Variabilité spatio-temporelle de la composition isotopique de l'eau ( 18O, 2H) dans le continuum sol-plante-atmosphère 2. Approche en conditions naturelles

Abstract

We have made intensive sampling for isotopic measurements ( 2 H 1 H and 18 O 16 O ) conducted on water in the soil-plant-atmosphere system. During two daily cycles, we have carried out the measurement of the isotopic composition of water in soil, collar, leaf and atmospheric vapor under different climatic conditions (sunny and cloudy days). We want to identify the processes that alter the isotopic composition of the leaf water in the water flow in a maize crop field (station of Bioclimatology, INRA, Grignon, Yvelines, France). In all cases, the isotopic compositions of leaf water show a periodic evolution with a maximum in the early afternoon and a minimum at the end of the night. This daily evolution is in response to the daily variations observed in relative humidity. Experimental verifications of the isotopic model of the transpiration process enable one to establish a physical model which shows a satisfactory agreement between the measured isotopic compositions in the leaf water and the model calculations. We modelled the changes in the 2 H 1 H and 18 O 16 O ratios of leaf water in response to the changes in the climatic factors (temperature and hygrometric deficit of saturation of the atmosphere), the 2 H 1 H and 18 O 16 O ratios of the atmospheric water vapor and of the source water (i.e. water from soil), and the turnover of water in the leaf. At first, this simple physical model consists of two compartments (a well-mixed leaf water reservoir and air water vapor). A diffusional boundary layer in the leaf stomata separates these two compartments. We try to explain the discrepancies between the measured and the calculated values by the effects of an incomplete mixing of water in the leaf. Taking into account the differences between the isotopic compositions of the water pools in the leaf enables one to calculate the daily evolution of the volume of leaf water marked by the transpiration process. Vein water and apoplasmic water are unfractionated concerning the soil water. We have found no linear covariance between the isotopic composition of the leaf water and the leaf water potential in this study. Taking into account the different origins of water supply in the leaf (soil and internal reserves of the leaf) and the amounts of water lost then recovered by the leaf enables one to explain the hysteresis loop observed between the isotopic composition of the leaf water and leaf water potential. With experimental data on the actual evapotranspiration flux, the isotopic model of transpiration allows a precise determination of the nature (transient or stationary) of the isotopic state of the water in the leaf with an increasing accuracy. During the day, the isotopic composition of the leaf water fitted a steady-state model because of the high values of the turnover of the foliar water. In this case, the value of the kinetic enrichment factor was obtained to be 28.5‰ ( 18O). These values agree well with theoretical predictions in pure diffusive conditions in the stomata chamber. Examining the relationship of the two isotopes tracers in the leaf water, following previous investigators, the intersection between the steady-state transpiration line and the meteoric line cannot be used to determine the isotopic composition of the source water in the plant in a δ 2H δ 18O diagram. There is no fractionation during water uptake by the roots. Then the roots density in the soil and the isotopic profile of the soil water show considerable heterogeneities with depth. The effects of the root system and the mechanisms involved for the water transport from the soil layers to the active roots increase the difficulties of sampling and analysis. The isotopic composition of the source water determined at the collar level shows a periodic evolution with a minimum in the afternoon and a maximum at dawn. These changes may reflect the use by the plant of available water from the most active sites of root absorption in deeper soil layers during transpiration under natural conditions.