ETHYLENE BIOSYNTHESIS AND ITS INVOLVEMENT IN SENESCENCE OF BROCCOLI FLORETS

Broccoli (Brassica oleracea, var. italica) florets (flower buds) senes ce rapidly after harvest at ambient temperatures. During senescence of harvested broccoli kept at 20 degrees C, the rate of ethylene product ion significantly increased concurrent with a rapid yellowing ascribed to chlorophyll degradation with the rate reaching a maximum and decli ning thereafter. ACC (1-aminocyclopropane-1-carboxylic acid) synthase activity gradually de creased as senescence progressed and, in contras t, ACC content in florets increased from the low level at the early st age of senescence and reached the high level at a later stage. ACC oxi dase activity in florets rapidly increased to reach a peak, which was followed by a sharp decline to a low level as senescence progressed, i n parallelling the increased pattern of ethylene production. The incre ase in ACC oxidase activity was promoted by the continous supply of ex ogenous ethylene as well as the enhanced rate of senescence. On the co ntrary, the rise in ACC oxidase activity was strongly suppressed by th e treatment with 2,5-norbornadiene (NBD), an inhibitor of ethylene act ion; concurrently NBD strongly retarded the progress in senescence. Th ese results may indicate that endogenous ethylene that is involved in senescence of broccoli florets simultaneously regulates the induction of ACC oxidase activity.