ROOT CLUMPING MAY AFFECT THE ROOT WATER POTENTIAL AND THE RESISTANCE TO SOIL-ROOT WATER TRANSPORT

We have appraised for clumped root systems the widely-accepted view that the resistance to water flux from soil to roots ('soil resistance') is low under most field conditions, so that root water potential would closely follow the mean soil water potential. Three root spatial arrangements were studied, simulating either the regular pattern generally assumed in models, or two degrees of root clumping frequently observed in the field. We used a numerical 2-dimensional model of water transfer which assumes a control of evapotranspiration by root signalling. Calculations were carried out at two evaporative demands and for two contrasting soil hydraulic properties. The rate of soil depletion, the timing of the reduction in evapotranspiration and the difference between root water potential and mean soil water potential were all affected by the root spatial arrangement, with a greater effect at high evaporative demand and low soil hydraulic conductivity. Almost all the soil water reserve was available to plants without reduction in evapotranspiration in the regular case, while only a part of it was available in clumped cases. In the regular case, calculated 'soil resistances' were similar to those calculated using Newman's (1969) method. Conversely they were higher by up to two orders of magnitude in clumped root spatial arrangements. These results place doubt on the generality of the view that 'soil resistance' is low under common field conditions. They are consistent with the results of field experiments, especially with recent data dealing with root-to-shoot communication.