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 the concentration of carbon dioxide external to the leaf from 350 to 700 cm³ m^–3 was found to be extremely variable from day to day in the field in Glycine max , Hordeum vulgare and Triticum aestivum . It was found that the leaf-to-air water vapour pressure difference (LAVPD) during the midday measurements of the stomatal response to carbon dioxide affected the magnitude of the response. On days when LAVPD was low, no significant change in conductance occurred with the increase in carbon dioxide concentration. When LAVPD was higher, conductance decreased by 24–52% with the increase in carbon dioxide within a few minutes. The sensitivity of conductance was approximately linearly related to LAVPD in wheat and barley. Experiments with G. max in the field indicated that, on days with low LAVPD, increasing the LAVPD just around the measured portion of a leaflet made stomatal conductance responsive to increased carbon dioxide. This result was also obtained under laboratory conditions with G. max , Helianthus annuus and Amaranthus retroflexus . In G. max , it was determined that leaves in which conductance was not responsive to the increase in carbon dioxide could be made responsive even at low LAVPD by the injection of abscisic acid into their petioles. Because it is known that abscisic acid sensitizes stomata to carbon dioxide, these results are consistent with the idea that abscisic acid may be involved in the response of stomatal conductance to changes in LAVPD.