Exogenous SA or 6-BA maintains photosynthetic activity in maize leaves under high temperature stress

With global warming, high-temperature(HT) stress has become a major abiotic stress for crops, in particular summer maize in China. Photosynthesis is sensitive to HT. Salicylic acid(SA) and 6-benzyladenine(6-BA) can improve the adaptation of plants to various biotic and abiotic stresses. However, their contribution to maintaining photosynthetic activity and alleviating photoinhibition in maize leaves under HT stress is still unclear. The effects of exogenous SA or 6-BA on growth, photosynthesis capacity, photosystem Ⅱ(PSII) activity, subcellular ultrastructure, antioxidant system, and plant hormones in maize leaves under HT stress were investigated. Under HT conditions, application of SA or 6-BA up-regulated gibberellin and zeatin content in leaves, increasing leaf area index(LAI). It also expanded the stomata by reducing abscisic acid and jasmonic acid content in leaves, cooling them and increasing CO2supply to photosynthesis. A higher net photosynthetic rate, combined with increased activity of the antioxidant system, alleviated oxidative stress in maize plants sprayed with SA or 6-BA, allowing them to maintain their chloroplast ultrastructure and PSII activity, in particular electron transfer from QAto QB. The increased LAI and net photosynthetic rate per unit leaf area also resulted in the accumulation of more biomass.