Importance of the mitochondrial alternative oxidase (AOX) pathway in alleviating photoinhibition in cucumber leaves under chilling injury and subsequent recovery when leaves are subjected to high light intensity

Abstract

ABSTRACTThe aim of this study was to assess the impact of the mitochondrial alternative oxidase (AOX) pathway on energy metabolism in chloroplast, and evaluate the importance of the AOX pathway in alleviating photoinhibition in cucumber (Cucumis sativus L.) leaves during chilling and the subsequent recovery period. Chilling (12°C) induced up-regultaion of the AOX pathway, which is important for chilled leaves to avoid overreduction of PSII and maintain photosynthetic electron transport. High light (200–300 µmol m−2 s−1) induced the decreases of maximal quantum yield of PSII photochemistry (Fv/Fm) and nonphotochemical quenching (NPQ), which was aggravated by 1 mM salicylhydroxamic acid (SHAM; AOX inhibitor), in cucumber leaves during chilling. However, Fv/Fm did not change and NPQ increased significantly in SHAM-treated leaves by chilling under low light. During recovery periods after chilling under low light, Fv/Fm, qP, ΦPSII, NPQ, and Fv’/Fm’ could return to normal degree in chilled leaves with or without SHAM. In contrast, high light induced more decrease of chlorophyll fluorescence parameters in chilled leaves during the recovery period, and SHAM treatment further aggravated the decrease. These results implied that up-regulation of AOX pathway in chilled leaves plays a critical role in protection against photoinhibition in leaves under excess light energy condition during chilling and the subsequent recovery period. However, thermal energy dissipation was more important to prevent photoinhibition in leaves exposed to chilling under low light.