Soil water availability and plant competition affect the yield of spring wheat

This study was conducted to determine the effect of both inter-cultivar and intra-cultivar competition on the growth of three spring wheat (Triticum aestivum L.) cultivars released in different periods that had different root:shoot ratios, differed in water extraction from dry soil, and differed in height. Two water regimes were imposed to compare competitive effects under irrigated and rainfed conditions. Our main hypotheses were that the different distribution of biomass between shoot and root in old and new wheat cultivars will alter their competitive ability, that differences in root size will alter their competitive ability through different water uptake patterns, and differences in the gradient for water uptake will alter their competitive ability in different environments by enabling water extraction from drier soil. In monoculture, the recent cultivars had significantly more grain yield and higher water use efficiency for grain (WUEG) than the old cultivar. Under the two water regimes the old cultivar had more root biomass, and extracted water in deeper soil layers, whereas the modern cultivars extracted more water in dry soil layers. The old cultivar benefited from inter-cultivar competition in terms of both grain yield and above-ground biomass accumulation at the expense of the modern cultivars, which showed significantly reduced growth in mixtures compared to in monoculture. Our study suggested that the below-ground competitive ability of cultivars may have a negative relationship with the grain yield and WUEG in monoculture. The yield superiority of modern and recent cultivars was mostly due to increased above-ground biomass, kernel number and WUEG and a smaller proportion of root biomass. Our results demonstrate that inter-plant competition is an important factor affecting spring wheat productivity in contrasting environments, but the extent and intensity of these effects depend on the adaptation of root traits to available soil water. Reducing root growth redundancy and enhancing the ability to deplete more soil water are clearly adaptive features for wheat for water-limited conditions.