Root efficiency and water use regulation relating to rooting depth of winter wheat

It is well-recognized that deep roots play a vital role in increasing soil water availability to crop water use, their ability in regulating the allocation of the crop water use during the growing season would also influence crop yield and water productivity. Experiments were conducted using tubes of 0.5, 1.0, 1.5 and 2.0 m in depth (with an inner diameter of 30.5 cm) and irrigation amounts varying from 90 to 500 mm for two seasons of winter wheat (2019–2020 and 2020–2021). The combinations of the different tube depth and irrigation created different situations in terms of maximum rooting depth and soil water availability to assess the functions of deep roots on regulation of soil water availability to crops, and if the deep root growth being economic or not in relating to crop production and water productivity (WP). The results showed that under the same seasonal evapotranspiration (ET), the shallower the root depth was, the lower the yield. Higher WP was achieved with deep root systems. Further analysis showed that higher proportion ET occurred during the reproductive stage for the treatments with deep roots, which increased the leaf photosynthetic rate and the duration of the greenness, resulting in higher harvest index and WP. Deep roots not only increased the soil water availability for crop water use, but also regulated the timing of the soil water use to favor an increased proportion of water use during the reproductive stage. The root: shoot ratio was slightly increased from the 0.5 m tube to 1.0 m tube, but under a sufficient water supply, root: shoot ratio gradually decreased in the 1.5 m and 2.0 m tubes, resulting in less root length corresponding to the unit grain production. The treatments with deep root systems increased the root efficiency. The results indicated that both the total available water and the allocation of the water consumption during the crop growing season influenced the yield and water productivity. Increasing rooting depth was an efficient regulation measure to optimize the allocation of water consumption and increase soil water availability to crops without increasing metabolic input in root growth.