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

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.