded over control Reference Osmohardening KCl Osmohardening CaCl2 Hardening 24 h Hardening 24 h Hardening 24 h Hydropriming 48 h Ascorbate priming Osmohardening KCl Osmohardening CaCl2 Hardening 24 h Hydropriming 48 h Ascorbate priming Osmohardening KCl Osmohardening CaCl2 Hardening 24 h Coarse Coarse Coarse Coarse Fine Coarse Coarse Coarse Coarse Coarse Fine Fine Fine Fine Fine KS-282 KS-282 KS-282 KS-282 Basmati-385 KS-282 KS-282 KS-282 KS-282 KS-282 Super-Basmati Super-Basmati Super-Basmati Super-Basmati Super-Basmati 6.60 units 5.80 units 5.4 units 6.60 units 8.05 units 4.20 units 4.10 units 6.40 units 5.30 units 4.31 units 4.00 units 3.56 units 4.25 units 5.52 units 3.94 units Farooq et al. (2006k) Farooq et al. (2006k) Farooq et al. (2006k) Basra et al. (2006b) Basra et al. (2005b) Farooq et al. (2006c) Farooq et al. (2006c) Farooq et al. (2006c) Farooq et al. (2006c) Farooq et al. (2006c) Farooq et al. (2006l) Farooq et al. (2006l) Farooq et al. (2006l) Farooq et al. (2006l) Farooq et al. (2006l) ∗ One unit of the enzyme’s activity is the amount of enzyme that released 1 µmol of maltose by 1 mL original enzyme solution in 1 minute. 163 164 M. Farooq et al. the vigor and proteinase, amylase, and lipase activities of seeds (Zhang et al. 2005). Promotion of germination in a highly dormant rice cultivar, Urucuia, was primarily related to an increase in α-amylase activity (Vieira et al., 2002). Likewise, indole acetic acid-soaked germinating rice seeds showed greater stimulation of α-amylase activity than gibberellic acid (Kim et al., 2006). In another study too, lanthanum nitrate enhanced the activities of α-amylase, proteinase, lipase, and other hydrolytic enzymes, and the contents of plant hormones such as indole acetic acid, gibberellic acid, and cytokinin, but abscisic acid contents changed a little (Fashui et al., 2003). Aged rice seed treated with lanthanum nitrate enhanced the respiratory rate and activities of superoxide dismutase, catalase, and peroxidase, and declined O2 – contents and plasma membrane permeability (Fashui, 2002). Rice seed treatment with increased concentrations of gibberellic acid enhanced the activities of superoxide dismutase and catalase by 37.9 and 22.8%, respectively (Deshpande et al., 2003). 4.1.2 Metabolites Carbohydrates constitute the major storage compounds in rice seeds. They are stored in the form of starch, which can not be consumed directly by the growing embryo. By the action of hydrolases, starch is converted into soluble sugars. Higher contents of soluble sugars are thus directly responsible for improved seed performance. In this context, the increased contents of total and reducing sugars and reduced nonreducing sugars in osmoprimed and hardened rice seeds are directly related to the activities of sugar hydrolyzing enzymes, as well as germination and seedling vigor (Table 5; Lee and Kim, 2000; Basra et al., 2005b, 2006b). Lee and Kim (1999) reported that optimum osmoconditioning of rice seed results in the disintegration of larger starch grains into tiny ones with the production of small holes in the starch granules and cavity between the embryo and endosperm. Higher levels of sucrose and lower levels of total soluble sugars and fructose were observed in the primed hybrid rice seeds. Significant negative correlations between the fructose content in primed seeds and the fructose and total soluble sugars contents in stressed seedlings were also found. Priming decreased fructose, and increased free proline contents in seeds and fructose content in seedlings, and thus improved the salt tolerance of seedlings (Ruan et al., 2003). Hormonal priming of seeds also increases the level of soluble sugars in the rice grains. Increased contents of soluble sugars were recorded in cytokinin (Deshpande et al., 2003), gibberellic acid, and to a greater extent in indole acetic acid-treated rice seeds (Kim et al., 2006). Osmolytes, including amino acids and derivatives, polyols and sugars, methylamines, and tertiary and quaternary ammonium compounds, are small low molecular weight organic solutes that are nontoxic and capable of maintaining cell and tissue water balance. All known osmolytes are compatible, do not perturb macromolecules, and more importantly stabilize membranes. Little work has been done on the changes in osmolyte levels in primed rice seeds. Reports show that a higher level of proline was observed in the primed seeds than in the control seeds, which also improved salt tolerance in hybrid rice seedlings (Ruan et al., 2003). In another Rice Seed Invigoration 165 study, Deshpande et al. (2003) recorded a 24.5% increase in free proline content over the control group in naphthalene acetic acid primed hybrid rice seeds. In nutshell, the production of sugars that can metabolize constitutes an important consequence of rice seed priming. These sugars provide a ready source of energy for the earlier production and establishment of seedlings. A build-up in the levels of free proline appears to be an important strategy, especially under suboptimal conditions. However, further studies are imperative on this aspect of seed priming in rice. 4.2 Molecular Basis Studies associated with protein and nucleic acid synthesis fail to discriminate between the