The mechanism of water absorption by roots II. The role of hydrostatic pressure gradients across the cortex

Using methods described in Part I differences of pressure were applied between the external medium and the xylem elements of tomato root systems. The pressure permeability coefficient $\text{k}_{p}$ (the water flux per unit pressure difference), and the osmotic permeability coefficient $\text{k}_{0}$ (the water flux per unit difference of osmotic potential between the external medium and the xylem elements) were compared under similar conditions of applied pressure. Also $\text{k}_{0}$ was measured in the presence and absence of pressure gradients. The most striking effect of applying a pressure gradient was an increase in $\text{k}_{0}$, 2 atm commonly causing a five-fold increase. Thus diffusion of water across the cortex was greatly facilitated by a pressure gradient. In addition it was found that under similar pressure conditions $\text{k}_{p}$ was consistently greater than $\text{k}_{0}$, $\text{k}_{p}$/$\text{k}_{0}$ being about 1.3 with an applied pressure difference of 2 atm. This indicated a mass flow of about a quarter of the total flux under these conditions. This dual nature of the water movement was confirmed by the action of cyanide and anaerobic conditions which caused a greater reduction in $\text{k}_{0}$ than $\text{k}_{p}$ showing that the diffusional pathway was more sensitive to decreased metabolism than the mass flow pathway. The effect of pressure gradients on diffusional water movement and the possible location of the mass flow pathway are briefly discussed as are their implications with reference to transpirational tensions and the uptake of solutes from the soil.