Wheat yield improvement is associated with altered root systems during cultivar replacement

Root morphology and anatomical traits that occurred during the cultivar selection process are important for understanding improvements in grain yield and water use efficiency. However, little is known about the changes in root morphology and root anatomy caused by cultivar replacement and their effects on grain yield and water use efficiency. Field and pot experiments were conducted to investigate the changing trends in root morphological and anatomical traits of six dryland winter wheat cultivars widely cultivated in Shaanxi Province of China, from 1949 to 2014, and determine their relationships with grain yield and water use efficiency. The results illustrated that compared with old cultivars, the grain yield and water use efficiency of modern cultivars increased by 35.7% and 48.4% respectively, and were closely related to root system changes. With cultivar succession, root biomass, root surface area, root length and root length density all decreased in the topsoil under irrigated or rainfed conditions, whereas these traits increased in the subsoil. The subsoil root biomass, root surface area, root length and root length density of cultivars released after 2000 increased by 26.1%, 11.7%, 25.4% and 23.2% respectively compared with the older cultivars before 2000. Cultivar replacement also increased the root cortex thickness by 29.2%, but reduced the root xylem center vessel diameter by 13.2%, without obvious changes in root diameter. Our study demonstrated that cultivar replacement altered root morphology and root anatomy, thereby improving grain yield and water use efficiency. The future challenge for wheat breeding is to increase grain yield and water use efficiency under water-deficit conditions, which may be achieved by further promoting subsoil root development and optimizing root anatomy.