| Abstract: | Two standard methods for the determination of osmotic potentialsin plant organs, thermocouple hygrometry of press saps and offreeze-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 presentedthat osmotically-active solutes are not completely extractedduring 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 responsiblefor the differences. The presence of the cutinized epidermisresults in a comparatively long equilibration time for leafdiscs, which may be reduced by appropriate measures. Key words: Thermocouple psychrometry, vapour pressure equilibration, leaf discs, press saps, solute extraction |
