Water-content components in bryophytes: analysis of pressure-volume relationships

Abstract

ing full-turgor water content for physiological work on water-stress responses in bryophytes is emphasized. The water associated with a bryophyte can be divided into (a) apoplast water held in cell-wall capillary spaces Key words: Thermocouple psychrometry, apoplast fraction, and by matric forces, (b) osmotic (symplast) water, and relative water content, osmotic potential, poikilohydry. (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 Introduction water content (RWC) values physiologically comparable with those for vascular-plant leaves. Pressure‐ In contrast to vascular plants, bryophytes at full turgor volume (P‐V) curves are presented and water-relations typically carry substantial and variable amounts of extraparameters estimated for bryophytes, including cellular water. Many mosses have their main pathways species with large thin-walled cells (Hookeria lucens of water movement outside rather than inside the plant and three marchantialian thalloid liverworts), species (Buch, 1945, 1947; Proctor, 1979, 1982). This makes with notably thick cell walls (Neckera crispa), and some standard methods for studying the water relations species with wettable surfaces and well-developed of vascular plants diYcult or impossible to apply to external capillary water conduction (Tortula ruralis, bryophytes, and uncritical transfer of concepts and techAnomodon viticulosus), and for the lichen Cladonia niques from vascular-plant physiology can lead to serious convoluta. Full-turgor water content ranged from c. misunderstanding and error. In particular, ‘relative water 110% DW. in T. ruralis and Andreaea alpina to 1400% contents’ based on notional ‘saturated’ water contents of DW. or more in Dumortiera hirsuta and Conocephalum bryophytes are not physiologically comparable to RWC conicum. Osmotic potential (Y p ) at full turgor was as generally understood in vascular plant physiology. between ’1.0 and ’2.0 MPa in most species, but Dilks and Proctor (1979) considered the water content substantially less negative values were found in the of a bryophyte as divisible into three parts—external thalloid liverworts (’0.35 to ’0.64 MPa). The x-inter- capillary water, symplast water within the cells, and cept of the P‐V curve is not a reliable estimate of apoplast water in the cell walls—and emphasized that apoplast volume and may give negative values; better the external capillary water is an essential functional estimates of apoplast volume may be obtained by component in the physiology of many bryophytes. The vapour equilibration at known low water potentials. external water is held at relatively high (near-zero) water Blotting external water from shoots usually gave full- potentials related to the size of the capillary spaces turgor water content estimates in reasonable agree- provided by the morphology of the plant; much of the ment with those obtained by analysis of P‐V curves, physiologically-important part of it is likely to be held but for different reasons they could be either higher between ’0.01 and ’0.5 MPa. The symplast water declines over a range of water potentials related to Y p , or lower than the true value. The importance of know