Auxin-induced hydrogen ion excretion: correlation with growth, and control by external pH and water stress

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Abstract
The acid-growth theory predicts that the rates of auxin-induced cell elongation and H+-excretion should be closely correlated as long as the experimental conditions remain fairly constant. To test this, Avena coleoptiles have been induced to elongate at different rates by varying the concentration of auxin, the age of the tissue, or by addition of metabolic inhibitors. As predicted, in each case there was a close correlation between the rates of H+-excretion and growth. The rate and direction of movement of H+ between the coleoptile and the external medium is regulated by the external pH. Coleoptiles take up H+ passively from acidic solutions and excrete H+ into basic solutions. In the absence of auxin, uptake and excretion are at equilibrium when the solution pH is near 5.7, a pH too high to allow rapid cell wall loosening. Auxin stimulates the excretion, but as the external pH drops the excretion is inhibited and a new equilibrium is established near 5.0. This allows amlows maximum wall loosening without causing toxic side-effects. H+-excretion is also affected by water stress. Increasing water stress induced by mannitol decreases H+-excretion in auxin-treated tissues but stimulates H+-excretion in the absence of auxin. At incipient plasmolysis H+-excretion is insensitive to auxin, suggesting that even if H+-excretion is mediated by an auxin-activated ATPase it may be impossible to demonstrate an effect of auxin on this enzyme in vitro. Three types of H+-excretion have been recognized in coleoptiles; auxin-induced, stress-induced, and basal level. All appear to require ATP, but only basal level H+-excretion does not also require continual protein synthesis.