Role of Auxin and Gibberellin in Differentiation of Primary Phloem Fibers

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Abstract
The hypothesis that auxin and GA3 control the differentiation of primary phloem fibers is confirmed for the stem of Coleus blumei Benth. IAA sufficed to cause the differentiation of a few primary phloem fibers. In long-term experiments, auxin induced a considerable number of fibers in mature internodes. GA3 alone did not exert any effect on fiber differentiation. Combinations of IAA with GA3 completely replaced the role of the leaves in primary phloem fiber differentiation qualitatively and quantitatively. Although the combined effect of the 2 growth hormones diminished considerably with increasing distance from the source of induction, auxin with GA3 or IAA alone induced fibers in a few internodes below the application site. When various combinations of both hormones were applied, high concentrations of IAA stimulated rapid differentiation of fibers with thick secondary walls; high GA3 levels resulted in long fibers with thin walls. The size of the primary phloem fibers correlated with the dimensions of the differentiating internode, thereby provinding evidence that both growth regulators figure in the control of stem extension. High IAA/low GA3 concentrations have an inhibitory effect on internode elongation, whereas low IAA/high GA3 concentrations promote maximal stem elongation.
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