Modeling the Dynamics of Water Flow through Plants, Role of Capacitance in Stomatal Conductance, and Plant Water Relations

Abstract

A model that couples stomatal conductance, photosynthesis, leaf energy balance, and transport of water through the soil-plant-atmosphere continuum has been developed. In this model, stomatal conductance depends on light, temperature, and intercellular CO2 concentration via photosynthesis and on leaf water potential. Leaf water potential is a function of soil water potential, plant hydraulic resistances, and the rate of water flow through the soil and plant. The net canopy photosynthesis is calculated by means of the detailed biochemical model of photosynthesis for C3 plants proposed by Farquhar and widely employed in both leaf and canopy-scale gas studies. Plant water relations are modeled as an analogue to a simple electrical circuit including plant hydraulic resistances and plant capacitance. Water uptake by roots is controlled by the water potential gradient between the absorbing root surface and a cylindrical soil element adjacent to the roots. Absorbing roots are assumed to be uniformly distributed throughout the entire soil volume. Water transport from soil to roots is simulated through solution of the Richards equation. Model calculations for a soil dry-down cycle of 15 d show the effects of plant water storage in the soil–plant–atmosphere continuum model on the stomatal conductance, leaf water potential, and water fluxes (water uptake from the soil, plant water storage, and transpiration). The model successfully captures the daytime asymmetry in conductance under water stress conditions, and it successfully describes the observed hysteresis in stomatal conductance versus leaf water potential. Comparison between modeled and measured average soil water content during an experimental period under controlled environmental conditions shows that the model is a reliable simulation tool. The accuracy of the model is also tested by comparing computations of mass and energy flux densities (water vapor and sensible heat) against eddy covariance measurements over wheat canopy. This first comparison with experimental data gives greater confidence in the proposed model. However, to test more specific responses of the model, it would be useful to set up new experiments with different canopies and more specific measurements of plant physiology.