Effect of Gibberellic Acid on Elongation and Longevity of Coleus Petioles

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Abstract
The effects of gibberellic acid on the longevity and elongation of variously aged, debladed petioles of Coleus blumei were studied, with particular reference to the hypotheses 1) that auxin increases longevity by increasing growth, and 2) that gibberellic acid acts by increasing the endogenous levels of auxin. Gibberellic acid, substituted for the leaf blades, significantly decreased longevity of younger petioles, as measured by days or hours to abscission. Gibberellic acid also decreased the longevity resulting from 0.1% indoleacetic acid. This is the opposite of the effect expected if it is increasing auxin levels in the petiole. In its effect on elongation of younger petioles, however, gibberellic acid did act in the direction expected if it were increasing effective levels of auxin in the petiole. The elongation rate from 0.1% gibberellic acid plus 0.1% indoleacetic acid in lanolin was as large or larger than that for 1.0% indoleacetic acid. Petioles which were 10 or more weeks old (i.e., at positions 5+ below the apical bud were not affected by 0.1% gibberellic acid in either longevity or rate of elongation, with or without 0.1% indoleacetic acid. Since 1.0% indoleacetic acid increases both longevity and elongation rate of these petioles over 0.1% indoleacetic acid, gibberellic acid is clearly not acting on older petioles as if it were increasing effective auxin levels).