WATER TRANSPORT-SYSTEM AND ITS COMPONENTS IN HIGHER-PLANTS - 4 - ON THE MECHANISM OF ROOT PRESSURE FORMATION IN INTACT PLANTS

A concept of the formation of elevated water potential (psi(2)) in the stele of plant roots exceeding that of the external medium (psi(o)) i s presented. The separation of the root compartment by Casparian strip s into peripheral zone 1 (cortex) and central zone 2 (stele) was shown to be the physical basis for the formation of elevated psi(2). It is suggested that more active metabolism in zone 1 results in higher osmo tic pressure in this zone as compared to zone 2. Upon equilibration of osmotic pressures between zone 1 and the external medium, this zone i ncreases its turgor and acts as a ''pressing cuff'' for zone 2. The el evated turgor P and low osmotic pressure in in pi(2)(in) in zone 2 det ermine a higher water potential of this zone (psi(2) = P - pi(2)(in) > psi(o)), which is the motive force of root pressure.