Trait-Based Optimization of Plant Density in Drip-Fertigated Wheat: Yield Formation and Nitrogen–Radiation–Water Use Efficiency Responses of Varieties Contrasting in Individual Spike Productivity

Optimizing plant density is critical for improving wheat yield and resource-use efficiency, but whether a single density recommendation applies to varieties differing in individual spike productivity under drip fertigation remains unclear. A two-year field experiment (2023–2024 and 2024–2025) was conducted with two winter wheat varieties contrasting in spike type: a multi-spike type (Jimai23, MS) and a large-spike type (Jimai24, LS). Four target densities (200, 300, 400, and 500 plants m−2) were evaluated under drip fertigation to quantify yield formation, dry matter production, radiation interception and use, N uptake and nutritional status, and water use. Grain yield responses to density differed markedly between varieties. MS showed an increase–plateau–decline pattern, with the highest yields at 300–400 plants m−2 (10.13–10.97 t ha−1), whereas LS increased to 400 plants m−2 and remained relatively stable at 500 plants m−2 (9.97–10.55 t ha−1). Increasing density increased spike number, LAI, intercepted solar radiation (ISR), and soil water consumption but decreased grains per spike, grain weight, and yield per spike in both varieties. Yield variation was more strongly associated with post-anthesis dry matter production than with grain number. Although MS intercepted more radiation, its radiation use efficiency (RUE), post-anthesis N uptake, N nutrition index (NNI), harvest index, agronomic N-use efficiency, fertilizer N recovery efficiency, and water use efficiency (WUE) declined sharply at high density. In contrast, LS maintained relatively stable RUE, higher NNI, stronger N uptake, and higher WUE at medium-to-high densities. These results demonstrate that optimal density under drip fertigation is variety-dependent and should be determined using a trait-based framework integrating nitrogen–radiation–water use efficiency.