WATER-CONTENT COMPONENTS IN BRYOPHYTES - ANALYSIS OF PRESSURE-VOLUME RELATIONSHIPS

The water associated with a bryophyte can be divided into (a) apoplast water held in cell-wall capillary spaces and by matric forces, (b) os motic (symplast) water, and (c) external capillary water. In many bryo phytes (c) is a large and variable component, preventing easy determin ation of full-turgor water content and of relative water content (RWC) values physiologically comparable with those for vascular-plant leave s. Pressure-volume (P-V) curves are presented and water-relations para meters estimated for bryophytes, including species with large thin-wal led cells (Hookeria lucens and three marchantialian thalloid liverwort s), species with notably thick cell walls (Neckera crispa), and specie s with wettable surfaces and well-developed external capillary water c onduction (Tortula ruralis, Anomodon viticulosus), and for the lichen Cladonia convoluta. Full-turgor water content ranged from c. 110% DW. in T. ruralis and Andreaea alpina to 1400% DW. or more in Dumortiera h irsuta and Conocephalum conicum. Osmotic potential (Psi(pi)) at full t urgor was between -1.0 and -2.0 MPa in most species, but substantially less negative Values were found in the thalloid liverworts (-0.35 to -0.64 MPa). The x-intercept of the P-ll curve is not a reliable estima te of apoplast volume and may give negative values; better estimates o f apoplast volume may be obtained by vapour equilibration at known low water potentials. Blotting external water from shoots usually gave fu ll-turgor water content estimates in reasonable agreement with those o btained by analysis of P-V curves, but for different reasons they coul d be either higher or lower than the true value. the importance of kno wing full-turgor water content for physiological work on water-stress responses in bryophytes is emphasized.