Hydraulic Resistance of Plants. I. Constant or Variable?

Abstract

Two popular equations for describing the flow of water through plants are discussed and criticized. They are the Ohm's Law analogue, in which the flow rate is assumed to be proportional to the difference in water potential across the plant, between leaf and soil; and an equation, used widely to describe transmembrane fluxes of single cells, in which the flow rate is assumed to be the sum of two components, one proportional to the difference in hydrostatic pressure, the other proportional to the difference in osmotic pressure, between specified points in the plant. The well-established theoretical inadequacies of the Ohm's Law analogue are reviewed, especially the poorly recognized fact that it is of little use if substantial amounts of solutes are flowing with the water. The other, composite, equation does not have the same grave theoretical inadequacies, but its failure to agree with published data is highlighted.