Genetic improvement of grain yield and nitrogen use efficiency of winter wheat cultivars in the Guanzhong Plain since the 1950s

Genetic improvement has substantially increased grain yield of winter wheat, yet the mechanism underlying nitrogen (N) transport and utilization remain unclear. This study aimed to quantify long-term genetic gains in yield and N use efficiency (NUE) and to evaluate their responses to water-N management. Field experiments were conducted using eight winter wheat cultivars released between 1950s and 2018 under three irrigations regimes viz no irrigation (W0) single irrigation at overwintering (W1), and two irrigations at overwintering and jointing stages (W2), (each irrigation at 600 m -3 ha -1 ) and three N application rates viz no nitrogen (N0), 120 kg ha -1 (N1), and 240 kg ha -1 (N2). Grain yield increased significantly with variety replacement, showing a genetic gain of 108.32 kg ha -1 yr -1 (1.59% yr -1 ) and cumulative improvements of 50-99% across treatments. Yield enhancement was mainly associated with increase spike number (SN) (3.42 spikes m -2 yr -1 ), while grain number (GN) showed no significant genetic improvement. Harvest index increased from 0.32 to 0.47-0.52 and thousand grain weight (TKW) showed modest gains but plateaued after 2010 under W0N1, W0N2, and W2N0 treatments. Nitrogen accumulation traits (NAA, NAM, and NAG) showed significant linear genetic gains, but NutE showed declined under most treatments. Increasing N rates reduced N partial factor productivity (N1 and N2 decreased by 20-36% compared to N0), although partially alleviated this reduction (W2>W1>W0). Structural equation model (PLS-SEM, R 2 =0.943) indicated that genetic improvement had the largest total effect on yield (0.94), with indirect effects produced by increasing WUE (0.73) and NUE (0.37), while TKW (0.72) contributed most directly to yield formation. The effect of GN is not significant, reflecting the source-sink balance mechanism. The comprehensive evaluated determined optimal combination W1N2 (overwintering irrigation and 240 kg N ha -1 ) and W2N1 (two irrigation and 120 kg N ha -1 ) with genetic yield gains of 73.90 and 73.77 kg ha -1 yr -1 , respectively. These results highlight a trade-off between improved N transport and reduced NUE, providing a theoretical basis for future breeding strategies that synergistically improve yield and resource use efficiency in the Guanzhong Plain.