Stomata regulate the transpiration flux according to environmental conditions. Among the parameters affecting stomatal aperture, abscisic acid (ABA) and CO2 are of major interest. ABA plays a crucial role in plant adaptation to water stress (Giraudat 1995). Synthesized with some delay, during a water stress, ABA induces a decrease in guard cell turgor resulting from a concerted modulation of ion channel activities (Blatt & Armstrong 1993; Pei et al. 1997), in order to limit water losses. CO2 is a second signal resulting in a reduction of stomatal aperture, but the mechanisms underlying this response are still debated (Willmer & Fricker 1996). It has been proposed that the apoplastic malate pool reflects the ambient CO2, and that a high malate concentration in the apoplast could lead to stomatal closure through the activation of anion channels in the guard cell plasma membrane (Hedrich et al. 1994). There are few studies and controversial results concerning a putative interaction between ABA and CO2 sensing in regulating stomatal movements. Mansfield (1976) observed a total independence of the ABA and CO2 responses in Xanthium strumarium. In contrast, the absence of CO2 has been reported to inhibit the effect of externally applied ABA in the same species (Raschke 1975) and in Solanum melongena (Eamus & Narayan 1989). Raschke & Hedrich (1985) have shown that the sensitization of guard cells to CO2 by ABA may depend on species and on the degree of stomatal aperture. More recently, an ABA-enhanced response to CO2 has been observed in soybean (Bunce 1998).
Previous studies have shown that stomatal response to growth in high CO2 was highly variable, depending on species and experimental conditions (Drake, Gonzalèz-Meler & Long 1997). In the context of constant increase in atmospheric CO2, these interactions between ABA and CO2 sensing at the level of stomata must be better understood to accurately predict the effects of increasing CO2 concentrations on plant water use. Thus, the aim of this work was (i) to investigate the dependence of stomatal response upon osmotic stress and ABA in different CO2 concentrations in A. thaliana, a species of fundamental genetic interest (ii) to evaluate whether this relation was altered by the CO2 concentration prevailing during growth. To address these questions, changes in mean leaf conductance triggered by osmotic stress or ABA were monitored in A. thaliana by gas exchange techniques at different CO2 levels for plants grown at normal or double concentration of CO2. Additionally, dose–response curves of stomatal response to ABA from plants grown in normal or elevated CO2 were established in epidermal strip experiments.