physiological events occurring during priming and those consequent to germination (McDonald, 2000). Therefore, it is important that due consideration be given to the understanding of the molecular basis of seed priming in terms of functional genomics (transcriptomics, proteomics, and metabolomics). Protein expression, taking place after the transcript synthesis, is commonly used by proteomics researchers to denote the presence or abundance of one or more proteins in a particular cell or tissue. Such reports are virtually lacking in the case of rice seed priming, although quite a few studies are available in other plant species (Gallardo et al., 2001; Wahid et al., 2008). Anuradha and Rao (2001) reported that the improvement of salinity tolerance in rice by brassino steroid seed treatments was associated with enhanced levels of nucleic acids and soluble proteins. 5 Seed Priming and Dormancy Management The dormancy of seed results in arrested germination and various priming treatments have proven their worth in overcoming this physiological phenomenon. The dormancy breakdown can be accomplished by priming with salts, hormones, or other substances. Seed-germination tests of 18 accessions representing 16 rice species were conducted under a series of dormancy-breaking treatments, including hull removal, use of salts, hydrogen peroxide, and temperature regimes. These data revealed that (1) removal of the seed hull was extremely effective for breaking seed dormancy; (2) species responded differently to various temperature regimes, and no single regime was consistently effective in breaking seed dormancy in all species (although heat treatment generally promoted germination of the species); and (3) some species responded to certain chemical treatments effectively under the optimum temperature regimes. An appropriate combination of seed-hull removal, dry heat, or chemical treatments, and germination under the optimum temperature regimes for individual rice species provided the best results for breaking seed dormancy (Naredo et al., 1998). Among the plant hormones, gibberellins are well-known for breaking the dormancy of dormant seeds and improving the germination of recalcitrant seeds 166 M. Farooq et al. (Srivastava, 2002). Evaluation of gibberellic acid in breaking the seed dormancy of the highly dormant rice cultivar, Urucuia, subjected to predrying in a forced air circulation chamber (40◦ C) for seven days or soaked in 60 mg gibberellic acid L–1 concentrations at 30◦ C for 2, 24, or 36 h, revealed that all the treatments significantly reduced the dormancy of seed, which was tightly linked to an increase in α-amylase activity and seed germination. This further suggested that α-amylase activity is an efficient marker to study the seed dormancy in rice (Vieira et al., 2002). In essence, the seed dormancy in rice can be broken by employing a wide range of seed treatments. However, priming with gibberellic acid, inorganic salts (particularly KNO3 ), and thermal treatments, are more effective. Further work utilizing novel growth-promoting substances is imperative in rice. 6 Rice Seed Priming and Stress Tolerance As such, rice carries an odd portfolio of tolerances and susceptibilities to stresses compared to other crops. It thrives in waterlogged soil and can tolerate submergence at levels that would kill other crops. It is moderately tolerant of salinity and soil acidity, but highly sensitive to drought and cold even where rice response to stress is superior to other crops. However, many rice-growing environments demand still greater tolerance than is found in the improved germplasm (Lafitte et al., 2004). Like other crops, rice is affected by various environmental constraints. Available literature on these lines is reviewed below: 6.1 Drought Drought is generally avoided in irrigated rice-production systems, but it is a consistent feature across much of the 63.5 Mha of rainfed rice sown annually, most of which is in tropical Asia, Africa, and Latin America (Narciso and Hossain, 2002). In the newly introduced aerobic rice culture, the frequency and intensity of drought may increase manifold. Du and Tuong (2002), while testing the effectiveness of different osmotica to improve the performance of direct-seeded rice, noted that osmopriming with 14% potassium chloride solution and saturated calcium phosphate solution was successful in improving the seedling emergence, stand establishment, and yield under water-deficit conditions. Harris et al. (2002) reported that in drought-prone areas, primed rice seeds germinated well and seedlings emerged faster and more uniformly leading to increased yield. A germination trial of 11 varieties of upland rice under limited water conditions revealed early and synchronized emergence owing to seed priming (Harris and Jones, 1997). In summary, the priming of rice seeds might be a useful way for better seedling establishment under water-limited soil conditions (Lee et al., 1998a). Rice Seed Invigoration 167 6.2 Salinity Salt stress is a major debacle to cereal production worldwide. Rice is a salt-sensitive crop, but at the same time it is the only cereal that has been recommended as a desalinization crop because of its ability to grow well under flooded conditions. This