Impact of Post-Anthesis Heat Stress on Nitrogen Use Efficiency and Yield Components in Wheat

This study aimed to investigate the impact of post-anthesis heat stress on yield formation and nitrogen assimilation capacity in wheat genotypes differing in heat tolerance, so as to provide a theoretical foundation for resilient and stable-yielding cultivation strategies.

Field experiments were conducted from 2022 to 2024 at the National High-Tech Agricultural Park of Anhui Agricultural University using a split-plot design. Temperature was set as the main factor with two levels, including high temperature stress (HT) and ambient control (CK), while three nitrogen rates (no nitrogen (N0), 112.5 kg·hm^-2 (N1), and 225.0 kg·hm^-2 (N2)) and six wheat cultivars (three heat-tolerant: HM33, LK1109, AN1589; three heat-sensitive: FM5, TN19, WK1702) were arranged as subplot factors. High temperature stress was applied after anthesis. Grain yield, photosynthetic parameters (LI-6800), and activities of key nitrogen metabolism enzymes (spectrophotometry) were determined.

High-temperature stress after anthesis significantly inhibited wheat growth, development, and physiological metabolism, leading to a marked decrease in yield, dry matter weight, relative chlorophyll content (SPAD), leaf area index (LAI), net photosynthetic rate (P_n), stomatal conductance (G_s), transpiration rate (T_r), nitrate reductase (NR) activity, and glutamine synthetase (GS) activity, as well as nitrogen accumulation, nitrogen use efficiency, nitrogen absorption efficiency, and nitrogen fertilizer use efficiency. The yield reduction due to high-temperature stress exhibited significant varietal heat tolerance differences and nitrogen application effects: the two-year average yield reduction of heat-tolerant varieties of HM33, LK1109, and AN1589 under N0, N1, and N2 treatments were 9.71%-6.13%, 9.91%-6.24%, and 11.87%-6.42%, respectively, lower than the reductions in sensitive varieties of FM5, TN19, and WK1702, which were 15.26%-10.38%, 12.56%-9.84%, and 12.93%-11.17%, respectively. In terms of physiological mechanisms, high-temperature stress after anthesis significantly reduced P_n across all nitrogen treatments, with the reduction showing a decreasing gradient as nitrogen levels increased: N0>N1>N2. The changes in G_s and T_r followed the same pattern as P_n. Furthermore, high-temperature stress significantly reduced NR and GS activity in the flag leaves across all nitrogen treatments, but increased nitrogen fertilization effectively and alleviated the decline in enzyme activity, with the highest enzyme activity observed under the N2 treatment. The reduction in nitrogen use efficiency under high-temperature stress also showed varietal differences: the nitrogen use efficiency of the heat-tolerant variety HM33 decreased the least under the N2 treatment (by 20.00%), while the sensitive variety FM5 exhibited the greatest decrease (31.55%) under the same treatment.

Post-flowering high temperature stress significantly reduced wheat yield, while the application of nitrogen fertilizers (112.5 and 225.0 kg·hm^-2) effectively mitigated this decline. Heat-tolerant varieties minimized yield losses by maintaining the activity of key nitrogen-metabolizing enzymes and optimizing nitrogen assimilation and transport. Increased nitrogen application, particularly at the 225.0 kg·hm^-2, significantly enhanced nitrogen use efficiency and promoted the accumulation of assimilates in grains. These results demonstrated that combining heat-tolerant genotypes with appropriate nitrogen management could improve thermotolerance in wheat, which provided a theoretical foundation for cultivating high and stable yields under heat stress.