[C]Ethylene Metabolism during Leaf Abscission in Cotton.

Changes in (14)C(2)H(4) metabolism in the abscission zone were monitored during cotton (cv. Deltapine 16) leaf abscission. Rates of (14)C(2)H(4) oxidation to (14)CO(2) and tissue incorporation in abscission zone segments cut from the second true leaf of nonabscising leaves of intact plants were similar (about 200 disintegrations per minute per 0.1 gram dry weight per 5.5 hours) and relatively constant over a 5-day period. Deblading to induce abscission caused a dramatic rise in (14)C(2)H(4) oxidation, but tissue incorporation was not markedly affected. This rise occurred well before abscission, reaching a peak of 1,375 disintegrations per minute per 0.1 gram dry weight per 5.5 hours 2 days after deblading when abscission was 40%. The rate then gradually declined, but on day 5 when abscission reached completion, it was still nearly three times higher than in segments from nonabscising leaves. Application of 0.1 millimolar abscisic acid in lanolin to the debladed petiole ends increased the per cent abscission slightly and initially stimulated (14)C(2)H(4) oxidation. In contrast, naphthaleneacetic acid applied in a similar manner delayed and markedly inhibited both abscission and (14)C(2)H(4) oxidation.Petiole segments cut 1 centimeter from the abscission zone of intact second true leaves also incorporated and oxidized (14)C(2)H(4) to (14)CO(2) but at rates two and six times higher, respectively, than that of comparable adjacent abscission zone segments. However, in marked contrast to the abscission zone segments, no changes in oxidation were observed when the leaves were debladed to induce abscission.THESE RESULTS DEMONSTRATE THAT: (a) prior to abscission, the ethylene oxidation, but not the tissue incorporation pathway, rapidly increases in the abscission zone; (b) this increase does not occur in adjacent petiole tissue; and (c) changes in the rate of oxidation and per cent abscission brought about by hormone treatments parallel one another. The possible significance of these changes in ethylene metabolism is discussed with respect to the hypothesis that ethylene action and metabolism are directly related.