is because the standing water in rice fields can help leach the salts from the topsoil to a level low enough for subsequent crops (Bhumbla and Abrol, 1978). Despite its high sensitivity to salinity, considerable variation in tolerance was observed in rice (Akbar et al., 1972; Flowers and Yeo, 1981). Various strategies can be used to minimize the effect of salinity on the germination of rice seed and emerging seedlings. For instance, addition of putrescine (0.01 mM) to NaCl solution (150 mM) can reduce net accumulation of sodium and chloride ions in seeds and increase water uptake. This suggests that putrescine can alleviate the adverse effects of sodium chloride salinity during the germination and early seedling growth of rice (Prakash and Prathapasenan, 1988). In their study, Kim et al. (2006) reported that although gibberellic acid and indole acetic acid improved the salt tolerance of dehulled rice seeds, indole acetic acid was more effective. Brassinosteroids can also be used to induce stress tolerance. For instance, rice-seed treatment with brassinosteroids can reverse the inhibitory effect of salinity on germination and seedling growth (Anuradha and Rao, 2001). Osmopriming with mixed salts also improved the salinity tolerance in rice (He et al., 2002; Ruan et al., 2003). The above information suggests that improvement for salt tolerance is feasible in rice by a variety of priming techniques. 6.3 Low Temperature The prevalence of low temperature at sowing results in poor rice-seed germination, seedling establishment, and vigor in several temperate rice-growing countries. Various strategies can be adopted to overcome the adversary of low temperature. For example, in a laboratory experiment, soaking rice seed in various concentrations of proline, betaine, putrescine, spermidine, and spermine increased germination and vigor at low temperatures. These compounds increased shoot growth by about 9 to 27% compared to growing the seedlings in water alone. Furthermore, the most effective concentrations to obtain an increase in shoot growth were 0.5, 2, 0.5, 0.05, and 0.05 mM for proline, betaine, putrescine, spermidine, and spermine, respectively (Naidu and Williams, 2004). Sasaki et al. (2005) noted that growth was promoted by hydrogen peroxide treatment under low temperature in a greenhouse. Soaking rice seeds in various combinations of indole acetic acid and glycinebetaine was also effective in inducing low-temperature tolerance in rice. Furthermore, the combined application of both was more effective in rice seed performance than their singular effects (Chen et al., 2005). In another study, sand priming improved the cold tolerance in direct-seeded rice (Zhang et al., 2006). He et al. (2002) also reported significantly higher and faster germination of osmoprimed rice seeds under 168 M. Farooq et al. low-temperature (5◦ C) stress. This suggested that application of osmoprotectants is more effective in inducing low-temperature tolerance during seed germination. 6.4 Submergence and Water Logging Excess water is a common constraint throughout the rain-fed rice production areas, such as South Asia and Southeast Asia and tropical Africa. Out of 40 Mha in Asia grown under rain-fed lowlands, about 15 Mha are frequently damaged by submergence (Huke and Huke, 1997). Submergence stress can also damage crops in irrigated areas due to high rainfall and/or impeded drainage, particularly early in the season. The annual average yield loss from submergence is estimated at about 80 kg ha–1 (Dey and Upadhaya, 1996). Although rice is adaptable to waterlogged conditions due to its capacity to develop aerenchyma, complete submergence can be lethal. Ruan et al. (2002b) reported improved seedling vigor index, seedling emergence, and stand establishment in flooded soil by osmopriming with calcium and sodium salts. Another study reported that rice-seed treatment with hydrogen peroxide can be used effectively to improve the submergence and flooding tolerance (Sasaki et al., 2005). To sum up, seed priming with inorganic salts, polyamines, osmoprotectants, plant growth regulators, and hydrogen peroxide in optimum concentrations can be effectively used to improve tolerance against different stresses, including salinity, low temperature, and waterlogging/submergence. However, to accustom the direct seedling rice without puddling, a better understanding of the phenomena involved in drought and high-temperature tolerance at seedling development are imperative. 