Impact of UV-B irradiation on photosynthetic performance and chloroplast membrane components in Oryza sativa L.

The impact of UV-B radiation on photosynthetic related parameters was studied in Oryza sativa L. cv. Safari plants, after an UV-B irradiation performed 1h per day for 7days (between 8 and 14days after germination) with a ten narrow-band (λ 311nm) that resulted in a total biological effective UV-B (UVB(BE)) of 2.975kJm(-2)day(-1) and a total of 20.825kJm(-2). Gas exchange measurements were severely affected, showing reductions higher than 80% in net photosynthesis (P(n)), stomatal conductance and photosynthetic capacity (A(max)), 1day after the end of the 7-days UV-B treatment. Similarly, several fluorescence parameters (F(o), F(v)/F(m), Fv'/Fm', ϕ(e), q(P) and q(E)) and thylakoid electron transport (involving both photosystems) were also severely reduced. Concomitantly, a decline of xanthophylls, carotenes, Chl a, Chl (a+b) and Chl (a/b) values was accompanied by the increase of the lipoperoxidation level in chloroplast membranes, altogether reflecting a loss of protection against oxidative stress. Seven days after of the end of UV-B treatment, most fluorescence parameters recovered, but in P(n), A(max), thylakoid electron transport rates, Chl a and lipid classes, as well as the level of lipoperoxidation, the impacts were even stronger than immediately after the end of stress, denoting a clear loss of performance of photosynthetic structures. However, only a moderate impact on total lipids was observed, accompanied by some changes in the relative weight of the major chloroplast membrane lipid classes, with emphasis on the decrease of MGDG and the increase of phospholipids. That suggested an ability to de novo lipid synthesis allowing qualitative changes in the lipid matrix. Notably, the leaves developed after the end of UV-B irradiation showed a much lower impact, with significantly decreased values only in P(n) and g(s), rises in several fluorescence parameters, thylakoid electron transport, photosynthetic pigments (xanthophylls and chls) and DEPS, while lipid classes presented values close to control. The results showed a global impact of UV-B in the photosynthetic structures and performance in irradiated leaves, but revealed also a low impairment extent in the leaves entirely developed after the end of the irradiation, reflecting a remarkable recovery of the plant after the end of stress, what could constitute an advantage under occasional UV-B exposure events in this vital worldwide staple food crop.