Image_2_Role of Hydraulic Signal and ABA in Decrease of Leaf Stomatal and Mesophyll Conductance in Soil Drought-Stressed Tomato.JPEG

Drought reduces leaf stomatal conductance (gs) and mesophyll conductance (gm). Both hydraulic signals and chemical signals (mainly abscisic acid, ABA) are involved in regulating gs. However, it remains unclear what role the endogenous ABA plays in gm under decreasing soil moisture. In this study, the responses of gs and gm to ABA were investigated under progressive soil drying conditions and their impacts on net photosynthesis (An) and intrinsic water use efficiency (WUEi) were also analyzed. Experimental tomato plants were cultivated in pots in an environment-controlled greenhouse. Reductions of gs and gm induced a 68–78% decline of An under drought conditions. While soil water potential (Ψsoil) was over −1.01 MPa, gs reduced as leaf water potential (Ψleaf) decreased, but ABA and gm kept unchanged, which indicating gs was more sensitive to drought than gm. During Ψsoil reduction from −1.01 to −1.44 MPa, Ψleaf still kept decreasing, and both gs and gm decreased concurrently following to the sustained increases of ABA content in shoot sap. The gm was positively correlated to gs during a drying process. Compared to gs or gm, WUEi was strongly correlated with gm/gs. WUEi improved within Ψsoil range between −0.83 and −1.15 MPa. In summary, gs showed a higher sensitivity to drought than gm. Under moderate and severe drought at Ψsoil ≤ −1.01 MPa, furthermore from hydraulic signals, ABA was also involved in this co-ordination reductions of gs and gm and thereby regulated An and WUEi.</p

Image_1_Role of Hydraulic Signal and ABA in Decrease of Leaf Stomatal and Mesophyll Conductance in Soil Drought-Stressed Tomato.JPEG

Drought reduces leaf stomatal conductance (gs) and mesophyll conductance (gm). Both hydraulic signals and chemical signals (mainly abscisic acid, ABA) are involved in regulating gs. However, it remains unclear what role the endogenous ABA plays in gm under decreasing soil moisture. In this study, the responses of gs and gm to ABA were investigated under progressive soil drying conditions and their impacts on net photosynthesis (An) and intrinsic water use efficiency (WUEi) were also analyzed. Experimental tomato plants were cultivated in pots in an environment-controlled greenhouse. Reductions of gs and gm induced a 68–78% decline of An under drought conditions. While soil water potential (Ψsoil) was over −1.01 MPa, gs reduced as leaf water potential (Ψleaf) decreased, but ABA and gm kept unchanged, which indicating gs was more sensitive to drought than gm. During Ψsoil reduction from −1.01 to −1.44 MPa, Ψleaf still kept decreasing, and both gs and gm decreased concurrently following to the sustained increases of ABA content in shoot sap. The gm was positively correlated to gs during a drying process. Compared to gs or gm, WUEi was strongly correlated with gm/gs. WUEi improved within Ψsoil range between −0.83 and −1.15 MPa. In summary, gs showed a higher sensitivity to drought than gm. Under moderate and severe drought at Ψsoil ≤ −1.01 MPa, furthermore from hydraulic signals, ABA was also involved in this co-ordination reductions of gs and gm and thereby regulated An and WUEi.</p

DataSheet_1_Relationships between stable isotope natural abundances (δ13C and δ15N) and water use efficiency in rice under alternate wetting and drying irrigation in soils with high clay contents.pdf

Natural abundance of the stable isotope (δ13C and δ15N) in plants is widely used to indicate water use efficiency (WUE). However, soil water and texture properties may affect this relationship, which remains largely elusive. Therefore, the purpose of this study was to evaluate δ13C as affected by different combinations of alternate wetting and drying irrigation (AWD) with varied soil clay contents in different organs and whole plant and assess the feasibility of using δ13C and δ15N as a physiological indicator of whole-plant water use efficiency (WUEwhole-plant). Three AWD regimes, I100 (30 mm flooded when soil reached 100% saturation), I90 (30 mm flooded when reached 90% saturation) and I70 (30 mm flooded when reached 70% saturation) and three soil clay contents, 40% (S40), 50% (S50), and 60% (S60), were included. Observed variations in WUEwhole-plant did not conform to theoretical expectations of the organs δ13C (δ13Corgans) of plant biomass based on pooled data from all treatments. However, a positive relationship between δ13Cleaf and WUEET (dry biomass/evapotranspiration) was observed under I90 regime, whereas there were no significant relationships between δ13Corgans and WUEET under I100 or I70 regimes. Under I100, weak relationships between δ13Corgans and WUEET could be explained by (i) variation in C allocation patterns under different clay content, and (ii) relatively higher rate of panicle water loss, which was independent of stomatal regulation and photosynthesis. Under I70, weak relationships between δ13Corgans and WUEET could be ascribed to (i) bigger cracks induced by water-limited irrigation regime and high clay content soil, and (ii) damage caused by severe drought. In addition, a negative relationship was observed between WUEwhole-plant and shoot δ15N (δ15Nshoot) across the three irrigation treatments, indicating that WUEwhole-plant is tightly associated with N metabolism and N isotope discrimination in rice. Therefore, δ13C should be used cautiously as an indicator of rice WUEwhole-plant at different AWD regimes with high clay content, whereas δ15N could be considered an effective indicator of WUEwhole-plant.</p