Responses of individual stomata in Ipomoea pes-caprae to various CO2 concentrations

The responses of individual stomata to CO2 concentrations ranging from 0 to 900 micromol mol(-1) air were analysed in Ipomoea pes-caprae L. Sweet (Convolvulaceae). The stomata were directly observed using a measurement system that permitted continuous observation of stomatal movement under controlled light and CO2 conditions. A CO2 concentration of 350 micromol mol(-1) or higher induced stomatal closure, whereas concentrations below 350 micromol mol(-1) did not. The time lag before stomatal closure decreased with increasing CO2 concentration, as did the steady-state aperture of the stomata after a change in CO2 concentration. However, the rate of stomatal closure increased with increasing CO2 concentration. Therefore, not only the stomatal closure rate but also the time from the CO2 concentration change to the beginning of stomatal closure changed with increasing CO2 concentration. These results suggest that atmospheric CO2 may be the stimulus for the closure of guard cells. No significant differences were observed between adaxial and abaxial stomata in terms of their responses to CO2. However, heterogeneous responses were detected between neighbouring stomata on each leaf surface.