Evaluating water fluxes of field-grown alfalfa from diurnal observations of natural isotope concentrations, energy budget and ecophysiological parameters

Abstract

In this work, the isotopic composition of water in soil, plant and atmospheric vapour has been used to investigate the flow processes in an alfalfa canopy during a typical summer day under Mediterranean climate. In addition to the stable isotopes of water, energy budget, leaf water potential and stomatal conductance were used in an integrated approach to a better understanding of the mechanisms of water transfer. The daily enrichment cycle of 18O and 2H in the leaves can be described by a physical model assuming that one of the important factors influencing the daily variation of 18O and 2H content is the daily humidity cycle. The isotopic composition of the input water can be either determined by the collar water or by the water of the soil layers supplying water to the active roots. There is no fractionation during water uptake by the roots. Total resistance to water flow in alfalfa calculated using an Ohm's law analogue was similar to other reported values. However, alfalfa contains internal stores of water. Differences between transpiration and root water uptake can be interpreted as indicating that these internal stores contribute substantially to transpiration during the day. Upshifts and downshifts in the isotopic composition of leaves are due to contributions from these internal stores and can probably result from a modification of the values of the kinetic fractionation factor in relation to the changes in the aerodynamical regime and the stomatal opening.