Root Water Uptake and Ideotypes of the Root System: Whole‐Plant Controls Matter

Core Ideas The threshold soil water potential for water uptake depends on stomatal control. Whole‐plant transpiration increases hydraulic conductance in roots and shoots. It decreases the rhizosphere hydraulic conductance, resulting in feedback loops. Root system ideotypes for water uptake are not always associated with the best yields. Models integrating controls in shoots and roots are needed to optimize root traits. Simulations of plant water uptake in soil science are based on the interplay between soil and root properties, with an imposed flux or water potential at the stem base. The dialogue between roots and shoots is important in water uptake. The threshold soil water potential for water uptake represents the soil water potential at which stomatal control stops transpiration over 24 h. Measurements show that it has a large variability among species and cultivars. Isohydric plants prevent low leaf water potentials via stomatal control, so their threshold soil water potential is high. Anisohydric plants allow low leaf water potentials, resulting in lower thresholds. These behaviors have a genetic control and can be simulated via whole‐plant models. In studied species, the hydraulic conductance in roots and shoots depends on the whole‐plant transpiration rate. In particular, there is a “dialogue” between the daily alternations in the transpiration rate and the circadian oscillations in root hydraulic conductance that affect the hydraulic conductance of the rhizosphere, with appreciable consequences on water uptake. Root traits such as length, branching, or depth interact with shoot traits such as leaf area or stomatal control, thereby generating feedbacks. As a consequence, optimum root systems for water uptake at a given time are not always those associated with the best yields. Models that take these whole‐plant results into account bring an extra level of complication but are probably indispensable whenever the aim is to optimize root traits in view of improved drought tolerance.