Novel Evidence for a Lack of Water Vapour Saturation Within the Intercellular Airspace of Turgid Leaves of Mesophytic Species

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Abstract
Ward, D. A. and Bunce, J. A. 1986. Novel evidence for a lack of water vapour saturation within the intercellular airspace of turgid leaves of mesophytic species—J. exp. Bot. 37: 504– By utilizing a dual-surface leaf chamber evidence was obtained suggesting that the water vapour pressure within the intercellular airspace of turgid leaves of mesophytic species can deviate significantly from the saturation vapour pressure at the leaf temperature. When the water vapour pressure of the air surrounding the lower leaf surface of sunflower was maintained constant and high, suddenly exposing the upper leaf surface to air with a low water vapour content caused the lower leaf surface to exhibit a negative rate of transpiration (i.e. an apparent uptake of water vapour). Since the vapour pressure of the air surrounding the lower (moist) surface was less than the saturation vapour pressure at the leaf temperature, the occurrence of negative transpiration indicated that the vapour pressure of the leaf airspace deviated from saturation under the conditions of measurement used. For both soybean and sunflower it was also found that if the humidity around the upper surface was maintained high and constant, a stepwise decrease in lower surface humidity caused substantial reductions in the transpiration rate and apparent conductance of the upper surface without any concomitant change in its photosynthetic rate. In contrast, both the photosynthetic rate and conductance of the lower surface were greatly reduced. The relative reductions of photosynthetic rate and conductance at the lower surface were the same. These responses are most easily explained in terms of a deviation from water vapour saturation within the intercellular airspace, which gives rise to spurious values of conductance.