Nitrogen Topdressing Rate Alters Starch and Protein Properties in Grains at Different Spike Positions Under Long-Term Field Conditions

Nitrogen (N) is a key nutrient influencing wheat growth, grain yield, and quality. A long-term field experiment was conducted using cultivar Zhengmai 1860 to clarify the effects of N topdressing on grain protein composition, starch accumulation, and yield. Treatments included a basal N application of 150 kg ha^-1 (N1) combined with four topdressing rates at jointing: 37.5, 75, 112.5, and 150 kg ha^-1 (N1 + 37.5, N1 + 75, N1 + 112.5, N1 + 150). Nitrogen topdressing significantly affected the physiological and biochemical characteristics of grains at different spike positions. Amylopectin, globulin, soluble starch (SS), and soluble starch synthase (SSS) accumulated most under 75-112.5 kg ha^-1, with N1 + 75 showing the strongest response in basal and middle spike grains. Amylose and granule-bound starch synthase (GBSS) peaked at the middle spike under N1 + 112.5. Protein component (gliadin, glutelin, albumin), amino acids, glutamate synthase (GOGAT), and glutamine synthetase (GS) increased progressively with higher N rates, with maximum accumulation at N1 + 150. Nitrogen topdressing also enhanced spike number (5.05-37.13%), grains per spike (3.86-16.22%), and 1000-grain weight (2.72-5.79%), with the highest yield (9451.7 kg ha^-1) at N1 + 112.5. These results highlight the critical role of optimized N management in improving grain composition and yield in wheat.