Variation of Stomatal Diffusive Resistance With Ambient Humidity in Sunflower (Helianthus annuus)

Abstract

Experiments with sunflower were set up to supply quantitative information on the relationship between transpiration diffusive resistance and evaporative demand as affected by changes in ambient humidity. The experiments were run on whole plants with the roots in nutrient solution to ensure an adequate supply of water. Particular attention was paid to relating calculated diffusive resistance values with measured diffusion values and visual observations of the stomata. The transpiration diffusive resistance of the stomata was found to increase in a linear manner as the water vapour concentration difference between leaf and surrounding air increased. Changes in calculated diffusive resistance were paralleled by changes in the diffusion of nitrous oxide through the stomata. Visual observation showed that the stomata closed as resistance increased and vice versa. Stomata from both adaxial and abaxial surfaces of the leaf demonstrated increases in diffusive resistance with increasing evaporative demand but the response of the adaxial surface change was not always concurrent with that of the abaxial surface. Leaf water content did not change as evaporative demand was changed, reflecting the observation that uptake of water through the roots equalled the amount lost by transpiration. It was concluded that changes in diffusive resistance were directly related to the stomatal aperture and that the primary response was at the level of the stomatal apparatus. There was no evidence of stomatal control resulting from lowered tissue water content. Anomalous results may be obtained in gas exchange experiments where a constant area of leaf is sealed into a cuvette. Tests indicate that errors arise where the leaf is held too firmly.