The effects of water-stress on leaf H218O enrichment

Summary Water-stress experiments withPhaseolus vulgaris L. were undertaken to determine the transpiration rate dependency of the naturally occurring leaf H₂¹⁸O fractionation process. Water-stress leaf H₂¹⁸O 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 H₂¹⁸O fractionation, the respective time courses for water-stressed and control leaf H₂¹⁸O accumulations were compared. Water-stressed leaves displayed a slower rate of isotopic enrichment relative to controls, as was predicted from their reduced transpiration rates. In an absolute sense, however, both control and water-stress leaf H₂¹⁸O 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 H₂¹⁸O discrimination. Several modifications of the theory are also considered.