7 Conclusion Seed invigoration tools have a great potential to improve the emergence and stand establishment under a wide range of field conditions. Among various techniques, osmopriming, osmohardening, hormonal priming and use of highly soluble and low molecular weight chemicals are of special consideration. These techniques can be effectively employed to enhance the crop performance under saline, submerged, and drought conditions. Seed invigoration techniques can also be employed to enhance the rice performance in direct-seeded cultures. There is a great variation in rice species, varieties/genotypes/hybrids and rice types regarding their responses to various priming treatments, which suggests that a lot of work still has to be done regarding the specific behavior of the rice material. Therefore, more precise invigoration techniques should be developed, using a range of salts, plant growth regulators, jasmonates, and osmolytes at varying concentration and for different durations. Optimal water potential, temperature range, and requirement for oxygenation should also be investigated. More research should focus on the use of commercial fertilizers as priming and seed-coating agents. Performance of invigorated seeds should be evaluated under a wide range of field conditions. These strategies should be employed Rice Seed Invigoration 169 to improve tolerance against biotic and abiotic stresses. Thermal treatments with alternate cycles of low and high temperature should be studied in detail. Prolonged storage of primed and hardened seeds may be another critical factor in the technology transfer and marketing of primed rice and other crops’ seeds. Therefore, more work should also be done to study the storage potential of primed seeds. Studies on the possibility of integration of different invigoration tools should be done. Mechanisms of rice seed priming, particularly related to enzymatic activities, should be revealed. Moreover, the regulation of α-amylase by calcium and potassium ions during the priming may need to be investigated. Stress tolerance during germination is important. Thus, the pathway of anti-oxidant biosynthesis should be investigated. Studies on functional genomics of seed priming may pay rich dividends. Although baseline information is available, functional analyses of individual genes and proteins associated with the improved performances in seed priming are becoming increasingly important. The regulation/expression of genes/proteins involved is highly imperative to assessing the seed priming-based invigoration. References Agboma P., Jones M.G.K., Peltonen-Sainio P., Rita H., Pehu E. (1997) Exogenous glycinebetaine enhances grain yield of maize, sorghum and wheat grown under two supplementary water regimes, J. Agron. Crop Sci. 178, 29–37. Akbar M., Yabuno T., Nakao S. (1972) Breeding for saline resistant varieties of rice I. Variability for salt tolerance among some rice varieties, Jpn. J. Breed. 22, 277–284. Anuradha S., Rao S.S.R. (2001) Effect of brassinosteroids on salinity stress induced inhibition of seed germination and seedling growth of rice (Oryza sativa L.), Plant Growth Regul. 33, 151–153. Austin R.B., Longden P.C., Hutchinson J. (1969) Some effects of hardening on carrot seed, Ann. Bot. 33, 883–895. Bakare S.O., Ukwungwu M.N., Fademi A.O., Harris D., Ochigbo A.A. (2005) Adoption study of seed priming technology in upland rice, Global Appr. Extension Prac. 1, 1–6. Balasubramanian V. Hill, J.E. (2002) Direct seeding of rice in Asia: emerging issues and strategic research needs for 21st century. In: Pandey S., Mortimer M., Wade L., Tuong T.P., Lopes K., Hardy B. (Eds.), Direct seeding: Research strategies and opportunities. International Research Institute, Manila, Philippines, pp: 15–39 Barkosky R.R., Einhelling F.A. (1993) Effects of salicylic acid on plant water relationship, J. Chem. Ecol. 19, 237–247. Basra A.S., Singh B., Malik C.P. (1994) Priming-induced changes in polyamine levels in relation to vigor of aged onion seeds, Bot. Bull. Acad. Sin. 35, 19–23. Basra S.M.A., Farooq M., Hafeez K., Ahmad N. (2004) Osmohardening: A new technique for rice seed invigoration, Int. Rice Res. Notes 29, 80–81. Basra S.M.A., Farooq M., Hussain M. (2005a) Influence of osmopriming on the germination and early seedling growth of coarse and fine rice, Pak. J. Seed Technol. 6, 33–42. Basra S.M.A., Farooq M., Khaliq A. (2003) Comparative study of pre-sowing seed enhancement treatments in fine rice (Oryza sativa L.). Pak. J. Life Soc. Sci. 1, 5–9. Basra S.M.A., Farooq M., Tabassum R. (2005b) Physiological and biochemical aspects of seed vigor enhancement treatments in fine rice (Oryza sativa L.), Seed Sci. Technol. 33, 623–628. Basra S.M.A., Farooq M., Tabassum R., Ahmed N. (2006b) Evaluation of seed vigor enhancement techniques on physiological and biochemical basis in coarse rice, Seed Sci. Technol. 34, 741–750. 170 M. Farooq et al. Basra S.M.A., Farooq M., Wahid A., Khan M.B. (2006a) Rice seed invigoration by hormonal and vitamin priming, Seed Sci. Technol. 34, 753–758. Basu R.N. (1994) An appraisal of research on wet and dry physiological seed treatments and their applicability with special reference to tropical and sub-tropical countries, Seed Sci. Technol. 22, 107–126. Beckman J.J., Moser L.E., Kubik K., Waller S.S. (1993) Big bluestem and switch grass establishment as influenced by seed priming. Agron. J. 85, 199–202. Bewley J.D. (1997) Seed germination and dormancy, Plant Cell 9, 1055–1 066. Bewley J.D., Black M. (1982) Physiology and Biochemistry of Seeds in Relation to Germination. Vol. 2, Viability, Dormancy and Environmental Control. Springer-Verlag, New York. Bhumbla D., Abrol I. (1978) Soils and Rice. International Rice Research Institute, Los Baños, Philippines. Borsani O., Valpuesta V., Botella M. (2001) Evidence for role of salicylic acid in the oxidative damage generated by NaCl and osmotic stress in Arabidopsis seedlings, Plant Physiol. 