Responses of photosystem II photochemistry and the alternative oxidase pathway to heat stress in grape leaves

To study the photochemical responses of photosystem II (PSII) and mitochondrial alternative oxidase (AOX) to heat stress in grapevine (Vitis vinifera L. cv. ‘Cabernet Sauvignon’), we exposed grape leaves to a range of temperatures (25, 30, 35, 40, and 42.5 °C) in the light (600 µmol m−2 s−1). At temperatures >35 °C, the rate of photosynthetic O2 evolution and the maximal photochemistry efficiency of PSII (Fv/Fm) decreased significantly. We observed that the relative variable fluorescence increased at the J step, and there was also an accumulation of H2O2. In addition, there was an increase in both the non-photochemical quenching (NPQ) and the capacity of the alternative oxidase pathway (AOX) at higher temperatures. When the leaves were treated with the AOX inhibitor salicylhydroxamic acid, a significant decrease in the PSII electron transport rate and NPQ and an increase in H2O2 content occurred. Our results suggest that high temperature limits electron transport beyond QA−, resulting in a strong accumulation of H2O2 in grape leaves. Inhibition of the AOX pathway increases the formation and accumulation of H2O2 in grape leaves, and may thereby increase photoinhibition in leaves experiencing heat stress.