CO2-triggered chloride release from guard cells in intact fava bean leaves.: Kinetics of the onset of stomatal closure

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 muL L-1, an increase of 250 muL L-1 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 muL L-1 upon "light off." The response to an increase in CO2 within the cuvette from 250 to 500 muL L-1 in dark-adapted leaves was equivalent to the response to an increase from 350 to 600 muL L-1 in the light. No Cl- efflux was triggered by 2-min CO2 pulses (150-800 muL L-1). After a switch from 350 muL L-1 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.