Treatment with salicylic acid decreases the effects of paraquat on photosynthesis

The present study investigated the possible mediatory role of salicylic acid (SA) in paraquat (Pq) toxicity on photosynthesis. Twelve-day-old barley seedlings were supplied with 500 μmol/L salicylic acid (SA) or 10 μmol/L paraquat (Pq) via the transpiration stream and kept in the dark for 24 h. Then they were exposed to 100 μmol m−2 s−1 PAR and samples were taken 1,2,3, and 6 h after the light exposure. Leaf gas exchange parameters, the activity of ribulose-1,5 bisphosphate carboxylase (RUBPC, EC 4.1.1.39) and of the photorespiratory enzymes phosphoglycolate phosphatase (PGP, EC 3.1.3.18), glycolate oxidase (GO, EC 1.1.3.1), and catalase (EC 1.11.1.6) were determined. Treatment of seedlings with SA alone resulted in decreased levels of chlorophyll, CO2 assimilation and transpiration rates. Pq treatment led to a decrease in chlorophyll content and to a very strong inhibition of photosynthesis, the effects were manifested by 1 h of illumination. Neither SA nor Pq caused changes in RUBPC activity, but instead greatly increased the activity of the photorespiratory enzymes. Pre-treatment of seedlings with SA provided protection against subsequent Pq-induced stress. This observation was confirmed by gas exchange parameters, chlorophyll content and by changes in some important parameters associated with oxidative stress, namely H2O2 production, lipid peroxidation and electrolyte leakage. Pre-treatment with SA fully blocked the inhibitory effect of Pq on photosynthesis and reduced Pq-induced H2O2 production, lipid peroxidation, and electrolyte leakage. The data suggest that SA might protect cells against oxidative damage and prevent photosynthesis upon paraquat treatment.