Physical Aspects of the Internal Water Relations of Plant Leaves

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Abstract
Relations between the water potential, osmotic potential, pressure potential, and relative water content have been derived on the basis of 2 simple assumptions. Measurements of these quantities were made on leaves of cotton, sunflower, pepper, and birds-foot trefoil to test the validity of these relations. Within the precision of the data, the assumption that the osmotic potential is inversely pro-protional to the relative water content of the leaf was satisfied. The turgor pressure was not related to water content in a simple linear fashion but the data could be represented adequately by 2 straight-line segments. A marked change in the modulus of elasticity of the leaves occurred at a turgor pressure of about 2 bars, corresponding to a water potential of about -12 bars. The so-called permanent wilting phenomenon is explained in terms of the variation in elastic modulus. The major change in leaf dimensions with changing water content occurred in the leaf thickness with only a modest variation in the leaf area.