Abscission: the initial effect of ethylene is in the leaf blade.

The leaf blade of cotton (Gossypium hirsutum L. cv. Stoneville 213) was investigated as the initial site of ethylene action in abscission. Ethylene applied at 14 mul/l to intact 3-week-old plants caused abscission of the third true leaf within 3 days. However, keeping only the leaf blade of this leaf in air during ethylene treatment of the rest of the plant completely prevented its abscission for up to 7 days. This inhibition of abscission was apparently the result of continued auxin production in the blade since (a) the application of an auxin transport inhibitor to the petiole of the air-treated leaf blade restored ethylene sensitivity to the leaf in terms of abscission; (b) repeated applications of naphthaleneacetic acid to the leaf blade of the third true leaf, when the entire plant was exposed to ethylene, had the same preventive effect on abscission of this leaf as keeping its leaf blade in air; and (c) the inhibitory effect of ethylene on auxin transport in the petiole, which is reduced by auxin treatment, was also reduced by placing the leaf blade in air.The reverse treatment of exposing only the leaf blade of the third true leaf to 14 mul/l of ethylene, while the rest of the plant was kept in air, also did not cause abscission for up to 5 days. Auxin transport in the petioles of these leaves, however, was inhibited over 80% within 2 days and this effect presumably accounted for their increased sensitivity to ethylene during the subsequent exposures of the whole leaf to the gas.These results suggest that an initial and essential function of applied ethylene in abscission is to reduce the amount of auxin transported out of the leaf blade. This reduction together with the inhibitory effect of ethylene on auxin transport in the petiole reduces the auxin level at the abscission zone to a point where the cells in this region become responsive to the more direct action of the gas (e.g., enzyme induction and secretion). This sequence of events accounts for the lack of abscission unless ethylene is applied to both the leaf blade and the abscission zone.