DOES AUXIN INHIBIT THE ABSCISSION OF COLEUS LEAVES BY ACTING AS A GROWTH HORMONE

The hypothesis that auxin prevents abscission, in Coleus blumei, by causing growth has been confirmed in a number of different ways: (1) in the intact plant, petioles grow until just before abseission; (2) excising the blades causes uniformly fast abseission, which is correlated with uniform absence of elongation; (3) if one stimulates the debladed petioles to renewed growth by substituting IAA for the leaf-blades, one can restore their longevity to that of the intact leaves; (4) increasing the concentration of IAA added to debladed petioles increases both the elongation and the longevity. However, the parallel between elongation and longevity was not exact: IAA concentrations giving full replacement of the blades in preventing abscission gave less than fult replacement of elongation in petioles 2 and 3 and more than full replacement in petioles 5-8. Following the time-course revealed that if an IAA-treated debladed petiole elongates as much or more than normal during the first week after deblading, then it will have normal longevity. IN A RECENT review of internal factors controlling abscission, Jacobs (1962) discussed the evidence that auxin, whether substituted for the leaf blade or normally produced by the blade, inhibits abscission primarily by causing growth and only indirectly by affecting abscission. The advantages of this interpretation had been pointed out earlier (Wetmore and Jacobs, 1953): the abscission-retarding effects of auxin can be explained by the already well-known growthstimulating effects of auxin, without necessitating the assumption of an entirely new type of effect. Laibach (1933) and Mai (1934), in tirst reporting the abscission-inhibiting effect of a natural source of auxin, noted the concomitant stimulation of growth. The question as to the primary locus of auxin action-whether directly on the abscission-layer, or on the growth of the distal petiole-seems important enough and unresolved enough to warrant more exact investigation (Jacobs, 1962). Accordingly, we looked for quantitative answers to the tollowing questions: (1) What are the relations between growth and abscission of intact untreated leaves? (2) Does excising the native source of auxin cause a decrease in the growth of the debladed petiole, parallel to the 1 Received for publication March 27, 1964. 2 Some of these results were reported in 1962 at the 16th International Horticultural Congress (Jacobs, 1964). 3 The contributions of Mr. Rochmis were incorporated in his 1960 experimental senior thesis at Princeton College, entitled "Studies in abscission: relationship of auxininduced growth to retention in Coleus petioles." Dr. Kaushik's contributions are represented in his Ph.D. thesis (1962) at Princeton University entitled "The physiology of abscission." Report of work supported by a grant from the National Science Foundation and in The Higgins Trust Fund. already known decrease in its longevity? (3) To what extent does substituting synthetic auxin for the leaf-blade restore both the longevity and growth of the petiole? MATERIALS AND METHODS-Plants of Coleus blumei Benth., of the clone previously used and described by Jacobs (1952, 1955, 1958}, Wetmore and Jacobs (1953), and Rossetter and Jacobs (1953), were used in all experiments. Culture methods and techniques were in general as in earlier papers. The deblading pattern used was either 2-sided or spiral, as described by Rossetter and Jacobs (1953). Plants with all leaves intact were used as controls. Axillaries were removed from all plants at the beginning of the experiments and all newly forming axillary buds removed as they developed. Fresh applications of IAA-lanolin paste were made once a week after removing the previouas application by cutting a very thin slice from the old cut end of the petioles. No correction was made for this effect in the measurements of elongation. Petioles were checked daily at a given time for abscission, following Jacobs' procedure (1955), and measurements for growth of petiole were taken weekly until the petioles abscised. Coleus leaves of various ages produce auxin in different amounts, judging by collections of diffusible auxin from this same clone (Wetmcore and Jacobs, 1953), yet, earlier investigators added IAA in the same concentration to all debladed leaves. In an attempt to more exactly match the normal gradient of auxin production, we applied IAA in a gradient of concentrations to petioles of differing age, estimating the steps from Fig. 6 of Wetmore and Jacobs. A "1% gradient" (as in Fig. 3) refers to 1% IAA in lanolin on No. 2 petioles, 0.6%o on No. 1, 0.5%0 on No. 3, 0.4%0 on