Effects of salicylic acid on the photosystem 2 of barley seedlings under osmotic stress

The effects of exogenous salicylic acid (SA) on photosystem 2 (PS 2) in barley (Hordeum vulgare L.) seedlings were investigated. SA pretreatment provided protection against subsequent osmotic stress. The highest protective effect of 0.25 mM SA was confirmed by determination of chlorophyll fluorescence, electrolyte leakage, malonyldialdehyde contents, PS 2 mRNAs and proteins. SA pretreatment increased reactive oxygen species (ROS), decreased net photosynthetic rate and stomatal conductance immediately, but prevented ROS accumulation during subsequent osmotic stress by activating antioxidant enzymes. Elimination of H₂O₂ during SA pretreatment inhibited almost all above mentioned SA effects. Therefore, SA pretreatment enhanced osmotic stress tolerance in barley seedlings mainly through ROS signals, rather than SA itself. The only SA-dependent and ROS-independent effect of exogenous SA on PS 2 was reduction of non-photochemical quenching.