Abstract

We show that differences in the oxygen isotope ratio of leaf water between C₃ and C₄ grasses (five species of each photosynthetic type) become less distinct as relative humidity increases, and that ¹⁸O leaf water differences translate directly to the oxygen isotope ratio of leaf cellulose. A conceptual model is presented that is based on grass blade growth characteristics and observed patterns of progressive enrichment in grasses. The Barbour and Farquhar (2000) model was capable of explaining the oxygen isotope ratio of bulk leaf cellulose of C₃ and C₄ grasses grown under a variety of growth conditions.