Effect of Changes in Leaf Water Oxygen Isotopic Composition on Discrimination Against C¹⁸O¹⁶O During Photosynthetic Gas Exchange

Abstract

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 ¹³CO₂ and C¹⁸O¹⁶O. Leaves were maintained at different vapour pressure gradients in order to generate a range of leaf water ¹⁸CO/¹⁶CO ratios. The ¹⁸CO content of leaf water increased when plants were exposed to higher vapour pressure deficits. The observed C¹⁸O¹⁶O discrimination values also increased with an increase in the leaf-air vapour pressure gradient and the associated change in leaf water ¹⁸/CO¹⁶CO values. In addition, the observed C¹⁸O¹⁶O discrimination values were strongly correlated with values predicted by a mechanistic model of isotopic fractionation.