Hydraulic processes in roots and the rhizosphere pertinent to increasing yield of water-limited grain crops: a critical review

To a first-order approximation, the yield of a water-limited grain crop depends (i) on how much water is available to the crop and (ii) on how crop water use is partitioned during the growing season. The latter determines the harvest index of the crop, that is, the proportion of the crop's above ground biomass that is converted into grain, which is typically optimal if about 30% of the seasonal available water supply is used during flowering and grain filling. Here, we review the role of roots in extracting water from the soil in both the amount and the timing that may lead to maximal grain yield, and the various mechanisms underlying this activity. These include architectural and anatomical traits; the biophysics of water movement from soil through roots to the leaves including especially the properties of and processes within the interface between roots and soil and the role of mucilage therein; and the physiological role of the roots in influencing the growth and transpiration of the crop canopy, which can optimize the seasonal pattern of water use. These various properties and mechanisms are discussed in the context of improving grain yield in strongly water-limited, especially semiarid, environments.