Nature New Biology 244, 285-288 (1973)
by P.H. Rubery and A.R. Sheldrake

The uptake of the auxin indol-3-yl acetic acid (IAA) into plant cells is of interest not only because this compound is a hormone, but also because its movement across the plasma membrane is probably involved in the polar transport of auxin. The plasma membrane contains auxin binding sites and may be a primary site of hormone action.

IAA partitions into non-polar solvents from acidified aqueous solutions because the undissociated acid is more soluble in such lipid solvents than in water. There is known to be a passive, non-metabolic component of the uptake of IAA and of the synthetic auxin 2,4-dichlorophenoxyacetic acid (2,4-D) into plant tissue which has been ascribed to the diffusion of the undissociated acid across the plama membrane. A carrier-mediated mechanism for auxin anion uptake is also possible but has not been conclusively demonstrated.

Uptake by the diffusion mechanism is linearly related to the concentration of the undissociated acid which is a function of the acid's pK and the pH of the incubation medium. If the pH of the medium is lower than that of the cells, the cells accumulate weak acid; the equation requires that the concentration of undissociated acid should be the same in each compartment. Thus the relation between the initial rate of uptake and pH should resemble a dissociation curve with a midpoint at the pK of the weak acid. This prediction is realized for the uptake of benzoic acid (pK=4.2) by yeast but not by the bacterium Proteus vularis, when, although the curve is still that of a dissociation, its midpoint is displaced by 1 pH unit above the pK of benzoic acid. Such displacement seems fairly widespread. By collating data from ninety experiments on pH dependence of biological effects of weak acids, a composite curve is obtained relating pH to log concentration of acid required to give a standard response; the midpoint of the curve is at a higher pH than the pK. Data on IAA and 2,4-D uptake reveal a similar effect. Here we suggest an explanation of this displacement which may be of general biological significance.