Physical Constraints and Biological Control of Root Water Uptake

How plant adapt their hydraulic conductance under heterogeneous and dynamics environment is a key trait for their resilience against drought stress. Plants are continuously subject to changes in atmospheric evaporative demand and soil moisture and have to continuously adapt their structure and function to improve their access to soil water and protect against dehydration and cavitation. These adaptation mechanisms take place locally at very contrasted temporal resolutions from seconds (for stomatal closure for instance) to weeks (for growth and development). Complex modeling tools exist, which simulate the soil-root system hydraulics at local and plant scales. Based on physical laws these models integrate the impact of the plant regulation on internal resistances and the control of the soil and of the rhizosphere on water stream. These multiscale models help understand how local change in conductance might affect the distribution of the root water uptake and transpiration rate. However, they are not efficient enough yet to be used in crop models. Simplified, upscaled models are therefore needed, which estimate plant behavior at low computational costs. Here, we present a new approach to characterize the sensitivity of plant systems to hydric conditions. We developed a simplistic mathematical function based on physical laws that characterizes the plant-soil hydraulic behavior. By comparing the model results to experimental data from the literature, we demonstrate that it captures key features of plant functioning. It also lead us to propose new hypothesis on optimal stomatal behavior. Finally, we use the model to investigate how adaptations of plant structural and functional characteristics affect its ability to extract soil water and to propose above- and below- strategies that plants can develop to mitigate the soil hydraulic constraints.