Melatonin enhances the low-temperature combined low-light tolerance of pepper (Capsicum annuum L.)seedlings by regulating photosynthesis, carotenoid, and hormone metabolism

Low temperature combined with low light (LL)is severe abiotic stress to plant growth and yield. However, little is known about the physiological mechanisms of the melatonin (MT)involved in the adaptation of pepper (Capsicum annuum L.)to LL. In this study, pepper seedlings subjected to LL stress (15 ?/5 ?, 100 µmol photons m?2 s?1)were pre-sprayed with 200 µmol L?1 MT to investigate the protective role of MT in photosynthesis, carotenoid metabolism, as well as hormones of pepper (Capsicum annuum L.)seedling. Our results indicated that the application of exogenous MT reduced the negative effect on plant growth inhibition of pepper seedlings caused by LL stress, significantly increased chlorophyll contents and photosynthetic capacity as result of improved photosynthesis rate, upregulated the relative expressions of PS? reaction center proteins genes (CaPsb A, CaPsb B, CaPsb C, and CaPsb S), reduced NPQ, 1-qP, and qN, as well as increased Fv/Fm, qP, Y(?), and electron transport rate (ETR)after 7 days. In addition, MT significantly enhanced abscisic acid metabolism and biosynthesis of indoleacetic acid, gibberellin 3, and zeatin-type cytokinins after 6 h, 12 h, 24 h, 48 h, and 168 h. Markedly, MT increased the contents of zeaxanthin, which is induced by the down-regulation of CaZEP, by 149.55 percent. Our findings confirmed the mitigating potential of MT application for LL stress by maintaining plant growth, improving the photosynthetic characteristics, regulating carotenoids, and hormone metabolism.