Early signals of water limitations begin at the root-soil interface: linking rhizosphere drying to water uptake decline.

Understanding when and where drought stress originates in the soil-plant continuum is essential for predicting plant responses to climate change. While stomatal closure is a well-known reaction to declining soil moisture, the precise hydraulic trigger remains unresolved. We investigated whether the initial reduction in root water uptake is concomitant with a localized depletion of water near the root surface. Using high-resolution neutron radiography, we visualized dynamic changes in water distribution near maize (Zea mays L.) roots under controlled drying. We quantified the shift in water uptake patterns and their impact on whole-plant water use. Under wet conditions, roots primarily extracted water from the bulk soil. As soil moisture declined below a texture-dependent threshold, hydraulic conductivity dropped, preventing water flow from the bulk soil into the rhizosphere. This caused a shift in water uptake to the rhizosphere, coinciding with reduced transpiration and stomatal downregulation. The transition occurred c. -5 kPa in sandy soils and -200 kPa in loamy soils. These results provide direct evidence that an early hydraulic limitation during soil drying occurs in the rhizosphere, particularly in sandy soils. This redefines the rhizosphere as a dynamic control zone that mediates early drought responses and links microscale hydraulic behavior with whole-plant function.