Exogenously applied indole-3-acetic acid (IAA) strongly promoted stem elongation over the long term in intact light-grown seedlings of both dwarf (cv Progress No. 9) and tal1 (cv Alaska) peas (Pisum safivum l.), with the relative promotion being far greater in dwarf plants. In dwarf seedlings, solutions of IAA (between 10-4 and 10-3 M), when continuously applied to the uppermost two internodes via a cotton wick, increased whole-stem growth by at least 6-fold over the first 24 h. The magnitude of growth promotion correlated with the applied IAA concentration from 10-6 to 10-3 M, particularly over the first 6 h of application. IAA applied only to the apical bud or the uppermost internode of the seedling stimulated a biphasic growth response in the uppermost internode and the immediately lower internode, with the response in the latter being greatly delayed. This demonstrates that exogenous IAA effedively promotes growth as it is transported through intact stems. IAA withdrawal and reapplication at various times enabled the separation of the initial growth response (ICR) and prolonged growth response (PCR) induced by auxin. The ICR was inducible by at least 1 order of magnitude lower IAA concentrations than the PCR, suggesting that the process underlying the ICR is more sensitive to auxin indudion. In contrast to the magnitude of the IAA effect in dwarf seedlings, applied IAA only doubled the growth in tal1 seedlings. These results suggest that endogenous IAA is more growth limiting in dwarf plants than in tal1 plants, and that auxin promotes stem elongation in the intact plant probably by the same mechanism of adion as in isolated stem segments. However, since dwarf plants to which IAA was applied failed to reach the growth rate of tal1 plants, auxin cannot be the only limiting factor for stem growth in peas.

Free indole-3-acetic acid levels were measured by gas chromatography-mass spectrometry in three ultra-tall 'slender' Pisum sativum L. lines differing in gibberellin content. Measurements were made for apices and stem elongation zones of light-grown plants and values were compared with wild-type, dwarf, and nana phenotypes in which internode length is genetically regulated, purportedly via the gibberellin level. lndole-3-acetic acid levels of growing stems paralleled growth rates in all lines, and were high in all three slender genotypes. Growth was inhibited by pchlorophenoxyisobutyric acid, demonstrating the requirement of auxin activity for stem elongation, and also by the ethylene precursor 1-aminocyclopropane-1 -carboxylic acid. It is concluded that the slender phenotype may arise from constant activation of a gibberellin receptor or transduction chain event leading directly or indirectly to elevated levels of indole-3-acetic acid, and that increased indole-3-acetic acid levels are a significant factor in the promotion of stem elongation.

Stem segments excised from light‐grown Pisum sativum L. (cv. Little Marvel) plants elongated in the presence of indole‐3‐acetic acid and its precursors, except for L‐tryptophan, which required the addition of gibberellin A, for induction of growth. Segment elongation was promoted by D‐tryptophan without a requirement for gibberellin, and growth in the presence of both D‐tryptophan and L‐tryptophan with gibberellin A3, was inhibited by the D‐aminotransferase inhibitor D‐cycloserine. Tryp‐tophan racemase activity was detected in apices and promoted conversion of L‐tryptophan to the D isomer; this activity was enhanced by gibberellin A3. When applied to apices of intact untreated plants, radiolabeled D‐tryptophan was converted to indole‐3‐acetic acid and indoleacetylaspartic acid much more readily than L‐tryptophan. Treatment of plants with gibberellin A3, 3 days prior to application of labeled tryptophan increased conversion of L‐tryptophan to the free auxin and its conjugate by more than 3‐fold, and led to labeling of N‐malonyl‐D‐tryptophan. It is proposed that gibberellin increases the biosynthesis of indole‐3‐acetic acid by regulating the conversion of L‐tryptophan to D‐tryptophan, which is then converted to the auxin.

Two-week-old dwarf peas (Pisum sativum cv Little Marvel) were sprayed with gibberellic acid (GA3), and after 3 or 4 days the upper stem and young leaf samples were analyzed for indole-3-acetic acid (IAA) and indole-3-acetyl aspartic acid by an isotope dilution high performance liquid chromatography method. GA3 increased IAA levels as much as 8-fold and decreased indole-3-acetyl aspartic acid levels.IAA is thought to have a major role in the regulation of elongation in young stems, and application of gibberellins may result in greatly increased stem elongation rates, particularly in dwarf varieties. Lantican and Muir (4) RESULTS AND DISCUSSION Typical determinations of IAA in pea stem segments are illustrated in Table I for three injections of 45 p.l from the final 2-ml sample. If the IAA or IAAsp peak overlapped other peaks, separation could be achieved by a slight adjustment of the pH. On occasion, confirmation that the peaks were IAA or IAAsp was obtained as follows. (a) Samples and standards were analyzed at a lower electrochemical cell potential and the peak area was divided by the peak area at the higher cell potential. These peak area ratios were the same for standards and samples. (b) Other elution solvents were sometimes used to examine standards and samples and in all cases the retention time of standards and samples was the same. (c) Samples and standards were sensitive to 3 N HCI at 60°C for 20 min but were not sensitive to 3 N NH40H under the same conditions. (d) In one case, methyl esters were made with diazomethane which chromatographed like the methylated standards on HPLC (50% methanol buffer, pH 5.0).GA3 induced a large increase in IAA levels in the stem as well as in the apex and young leaves (Table II). The greatest increase occurred in the rapidly elongating upper stem sections, but free IAA levels were still up to 3-fold greater in the lower stem segments of treated plants compared to untreated ones. The young leaves, plus apical meristems, should include the sites of most IAA synthesis, and levels of IAA are increased up to 5-fold by GA3 in this sample. It is interesting that the less responsive plants (experiment B) also gave a lower GA3-induced IAA increase (Table II)

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