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Aims: The putative role of the rhizosphere in controlling root water uptake is receiving increasing attention. Recent experiments showed that the rhizosphere turned temporarily hydrophobic after drying and subsequent rewetting. Our objective was to investigate whether the rhizosphere hydrophobicity influences the hydraulic conductivity of the rhizosphere-root continuum. Methods: Lupins were grown in aluminium containers filled with a sandy soil. When the plants were 30 days-old, the soil was let dry to a water content of 2-4 % and then it was irrigated. The soil water content during irrigation was imaged using a time-series neutron radiography. By image processing, we quantified the increase in the volume of water in the roots upon irrigation. Using this information and additional measurements of the root pressure after irrigation, we calculated the water flow into the roots and the total hydraulic conductance of the rhizosphere-root continuum, K. Results: The radiographs showed that: 1) the rhizosphere stayed temporarily dry after irrigation; 2) as the rhizosphere slowly rewetted, the roots rehydrated of 63 %. During 2 to 3 h subsequent to irrigation, K increased from 1.36 ± 1.09 × 10 m s MPa to 5.02 ± 2.13 × 10 m s MPa, approaching the conductance of lupin roots in wet soils measured in previous experiments. Based on our calculations, these values of K correspond to a rhizosphere conductivity of 3.87 ± 1.91 × 10 m s (shortly after irrigation) and 1.09 ± 1.64 × 10 m s (2-3 h after irrigation). Conclusions: We conclude that during a drying/wetting cycle, the conductivity of the root-soil interface is a temporary limit to root water uptake. We postulate that the temporary reduced hydraulic conductivity is primarily caused by the rhizosphere water repellency. [ABSTRACT FROM AUTHOR]

To investigate the extent and size of root-soil air gaps that develop during soil drying, resign casts of roots of the desert succulent Agave deserti Engelm, were made in situ for container-grown plants and in the field. Plants that were droughted in containers for 7 and 14 had 24 and 34% root shrinkage, respectively, leading to root-soil air gaps that would reduce the hydraulic conductivity at the root-soil interface by a factor of about 5. When containers were vibrated during drought, root-soil air gaps were greatly diminished, and the predicted conductivity at the interface was similar to that of the control (moist soil). For plants in the field (4 and 6 wk after the last rainfall), root shrinkage was grater than for container-grown plants, but root-soil contact on the root periphery was greater, which led to a higher predicted hydraulic conductivity at the root-soil contact on the root periphery was greater, which led to a higher predicted hydraulic conductivity at the root-soil interface. To test the hypothesis that root-soil air gaps would help to limit water efflux from roots in drying soil, the water potentials of the soil, root, and shoot of plants from vibrated containers (with gaps eliminated or reduced) and non-vibrated containers were compared. The soil water potential was lower for vibrated containers after 14 d of drought, suggesting more rapid depletion of soil water due to better root-soil contact, and the root water potential was lower as well, suggesting greater water loss by roots in the absence of root-soil air gaps. Thus air gap could benefit A. deserti by helping to maintain a higher root water potential in the early stages of drought and later by limiting root water loss at the root-soil interface when the water potential exceeds that of the soil. [ABSTRACT FROM AUTHOR]

A common assumption in models of water flow from soil to root is that the soil can be described in terms of its representative or effective behavior. Microscale heterogeneity and structure are thereby replaced by effective descriptions, and their role in flow processes at the root‐soil interface is neglected. Here the aim was to explore whether a detailed characterization of the microscale heterogeneity at the scale of a single root impacts the relation between flow rate and pressure gradient. Numerical simulations of water flow toward a root surface were carried out in a two‐dimensional domain with a randomized configuration of spatially variable unsaturated hydraulic conductivities and varying boundary conditions, that is, increasing and decreasing root water uptake rates. By employing Matheron's method, the soil hydraulic properties were varied, while the effective hydraulic conductivity (corresponding to the geometric mean) remained unchanged. Results show that domains with a uniform conductivity could not capture important features of water flow and pressure distribution in spatially variable domains. Specifically, increasing heterogeneity at the root‐soil interface allowed to sustain higher root water uptake rates but caused a slower recovery in xylem suction after transpiration ceased. The significance of this is that, under critical conditions, when pressure gradients and flow rates are high, microscale heterogeneity may become an important determinant and should not be neglected in adequate descriptions of water flow from soil to root in dry soil. Key Points: At the scale of a single root soil heterogeneity impacts the relation between flow rate and pressure headHere the emergent relation between water flow and pressure head gradient cannot be described by a single effective conductivityExplicitly accounting for soil heterogeneity might give new insights on water flow in soil‐plant‐atmosphere systems [ABSTRACT FROM AUTHOR]