Rapid Auxin-induced Decrease in Free Space pH and Its Relationship to Auxin-induced Growth in Maize and Pea

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Abstract
A pH microelectrode was used to investigate the auxin effect on free space pH and its correlation with auxin-stimulated elongation in segments of pea (Pisum sativum) stem and maize (Zea mays cv. ''Bear Hybrid'') coleoptile tissue. Auxin induces a decrease in free space pH in both tissues. In maize coleoptiles, free space pH begins to fall within about 12 min of exposure to auxin and decreases by about 1 pH unit by approximately 30 min. In pea, pH begins to decrease within an average of 15-18 min of exposure to auxin and falls by about 0.9 pH unit by approximately 40 min. Auxin-stimulated elongation, measured in the same 2 tissues similarly prepared, appears in maize at the earliest 18 min after auxin application, while in pea it appears at the earliest 21-24 min after auxin application. The auxin analogs p-chlorophenoxyisobutyric acid and phenylacetic acid do not stimulate elongation above control levels in maize or pea tissue segments and do not cause a decrease in free space pH in either tissue. These findings are consistent with the acid secretion theory of auxin action.
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