The effects of water-stress on leaf H2(18O) enrichment.

Water-stress experiments with Phaseolus vulgaris L. were undertaken to determine the transpiration rate dependency of the naturally occuring leaf H2(18O) fractionation process. Water-stress leaf H2(18O) levels were observed to be unexpectedly higher than controls. Speculations on the cause of this phenomenon are discussed. Since transpiration rate variations should theoretically affect only the rate and not the extent of leaf H2(18O) fractionation, the respective time courses for water-stressed and control leaf H2(18O) accumulations were compared. Water-stressed leaves displayed a slower rate of isotopic enrichment relative to controls, as was predicted from their reduced transpiration rate. In an absolute sense, however, both control and water-stress leaf H2(18O) fractionation rates were markedly greater than projected values from the existing model. Consequently, transpiration rates cannot be derived accurately at present from the observed rates of leaf H2(18O) discrimination. Several modifications of the theory are also considered.