The Hydraulic Pathways in Nicotiana glauca (Grah.) and Tradescantia virginiana (L.) Leaves, and Water Potentials in Leaf Epidermes

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Abstract
The hydraulic conductances of leaves of a species which exhibits stomatal responses to humidity (Nicotiana glauca) are significantly lower than the conductances in a species which does not exhibit such responses (Tradescantia virginiana). This difference could at least partly account for their difference in stomatal response to humidity. In both species, the hydraulic conductance between the leaf bulk and its epidermis is much lower than the conductance in any other part of the pathway. The apparently conflicting results, reported in recent literature, on the hydraulic conductances and water pathways in leaves are reinterpreted, and shown to be due to misinterpretation of results. The recently published criticisms of a technique used to measure hydraulic conductivity are commented on and refuted. An examination of the factors that influence the water potential at the sites of evaporation from the inner walls of the epidermis near stomatal pores showed that the water potential at these sites is lower than the bulk epidermal water potential. The water potential at these sites changes in a complex way as stomatal aperture changes. As it is reduced the ratio of: ‘water potential at sites of evaporation on the inner walls of the epidermis near stomatal pores/bulk leaf water potential‘ increases. The positive feedback effect of this phenomenon, which tends to keep stomatal water potential constant as the stomata close and therefore enhances closure, and two other ‘passive’ positive feedback effects on the water potential at sites of evaporation near stomata that have been reported in the literature are briefly discussed.