Resistance to Water Transport in Plants—a Misconception

Abstract

IN 1948, van den Honert1 directed attention to Gradmann's idea of applying an analogue of Ohm's law to water movement in plants, and others have made the same suggestion (see Ray2 and Monteith3). Using this analogy, Gradmann concluded that the major resistance to water flow in the plant is in the gaseous phase above the transpiring surface, since almost all the potential drop occurs there. Van den Honert reconsidered this idea and concluded that the “permeability” of a free water surface appears to be 20 to 500 times smaller than protoplasmic permeability. He also concluded that regulators of water transport can be in the gaseous phase only, and if, for example, resistance to the passage of water in the roots is increased, it will have little effect on the overall rate of water transport.