Effects of salicylic acid on photosynthetic activity and chloroplast morphology under light and prolonged darkness

Salicylic acid (SA) is a key component of plant defence, which exerts a concentration-dependent effect on photosynthesis under multi-faceted influence of light. Photosynthetic activities and chloroplast morphology were studied in tomato plants after treatment with a sublethal, 0.1 mM, and a cell death-inducing, 1 mM concentrations of SA under normal photoperiod during light phase and after a prolonged dark phase. SA (1 mM) decreased the maximal (Fv/Fm) and effective quantum yields of PSII [Y(II)] and PSI [Y(I)] under both environmental conditions, however, the photoprotective processes were not significantly different between light and dark samples. Decrease in grana height, thylakoid dilation and deformation of lumen were also observed in the light. In contrast to illuminated samples, 0.1 mM SA decreased Y(II) and Y(I) after dark incubation, but nonphotochemical energy dissipation and cyclic electron flow increased, suggesting that the photoprotective mechanisms could be activated in plants exposed to prolonged darkness.