Nitrogen Alleviates Heat-Induced Damage in Rice by Coordinating Hormonal Balance and Carbohydrate Metabolism to Maintain Reactive Oxygen Species Homeostasis

Nitrogen plays a critical role in regulating rice growth and stress resistance, yet its influence on heat tolerance at the seedling stage remains poorly understood. To clarify the physiological mechanisms involved, this study subjected rice seedlings to a transient range of temperature treatments (30, 35, 40, and 45 °C) under varying nitrogen levels. We systematically evaluated plant growth and analyzed key metabolic responses related to carbohydrates, phytohormones, and reactive oxygen species (ROS). The results demonstrated that temperatures of 40 °C and 45 °C significantly suppressed seedling growth, while elevated nitrogen supply effectively mitigated heat-induced damage, as evidenced by reduced leaf wilting and higher chlorophyll retention. Under high-temperature stress, seedlings receiving high nitrogen maintained superior carbohydrate reserves, higher levels of hormones such as zeatin ribosides, indole-3-acetic acid, and gibberellins, as well as a greater activity of key nitrogen metabolism enzymes compared to those under low nitrogen. Furthermore, high nitrogen enhanced the activity of antioxidant enzymes (superoxide dismutase and catalase) and significantly lowered the accumulation of malondialdehyde and hydrogen peroxide. Collectively, these findings indicate that appropriate nitrogen application enhances heat tolerance in rice seedlings through an integrated regulation of carbohydrate and hormone metabolism coupled with strengthened antioxidant capacity and improved ROS homeostasis.