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 sto matal closure. Their putative interaction in stomatal regulation was invest igated in well-watered air-grown or double CO2-grown Arabidopsis thaliana p lants, using gas exchange and epidermal strip experiments. With plants grow n in normal air, a doubling of the CO2 concentration resulted in a rapid an d transient drop in leaf conductance followed by recovery to the pre-treatm ent 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 conductan ce, their stomatal responses to an osmotic stress (0.16-0.24 MPa) were diff erent. The decrease in leaf conductance in response to the osmotic stress w as strongly enhanced at elevated CO2, Similarly, the drop in leaf conductan ce 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 doubl e CO2, Levels of leaf conductance and carbon assimilation rate measured at double CO2 were similar for air-grown and elevated CO2-grown plants. An enh anced response to ABA was still observed at high CO2 in pre-conditioned pla nts. It is concluded that: (i) in the absence of stress, elevated CO2 sligh tly affects leaf conductance in A. thaliana; (ii) there is a strong interac tion in stomatal responses to CO2 and ABA which is not modified by growth a t elevated CO2.