Triple oxygen isotope composition of leaf waters in Mpala, central Kenya

Variations in triple oxygen isotopes have been used in studies of atmospheric photochemistry, global productivity and increasingly in studies of hydroclimate. Understanding the distribution of triple oxygen isotopes in plant waters is critical to studying the fluxes of oxygen isotopes between the atmosphere and hydrosphere, in which plants play an important role. In this paper we report triple oxygen isotope data for stem and leaf waters from Mpala, Kenya and explore how Δ17O, the deviation from an expected relationship between O17/O16 and O18/O16 ratios, in plant waters vary with respect to relative humidity and deuterium excess (d-excess). We observe significant variation in Δ17O among waters in leaves and stems from a single plant (up to 0.16‰ range in Δ17O in leaf water in a plant over the course of a signal day), which correlates to changes in relative humidity. A steady state model for evaporation in leaf water reproduces the majority of variation in Δ17O and d-excess we observed in leaf waters, except for samples that were collected in the morning, when relative humidity is high and the degree of fractionation in the system is minimal. The data and the steady state model indicate that the slope, λtransp, that links δ17O and δ18O values of stem and leaf waters and characterizes the fractionation during transpiration, is strongly influenced by the isotopic composition of ambient vapor when relative humidity is high. We observe a strong, positive relationship between d-excess and Δ17O, with a slope 2.2±0.2permeg‰−1, which is consistent with the observed relationship in tropical rainfall and in water in an evaporating open pan. The strong linear relationship between d-excess and Δ17O should be typical for any process involving evaporation or any other fractionation that is governed by kinetic effects.