EFFECTS OF HUMIDITY ON SHORT-TERM RESPONSES OF STOMATAL CONDUCTANCE TO AN INCREASE IN CARBON-DIOXIDE CONCENTRATION

The magnitude of the response of stomatal conductance to a change in t he concentration of carbon dioxide external to the leaf from 350 to 70 0 cm(3) m(-3) was found to be extremely variable from day to day in th e field in Glycine max, Hordeum vulgare and Triticum aestivum. It was found that the leaf-to-air water vapour pressure difference (LAVPD) du ring the midday measurements of the stomatal response to carbon dioxid e affected the magnitude of the response, On days when LAVPD was low n o significant change in conductance occurred with the increase in carb on dioxide concentration. When LAVPD was higher, conductance decreased by 24-52% with the increase in carbon dioxide within a few minutes, T he sensitivity of conductance was approximately linearly related to LA VPD in wheat and barley, Experiments with G. max in the field indicate d that, on days with low LAVPD, increasing the LAVPD just around the m easured portion of a leaflet made stomatal conductance responsive to i ncreased carbon dioxide, This result was also obtained under laborator y conditions with G. max, Helianthus annuus and Amaranthus retroflexus . In G. max, it was determined that leaves in which conductance was no t responsive to the increase in carbon dioxide could be made responsiv e even at low LAVPD by the injection of abscisic acid into their petio les, Because it is known that abscisic acid sensitizes stomata to carb on dioxide, these results are consistent with the idea that abscisic a cid may be involved in the response of stomatal conductance to changes in LAVPD.