Effects of Gibberellic Acid on Utilization of Auxin Precursors by Apical Segments of the Avena Coleoptile

It has been shown recently that treatment with gibberellic acid (GA) results in an increase in diffusible auxin in the pea (7), sunflower (7) and Centaurea (8). Treatment with GA also increases the amount of extractable auxin in peas (14), the potato tuber (18), Rhus typhina (13) and Trifoliuws ochroleucunm (15). Further it has been (lenmonstrated that in the dwarf pea and Cenitauirea the increase in diffusible auxin occurs before the elongation of the stem or the bolting response to GA treatment (9). In the tomato ovary at anthesis there is no diffusible auxin but, following GA treatment, (liffusible auxin is obtained and the amount increases with time up to 22 (lay-s concomitant wvith the growth of the ovary (16). These observations show that GA has a role in auxin formation. Furthermiiore, enzyme preparations from plant tissues treate(d with GA have a greater efficiency for conversion of tryptophan into auxin than those obtained fromii nontreated tissue (12). Tryptophan (19), tryptamine (17), inidole-3-acetaldlehyde (10) and indole-3-acetonitrile (2) have all been shown to be precursors of auxini in coleoptile tissue. To assay the auxin fornme(d and to examine the interaction of the precursors with GA a modified straight growth test was devised. In this test the segments of coleoptile tissue included the tips as they have been shown to contain the enzyme systems capable of converting the auxin precursors (10, 19).