What matters more, biomass accumulation or allocation, in yield and water productivity improvement for winter wheat during the past two decades?

Continuous increases in crop production and water productivity (WP) are important for agriculture that is facing fresh-water shortages globally. Biomass production and the allocation of biomass to grains are two critical factors influencing crop production and WP. A long-term field experiment was conducted at the Luancheng Agro-Eco-Experimental station in the North China Plain (NCP) for twenty winter wheat growing seasons (2002-2022) under six irrigation treatments to examine the contributions of different factors to yield and WP improvement. The six irrigation treatments were classified into four levels of water supply, i.e., adequate water supply (AW), light water deficit (LD), moderate water supply (MD), and serious water deficit (SD). The results showed that the average yield of winter wheat for all the treatments increased at a rate of 79.1 kg/ha/yr, while the corresponding seasonal evapotranspiration (ET) increased by 2.0 mm/yr, resulting in an improvement in WP at a rate of 0.012 kg/m3/yr over the past 20 years. The maximum dry matter accumulation and the highest yield improvement were observed for the AW treatment, which increased by 119.0 kg/ha/yr. On average, the highest WP was achieved at 1.84 kg/m3 under MD conditions during the past two decades. Under various water supply condi-tions, the leaf photosynthesis rate and leaf area of a single plant were relatively stable. The plant density showed an increasing trend, and the anthesis date gradually advanced with cultivar renewal, which contributed to the increased biomass accumulation during reproductive stages. The biomass allocation had less of an effect on the final yield in recent years since the harvest index (HI) has nearly approached the upper limit, and the increase in biomass accumulation contributed more to a sustained increase in the yield and WP of winter wheat. Breeding cultivars that could tolerate high density and be less sensitive to water stress would benefit biomass production to improve the grain yield of winter wheat in future.