Abscission

Abstract

A 1-hour application of indole-3-acetic acid to bean (Phaseolus vulgaris L. cv. Red Kidney) explants inhibited abscission for an 8-hour aging period. Use of indole-3-acetic acid-'4C showed that the applied indole-3-acetic acid was conjugated within explant tissue and that this conjugation mechanism accounts for loss of effectiveness of indole-3-acetic acid in inhibiting abscission after 8 hours. Reapplication of indole-3-acetic acid to an explant at a later time, before the induced aging requirement was completed reinhibited abscission. 2,4-Dichlorophenoxyacetic acid, which is not destroyed or conjugated by this system, did not lose its ability to inhibit abscission. It was concluded that indole-3-acetic acid destruction is one of the processes involved in the aging stage of abscission in explants.Indoleacetic acid has been demonstrated to effectively inhibit abscission in many plant systems if it is applied to the distal cut surface of petioles (10,14,15,17). The applied auxin has been supposed to replace the continuous auxin supply from the leaf (14,20). However, the mechanism by which IAA exerts its inhibition properties on abscission is unknown. Research has been primarily concerned with auxin transport systems, auxin balances, and auxin concentrations. Such work is well summarized in recent review articles (2, 12).There has been no correlation between time of application, location, or form of auxin at the time it functions to inhibit abscission processes. This paper presents a series of experiments designed to determine movement and conjugation of IAA in explants as correlated with initiation of abscission. MATERIALS AND METHODSPlant Material. Bean plants (Phaseolus vulgaris L. cv. Red Kidney), were grown at 24 + 2 C, 64% relative humidity, and a 12-hr photoperiod from fluorescent and incandescent lamps with 1200 ft-c at plant level. Petiole abscission zone explants from the primary leaf were excised from 14-day-old plants and inserted into a 3-mm layer of 1.5% agar contained in 43 + 2-ml gas collection bottles (5, 6). During the experiments, the explants were exposed to 25 C and 400 ft-c of continuous fluorescent light. Unless otherwise noted, each experimental datum represents experi- ments repeated on three different occasions with three sets of 10 explants.Application of Chemicals. Indole-3-acetic acid, the conjugates and derivatives of IAA (indoleacetyl aspartate, indoleacetamide, and ethyl indoleacetate), and 2,4-D were applied by placing a 50-Al drop of 1.5% agar containing the compound on the distal cut surface of the explant. In instances where the agar was subsequently removed, the explant was wiped with a moist tissue (10). The concentration of growth substance supplied in the agar was 50 M for IAA and its derivatives and 500 AM for 2,4-D.These concentrations of IAA and 2,4-D were selected as being effective for inhibition of abscission (1, 8, 10).Radioactive Assays. In studies on the movement of conjugation mechanisms of IAA in explants, IAA-1-14C (specific radioactivity 25.6 mc/mmole) o...