Combined high light and heat stress induced complex response in tomato with better leaf cooling after heat priming

Light and temperature are two primary environmental factors for plant growth and development. The response of plants to multiple stresses of high light intensity and heat stress are complex. The priming effects of high light and heat stress on improving heat tolerance of plants need to be further illuminated. This study aimed to explain the effect of high light intensity, high temperature and their combination on tomato and clarify the response of tomato to heat stress after priming. Tomato plants were treated under control, high light, heat stress and the combination for the first-round treatments, followed by recurring heat stress as the second-round treatments. For the first-round treatments, the net photosynthetic rate (P_N) of the plants at individual high light and individual high temperature on day four significantly increased and decreased, respectively, as compared with control. Combined stress caused significant reduction in F_v/F_m (maximum quantum efficiency of photosystem II) and chlorophyll content as well as increase in carotenoids and carbohydrates content. No significant difference in the P_N was observed in tomato with and without priming; however, heat priming did improve the heat avoidance ability by increasing evaporation and decreasing leaf temperature. Overall, the high light affected the physiological response of tomatoes at heat stress. The tomato plants developed their defense systems including chlorophyll loss and synthesis of carotenoids to protect themselves from multiple stresses. Our work provided new insights into the understanding of plants response to high light and heat stress.