Root size, distribution and soil water depletion as affected by cultivars and environmental factors

Field experiments were carried out in two parts at the Luancheng Experimental Station in the North China Plain. Part I, which was a continuous experiment that ran from 1990 to 2008, investigated the change of grain yield, evapotranspiration (ET), root size and soil water utilization of winter wheat (Triticum aestivum L.) under three water regimes (rain-fed, two and four irrigation applications). Part II used 10 cultivars released from 1970 to 2000 that were grown under the same condition for two seasons (2005/2006 and 2006/2007) to compare root size, root:shoot ratio and soil water depletion (SWD). The results of testing in Part I showed that the yield and ET of winter wheat gradually increased from 1990 to the present. There was no consistent change in total root length (TRL) over time. The difference in root size among seasons and irrigation treatments mainly occurred in the upper soil profile, where the root length density (RLD) was greater. No direct relationship was found between root size and soil water use. Thus, TRL was not a factor that indicated the water extracting capacity of crops. The results from Part II revealed that the seasonal ET of earlier released cultivars (ERC) was similar to that of recently released cultivars (RRC) under well-watered conditions. However, ET was slightly increased from ERC to RRC under water deficit conditions, indicating high soil water depletion by RRC. TRL decreased from ERC to RRC and was significantly correlated with plant height. The breeding of winter wheat that reduced plant height not only increased harvest index, but also reduced root size, resulting in a smaller root:shoot ratio. The reduction in TRL from ERC to RRC mainly occurred in the top soil profile. The results from both tests indicate that TRL is not a factor that determines soil water use; rather, the distribution of RLD along the soil profile plays more important role in soil water utilization. Smaller root size in the upper soil layer did not affect soil water uptake, and might be more economical in terms of production efficiency.