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

Abstract

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.Abbreviations: ABA, abscisic acid; PIP, plasma membrane intrinsic protein; PRD, partial root drying.The root system is the first actor for plant water uptake through its spatial architecture and the distribution of hydraulic conductance along root axes and branching orders. As a consequence, most models of water uptake are based on a dialogue between soil and root hydraulic properties, with an imposed flux or water potential at the base of the stem considered as the boundary of the system (Gardner, 1960;Nimah and Hanks, 1973;Feddes et al., 1976). This has resulted in models that couple soil water depletion and root water uptake (Doussan et al., 2006;Javaux et al., 2008;Schneider et al., 2010). The rationale for imposing boundary conditions at the stem base is that the water flux though the plant is primarily controlled by stomatal conductance. Indeed, the latter is several orders of magnitudes lower than any hydraulic conductance in the soil or in the plant even when stomata are fully opened. This can be visualized in plants that have defective stomatal control and wilt even under well-watered conditions (Borel et al., 2001;Dodd et al., 2009). Furthermore, root hydraulic conductance has no effect on transpiration when it is manipulated via chemical compounds (Ehlert et al., 2009).However, transpiration is controlled via feedbacks involving both roots and shoots, namely chemical and/or hydraulic messages in the short term and soil water depletio...

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