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

• Published in: Plant, cell and environment Vol. 44; no. 2; pp. 425 - 431
• Main Authors: Abdalla, Mohanned, Carminati, Andrea, Cai, Gaochao, Javaux, Mathieu, Ahmed, Mutez Ali
• Format: Journal Article
• Language: English
• Published: Chichester, UK John Wiley & Sons, Ltd 01.02.2021; Wiley Subscription Services, Inc
• Subjects: belowground hydraulic; Closures; Conductance; Drought; Drying; environment; hydraulic conductivity; Hydraulic models; hydraulic resistance; Hydraulics; leaf water potential; leaves; Moisture content; Nonlinear systems; Pressure chambers; Resistance; root pressure; soil drying; Soil investigations; Soil resistance; Soil water; Soil water potential; Soils; Solanum lycopersicum L; Stomata; stomatal movement; Tomatoes; Transpiration; Water potential; water stress; Xylem; xylem water potential; belowground hydraulic; hydraulic conductivity; transpiration; leaf water potential; soil drying; water stress; Solanum lycopersicum L
• ISSN: 0140-7791; 1365-3040; 1365-3040
• DOI: 10.1111/pce.13939

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 (Ksp) on transpiration (E) and stomatal regulation. We used a root pressure chamber to measure the relation between E, leaf xylem water potential (ψleaf‐x) and soil water potential (ψsoil) in tomato. Additional measurements of ψleaf‐x were performed with unpressurized plants. A soil–plant hydraulic model was used to simulate E(ψleaf‐x) for decreasing ψsoil. In wet soils, E(ψleaf‐x) had a constant slope, while in dry soils, the slope decreased, with ψleaf‐x rapidly and nonlinearly decreasing for moderate increases in E. The ψleaf‐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 Ksp 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 ψleaf‐x caused by a decrease in Ksp and elucidate a strong correlation between stomatal regulation and belowground hydraulic limitation. What triggers stomatal closure during soil drying? We investigated this fundamental question in tomato and demonstrated that, as the soil dried, the relation between leaf xylem water potential and transpiration rate became markedly nonlinear, indicating a drop in soil–plant hydraulic conductance. The loss of soil–plant hydraulic conductance, which was concomitant with stomatal closure, was primarily explained by a decrease in soil–root conductance.