Low carbon dioxide concentrations can reverse stomatal closure during water stress

Leaf water potentials below threshold values result in reduced stomatal conductance (g_s). Stomatal closure at low leaf water potentials may serve to protect against cavitation of xylem. Possible control of g_s by leaf water potential or hydraulic conductance was tested by drying the rooting medium in four herbaceous annual species until g_s was reduced and then lowering the CO₂] to determine whether g_s and transpiration rate could be increased and leaf water potential decreased and whether hydraulic conductance was reduced at the resulting lower leaf water potential. In all species, low CO₂] could reverse the stomatal closure because of drying despite further reductions in leaf water potential, and the resulting lower leaf water potentials did not result in reductions in hydraulic conductance. The relative sensitivity of g_s to internal CO₂] in the leaves of dry plants of each species averaged three to four times higher than in leaves of wet plants. Two species in which g_s was reputed to be insensitive to CO₂] were examined to determine whether high leaf to air water vapor pressure differences (D) resulted in increased stomatal sensitivity to CO₂]. In both species, stomatal sensitivity to CO₂] was indeed negligible at low D, but increased with D, and low CO₂] partly or fully reversed closure caused by high D. In no case did low leaf water potential or low hydraulic conductance during drying of the air or the rooting medium prevent low CO₂] from increasing g_s and transpiration rate.