Comparative Stimulation of Parthenocarpy in the Tomato by Various Indole Compounds

Abstract

The stimulatory effect of indole compounds on fruit-setting of several horticultural crops (7,8,18) was soon established following the identification of indole-3-acetic acid as a plant growth hormone (12). Strangely enough, however, for the stimulation of parthenocarpy many related compounds, some analogs of indole-3-acetic acid, were found more effective (7,8,11,23,25,27,32). MIeanwhile, extracts of corn kernels in the milk stage were described as having a fruit-setting potency much greater than could be accredited to indole-3-acetic acid alone (9,26). Recently the fruit-setting factor derived from the ethanol extracts of immature corn kernels was characterized by REDEMANN et al. (16) as the ethyl ester of indole-3-acetic acid and was found to induce parthenocarpic fruit development in the tomato when applied in dilutions of 1 part in 10,000. Thus, the fruitsetting activity was multiplied almost 100-fold compared with the free acid or the salts of heteroauxin.Recent studies concerned with the relation of chemical structure to activity, the relative instability of indole-3-acetic acid, the enhancement of activity by the addition of chlorine to the ring structure of certain growth substances, the role of growth substances in in vitro culture of tomato fruits (14) and finally, the scant attention directed toward halogenated or otherwise substituted indole compounds in biological activity testing suggested a more detailed evaluation of this chemical group of compounds with respect to the comparative induction of parthenocarpic fruit development in the tomato.Methods A known quantity of the compound was dissolved in anhydrous, peroxide-free ether and diluted to the desired concentration. An aliquot of this solution containing the required amount of the compound was added to a known quantity of anhydrous lanolin and stirred until solution was complete. The ether was removed by immersing the containers of the solution in a hot water bath. For higher dilutions the same procedure was followed with smaller aliquots.Estimation of the fruit-setting activity of the various indole compounds consisted of applying 1, 0.1, 0.01, and 0.001% lanolin solutions of each chemical to the ovary of the tomato flower from which the stamens had been 1 Journal article no. 1385 of the Michigan Agricultural Experiment Station.

Correlation of structural and physico-chemical parameters with the bioactivity of alkylated derivatives of indole-3-acetic acid, a phytohormone (auxin)

“…Similar results were obtained in the Avena coleoptile straight-growth test (Muir et al, 1949;Muir & Hansch, 1953) and the split pea-stem curvature assay (Hoffmann et al, 1952;Porter & Thimann, 1965). In the tomato, 2-Me-IAA induced parthenocarpy (Sell et al, 1952) if applied in suf®ciently high concentrations and 5-Me-IAA was at least as effective as IAA in causing cell elongation and cell proliferation in stems and petioles (Hoffmann et al, 1952).…”

Effects of chlorosubstituted indoleacetic acids on root growth, ethylene and ATP formation

“…Synthetic chloroindoleacetic acids were tested as auxins already in the 1950s when no one suspected that they were natural compounds {Hoffmann et al 1952, Muir and Hansch 1953, Sell et al 1953, Porter and Thimann 1965. Several of the monochlorosubstituted indoleac-etic acids are highly active auxins, while most dichlorosubstituted indoleacetic adds are less active auxins or, perhaps, anti-auxins.…”

Some Effects of Gibberellin on Flowering and Fruit Setting.

“…Whereas the comparisons are relative, the greater effectiveness of GB at very low concentrations is apparent, and equals or exceeds that of any known indole compound (10). This is one additional bio-assay in which the effects of GB on growth resemble those of auxin (3, 4, 7).…”