Physiological mechanisms of methyl jasmonate (MeJA) alleviating the effects of heat stress on ear differentiation in maize

Global warming has increased the frequency and severity of heat stress during the maize growing season in the Huang-Huai-Hai Plain of China, which can markedly hinder young ear development and cause substantial yield losses in summer maize. Methyl jasmonate (MeJA) is a biologically active compound that can alleviate abiotic stress and promote reproductive development, and thus represents a promising exogenous regulator for improving crop stress responses. This study aimed to clarify the effects of MeJA on young ear differentiation, fertilization, and seed-setting traits during the floret differentiation stage under heat stress. The summer maize cultivar Xianyu 335 was used as the experimental material. Exogenous MeJA was applied by foliar spraying at two key stages of ear differentiation (V9, 9th leaf stage; V12, 12th leaf stage) under simulated heat-stress conditions. By quantifying endogenous hormones and antioxidant responses, we elucidated how MeJA regulates ear morphology, floret number, and fertilization and seed-setting characteristics under heat stress. Exogenous MeJA significantly increased the contents of trans-zeatin+trans-zeatin riboside, salicylic acid, and jasmonic acid in young ears while reducing abscisic acid content. Compared with the control, MeJA also increased superoxide dismutase, peroxidase, and catalase activities in young ears and reduced malondialdehyde accumulation, thereby enhancing stress tolerance and mitigating heat stress-induced damage. Accordingly, MeJA increased young ear length, diameter, and floret number by 5.54%-48.88%, 12.34%-24.41%, and 2.51%-25.51%, respectively, and reduced the length of the undeveloped ear portion by 10.26%-50.00%. In addition, MeJA significantly increased the number of exposed silks and the floret fertilization rate while decreasing the grain abortion rate, resulting in marked increases in kernels per ear (48.36%-57.23%) and grain yield (49.14%-66.79%). Overall, MeJA alleviated heat stress-induced inhibition of ear differentiation by modulating endogenous hormone status and activating the antioxidant system in young ears, thereby reducing oxidative damage and improving fertilization and seed-setting capacity. These findings provide a theoretical basis for developing heat stress-resilient cultivation practices and stabilizing summer maize yield.