11.3: Root water uptake

The rate of root water uptake can be limited by either the hydraulic conductivity of the rhizosphere soil, i.e. the soil immediately adjacent to the roots, or by the water potential gradient between the soil and the roots. The traditional conceptual model of root water uptake held that root water uptake lowered the water content of the rhizosphere soil, which increased the hydraulic gradient between the rhizosphere soil and the bulk soil. This increased gradient would act to increase water flow toward the rhizosphere and roots. However, the lowered water content of the rhizosphere soil also decreased the soil hydraulic conductivity (recall Fig. 4‑7), which would act to decrease water flow to the roots.

As long as the effect of the increased gradient was adequate to offset the effect of the decreased hydraulic conductivity, then root water uptake could proceed at a steady rate. But, inevitably, a time would come when the water potential of the rhizosphere soil reached near-equilibrium with the water potential of the roots. From that time on, the hydraulic gradient could only decrease and the hydraulic conductivity would continue to decrease. Thus, the rate of root water uptake would start to decline sharply. This is analogous, in many ways, to the transition from the constant-rate stage of evaporation to the falling-rate stage.

This traditional view of root water uptake is being transformed by new data resulting from advances in our ability to monitor the soil water distribution in the rhizosphere and adjacent soil.It is well known that plant roots often exude organic compounds into the rhizosphere, and those root exudates often bind to the soil particles in the immediate vicinity of the roots, acting as a type of glue binding soil particles and aggregates together(Fig. 11-4)

Figure 11-4. Evidence of attachment of root exudates: (a) rhizosheath of a desert grass from the Kalahari Desert; (b) SEM image of glass-beads glued together by deposition of synthetic exudate during drying; and (c) synthetic sand aggregates glued together by synthetic exudate. Reproduced from Ghezzehei and Albalasmeh (2015).

Exudate concentrations up to 15,000 mg kg-¹ have been measured within the first few millimeters of the rhizosphere surrounding plant roots [2]. In addition to their impacts on soil microbiology, these exudates appear to play a dual role in root water uptake. Evidence suggests that: 1) these exudates lower the osmotic potential of the water in the rhizosphere relative to that of the bulk soil, creating a “built in” water potential gradient toward the roots and 2) these exudates form a gel-like substance that can retain water via capillary effects during drying, resulting in greater water retention near the roots than in the bulk soil. By these mechanisms, it appears that plants alter the rhizosphere in ways that result in the rhizosphere soil being wetter than the bulk soil, even while root water uptake continues and water flows from the bulk soil to the rhizosphere. Visual evidence consistent with this hypothesis is provided in the photograph below (Fig. 11-5)

Figure 11-5. Intact soil sample from the 5-cm depth of a clay loam soil with native grassland vegetation near Stillwater, Oklahoma, USA on 31 Aug. 2020, in late morning. The rhizosphere surrounding the bulbous root is notably wetter than the bulk soil.