126, 1024–1030. Bouchereau A., Aziz A., Larher F., Tanguy M. (1999) Polyamines and environmental challenges: Recent development, Plant Sci. 140, 103–125. Bradford K.J. (1986) Manipulation of seed water relations via osmotic priming to improve germination under stress conditions, Hort Sci. 21, 1105–1112. Bradford K.J., Haigh A.M. (1994) Relationship between accumulated hydrothermal time during seed priming and subsequent seed germination rates, Seed Sci. Res. 4, 63–69. Bray C.M. (1995) Biochemical process during the osmopriming of seeds. In: Kigel J., Galili G., (Eds.), Seeds Development and Germination, Marcel Dekker, New York, pp. 767–789. Bray C.M., Davison P.A., Ashraf M., Taylor R.M. (1989) Biochemical changes during osmopriming of leek seeds, Ann. Bot. 63, 185–193. Brocklehurst P.A., Dearman J. (1983) Interactions between seed priming treatments and nine seed lots of carrot, celery and onion. I. laboratory germination, Ann. Appl. Biol. 102, 577–584. Brocklehurst P.A., Dearman J., Drew R.L.K. (1984) Effects of osmotic priming on seed germination and seedling growth in leek, Sci. Hort. 24, 201–210. Burgass R.W., Powell A.A. (1984) Evidence for repair processes in the invigoration of seeds by hydration, Ann. Appl. Biol. 53, 753–757. Chen D., Gunawardena T.A., Naidu B.P., Fukai S., Basnayake J. (2005) Seed treatment with gibberellic acid and glycinebetaine improves seedling emergence and seedling vigour of rice under low temperature, Seed Sci. Technol. 33, 471–479. Corbineau F., Côme D. (2006) Priming: A technique for improving seed quality, Seed Testing Int. 132, 38–40. Corbineau F., Picard M.A., Côme D. (1994) Germinability of leek seeds and its improvement by osmopriming, Acta Hort. 371, 45–52. Dadlani M., Seshu D.V. (1990) Effect of wet and dry heat treatment on rice seed germination and seedling vigor, Int. Rice Res. Newslet. 15, 21–22. Deshpande V.N., Waghmode B.D., Dalvi V.V., Vanave P.B. (2003) Naphthaleneacetic acid holds promise in hybrid rice seed production, Ind. J. Genet. Plant Breed. 63, 2, 157–158. Dey M., Upadhaya H. (1996) Yield loss due to drought, cold and submergence in Asia. In: Evenson R., Herdt R., Hossain M. (Eds.), Rice Research in Asia: Progress and Priorities, CAB International and IRRI: Wallingford, UK. Ding C.K., Wang C. (2003) The dual effects of methyl salicylate on ripening and expression of ethylene biosynthetic genes in tomato fruit, Plant Sci. 164, 589–596. Du L.V., Tuong T.P. (2002) Enhancing the performance of dry-seeded rice: effects of seed priming, seedling rate, and time of seedling. In: Pandey S., Mortimer M., Wade L., Tuong T.P., Lopes K., Hardy B., (Eds.) Direct seeding: Research strategies and opportunities, International Research Institute, Manila, Philippines, pp: 241–256. Farooq M., Basra S.M.A. (2005) Rice Cultivation by Seed priming, Daily Dawn, Lahore, Pakistan. August 28, 2005. Rice Seed Invigoration 171 Farooq M., Basra S.M.A., Afzal I., Khaliq A. (2006 g) Optimization of hydropriming techniques for rice seed invigoration, Seed Sci. Technol. 34, 507–512. Farooq M., Basra S.M.A., Ahmed N. (2005a) Rice seed priming, Int. Rice Res. Notes 30, 45–48. Farooq M., Basra S.M.A., Ahmad N. (2007a) Improving the performance of transplanted fine rice by seed priming, Plant Growth Regul. 51, 129–137. Farooq M., Basra S.M.A., Ahmad N., Warriach E.A. (2005d) Seed germination, seedling vigor and electrical conductivity of seed leachates in coarse and fine rice as affected by dry heat and chilling treatments, Euro-Asian J. Appl. Sci. 1, 18–35. Farooq M., Basra S.M.A., Cheema M.A. Afzal I. (2006b) Integration of pre-sowing soaking, chilling and heating treatments for vigor enhancement in rice (Oryza sativa L.), Seed Sci. Technol. 34, 499–506. Farooq M., Basra S.M.A., Hafeez K. (2006a) Seed invigoration by osmohardening in fine and coarse rice, Seed Sci. Technol. 34, 181–187. Farooq M., Basra S.M.A., Hafeez K., Ahmad N. (2005b) Thermal hardening: a new seed vigor enhancement tool in rice, J. Integr. Plant Biol. 47, 187–193. Farooq M., Basra S.M.A., Hafeez K., Ahmad N. (2005c) Use of commercial fertilizers as osmotica for rice priming, J. Agric. Soc. Sci. 1, 172–175. Farooq M., Basra S.M.A., Hafeez K., Warriach E.A. (2004b) Influence of high and low temperature treatments on the seed germination and seedling vigor of coarse and fine rice, Int. Rice Res. Notes 29, 75–77. Farooq M., Basra S.M.A., Karim H.A., Afzal, I. (2004a) Optimization of seed hardening techniques for rice seed invigoration, Emirates J. Agric. Sci. 16, 48–57. Farooq M., Basra S.M.A., Khalid M. Tabassum R., Mehmood T. (2006 k) Nutrient homeostasis, reserves metabolism and seedling vigor as affected by seed priming in coarse rice, Can. J. Bot. 84, 1196–1202. Farooq M., Basra S.M.A., Khan M.B. (2007b) Seed priming improves growth of nursery seedlings and yield of transplanted rice, Arch. Agron. Soil Sci. 53, 315– 326. Farooq M., Basra S.M.A., Rehman H. (2006e) Seed priming enhances emergence yield and quality of direct seeded rice, Int. Rice Res. Notes 31, 42–44. Farooq M., Basra S.M.A., Rehman H. (2008) Seed priming with polyamines improve the germination and early seedling growth in fine rice, J. New Seeds 9, 145–155. Farooq M. Basra S.M.A., Rehman H., Hussain M., Amanat Y. (2007c) Pre-sowing salicylate seed treatments improve the germination