Differential responses of grain yield, grain protein, and their associated traits to nitrogen supply in soft red winter wheat

Increased application of nitrogen fertilizers has significantly raised grain yield and protein concentration in wheat. However, only 30–50% of applied fertilizer nitrogen are usually utilized by the plant. In this study, four soft red winter wheat genotypes (Triticum aestivum L., IL07-4415, MD05W10208-11-8, OH06-150-57 and Sisson) were grown under three different nitrogen regimes (high, medium, and low) in a greenhouse, and grain yield, grain protein concentration, nitrogen use efficiency (NUE) and their associated traits were evaluated. Among the four genotypes, a high-yielding cultivar, Sisson, exhibited superior performance in terms of grain weight plant⁻¹ and NUE for yield (NUEY) at low nitrogen due to maintained grain number spike⁻¹ and harvest index. Significant yield losses due to nitrogen limitation were attributable to reduced spike number plant⁻¹ and grain number spike⁻¹ in the other genotypes. Interestingly, a linear relationship between NUEY and NUE for grain protein (NUEP) was detected at high (R² = 0.67) and low (R² = 0.42) nitrogen; both of these traits were positively correlated with grain number spike⁻¹, 1000-seed weight, and harvest index under nitrogen-limited conditions (R² = 0.35–0.48). These results suggest that simultaneous improvement of NUEY and NUEP could be achieved through the selection of the three yield components (grain number spike⁻¹, 1000-seed weight, and harvest index) at low nitrogen.