Impact of nitrogen on photosynthesis, remobilization, yield, and efficiency in winter wheat under heat and drought stress

Droughts and high temperatures often occur during the late reproductive periods of winter wheat, greatly threatening winter crop production. However, the potential of nitrogen (N) management to mitigate the adverse effects of heat and drought stress on winter wheat remains unclear. Therefore, a two-season wheat experiment was performed to examine the impacts of temperature, soil moisture, and N supply on the photosynthetic rate, N accumulation and remobilization efficiency, and water and N use efficiency. Drought and heat stress resulted in a significant reduction in gas exchange, which was exacerbated by combined stress. In the combined stress treatment (HD), the stomatal conductance and transpiration rate under low N (N1) application increased by approximately 20 % and 15 %, respectively, compared to those under high N (N3) application. The highest net photosynthetic rate and instantaneous water use efficiency were observed under medium N (N2) application. An appropriate reduction in N supply alleviated the impacts of combined stress on growth traits, with the aboveground dry mass and green leaf area under N1 application increasing by 8.49 % and 24.10 %, respectively, compared with those under N3. Drought and heat stress significantly reduced the mean grain-filling rate, grain N accumulation (GNA), and contribution to grain N by post-anthesis N uptake (CANU), especially under combined stress. Compared with that under N1, the GNA in the DH treatment under N2 increased by 7.16 %, whereas the CANU gradually decreased with increasing N supply. Compared with the control, combined stress slightly increased WUE-biomass but decreased WUE-yield, and 19.91 % and 10.77 % lower NUE-yield and NUE-biomass, respectively, were observed under combined stress. The yield, WUE-biomass, and NUE-biomass of the DH treatment with N1 application increased by 16.19 %, 9.27 %, and 6.97 %, respectively, compared with those of the N3 treatment, whereas the WUE-biomass and NUE-yield were greatest under the N2 treatment. Entropy-topsis analysis revealed that the DH treatment under the N1 supply had a greater composite evaluation index than did the N2 and N3 supplies. Reducing nitrogen application effectively mitigated yield suppression and improved water and nitrogen use efficiency in winter wheat under post-anthesis combined stress.