Implications of mucilage on pressure bomb measurements and water lifting in trees rooting in high-salinity water

Abstract

Various studies on mangroves and other tall trees rooting in high-salinity water have given compelling evidence that tension is not the only factor in water lifting as thought by plant physiologists. A characteristic feature of these trees is that the tissue cells, the apoplastic space and, in particular, the lumen and the inner walls of many xylem vessels of the roots, the trunk and the branches (up to the apex) contain mucilage. Data on single marine giant algal cells are presented that show that mucilage reduces the chemical activity of water. Longitudinal gradients in the chemical activity of water and interfacial forces are presumably the dominant forces for water lifting. In order to save water on its tortuous pathway to the uppermost foliage trees apparently use different strategies (as revealed by 1 H-NMR imaging), e.g. reduction of the conducting xylem area in the branches at intermediate height by mucilage or interruption of the xylem water columns by gas-filled segments and water lifting through mucilage networks and surface films. Pressure bomb experiments over the entire height of the trees revealed clearly that balancing pressure values cannot be taken as a measure for xylem tension. Such values can be used generally for an estimation of the chemical potential of water in the xylem of leafy twigs under atmospheric pressure, µ w,h=0 , provided that a species-specific "threshold pressure" (depending on wood density, elastic forces of the tissue, hydraulic coupling between xylem and tissue cells, intercellular spaces, cellular osmotic pressure etc.) is subtracted from the balancing pressure values. Transpiration increases the "threshold pressure" considerably and in an unpredictable way. Thus, as shown here, predawn balancing pressure data taken at various heights can yield information about the height dependence of µ w (measured at h=0) under field conditions, particularly when the water content of the xylem is simultaneously determined in a reliable manner (e.g. by the compression/decompression method in combination with centrifugation).

Mangrove root: adaptations and ecological importance

“…Accumulation of mucilage in xylem vessels is an important strategy of Trees mangrove trees to save water. The viscous, polymeric substances in the xylem sap limit the rate of water flow and decrease transpiration (Zimmermann et al 1994(Zimmermann et al , 2002. Combined with high water-use efficiency this mechanism slows the rate of water uptake and prevents salts from accumulating in the soil surrounding the roots.…”

Salt tolerance mechanisms in mangroves: a review

“…These mucopolysaccharides in the xylem sap are apparently involved in water transport in the xylem conduit of R. mangle (Zimmermann et al 1994). Accumulation of mucilage in xylem vessels is an important strategy of mangrove trees to save water on its tortuous pathway to the uppermost crown (Zimmermann et al 1994(Zimmermann et al , 2002. The viscous, polymeric substances in the xylem sap limit flow rate and decrease transpiration (Zimmermann et al 1994(Zimmermann et al , 2002.…”

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“…This effect is known to exist in electroosmotic flow and, although electro-osmotic flow is not under consideration here, the adhesion of the positive cations to the walls may still be relevant. Zimmermann U. et al (2002bZimmermann U. et al ( , 2004 have identified layers of mucilage adjacent to the xylem walls. This mucilage may be a consequence of damage to the xylem, as some physiologists maintain (e.g.…”