Evaluating kinetic fractionation factors used for reconstructions from oxgen and hydrogen isotope ratios in plant water and cellulose

Abstract

Oxygen and hydrogen kinetic isotope enrichments occur in plant leaf water during evapotranspiration. Kinetic fractionation factors for 18O derived empirically from a variety of isotope studies of plant water compare favourably with fractionation factors estimated from theoretical consideration of leaf energy balance and boundary layer dynamics. Kinetic fractionation during evapotranspiration is sensitive to the nature of the boundary layer, which is controlled by leaf size and morphology. Generally, plants with small segmented leaves have a lower component of turbidity in the leaf boundary layer, which results in higher Ck values, than do plants having large simple leaves and more turbulent boundary layers. Equivalent analysis of 2H enrichment in plant leaf water reveals less consistent kinetic relations with leaf size and morphology and an apparent temperature-dependent isotope effect acting in opposition to evaporative enrichment. Accounting for this temperature-dependent isotope effect helps to (1) reconcile the 2Ck inconsistencies for different leaves and (2) explain a temperature effect in white pine which White et al. (1994) attributed to variable biochemical fractionation during cellulose synthesis. Improved characterization of kinetic effects during evapotranspiration has implications for isotopic study of plant water use and for constraining interpretations of paleoclimate based on the isotopic composition of plant tissues.