A FINITE-ELEMENT MODEL OF RADIAL AND AXIAL CONDUCTIVITIES FOR INDIVIDUAL ROOTS - DEVELOPMENT AND VALIDATION FOR 2 DESERT SUCCULENTS

A morphologically explicit numerical model for analysing water uptake by individual roots was developed based on a conductor network, with specific conductors representing axial or radial conductivities for discrete root segments. Hydraulic conductivity (Lp; m s-1 MPa-1) was measured for roots of Agave deserti Engelm. and Opuntia ficus-indica (L.) Miller by applying a partial vacuum to the proximal ends of excised roots in solution. Lp was also measured for 40- to 80-mm segments along a root, followed by measurements of axial conductivity and calculation of radial conductivity. Predicted values of Lp for entire roots based on two to ten segments per root averaged 1.04±0.07 (mean±s.e. mean for n = 3) of the measured Lp for A. deserti and 1.06±0.10 for O. ficus-indica. The model also closely predicted the drop in water potential along the root xylem (Δψxylem); when a tension of 50 kPa was applied to the proximal ends of 0.2 m-long roots of A. deserti and O. ficus-indica, the measured Δψxylem to midroot averaged 30 kPa compared with a predicted decrease of 36 kPa. Such steep gradients in ψxylem suggest that the driving force for water movement from the soil to young distal roots may be relatively small. The model, which agreed with an analytical solution for a simple hypothetical situation, can quantify situations without analytical solutions, such as when root and soil properties vary arbitrarily along a root. © 1992 Annals of Botany Company.