Salicylic acid alleviates decreases in photosynthesis under heat stress and accelerates recovery in grapevine leaves

Background: Although the effect of salicylic acid (SA) on photosynthesis of plants including grapevines has been investigated, very little is yet known about the effects of SA on carbon assimilation and several components of PSII electron transport (donor side, reaction center and acceptor side). In this study, the impact of SA pretreatment on photosynthesis was evaluated in the leaves of young grapevines before heat stress (25 degrees C), during heat stress (43 degrees C for 5 h), and through the following recovery period (25 degrees C). Photosynthetic measures included gas exchange parameters, PSII electron transport, energy dissipation, and Rubisco activation state. The levels of heat shock proteins (HSPs) in the chloroplast were also investigated. Results: SA did not significantly (P < 0.05) influence the net photosynthesis rate (P-n) of leaves before heat stress. But, SA did alleviate declines in P-n and Rubisco activition state, and did not alter negative changes in PSII parameters (donor side, acceptor side and reaction center QA) under heat stress. Following heat treatment, the recovery of P-n in SA-treated leaves was accelerated compared with the control (H2O-treated) leaves, and, donor and acceptor parameters of PSII in SA-treated leaves recovered to normal levels more rapidly than in the controls. Rubisco, however, was not significantly (P < 0.05) influenced by SA. Before heat stress, SA did not affect level of HSP 21, but the HSP21 immune signal increased in both SA-treated and control leaves during heat stress. During the recovery period, HSP21 levels remained high through the end of the experiment in the SA-treated leaves, but decreased in controls. Conclusion: SA pretreatment alleviated the heat stress induced decrease in P-n mainly through maintaining higher Rubisco activition state, and it accelerated the recovery of P-n mainly through effects on PSII function. These effects of SA may be related in part to enhanced levels of HSP21.