The triple isotopic composition of oxygen in leaf water

Abstract

The isotopic composition of atmospheric O 2 depends on the rates of oxygen cycling in photosynthesis, respiration, photochemical reactions in the stratosphere and on δ 17O and δ 18O of ocean and leaf water. While most of the factors affecting δ 17O and δ 18O of air O 2 have been studied extensively in recent years, δ 17O of leaf water—the substrate for all terrestrial photosynthesis—remained unknown. In order to understand the isotopic composition of atmospheric O 2 at present and in fossil air in ice cores, we studied leaf water in field experiments in Israel and in a European survey. We measured the difference in δ 17O and δ 18O between stem and leaf water, which is the result of isotope enrichment during transpiration. We calculated the slopes of the lines linking the isotopic compositions of stem and leaf water. The obtained slopes in ln( δ 17O + 1) vs. ln( δ 18O + 1) plots are characterized by very high precision (∼0.001) despite of relatively large differences between duplicates in both δ 17O and δ 18O (0.02–0.05‰). This is so because the errors in δ 18O and δ 17O are mass-dependent. The slope of the leaf transpiration process varied between 0.5111 ± 0.0013 and 0.5204 ± 0.0005, which is considerably smaller than the slope linking liquid water and vapor at equilibrium (0.529). We further found that the slope of the transpiration process decreases with atmospheric relative humidity ( h) as 0.522–0.008 × h, for h in the range 0.3–1. This slope is neither influenced by the plant species, nor by the environmental conditions where plants grow nor does it show strong variations along long leaves.