[Show abstract][Hide abstract] ABSTRACT: The water supply to leaves of 25 to 60 m tall trees (including high-salinity-tolerant ones) was studied. The filling status of the xylem vessels was determined by xylem sap extraction (using jet-discharge, gravity-discharge, and centrifugation) and by (1)H nuclear magnetic resonance imaging of wood pieces. Simultaneously, pressure bomb experiments were performed along the entire trunk of the trees up to a height of 57 m. Clear-cut evidence was found that the balancing pressure (P(b)) values of leafy twigs were dictated by the ambient relative humidity rather than by height. Refilling of xylem vessels of apical leaves (branches) obviously mainly occurred via moisture uptake from the atmosphere. These findings could be traced back to the hydration and rehydration of mucilage layers on the leaf surfaces and/or of epistomatal mucilage plugs. Xylem vessels also contained mucilage. Mucilage formation was apparently enforced by water stress. The observed mucilage-based foliar water uptake and humidity dependency of the P(b) values are at variance with the cohesion-tension theory and with the hypothesis that P(b) measurements yield information about the relationships between xylem pressure gradients and height.
[Show abstract][Hide abstract] 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 1H-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, µw,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 µw (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).