The concomitance of water use regulations and loss in soil-plant hydraulic conductivities

Stomatal regulation, which governs water loss and hence plant water use, is a key feature facilitating plant adaptation to water-limited environments. Nevertheless, the underlying mechanisms governing stomatal closure remain disputed. Recent studies proposed that the loss in hydraulic conductivities within the soil-plant system is the main driver of stomatal closure. However, the primary hydraulic constraint along the system, being in the soil and/or within the plant, remains without consensus. Furthermore, simultaneous measurements of the hydraulic limitation and stomatal regulation, especially in intact plants, is challenging. Here, we reviewed the recent literature on the relationship between stomatal closure and the loss of hydraulic conductance of key elements across the soil-plant-atmosphere continuum: soil, root, root-soil interface, xylem and leaf. We observed higher correlation between stomatal closure and declining below-ground hydraulics rather than leaf and/or xylem hydraulics. This analysis confirms the notion that stomatal closure is triggered by the decline of the soil-plant hydraulic conductance, and that this decline has often a below-ground origin. Understanding the key regulatory role of below-ground hydraulics is critical for forecasting and managing plant behavior under drought.