ABSTRACT

Many aspects of plant water use -particularly in response to soil drought -may have as their basis the alteration of hydraulic conductance from soil to canopy. The regulation of plant water potential ( Ψ Ψ Ψ Ψ ) by stomatal control and leaf area adjustment may be necessary to maximize water uptake on the one hand, while avoiding loss of hydraulic contact with the soil water on the other. Modelling the changes in hydraulic conductance with pressure gradients in the continuum allows the prediction of water use as a function of soil environment and plant architectural and xylem traits. Large differences in water use between species can be attributed in part to differences in their 'hydraulic equipment' that is presumably optimized for drawing water from a particular temporal and spatial niche in the soil environment. A number of studies have identified hydraulic limits as the cause of partial or complete foliar dieback in response to drought. The interactions between root:shoot ratio, rooting depth, xylem properties, and soil properties in influencing the limits to canopy water supply can be used to predict which combinations should optimize water use in a given circumstance. The hydraulic approach can improve our understanding of the coupling of canopy processes to soil environment, and the adaptive significance of stomatal behaviour.

In this paper we describe how a model of stable isotope fractionation processes, originally developed by H. Craig and L.I. Gordon ([1965] in E Tongiorgi, ed, Proceedings of a Conference on Stable Isotopes in Oceanographic Studies and Paleotemperature, Spoleto, Italy, pp 9-130) for evaporation of water from the ocean, can be applied to leaf transpiration. The original model was modified to account for turbulent conditions in the leaf boundary layer. Experiments were conducted to test the factors influencing the stable isotopic composition of leaf water under controlled environment conditions. At steady state, the observed leaf water isotopic composition was enriched above that of stem water with the extent of the enrichment dependent on the leaf-air vapor pressure difference (VPD) and the isotopic composition of atmospheric water vapor (AWV). The higher the VPD, the larger was the observed heavy isotope content of leaf water. At a constant VPD, leaf water was relatively depleted in heavy isotopes when exposed to AWV with a low heavy isotope composition, and leaf water was relatively enriched in heavy isotopes when exposed to AWV with a large heavy isotope composition. However, the observed heavy isotope composition of leaf water was always less than that predicted by the model. The extent of the discrepancy between the modeled and observed leaf water isotopic composition was a strong linear function of the leaf transpiration rate.The stable isotopic composition of plant leafwater is altered during transpiration. Water vapor molecules containing the lighter isotopes of oxygen and hydrogen escape from the leaf more readily than do heavy isotope molecules, so that during transpiration, leaf water becomes enriched in heavy isotope molecules (19,24,28 photosynthetic gas exchange and plant water-use efficiency (10,11,20,24,28).Previous attempts to use the Craig and Gordon model in studies of leaf water isotopic enrichment have been complicated by two factors. First, there has been uncertainty about the value to use for the kinetic fractionation factor. Values for the relative rates of diffusion of water vapor molecules containing light and heavy isotopes of oxygen and hydrogen have been measured (16). These measured values for the kinetic fractionation factor are appropriate for molecular diffusion only, however, and need to be modified for turbulent conditions in a boundary layer. In previous studies there has been uncertain and inconsistent modification of the kinetic fractionation factor to account for turbulence in the water vapor diffusion pathway (3,7,11,14,20,22,23,25). Second, a major assumption in the derivation of the evaporative enrichment model is that isotopic steady state is reached. When leaf water is at isotopic steady state, the isotopic composition of transpiration water is the same as the source or stem water isotopic composition (24). The assumption of isotopic steady state has not been verified in most studies attempting to test the ability of the Craig and Gordon model to predict leaf wat...