AbstractAccording to the acid growth theory of auxin action, it has been proposed that auxin decreases root growth by inhibiting the proton pump, thus causing an alkalinization of the apoplast. This paper critically tests this hypothesis with corn (Zea maysL.) roots. It was found that: i) the pH‐growth curve for roots exhibits a broad optimum ranging from pH 4.5 to 9. ii) Any acid‐induced growth is of very short duration, iii) The low sensitivity of root growth to external pH is independent of both the pump activity and the buffer capacity of the bathing solution, iv) Neither incubation in acidic buffer nor stimulation of the proton pump reverts the auxin‐induced root growth inhibition. It is concluded that the auxin‐induced root growth inhibition is not mediated by cell wall alkalin