Genetic gains in wheat yield and its physiological determinants under contrasting growing conditions

Understanding the physiological bases behind genetic gains in wheat ( Triticum aestivum L.) yield as well as its interplay with environmental conditions are essential to guide breeding strategies for improvement and adaptability. In this study we investigated the physiological drivers of yield differences between traditional and modern bread wheat cultivars and assessed their responsiveness under contrasting growing conditions. Eleven cultivars representing different breeding eras were tested in three field experiments throughout the 2022/23 season in a high-yield condition and two low-yield environments differing in their water input (rainfed and irrigated) and photothermal conditions. Differences between genotypes and environment were significant whereas yield, grain number per m 2 , and harvest index showed stagnation after 2000 in favourable and resource-limited environments. Yield increases were strongly associated with higher grain number per m 2 , primarily driven by more grains per spike. In contrast, average grain weight and spike density showed little variation across breeding eras. Relative yield gains were 0.67 and 0.71% y⁻¹ under high-yielding irrigated and rainfed conditions, respectively. Gains in biomass accumulation and fruiting efficiency also contributed to yield improvements, with both traits expressing genetic progress even under low-potential conditions. Interestingly, crop responses differed between stress types despite similar yield penalties. Under rainfed conditions, genetic gains were maintained though reduced, whereas no consistent trend was detected under late sowing. This suggests that the expression of genetic gain may depend on the nature of the stress imposed, being maintained under resource limitation but not under stress associated with altered photothermal conditions and strong constraints.