Elevated CO₂ enhances stomatal responses to osmotic stress and abscisic acid in Arabidopsis thaliana

A , carbon assimilation rate ABA, abscisic acid Ci , intercellular space CO 2 concentration g , leaf conductance WUE, water use efficiency Carbon dioxide and abscisic acid (ABA) are two major signals triggering stomatal closure. Their putative interaction in stomatal regulation was investigated in well‐watered air‐grown or double CO 2 ‐grown Arabidopsis thaliana plants, using gas exchange and epidermal strip experiments. With plants grown in normal air, a doubling of the CO 2 concentration resulted in a rapid and transient drop in leaf conductance followed by recovery to the pre‐treatment level after about two photoperiods. Despite the fact that plants placed in air or in double CO 2 for 2 d exhibited similar levels of leaf conductance, their stomatal responses to an osmotic stress (0·16–0·24 MPa) were different. The decrease in leaf conductance in response to the osmotic stress was strongly enhanced at elevated CO 2 . Similarly, the drop in leaf conductance triggered by 1 μ M ABA applied at the root level was stronger at double CO 2 . Identical experiments were performed with plants fully grown at double CO 2 . Levels of leaf conductance and carbon assimilation rate measured at double CO 2 were similar for air‐grown and elevated CO 2 ‐grown plants. An enhanced response to ABA was still observed at high CO 2 in pre‐conditioned plants. It is concluded that: (i) in the absence of stress, elevated CO 2 slightly affects leaf conductance in A. thaliana ; (ii) there is a strong interaction in stomatal responses to CO 2 and ABA which is not modified by growth at elevated CO 2 .