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. Pressurevolume (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 (Ψπ) 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 fullturgor 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 know ing full-turgor water content for physiological work on water-stress responses in bryophytes is emphasized.