Declining Soil Hydraulic Conductivity Shifts Root Water Uptake from Bulk Soil to the Rhizosphere and Triggers Stomatal Closure

Soil water availability is a critical factor in determining how plants regulate their water relations, with drying soils imposing hydraulic constraints that affect root water uptake and stomatal behavior. As soils dry, their hydraulic conductivity is reduced, limiting water movement to the roots and ultimately impacting the flow of water within the soil-plant continuum. When root water uptake exceeds the flow rate allowed by the bulk soil, transpiration cannot be sustained for long. In theory, the critical point when root water uptake is no longer matched by soil water flow should be concomitant with a local depletion of water in the rhizosphere. However, such local depletion has never been observed.

In this study, we used a time-series neutron radiography performed at the ICON beamline of the Paul Scherrer Institute (Villigen PSI, Switzerland) to visualize and quantify root water uptake and soil water distribution in maize samples. Seedlings were grown under controlled conditions in rhizoboxes filled with sandy and loamy soils for two weeks, followed by a period of progressive drying. High-resolution imaging revealed a clear shift in water uptake patterns as the soil dried: initially, water was extracted predominantly from the bulk soil, but under drier conditions, uptake increasingly shifted to the rhizosphere. As soil drying progressed, the rate of water uptake from the rhizosphere became insufficient to meet the transpiration demand. The critical point when water uptake shifted from the bulk to the rhizosphere soil occurred at less negative water potentials in sandy soils (-4 to -5 kPa) than in loamy soils (-100 to -300 kPa), reflecting the differences in hydraulic properties between the two soil types.

These results show that under drought conditions, the rhizosphere serves as a primary water source for plants but cannot fully sustain transpiration over time, ultimately leading to stomatal closure and reduced water loss. By providing direct experimental evidence of how soil hydraulic limitations and rhizosphere water dynamics shape plant responses, this study provides new experimental evidence on the key role of rhizosphere water dynamics in regulating plant water use.