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

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