Modeling Active and Passive Uptake of Salts By Plants.

Plant salt stress is one of the major causes of crop productivity reduction, especially in irrigated areas or in (semi-)arid regions. Some models describe plant salt stress as  a function of soil osmotic potential, neglecting toxicity caused by ionic stress. Existing empirical models are limited and restricted to experimental conditions. Our objective is to describe solute uptake by roots using a model that accounts for active and passive mechanisms of uptake. Starting from the Michaelis-Menten (MM) equation to describe active and passive solute uptake mechanisms, we assumed that stress is caused by passive uptake only as active uptake is plant regulated. We developed an implicit numerical root water and solute extraction model to solve Richards and convection-dispersion equations, respectively, at a microscopic scale. In the solute extraction part of the model,  we linearized the low concentration part of MM equation to use it as boundary condition at the root surface, resulting in a soil concentration-dependent solute flux model. It can predict the solute concentration in the soil solution as a function of radial distance from the root and time. The advantage of this model, when compared to constant and zero solute flux models, is that it enables partitioning between passive and active uptake and, therefore, to determine the solute concentration inside the plant, which can be  linked to osmotic and ion-specific stress. This model can be merged into macroscopic crop growth models to predict the crop yield in different scenarios.