Hydraulically versus Hormonally Controlled Root Water Uptake from Partially Wet Root Zones

Drip irrigation may lead to non-uniform distributions of water in the root zone. Models that simulate plant root water uptake and transpiration should account for this spatial variation of root zone water content. In this contribution, we present a detailed mechanistic model that describes and couples water flow through the soil, to and through the plant root system and that resolves flow at the single root scale. The water potentials and water fluxes in the plant are determined by the soil water potentials, the architecture and hydraulic properties of the root system, and by the upper boundary conditions at the shoot or root collar, which could be either a prescribed transpiration flux or leaf water potential. In this concept, the soil-plant system is treated as a purely hydraulic system. However, plant physiologists have investigated and debated the role of plant hormones in the regulation of plant transpiration as a function of root zone water potential. Hormones, which are produced in the root zone as a function of the root water potential, are transported by the transpiration stream to the shoot where they regulate stomatal conductance and transpiration. We implemented hormonal signaling in the model and simulated root water uptake and plant transpiration reduction for different scenarios. In the scenarios, the degree of hormonal control, the wetted fraction of the root zone, and the transpiration rate were varied. The simulated plant transpiration rate could be related to an average root zone soil water potential. For purely hydraulically controlled root water uptake, a linear relation between a maximal possible transpiration rate and the average soil water potential was found. For hormonally controlled root water uptake, rather the ratio of actual to potential transpiration rate is a function of the average soil water potential. This indicates that transpiration reduction occurs whenever a part of the root zone is dry independent of the potential transpiration rate. The water potential in the root zone in the dry part decreases with increasing transpiration rate leading to a larger hormone production. But the higher transpiration stream and dilution compensate the higher hormone production so that the hormone concentration in the shoot is not affected