Two standard methods for the determination of osmotic potentials in plant organs, thermocouple hygrometry of press saps and of freeze-thawed leaf discs, were compared on leaves of ten species. Strictly parallel samples invariably showed lower (more negative) osmotic potentials for the leaf discs. Evidence is presented that osmotically-active solutes are not completely extracted during the preparation of press saps and that the discs contain, on average, a smaller amount of apoplastic water than the saps. Hydrolysis of macromolecules in the discs was not responsible for the differences. The presence of the cutinized epidermis results in a comparatively long equilibration time for leaf discs, which may be reduced by appropriate measures.
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Kikuta et al. (1992) studied this question.