Stable oxygen and hydrogen isotope composition of leaf water in C3 and C4 plant species under field conditions.

In this paper we make comparisons between the observed oxygen and hydrogen stable isotope composition of leaf water and the predictions of the Craig-Gordon model of evaporative isotopic enrichment. Comparisons were made among two C3 species (Chenopodium album and Helianthus annuus) and two C4 species (Amaranthus retroflexus and Kochia scoparia), when plants were exposed to natural environmental conditions in the field. There were significant differences among the species for the hydrogen and oxygen isotopic composition of leaf water at mid-day. The Amaranthus and Helianthus plants had lower leaf water δD and δ18O values than did Kochia and Chenopodium. The observed leaf water δ values were significantly lower than those predicted by the evaporative enrichment model for all the species. The degree of discrepancy between the observed and modelled leaf water isotopic compositions differed among species. There was a strong linear relationship between the oxygen and hydrogen isotopic compositions of stem water, observed leaf water and the modelled leaf water for all species. The observed leaf water isotopic composition for the different species occurred at different points along the line connecting the stem water isotopic composition and the modelled leaf water isotopic composition in a plot of δD and δ18O. We interpret these linear relationships as mixing lines between the unfractionated source or stem water isotopic composition and the isotopic composition of water at the evaporation sites within leaves (as defined by the evaporative enrichment model).