Advances in understanding plant root water uptake

Water deficit is one of the primary limitations to crop production. Here, we review the role of root and rhizosphere hydraulic processes that affect the ability of a plant to extract water from the soil. Prominent features of rhizosphere hydraulic properties are: root shrinkage, alteration of pore geometry in the rhizosphere, effect of mucilage on water retention, hydraulic conductivity and water repellency, root hairs, and mycorrhiza connecting the root surface to the soil matrix. All these factors are strongly dynamic, changing over time and with soil moisture conditions. Although our understanding of the mechanisms related to these factors has advanced significantly in the last ten years, the relative importance of these rhizosphere processes for the ability of crops to extract water from the soil and better tolerate drought is still largely unclear. We propose that the next research step is to investigate the implications of these rhizosphere processes on crop growth and water use economy and use this knowledge to grow more resilient crops that match to their environment.

The effect of root shrinkage on soil water inflow

As a soil dries, the roots growing in it may shrink and retain only partial contact with the soil. The steady-state model described here calculates the effect of root shrinkage on the water inflow across soils and roots with various hydraulic conductivities. Relative to standard conditions, typical of a secondary root in a loam at field capacity, the water inflow was very sensitive to the root radius, the root shrinkage, the root hydraulic conductivity and the water potential in the bulk soil and at the root endodermis. The inflow was insensitive to the density of rooting, and to the soil hydraulic conductivity except in dry sandy soil where this was lower than the root hydraulic conductivity. Loss of full contact can decrease the inflow by a factor of up to about three. In very dry conditions, vapour transfer across the air gap between the root and soil surfaces can contribute usefully to the total water inflow.

Resistance to water uptake by Douglas-fir seedlings in soils of different texture

Seedlings of Pseudotsugamenziesii (Mirb.) Franco were grown in fertilized silty clay, silt loam, and loamy sand in a growth chamber. Needle water potentials hardly changed as soil water potential, ψs, dropped to about −2.5 MPa. At ψs = −0.6 MPa, the effect of soil texture on water uptake rate was statistically significant (p = 0.01). Calculated water uptake resistance (from soil to foliage), R, was hardly affected by ψs between −0.5 and −1.0 MPa, but nearly doubled as ψs fell from −1.0 to −2.2 MPa. Plant water resistance is inferred to change relatively little over this range. Upper limits of soil resistance at ψs > −2.5 MPa, estimated (by Gardner's equation) for silt loam and silty clay, are too low to make a large contribution to R, or to the change in R with ψs, or to the large differences in average R among different textures at ψs values from −0.5 to −2.2 MPa. It is inferred that contact resistance, Rc, is large, varies significantly with ψs, and may vary with texture. Unsaturated hydraulic conductivity differences theoretically account for a relationship of Rc, with texture, and, together with possible root shrinkage, could account for a relationship of Rc, with ψs. Mycorrhizal development in these fertilized seedlings was too slight to justify consideration of hyphal resistance.