Regulation of ethylene biosynthesis during senescence of oat leaf segments

The evolution of endogenous ethylene, the conversion of 1-aminocylopropane-1-car-boxylic acid (ACC) to ethylene and the amounts of ACC (free and conjugated) have been followed during the senescence of oat ( Avena sativa L. cv. Victory) leaf segments. During the first three days of incubation of leaf segments in darkness, endogenous ethylene evolution and ACC-dependent ethylene production displayed a close relationship, both showing an increase followed by a decrease to the basal rate. However, unlike ethylene production, the level of ACC increased during the five days of incubation in the dark without any decline. It is concluded that ACC synthesis does not limit ethylene production, at least in the last stages of leaf senescence when ethylene production markedly decreased. The level of conjugated ACC increased and reached a plateau already at the first day of incubation. Yet, at the progressive stages of senescence, when the level af ACC gradually increased, no further conjugation of ACC could be detected. Thus, conjugation of ACC cannot account for ethylene drop at the last stages of oat leaf senescence.