Water Movement Through Plant Roots

Mathematical analysis of the hydraulics of water movement through plant roots, in terms of radial and axial resistances, has led to equations which provide new insights into the effects of the component resistan ces on water uptake by and movement through individual roots and root systems. The ratio of axial to radial resistance determines the optimum length of a root and its total resistance to water movement. The equations permit direct calculations of the plant water potentials necessary, at the base of the plant, to sustain given flow rates through root systems with given characteristics. Lateral spacing and the resistance of individual laterals are the dominant factors determining total flux per unit area into a root. When soil water potential increases with depth (surface layers drier) root resistance tends to decrease with increasing flow rate; the reverse occurs when the surface is wetter than the lower layers. Calculated patterns of water movement into and through roots, in relation to soil water potential and flow rate through the root, indicate efflux from root to soil under certain conditions. This is considered to reflect reality, although the fluxes are probably transient or intermittent. The equations presented should be combined with equations describing water movement through soil to define the behaviour of the whole root-soil system adequately.