Leaf Age and Ethylene-induced Abscission.

Ethylene has been generally credited with promoting the abscission of the oldest leaves on a plant first. Vegetative cotton (Gossypium hirsutum L.) seedlings are an exception to this generalization. Under some conditions the younger, apical, unexpanded, or partially expanded leaves abscise before the less young, basal leaves or cotyledons. The degree or extent of apical leaf abscission increases with ethylene concentration and with plant age from 2 to 5 weeks. The response is promoted by auxin transport inhibitors. Usually the leaves which abscise first are those which have just unfolded and ones apical to the opened but unexpanded leaves. With plants with eight or nine leaves and macroscopic leaf buds, after the initial loss of unexpanded leaves, abscission tends to progress downward from the youngest remaining leaves and upward from the oldest leaves. The findings indicate that some characteristic(s) of apical leaves increases their sensitivity to ethylene. The characteristic may be differences in the abscission process between expanded and unexpanded leaves or differences in the hormone complement of the different leaves. Work is under way to modify this young leaf abscission response in an effort to determine its cause.