Increased Grain Yield in Modern Genotypes of Spring Wheat for Dryland Cultivation in Northwest China is Associated with the Decreased Allocation of Carbon to Roots

The increase in wheat yield in modern cultivars compared with landraces previously grown by dryland farmers in northwest China is associated with a decrease in aboveground vegetative growth. It is not clear whether the reduction in aboveground growth is associated with increased or decreased root growth and carbon allocation. Using genotypes of wheat released over the last 125 years, we evaluated the changes in belowground biomass, structure and activity as the grain yield increased with modern genotypes. Fourteen spring wheat genotypes released in different eras were selected to study the relationships between root traits and grain yield in two dryland environments in northwest China. In the field, root biomass, root-to-shoot ratio, and root length density decreased, while specific root length and water uptake per unit root biomass increased as the grain yield increased in later-released genotypes. Leaf 13C pulse labeling was applied to six representative genotypes at the stem elongation and early grain-filling stages to measure the distribution of assimilated carbon between shoots, roots, the root rhizosphere, and losses by root respiration. The labeling with 13C showed that at stem elongation and early grain filling, the proportion of 13C-labeled dry matter allocated to aboveground biomass increased, and that allocated to roots decreased as grain yield increased with the later release of the genotypes. At stem elongation, across all genotypes 55% (genotypic range 46–66%) of the 13C was in the shoot biomass and 19% (13–25%) was used to build root biomass, while 16% (13–19%) was lost as root and soil respiration and 10% (8–11%) was in the root rhizosphere. At the grain filling stage, 75% (68–80%) of the 13C was in the shoots, 7% (5–9%) was in the roots, 12% (9–15%) was used for root and soil respiration, and 6% (5–7%) was in the root rhizosphere. In the modern high-yielding genotypes, a lower percentage of 13C was translocated to the roots, secreted into the rhizosphere and lost by root respiration than in the landraces. While crop breeders do not actively select and breed for root characteristics, we conclude that breeding and selection for higher-yielding genotypes of spring wheat in northwest China has inadvertently selected for smaller but more efficient root systems that have contributed to higher grain yield in the rainfed environment of the study region. The implications of this biomass distribution for grain yield in other dryland environments are discussed.