The application of mixed nitrogen increases photosynthetic and antioxidant capacity in rice (Oryza sativa) under heat stress at flowering stage

In recent years, heat stress has become a serious threat to rice production in China. It is well documented that nitrogen forms can regulate the heat tolerance of plants. However, its effect on the rice plants has not been described before. In this study, a pot experiment was performed with two restorer lines of indica rice, heat-tolerant 996 (HT 996) and heat-sensitive 343 (HS 343), to investigate whether nitrogen forms could mitigate heat stress at flowering stage and their physiological mechanism. The experiment received five treatments: (1) NN0: natural temperature level (32 celcius) with urea-N; (2) HN0: high temperature level (38 celcius) with urea-N; (3) HN1: high temperature level with NH4+-N; (4) HN2: high temperature level with mixed N [NH4+: NO3- at 50: 50 (w:w)]; and (5) HN3: high temperature level with NO3--N, respectively. The results showed that heat stress reduced seed setting, photosynthetic characteristics, leaf nitrogen content and antioxidant enzyme activities of rice. However, the application of mixed N significantly increased the 1000-grain weight and seed setting rate under heat stress, thus increasing yield. The increased yield was mainly caused by higher leaf N content, photosynthetic rate, transpiration rate and stomatal conductance, which led to a reduction in panicle and leaf temperatures. Meanwhile, the application of mixed N also improved the activities of antioxidant enzymes, and reduced the amount of reactive oxygen species, thereby mitigating the damage caused by exposure to high temperature. In conclusion, the application of mixed N can effectively alleviate the adverse consequences of high temperature, and could be further applied to provide strategies for the prevention and control of high temperature-prone areas during the rice flowering period.