Effects of Root Trace Nitrogen Reduction in Arid Areas on Sucrose–Starch Metabolism of Flag Leaves and Grains and Yield of Drip-Irrigated Spring Wheat

To investigate the effect of nitrogen (N) application on the carbon metabolism and yield of flag leaves and grains of spring wheat under drip irrigation in Xinjiang, a split-zone design was adopted from 2020 to 2021, with strong-gluten wheat, Xinchun 37 (XC37), and medium-gluten wheat, Xinchun 6 (XC 6), as the main zones and different nitrogen application rates as the sub-zones. Four nitrogen application rates of 0, 210, 255, and 300 kg·ha−1 (CK2, B1, A1, and CK1, respectively) were set to analyze and compare the nitrogen response of key enzyme activity, soluble sugar, and sucrose and starch content in flag leaves and grains to control yield formation. The results showed that with the increase in nitrogen application, the activities of sucrose phosphate synthase (SPS) and sucrose synthase (SS) in flag leaves; the activities of SS, adenosine diphosphate glucose pyrophosphorylase (ADPG-PPase), soluble starch synthase (SSS), granule-bound starch synthase (GBSS), and starch branching enzyme (SBE) in grains; the contents of soluble sugar and sucrose in the flag leaves; and the yield, all first increased and then decreased. There is a significant difference between A1 (255 kg·ha−1) and the CK1 (300 kg·ha−1), B1 (210 kg·ha−1), and CK2 (0 kg·ha−1) treatments under the above indicators, with increases of 8–158%, 9–155%, 8–53%, 5–63%, 3–86%, 3–57%, 9–79%, 9–197%, and 9–113%, as well as higher levels of amylose, amylopectin, and total starch content than other treatments by 2–30%, 11–84%, and 8–63%, respectively. Correlation and stepwise regression analyses indicated highly a significant positive correlation between the yield and soluble sugar and sucrose of flag leaves and grains, as well as their key enzymes and starch. Among them, soluble sugar in grains, amylopectin, and sucrose in grains have the greatest impact on the yield of XC37, determining 85% of its yield. SSS, soluble sugars in grains, amylopectin, and SBE have the greatest impact on the yield of XC 6, determining 80% of its yield. The starch showed a highly significant positive correlation with ADPG-PPase, SSS, GBSS, and SBE. There was a significant interaction effect between the nitrogen application rate and variety, with better performance observed in Xinchun 37 compared to Xinchun 6. Under drip irrigation conditions in arid areas, a nitrogen application of 255 kg·ha−1 can effectively regulate the metabolism of sucrose to starch in the flag leaves and grains of spring wheat, which is conducive to the