Abstract
Graded external osmotic pressures were used to examine the relationship of stomatal aperture to guard-cell turgor pressure for Vicia faba. Stomata were in floating epidermal strips and free of contact with intact epidermal cells. A number of errors were discovered in this and in the classical incipient-plasmolysis methods for measuring cell osmotic pressure. Unstirred layers in exosmosis slow equilibration markedly and rapid loss of internal solute by guard cells can occur; considerations of both water and solute fluxes and their interactions become important. However, it was possible to show that stomatal aperture under the above conditions was linearly related to guard-cell turgor pressure with a slope of 0.5 μm bar−1.

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