Stomatal CO2-responses at sub- and above-ambient CO2levels employ different pathways in Arabidopsis

Stomatal pores that control plant CO 2 uptake and water loss affect global carbon and water cycles. In the era of increasing atmospheric CO 2 levels and vapor pressure deficit (VPD), it is essential to understand how these stimuli affect stomatal behavior. It is unknown whether stomatal responses to sub-ambient and above-ambient CO 2 levels are governed by the same regulators and whether these responses depend on VPD. We studied stomatal conductance responses in Arabidopsis stomatal signaling mutants under conditions where CO 2 levels were either increased from sub-ambient to ambient (400 ppm) or from ambient to above-ambient levels under normal or elevated VPD. We found that guard cell signaling components involved in CO 2 -induced stomatal closure have different roles in the sub-ambient and above-ambient CO 2 levels. The CO 2 -specific regulators prominently affected sub-ambient CO 2 responses, whereas the lack of guard cell slow-type anion channel SLAC1 more strongly affected the speed of above-ambient CO 2 -induced stomatal closure. Elevated VPD caused lower stomatal conductance in all and faster CO 2 -responsiveness in some studied genotypes and CO 2 -transitions. Our results highlight the importance of experimental set-ups in interpreting stomatal CO 2 - responsiveness, as stomatal movements under different CO 2 concentration ranges are controlled by distinct mechanisms. Sometimes elevated CO 2 and VPD responses also interact. Hence, multi-factor treatments are needed to understand plant behavior under future climate conditions.