and early seedling growth in fine rice, Pak. J. Agric. Sci. 44, 16–23. Farooq M., Basra S.M.A., Rehman H., Mehmood T. (2006f) Germination and early seedling growth as affected by pre-sowing ethanol seed treatments in fine rice, Int. J. Agric. Biol. 8, 19–22. Farooq M., Basra S.M.A., Saleem B.A. (2006d) Direct seeding method popular among rice farmer, Daily Dawn, Lahore, Pakistan. June 05, 2006. Farooq M., Basra S.M.A., Saleem B.A. (2006h) Integrated rice growing system, Daily Dawn, Lahore, Pakistan. August 07, 2006. Farooq M., Basra S.M.A., Saleem B.A. (2006i) System of rice intensification: a beneficial option, Daily Dawn, Lahore, Pakistan. September 04, 2006. Farooq M., Basra S.M.A., Saleem B.A. (2006j) Integrated weed management in rice. Daily Dawn, Lahore, Pakistan. July 24, 2006. Farooq M., Basra S.M.A., Tabassum R., Afzal I. (2006l) Enhancing the performance of direct seeded fine rice by seed priming, Plant Prod. Sci., 9, 446–456. Farooq M., Basra S.M.A., Wahid A. (2006c) Priming of field-sown rice seed enhances germination, seedling establishment, allometry and yield, Plant Growth Regul. 49, 285–294. Fashui H. (2002) Study on the mechanism of cerium nitrate effects on germination of aged rice seed, Biol. Trace Element Res. 87, 191–200. Fashui H., Ling W., Chao L. (2003) Study of lanthanum on seed germination and growth of rice, Biol. Trace Element Res. 94, 273–286. 172 M. Farooq et al. Finnerty T.L., Zajicek J.M., Hussey M.A. (1992) Use of seed priming to bypass stratification requirements of three Aquilegia species, Hort Sci. 27, 310–313. Flematti G.R., Ghisalberti E.L., Dixon K.W., Trengove R.D. (2004) A compound from smoke that promotes seed germination, Science 305, 977. Flowers T., Yeo A. (1981) Variability in the resistance of sodium chloride salinity within rice (Oryza sativa L.) varieties, New Phytol. 88, 363–373. Fourest E., Rehms L.D., Sands D.C., Bjarko M., Lund R.E. (1990) Eradication of Xanthomonos campestris pv translucens from barley seed with dry heat treatment, Plant Dis. 74, 816–818. Gallardo K., Job C., Groot S.P.C., Puype M., Demol H., Vandekerckhove J., Job D. (2001) Proteomic analysis of Arabidopsis seed germination and priming, Plant Physiol. 126, 835–848. Gleick P.H. (1993) Water crisis: a guide to the world’s freshwater resources. Pacific Institute for Studies in Development, Environment, and Security. Stockholm Environment Institute.Oxford University Press, New York (USA). Gray D., Steckel J.R.A., Hands L.J. (1990) Responses of vegetable seeds to controlled hydration. Ann. Bot. 66, 227–235. Guedes A.C., Cantliffe D.J. (1980) Germination of lettuce seeds at high temperatures after seed priming, J. Amer. Soc. Hort. Sci., 105, 777–781. Hardegree S.P., Emmerich W.E. (1992a) Effect of matric-priming duration and priming water potential on germination of four grasses, J. Exp. Bot. 43, 233–238. Hardegree S.P., Emmerich W.E. (1992b) Seed germination response of four south-western range grasses to equilibration at sub-germination matric-potentials, Agron. J. 84, 994–998. Harris D. (2006) Development and testing of on-farm seed priming, Adv. Agron. 90, 129–178. Harris D., Jones M. (1997) On-farm seed priming to accelerate germination in rainfed, dry-seeded rice, Inter. Rice Res. Notes 22, 30. Harris D., Joshi A., Khan P.A., Gothkar P., Sodhi P.S. (1999) On-farm seed priming in semi-arid agriculture: Development and evaluation in maize, rice and chickpea in India using participatory methods, Exp. Agric. 35, 15–29. Harris D., Tripathi R.S., Joshi A. (2000) On-farm seed priming to improve crop establishment and yield in dry direct-seeded rice. Proc. Int. Workshop on Dry-seeded Rice Technol. 25–28 January, Bangkok, Thailand. Harris D., Tripathi R.S., Joshi A. (2002) On-farm seed priming to improve crop establishment and yield in dry direct-seeded rice. In: Pandey S., Mortimer M., Wade L., Tuong T.P., Lopes K., Hardy B., (Eds.), Direct seeding: Research strategies and opportunities. International Research Institute, Manila, Philippines, pp: 231–240. He C.Z., Hu J., Zhu Z.Y., Ruan S.L., Song W.J. (2002) Effect of seed priming with mixed- salt solution on germination and physiological characteristics of seedling in rice (Oryza sativa L.) under stress conditions, J. Zhejiang Univ. (Agric. Life Sci.), 28, 175–178. Heydecker W. (1977) Stress and seed germination: an agronomic view. In: Khan A.A., (Ed.), The physiology and biochemistry of seed dormancy and germination, Elsevier/North-Holland Biomedical Press, Amsterdam, pp. 237–282. Heydecker W., Coolbear P. (1977) Seed treatments for improved performance – survey and attempted prognosis, Seed Sci. Technol. 5, 353–425. Heydecker W., Higgins J., Turner Y.J. (1975) Invigoration of seeds. Seed Sci. Technol. 3, 881–888. Hu J., Zhu Z.Y., Song W.J., Wang J.C., Hu W.M. (2005) Effects of sand priming on germination and field performance in direct-sown rice (Oryza sativa L.), Seed Sci. Technol. 