Physiological evidence for auxin-induced hydrogen-ion secretion and the epidermal paradox

Peeled Avena coleoptile sections will respond to auxin only if the molarity of the incubation buffer at pH 6.2 is less than 5 mM. This inhibition of auxin-induced growth is not due to toxicity or to a reduction of turgor below the critical value needed for extension but rather appears to be related simply to buffering capacity. These data therefore serve as physiological evidence that H(+)-secretion is an intregal part of auxin-induced cell wall loosening. Other data obtained utilizing peeled plant sections and epidermal strips suggest that the epidermis does not directly control cell extension growth. A model is proposed to explain the curvature response in split-segments tests in terms of a H(+) gradient across the section. As far as tested this model appears to be an alternative to an older concept which implied that the curvature phenomenon in split sections was mediated by special properties of the epidermal layer. Our results suggest that the curvature response may be more directly attributable to the presence of the cuticle.