Stomatal closure of tomato under drought is driven by an increase in soil-root hydraulic resistance

The fundamental question as to what triggers stomatal closure during soil drying remains contentious. Thus, we urgently need to improve our understanding of stomatal response to water deficits in soil and atmosphere. Here, we investigated the role of soil-plant hydraulic conductance (K-sp) on transpiration (E) and stomatal regulation. We used a root pressure chamber to measure the relation between E, leaf xylem water potential (psi(leaf-x)) and soil water potential (psi(soil)) in tomato. Additional measurements of psi(leaf-x) were performed with unpressurized plants. A soil-plant hydraulic model was used to simulate E(psi(leaf-x)) for decreasing psi(soil). In wet soils, E(psi(leaf-x)) had a constant slope, while in dry soils, the slope decreased, with psi(leaf-x) rapidly and nonlinearly decreasing for moderate increases in E. The psi(leaf-x) measured in pressurized and unpressurized plants matched well, which indicates that the shoot hydraulic conductance did not decrease during soil drying and that the decrease in K-sp is caused by a decrease in soil-root conductance. The decrease of E matched well the onset of hydraulic nonlinearity. Our findings demonstrate that stomatal closure prevents the drop in psi(leaf-x) caused by a decrease in K-sp and elucidate a strong correlation between stomatal regulation and belowground hydraulic limitation.

Automated summary

Investigating the impact of soil-plant hydraulic conductance on plant transpiration and stomatal regulation, this study reveals the mechanism of stomatal closure in dry soils to prevent the decrease in leaf water potential. The findings suggest that shoot hydraulic conductance does not decrease during soil drying, and there is a strong correlation between stomatal regulation and belowground hydraulic limitation.