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24-epibrassinolide alleviate drought-induced photoinhibition in Capsicum annuum via up-regulation of AOX pathway

Hu, Wen-hai, Yan, Xiao-hong, He, Yan, Xi, Rei
Scientia horticulturae 2019 v.243 pp. 484-489
Capsicum annuum, NADP (coenzyme), chlorophyll, chloroplasts, drought, leaves, malate dehydrogenase (NADP), malates, oxaloacetic acid, pepper, photochemistry, photoinhibition, photosynthetic electron transport, photosystem II, polyethylene glycol, water content
The aim of this study was to explore whether application of BRs could alleviate drought-induced photoinhibition in pepper leaves by improving the photosynthetic electron transport via up-regulation of alternative oxidase (AOX) pathway, we examined the effects of 24-epibrassinolide (EBR, 0.1 μM) and salicylhydroxamic acid (SHAM, 1 mM) on the AOX pathway capacity, matale/oxaloacetate (OAA) shuttle, and chlorophyll fluorescence in pepper (Capsicum annuum L.) leaves under drought condition (treated by 15% polyethylene glycol, PEG). In this study, the leaf relative water content (LRWC) decreased from 92.0% to 47.0% by drought treatment for 4 d. Drought induced the up-regulation of AOX pathway and increased the activities of NADP-malate dehydrogenase (MDH) and NAD-MDH. Inhibition of AOX pathway decreased NADP-MDH and NAD-MDH activities and enhanced the increases of NADPH and NADPH/NADP+ ratio in leaves under drought condition. Meanwhile, inhibition of AOX pathway aggravated the decreases of maximal quantum efficiency of PSII (Fv/Fm), quantum efficiency of PSII (ФPSII), photochemical quenching coefficient (qP), and photosynthetic electron transport rate (ETR) in leaves under drought condition. These results indicated that up-regulation of AOX pathway could activate malate/OAA shuttle and dissipate the excess reducing equivalents in chloroplast, resulting in an optimization of PSII efficiency and photosynthetic electron transport. EBR treatment up-regulated the activity of AOX pathway in leaves under well-watered and drought conditions. EBR treatment also significantly alleviated the decreases of Fv/Fm, ФPSII, qP, and ETR in leaves with/without SHAM treatment under drought condition, however, the improvement degree was more efficiently in leaves with SHAM treatment. Furthermore, EBR increased the activities of NADP-MDH and NAD-MDH, and significantly decreased NADPH/NADP+ ratio in leaves with/without SHAM treatment under drought condition. These results suggested that EBR-induced protection against photoinhibition was partly related to the up-regulation of AOX pathway by dissipating excess reducing equivalents.