Root and Rhizosphere Traits Relevant to Increasing Yield of Water-limited Grain Crops: A Critical Review

Inadequate amounts of water can limit the yield of grain crops in many ways. To a first-order approximation, the yield of a water-limited grain crop depends on (1) how much water is available to the crop and (2) how that water 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 both in the amount and in 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 and processes of 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 optimise the seasonal pattern of water use. These various properties and mechanisms will be discussed within the context of optimising grain yield in a range of water-limited environments.