EFFECT OF CHANGES IN LEAF WATER OXYGEN ISOTOPIC COMPOSITION ON DISCRIMINATION AGAINST (COO)-O-18-O-16 DURING PHOTOSYNTHETIC GAS-EXCHANGE

Photosynthetic gas exchange measurements were combined with measurements of the carbon and oxygen stable isotopic composition of CO2 after it passed over a leaf of Phaseolus vulgaris or Senecio spp. plants held in a controlled environment chamber. Calculations were then made of discrimination by the leaf against (CO2)-C-13 and (COO)-O-18-O-16. Leaves were maintained at different vapour pressure gradients in order to generate a range of leaf water O-18/O-16 ratios. The O-18 content of leaf water increased when plants were exposed to higher vapour pressure deficits. The observed (COO)-O-18-O-16 discrimination values also increased with an increase in the leaf-air vapour pressure gradient and the associated change in leaf water 18/(OO)-O-16 values. In addition, the observed (COO)-O-18-O-16 discrimination values were strongly correlated with values predicted by a mechanistic model of isotopic fractionation.