Resistance to water flow in Fagus grandifolia leaves

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Abstract
The tempo of water efflux from single F. grandifolia leaves was measured. The resistance to pressure-driven water efflux was measured for normal leaves, Rs, and for leaves in which extracellular mesophyll spaces are infiltrated with water, Rs*. The ratio Rs*/Rs is about 0.4. The resistance to water flow through the xylem, Rx, was also measured and found to be a small part of Rs, i.e., Rx/Rs = 0.08. The activation energies for water efflux from normal and infiltrated leaves are both about 26 .+-. 4 kJ/mol. The membrane hydraulic conductivity of F. grandifolia leaves is roughly 10-6 cm s-1 bar-1 (1 bar = 100 kPa). Water travels the shortest path between the cell sap and the nearest xylem vessel, flowing in and out of mesophyll cells through the areas in contact between adjacent cells along the pathway.