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Detailed three-dimensional models of root water uptake have becomeincreasingly popular for investigating the process of root water uptake.However, they suffer from a lack of information on important parameters,particularly on the spatial distribution of root axial and radialconductivities, which vary greatly along a root system. In this paper weexplore how the arrangement of those root hydraulic properties and branchingwithin the root system affects modelled uptake dynamics, xylem waterpotential and the efficiency of root water uptake. We first apply a simplemodel to illustrate the mechanisms at the scale of single roots. By using twoefficiency indices based on (i) the collar xylem potential ("effort") and(ii) the integral amount of unstressed root water uptake ("water yield"),we show that an optimal root length emerges, depending on the ratio betweenroots axial and radial conductivity. Young roots with high capacity forradial uptake are only efficient when they are short. Branching, incombination with mature transport roots, enables soil exploration andsubstantially increases active young root length at low collar potentials.Second, we investigate how this shapes uptake dynamics at the plant scaleusing a comprehensive three-dimensional root water uptake model. Plant-scaledynamics, such as the average uptake depth of entire root systems, were onlyminimally influenced by the hydraulic parameterization. However, otherfactors such as hydraulic redistribution, collar potential, internalredistribution patterns and instantaneous uptake depth depended strongly onthe arrangement on the arrangement of root hydraulic properties. Root systemswere most efficient when assembled of different root types, allowing forseparation of root function in uptake (numerous short apical young roots) andtransport (longer mature roots). Modelling results became similar when thisheterogeneity was accounted for to some degree (i.e. if the root systemscontained between 40 and 80% of young uptake roots). The average collarpotential was cut to half and unstressed transpiration increased by up to25% in composed root systems, compared to homogenous ones. Also, the leastefficient root system (homogenous young root system) was characterized byexcessive bleeding (hydraulic lift), which seemed to be an artifact of theparameterization. We conclude that heterogeneity of root hydraulic propertiesis a critical component for efficient root systems that needs to be accountedfor in complex three-dimensional root water uptake models

hydrology and earth system sciences

Effect of parameter choice in root water uptake models – the arrangement of root hydraulic properties within the root architecture affects dynamics and efficiency of root water uptake