Modelling of the hydraulic architecture of root systems: An integrated approach to water absorption - Distribution of axial and radial conductances in maize

The 'Hydraulic Tree Model' of the root system simulates water uptake through root systems by coupling a root architecture model with laws for water flow into and along roots (Doussan, Pages and Vercambre, Annals of Botany 81: 213-223, 1998). A detailed picture of water absorption in all roots comprising the root system is thus provided. Moreover, the influence of different distributions of radial and axial hydraulic conductances in the root system on the patterns of water uptake can be analysed. Use of the model with Varney and Canny's data (1993) for flow along maize roots demonstrated that a constant conductance in the root system cannot reproduce the observed water flux profiles. Taking into account the existing data on hydraulic conductances in maize roots, we fitted the distribution of conductances in the root system to the observed flux data. The result is that, during root tissue maturation, the radial conductivity decreases by one order of magnitude while the axial conductance increases by about three orders of magnitude. Both types of conductance exhibit abrupt changes in their evolution. Due to the conductance distribution in the root system, appreciable water potential gradients may develop in the roots, in both the branch roots and main axes. An important point is that the conductance distribution in the branch roots described by the model should be related to the age of the tissue (and not the distance from the branch root tip) and is therefore closely related to the development process. Thus for branch roots, which represent about 90% of the calculated total water uptake in 43-d-old maize, water absorption will depend on the opening of the metaxylem in the axes, and on the time dependent variation of the conductances in the branch roots. (C) 1998 Annals of Botany Company.