Water uptake by plant roots: an integration of views

A COMPOSITE TRANSPORT MODEL is presented which explains the variability in the ability of roots to take up water and responses of water uptake to diff erent factors. The model is based on detailed measurements of 'root hydraul ics' both at the level of excised roots (root hydraulic conductivity, Lp(r) ) and root cells (membrane level; cell Lp) using pressure probes and other techniques. The composite transport model integrates apoplastic and cellula r components of radial water flow across the root cylinder. It explains why the hydraulic conductivity of roots changes in response to the nature (osm otic vs. hydraulic) and intensity of water flow. The model provides an expl anation of the adaptation of plants to conditions of drought and other stre sses by allowing for a 'coarse regulation of water uptake' according to the demands from the shoot which is favorable to the plant. Coarse regulation is physical in nature, but strongly depends on root anatomy, e.g. on the ex istence of apoplastic barriers in the exo- and endodermis. Composite transp ort is based on the composite structure of roots. A 'fine regulation' resul ts from the activity of water channels (aquaporins) in root cell membranes which is assumed to be under metabolic and other control.