33, 243–248. Huaqi W., Bouman B.A.M., Zhao D., Changgui W., Moya P.F. (2002) Aerobic rice in northern China: Opportunities and challenges. Paper presented at the workshop on water-wise rice production, 8–11 April 2002 at IRRI headquarters in Los Baños, Philippines. Huke R., Huke E. (1997) Rice area by type of culture: South, Southeast and East Asia. A revised and updated database. International Rice Research Institute, Los Baños, Philippines. Jeong Y.O., Cho J.L., Kang S.M. (1994) Priming effect of pepper (Casicum annum L.) as affected by aging and growth regulators treatments, J. Kor. Soc. Hort. Sci. 35, 407–414. Rice Seed Invigoration 173 Johnson S.E., Lauren J.G., Welch R.M., Duxbury J.M. (2005) A comparison of the effects of micronutrient seed priming and soil fertilization on the mineral nutrition of chickpea (Cicer arietinum), lentil (Lens culinaris), rice (Oryza sativa) and wheat (Triticum aestivum) in Nepal, Exp. Agric. 41, 427–448. Joule J.A., Mills K. (2000) Heterocyclic Chemistry. 4th ed. Blackwell Science Publishing, Oxford, UK. Kalita U., Suhrawardy J., Das J.R. (2002) Effect of seed priming with potassium salt and potassium levels on growth and yield of direct seeded summer rice (Oryza sativa L.) under rainfed upland condition, Ind. J. Hill Farm. 15, 50–53. Kang H.M., Saltveit M. (2002) Chilling tolerance of maize, cucumber and rice seedling leaves and roots are differentially affected by salicylic acid, Physiol. Plant 115, 571–576. Karssen C.M., Haigh A., Toorn P., Weges R. (1989) Physiological mechanisms involved in seed priming. In: Taylorson R.B. (Ed.), Recent Advances in the Development and Germination of Seeds, Plenum Press, New York, pp: 269–280. Kaur S., Gupta A.K., Kaur N. (2000) Effect of GA3 , kinetin and indole acetic acid on carbohydrate metabolism in chickpea seedlings germinating under water stress, Plant Growth Regul. 30, 61–70. Kaur S., Gupta A.K., Kaur N. (2002) Effect of osmo- and hydropriming of chickpea seeds on seedling growth and carbohydrate metabolism under water deficit stress, Plant Growth Regul. 37, 17–22. Khan A.A. (1992) Pre-plant physiological conditioning, Hort. Rev. 13, 131–181. Kim J.H., Lee S.C., Song D.S. (1989) Morpho-physiological studies on elongation of mesocotyl and seminal root in rice plant, Kor. J. Crop Sci. 34, 325–330. Kim J.K., Lee M.H., Oh Y.J. (1993) Effect of gibberellin seed-spray on seedling emergence and growth in dry-seeded rice, Kor. J. Crop Sci. 38, 297–303. Kim S.K., Son T.K., Park S.Y., Lee I.J., Lee B.H., Kim H.Y., Lee S.C. (2006) Influences of gibberellin and auxin on endogenous plant hormone and starch mobilization during rice seed germination under salt stress, J. Environ. Biol. 27, 181–186. Kulkarni M.G., Sparg S.G., Light M.E., Van Staden J. (2006) Stimulation of rice (Oryza sativa L.) seedling vigour by smoke-water and butenolide, J. Agron. Crop Sci. 192, 395–398. Lafitte H.R., Ismail A.M., Bennett J. (2004) Abiotic stress tolerance in rice for Asia: progress and the future. New directions for a diverse planet: Proceedings of the 4th International Crop Science Congress Brisbane, Australia, Sep. 26–Oct. 1, 2004. Lee S.S., Kim J.H. (1999) Morphological change, sugar content, and α-amylase activity of rice seeds under various priming conditions, Kor. J. Crop Sci. 44, 138–142. Lee S.S., Kim J.H. (2000) Total sugars, α-amylase activity, and germination after priming of normal and aged rice seeds, Kor. J. Crop Sci. 45, 108–111. Lee S.S., Kim J.H., Hong S.B. (1999) Effect of priming and growth regulator treatments of seed on emergence and seedling growth of rice, Kor. J. Crop Sci. 44, 134–137. Lee S.S., Kim J.H., Hong S.B., Kim M.K., Park E.H. (1998c) Optimum water potential, temperature, and duration for priming of rice seeds, Kor. J. Crop Sci. 43, 1–5. Lee S.S., Kim J.H., Hong S.B., Yun S.H. (1998a) Effect of humidification and hardening treatment on seed germination of rice, Kor. J. Crop Sci. 43, 157–160. Lee S.S., Kim J.H., Hong S.B., Yun S.H., Park E.H. (1998b) Priming effect of rice seeds on seedling establishment under adverse soil conditions, Kor. J. Crop Sci. 43, 194–198. Lee S.Y., Lee J.H., Kwon, T.O. (2002) Varietal differences in seed germination and seedling vigor of Korean rice varieties following dry heat treatments, Seed Sci. Technol. 30, 311–321. Mathew J., Mohanasarida K., Resmi O.N. (2004) Addressing water scarcity in dry seeded lowland rice, New directions for a diverse planet: Proceedings of the 4th International Crop Science Congress Brisbane, Australia, 26 Sep–1 Oct 2004. McDonald M.B. (2000) Seed priming. In: Black M., Bewley J.D., (Eds.), Seed Technology and Its Biological Basis. Sheffield Acad. Press, Sheffield, England. 174 M. Farooq et al. Mickelbart M.V., Peel G., Joly R.J., Rodes D., Ejeta G. (2003) Development and Characterization of near-isogenic lines of sorghum segregating for glycinebetaine accumulation, Physiol. Plant 118, 253–261. Miyoshi K., Sato T. (1997a) The effects of kinetin and gibberellin on the germination of dehusked seeds of indica and japonica rice (Oryza sativa L.) under anaerobic and aerobic conditions, Ann. Bot. 80, 479–483. Miyoshi K., Sato T. (1997b) The effects of ethanol on the germination of seeds of Japonica and Indica rice (Oryza sativa L.) under anaerobic and aerobic conditions, Ann. Bot. 79, 391–395. Mohanasarida K., Mathew J. (2005) Effect of seed hardening on growth and yield attributes and yield of semi-dry rice, Res. Crops 6, 26–28. Musa A.M., Johansen C., Kumar J., Harris D. (1999) Response of chickpea to seed priming in the high barid tract of Bangladesh, Inter. Chickpea Newslett. 