The use of stable isotopes of water in characterising the source of water in vegetation

The use of stable isotopes of water as tracers in characterising the source of water in vegetation is tried in three field situations. Initially, methodological aspects of the uptake of water into plant tissue, water movement from roots to tops and recovery from stem-wood by distillation are considered. Using tracers of deuterium and oxygen-18, diffusion of water into lettuce leaf tissue was demonstrated to occur as the whole molecule, equilibrating in 24 h. It is considered from the similar tissue density that diffusion of water into roots would be similar. Distillation in toluene or kerosene to recover water from various materials found small decreases in deuterium composition of the recovered water: −2.3‰ when recovering water from pine wood, −1.4‰ from potting mix and −3.2‰ from wood of native plants, while water recovered from lettuce leaf was not subject to such effects. These decreases are correlated to the proportion of organic matter in the materials, and may relate to decomposition of a component of this. Uptake of water by potted Hordeum vulgare, Atriplex nummularia, A. rhagodiodes, A. vesicaria, Callitris preissii, Casuarina glauca, Eucalyptus oleosa, E. citriodora and Maireana sedifolia was found to be relatively free of further fractionation.

In field studies, comparison of the isotopic composition of water from stems of Hordeum vulgare, Maireana sedifolia and Pinus radiata and from adjacent soil profiles suggested that the depth of extraction of water by roots varied with water availability and rooting depth. Examination of the stable isotopes of soil and sap water in addition to soil water contents provides a more realistic interpretation of plant water source than obtained by studying soil water content alone.