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) osmotic (symplast) water, and (c) external capillary water. In many bryophytes (c) is a large and variable component, preventing easy determination of full-turgor water content and of relative water content (RWC) values physiologically comparable with those for vascular-plant leaves. Pressure-volume (P-V) curves are presented and water-relations parameters estimated for bryophytes, including species with large thin-walled cells (Hookeria lucens and three marchantialian thalloid liverworts), species with notably thick cell walls (Neckera crispa), and species with wettable surfaces and well-developed external capillary water conduction (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 hirsuta and Conocephalum conicum. Osmotic potential () at full turgor 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-V curve is not a reliable estimate of apoplast volume and may give negative values; better estimates of apoplast volume may be obtained by vapour equilibration at known low water potentials. Blotting external water from shoots usually gave full-turgor water content estimates in reasonable agreement with those obtained by analysis of P-V curves, but for different reasons they could be either higher or lower than the true value. The importance of knowing full-turgor water content for physiological work on water-stress responses in bryophytes is emphasized.Key words: Thermocouple psychrometry, apoplast fraction, relative water content, osmotic potential, poikilohydry.