The Effects of Nitrogen Reduction on Photosynthetic Process, Antioxidant Enzyme Activities and Yield of Winter Wheat

According to the national dual carbon target, this study aimed to address a range of environmental issues caused by excessive nitrogen fertilizer application in winter wheat cultivation in Jiangsu Province. From a plant physiology perspective, the potential for reducing nitrogen during the production process of winter wheat was analyzed. The control group used the highest nitrogen application rate in the region (360 kg·hm^-2, N360), and four nitrogen reduction treatments were established: 300 (N300), 270 (N270), 240 (N240), and 180 (N180) kg·hm^-2. The main wheat variety "Zhenmai 12" in Jiangsu Province was selected as the experimental variety. At both heading and flowering stages, various photosynthetic parameters, chlorophyll content, and antioxidant enzyme activities of flag leaves were measured. The results reveal that compared to the N360 treatment, at the flowering stage there was a significant decrease in superoxide dismutase (SOD) and catalase (CAT) activities in flag leaves with N300 treatment; however no significant differences were observed for other parameters. In addition, net photosynthetic rate decreased by 16.25%, spike number decreased by 9.44%, and SOD enzyme activity decreased by 49.7% with N270 treatment; SPAD value and net photosynthetic rate decreased at heading stage but remained unaffected at flowering stage with N240 treatment; meanwhile peroxidase (POD) activity increased by 41.01%, malondialdehyde (MDA) content decreased by 44.92%, spike number decreased after maturity while thousand-grain weight increased by 7.82% with N240 treatment without significantly affecting yield; all parameters showed significant decreases with N180 treatment. The structural equation was constructed to examine the relationship between each parameter and yield in winter wheat production. Nitrogen exhibited dual effects on yield: firstly, nitrogen reduction influenced thousand-grain weight by modulating enzyme activity, subsequently impacting yield; secondly, nitrogen reduction affected the photosynthetic process, thereby influencing spike number and grains per spike, ultimately affecting yield. These findings demonstrate that in the winter wheat production area of Jiangsu Province, the nitrogen application rate for winter wheat could be reduced to 240 N·hm^-2 while maintaining a satisfactory level of yield. This outcome provides valuable insights for achieving nitrogen reduction and emission mitigation in agricultural ecosystems.