Mesophyll conductance decreases in the wild type but not in an ABA-deficient mutant (aba1) of Nicotiana plumbaginifolia under drought conditions

Under drought conditions, leaf photosynthesis is limited by the supply of CO2. Drought induces production of abscisic acid (ABA), and ABA decreases stomatal conductance (g(s)). Previous papers reported that the drought stress also causes the decrease in mesophyll conductance (g(m)). However, the relationships between ABA content and g(m) are unclear. We investigated the responses of g(m) to the leaf ABA content [(ABA)(L)] using an ABA-deficient mutant, aba1, and the wild type (WT) of Nicotiana plumbaginifolia. We also measured leaf water potential ((L)) because leaf hydraulics may be related to g(m). Under drought conditions, g(m) decreased with the increase in (ABA)(L) in WT, whereas both (ABA)(L) and g(m) were unchanged by the drought treatment in aba1. Exogenously applied ABA decreased g(m) in both WT and aba1 in a dose-dependent manner. (L) in WT was decreased by the drought treatment to -0.7MPa, whereas (L) in aba1 was around -0.8MPa even under the well-watered conditions and unchanged by the drought treatment. From these results, we conclude that the increase in (ABA)(L) is crucial for the decrease in g(m) under drought conditions. We discuss possible relationships between the decrease in g(m) and changes in the leaf hydraulics. This study aimed at investigating the possible involvement of ABA in decreasing mesophyll conductance under drought conditions. Here, we used the ABA deficient mutant of Nicotiana plumbaginifolia, aba1, and photosynthetic characteristics of this mutant and the wild type were compared. The experiments provided the clear evidence that the increase in leaf ABA decreased mesophyll conductance. Commentary: