Events preliminary to avocado (Persea americana Mill) fruitlet abscission include senescence of the nucellus and seed coat. The dynamics of nucellar deterioration and ethylene production leading to seed abortion and abscission in avocado was examined. Excised branches bearing clusters of fruit from 1'0-2-5 cm diameter were placed in humid chambers to reduce transpirational water loss. Fruitlets synchronously began nucellar and seed coat deterioration 27-33 h after excision and rapidly progressed through stages of increasing degradation culminating in abscission approximately 2 days later. The nucellus-seed coat produced a temporary burst of ethylene at the first visible sign of nucellar senescence followed by less ethylene production in the mesocarp approximately 12 h later. All fruit underwent nucellar degradation prior to abscission. Exogenously applied ethylene accelerated fruitlet abscission with concentrations as low as 1-0 //11~' and with maximum response at 100 /il 1~' or greater. Maximal response took 2 days. Afninoethoxyvinyl-glycine (AVG) at 30 ftM inhibited ethylene production and fruitlet abscission. The senescence process, however, was not affected in any way by ethylene or AVG treatments. Observations of attached fruit suggest that nucellar-seed coat senescence, concomitant ethylene production, and resulting abscission take place in a manner and within a time period similar to that observed on detached branches. It is concluded that nucellar and seed coat senescence is prerequisite to avocado fruitlet abscission, and the time required from the first indication of nucellar breakdown to abscission of that fruitlet appears to be approximately 2 days. The senescence process is responsible for a large, transient rate increase in ethylene production by the nucellus and perhaps seed coat. Ethylene is considered to be the result rather than the cause of nucellar-seed coat senescence. The ethylene thus produced induces fruit abscission.

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