ABSTRACT As photosynthesis is already known to be affected under various metal stresses, the aim of this study is to investigate uranium effects on photosynthetic parameters. Therefore, 18-day-old Arabidopsis thaliana seedlings were exposed to 50 μM uranium during 1–96 h. Uranium uptake, effects on growth parameters of roots and leaves and further responses on photosynthesis, pigment concentrations and lipid peroxidation in leaves were investigated. Uranium was highly taken up by the roots (50,352 ± 3383 μg g−1 DW at 96 h) causing complete growth arrest of the plants. Although uranium concentrations in the leaves remained low (15.0 ± 4.0 μg g−1 DW at 96 h), a remarkable photosynthetic response mechanism was observed. By chlorophyll fluorescence measurements it was observed that the potential photosynthetic efficiency (Fv/Fm) remained maximal while the effective efficiency of photosystem II (φPSII), which is a measure for the proportion of light absorbed by PSII used in photochemistry, even increased due to a decrease in non-photochemical quenching (NPQ), which indicates the conversion of excess energy into heat, but no alterations in non-regulated energy dissipation (NO). When measuring rapid light curves (RLC), giving the increase of the electron transport rate as function of irradiance, no differences were observed for the maximal electron transport rate (ETRmax) but an increase in α, representing the photosynthetic rate in the light-limited region of the RLC, was observed under uranium stress. We concluded that plant leaves start increasing their photosynthetic efficiency and decreasing their non-photochemical quenching under uranium stress.
