Direct quantification of leaf transpiration isotopic composition

The stable isotopic composition of plant transpired water (δT) is a powerful tracer used to characterize plant processes in the fields of ecology, plant physiology and hydrology. However, δT is rarely directly measured due to the general difficulty in traditional water vapor isotopic measurements. We report a new direct method with the potential to continuously monitor δT utilizing a commercially available laser-based isotope analyzer coupled to a transparent leaf chamber in a flow-through system arrangement. The method is based on the mass balance of both water vapor and water vapor isotopes inside the chamber. We present the theoretical bulk water vapor and isotope mixing equations, which we verify using simulated water vapor of known isotopic compositions, producing a precision of 1.6‰ and 1.0‰ for δ2H and δ18O respectively. We demonstrate the applicability of our method to field observations and capture rapid (minute time scale) δT responses to shifts in transpiration driven by variation in irradiance.