Responses of Net Photosynthesis and Conductance to Independent Changes in the Humidity Environments of the Upper and Lower Surfaces of Leaves of Sunflower and Soybean

Abstract

A dual-surface leaf chamber was used to investigate the responses of net photosynthesis and leaf conductance to independent changes in the humidity environments of the upper and lower surfaces of leaves of sunflower and soybean. In sunflower decreasing the humidity around the upper leaf surface while maintaining that of the lower surface constant and high reduced both the photosynthetic rate and the conductance of the lower surface. These reductions could not be attributed to changes in bulk leaf water potential since the transpiration rate of the whole leaf remained constant. Similarly, the reductions were not related to localized water deficits in the lower epidermis or lower mesophyll since the transpiration rate of the lower surface was reduced. Possible mechanisms whereby the gas exchange characteristics of the lower leaf surface of sunflower respond to the humidity environment of the upper surface are discussed. In contrast to sunflower, the photosynthetic rate of the lower surface of soybean was insensitive to the humidity environment of the upper surface. In leaves of sunflower grown under a moderate temperature and a medium light level, simultaneous decreases of humidity at both leaf surfaces reduced the photosynthetic rate of the whole leaf without affecting the substomatal partial pressure of CO₂. In contrast, with leaves developed under a cool temperature and a high light level, both the photosynthetic rate and the substomatal partial pressure of CO₂ were reduced. Evidently, the occurrence in sunflower of the response pattern suggesting a non-stomatal inhibition of photosynthesis by low humidity depends upon the environment during growth. The possibility that this non-stomatal inhibition may be an artifact due to an error in the assumption of water vapour saturation within the leaf airspace is considered.