Methyl jasmonate reduces the damage caused by low temperature combined with low light: By regulating photosynthetic capacity and hormone levels of pepper seedlings (Capsicum annuum L.).

Low temperature combined with low light (LL) stress is a major issue restricting the yield and economic benefits of protected vegetables. As a typical thermophilic vegetable, the growth and yield of Peppers (Capsicum annuum L.) are easily limited by LL conditions. Methyl jasmonate (MJ), a signal molecule, plays a vital role in plant stress responses. Therefore, this study investigated the regulatory mechanisms of Methyl jasmonate-induced cold tolerance in peppers. The results indicated that the 200 μmol L-1 MJ pretreatment significantly increased the leaf area of pepper seedlings under LL stress (10 °C/5 °C, 100 μmol m-2 s-1/0 μmol m-2 s-1, 12 h/12 h), and the net photosynthetic rate and stomatal conductance increased by 284.61% and 62.77%, respectively. MJ pretreatment induced stomatal opening of pepper leaves under LL stress, reduced Y (NPQ) and Y (NO), increased RC/CS, and up-regulated the expression of the CaPsb S gene encoding a chloroplast protein, thereby significantly increasing the net photosynthetic rate. Meanwhile, MJ significantly increased the contents of K, Ca, Mg, Fe, and Mn in pepper plants under LL stress, the contents of endogenous IAA, ZT, GA3, and JA in pepper leaves, induced the up-regulated expression of CaJMT and CaMJE genes, enhanced the activities of JMT and MJE enzymes, promoted the synthesis of JA, and then improved the stress resistance of pepper plants. The results showed that MJ could significantly affect chlorophyll content under low-temperature, low-light conditions, regulate hormone levels, induce the expression of jasmonic acid synthesis genes, and optimize the photosynthetic index, thereby improving the low-temperature, low-light tolerance of pepper seedlings. Using multi-index synergistic analysis, we assessed the synergistic effects among hormones, elements, and photosynthetic indices. This study is expected to improve pepper yield and quality in facility cultivation and provide insights for pepper resistance breeding.