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

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 CO2-grown Arabidopsis thaliana plants, using gas exchange and epidermal strip experiments. With plants grown in normal air, a doubling of the CO2 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 CO2 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 CO2, Similarly, the drop in leaf conductance triggered by 1 mu M ABA applied at the root level was stronger at double CO2. Identical experiments were performed with plants fully grown at double CO2, Levels of leaf conductance and carbon assimilation rate measured at double CO2 were similar for air-grown and elevated CO2-grown plants. An enhanced response to ABA was still observed at high CO2 in pre-conditioned plants. It is concluded that: (i) in the absence of stress, elevated CO2 slightly affects leaf conductance in A. thaliana; (ii) there is a strong interaction in stomatal responses to CO2 and ABA which is not modified by growth at elevated CO2.