Ethylene Biosynthesis and Its Involvement in Senescence of Broccoli Florets.

Abstract

Broccoli (Brassica oleracea, var. italica) florets (flower buds) senesce rapidly after har-vest at ambient temperatures. During senescence of harvested broccoli kept at 20°C, the rate of ethylene production significantly increased concurrent with a rapid yellowing ascribed to chlorophyll degradation with the rate reaching a maximum and declining thereafter. ACC (1-aminocyclopropane-1-carboxylic acid) synthase activity gradually de-creased as senescence progressed and, in contrast, ACC content in florets increased from the low level at the early stage of senescence and reached the high level at a later stage. ACC oxidase activity in florets rapidly increased to reach a peak, which was followed by a sharp decline to a low level as senescence progressed, in parallelling the increased pat-tern of ethylene production. The increase in ACC oxidase activity was promoted by the continous supply of exogenous ethylene as well as the enhanced rate of senescence. On the contrary, the rise in ACC oxidase activity was strongly suppressed by the treatment with 2, 5-norbornadiene (NBD), an inhibitor of ethylene action; concurrently NBD strongly retarded the progress in senescence. These results may indicate that endogenous ethylene that is involved in senescence of broccoli florets simultaneously regulates the induction of ACC oxidase activity.