Plant Physiol, October 2002, Vol. 130, pp. 940-950

CO₂-Triggered Chloride Release from Guard Cells in Intact Fava Bean Leaves. Kinetics of the Onset of Stomatal Closure¹

Botanisches Institut I, Justus-Liebig-Universität, D-35390 Giessen, Germany

The influence of CO₂ on Cl release from guard cells was investigated within the intact leaf by monitoring the Cl activity in the apoplastic fluid of guard cells with a Cl-sensitive microelectrode. In illuminated leaves adapted to a CO₂ concentration within the cuvette of 350 µL L¹, an increase of 250 µL L¹ CO₂ triggered a transient rise in the apoplastic Cl activity from 3 to 14 mM within 10 min. This Cl response was similar to the Cl efflux evoked by turning off the light, when the substomatal CO₂ was kept constant (CO₂ clamp). Without CO₂ clamp, substomatal CO₂ increased by 120 µL L¹ upon "light off." The response to an increase in CO₂ within the cuvette from 250 to 500 µL L¹ in dark-adapted leaves was equivalent to the response to an increase from 350 to 600 µL L¹ in the light. No Cl efflux was triggered by 2-min CO₂ pulses (150-800 µL L¹). After a switch from 350 µL L¹ to CO₂-free cuvette air, the guard cells were less sensitive to a rise in CO₂ and to light off, but the sensitivity to both stimuli partially recovered. Changes in CO₂ also caused changes of the guard cell apoplastic voltage, which were generally faster than the observed Cl responses, and which also promptly occurred when CO₂ did not initiate Cl efflux. The comparatively slow activation of Cl efflux by CO₂ indicates that an intermediate effector derived from CO₂ has to accumulate to fully activate plasma membrane anion channels of guard cells.