6, 20–22. Naidu B.P., Williams R. (2004) Seed treatment and foliar application of osmoprotectants to increase crop establishment and cold tolerance at flowering in rice. Report for the Rural Industries Research and Development Corporation. RIRDC Publication No, 04/004. Nakagawa A., Yamaguchi T. (1989) Seed treatments for control of seed-borne Fusarium roseum on wheat, Jpn. Apric. Res. Quater. 23, 94–99. Narciso J., Hossain M. (2002) World Rice Statistics. International Rice Research Institute, Los Baños, Philippines. Naredo M.E.B., Juliano A.B., Lu B.R., De Guzman F., Jackson M.T. (1998) Responses to seed dormancy-breaking treatments in rice species (Oryza L.), Seed Sci. Technol. 26, 675–689. Ota Y., Nakayama M. (1970) Effects of seed coating with calcium peroxide on germination under submerged conditions in rice plant, Proc. Crop Soc. Jpn. 39, 535–536. Pen Aloza A.P.S., Eira M.T.S. (1993) Hydration- dehydration treatments on tomato seeds (Lycopersicon esculentum Mill), Seed Sci. Technol. 21, 309–316. Perl M., Feder Z. (1981) Improved seedling development of pepper seeds (Capsicum annum) by seed treatment for pre-germination activities, Seed Sci. Technol. 9, 655–663. Pill W.G., Necker A.D. (2001) The effects of seed treatments on germination and establishment of Kentucky bluegrass (Poa pratensis L.), Seed Sci. Technol. 29, 65–72. Prakash L., Prathapasenan G. (1988) Putrescine reduces NaCl-induced inhibition of germination and early seedling growth of rice (Oryza sativa L.), Aust. J. Plant Physiol. 15, 761–767. Raskin I. (1992) Role of salicylic acid in plants, Ann. Rev. Plant Physiol. Plant Mol. Biol. 43, 439–463. Ross C., Bell R.W., White P.F. (2000) Phosphorus seed coating and soaking for improving seedling growth of Oryza sativa (rice) cv. IR66, Seed Sci. Technol. 28, 391–401. Ruan S., Xue Q., Tylkowska K. (2002a) Effects of seed priming on germination and health of rice (Oryza sativa L.) seeds, Seed Sci. Technol. 30, 451–458. Ruan S., Xue Q., Tylkowska K. (2002b) The influence of priming on germination of rice (Oryza sativa L.) seeds and seedling emergence and performance in flooded soils, Seed Sci. Technol. 30, 61–67. Ruan S.L., Zhong X.Q., Hua W.Q., Ruan S.L., Xue Q.Z., Wang Q.H. (2003) Physiological effects of seed priming on salt-tolerance of seedlings in hybrid rice (Oryza sativa L.), Sci. Agric. Sin. 36, 463–468. Sarma S., Dey S.C., Choudhuri A.K. (1993) Influence of seed priming and antitranspirant on physiological parameters in rice (Oryza sativa L.), Neo Bot. 1, 1–2. Sasaki K., Kishitani S., Abe F., Sato T. (2005) Promotion of seedling growth of seeds of rice (Oryza sativa L. cv. Hitomebore) by treatment with H2 O2 before sowing, Plant Prod. Sci. 8, 509–514. Song W.J., Hu J., Qiu J. Geng H.Y., Wang R.M. (2005) Primary study on the development of special seed coating agents and their application in rice (Oryza sativa L.) cultivated by direct seeding, J. Zhejiang Uni. (Agric. Life Sci.) 31, 368–373. Soon K.J., Whan C.Y., Gu S.B., Kil A.C., Lai C.J. (2000) Effect of hydropriming to enhance the germination of gourd seeds, J. Kor. Soc. Hort. Sci. 41, 559–564. Rice Seed Invigoration 175 Srivastava L.M. (2002) Plant Growth and Development: Hormones and Environment. Academic Press, London. Szalai G., Tari I., Janda T., Pestenacz A., Páldi E. (2000) Effects of cold acclimation and salicylic acid on changes in ACC and MACC contents in maize during chilling, Biol. Plant. 43, 637–640. Taiz L., Zeiger E. (2006) Plant Physiology, 4th ed. Sinaur Associates Inc. Massachusetts. Taylor A.G., Allen P.S., Bennett M.A., Bradford J.K., Burris J.S., Misra M.K. (1998) Seed enhancements, Seed Sci. Res. 8, 245–256. Taylorson R.B., Hendricks S.B. (1979) Overcoming dormancy in seeds with ethanol and other anesthetics, Planta 145, 507–510. Thornton J.M., Powell A.A. (1992) Short-term aerated hydration for the improvement of seed quality in Brassica oleracea, Seed Sci. Res. 2, 41–49. Vieira A.R., Vieira M.G., Fraga A.C., Oliveira J.A., Santos C.D., Vieira G.G., Santos C.D. (2002) Action of gibberellic acid (GA3 ) on dormancy and activity of alpha-amylase in rice seeds, Rev. Bras. Sementes 24, 43–48. Wahid A., Sehar S., Perveen M., Gelani S., Basra S.M.A., Farooq M. (2008) Seed pretreatment with hydrogen peroxide improves heat tolerance in maize at germination and seedling growth stages. Seed Sci. Technol. 36, 663–645. Watson M.B., Malmberg R.L. (1998) Arginine decarboxylase (polyamine synthesis) mutants of Arabidopsis thaliana exhibit altered root growth, Plant J. 13, 231–239. Welbaum G.E., Shen Z., Oluoch M.O., Jett L.W. (1998) The evolution and effects of priming vegetable seeds, Seed Technol. 20, 209–235. Yamauchi M. (2002) Reducing floating rice seedlings in wet direct sowing by increasing specific gravity of seeds with iron powder coating. Jpn. J. Crop Sci. 71, 150–151. (In Japanese.) Zhang J., Liu D., Huang Y., Liu X. (2005) Effects of seed soaking with La3+ on seed germination and seedling growth of rice, Chin. J. Ecol. 24, 893–896. Zhang S., Hu J.; Liu N.; Zhu Z. (2006) Pre-sowing seed hydration treatment enhances the cold tolerance of direct-sown rice, Seed Sci. Technol. 34, 593–601. Zhang X.G. (1990) Physiochemical treatments to break dormancy in rice, Int. Rice Res. Newslett. 15, 22.