Flower Abscission in Excised Inflorescences of Three Plectranthus Cultivars

Flower abscission induced by ethylene in three Plectranthus cultivars was investigated in order to characterise response to a range of inhibitory and antagonistic compounds. Excised inflorescences were exposed to 100 ml l⁻¹ ethylene gas or placed in various concentrations of ethephon (277, 27.7, 2.77, 0.277 and 0.0277 μM). Flower abscission in Plectranthus was readily induced by applying ethylene gas and by the 277 μM dose of ethephon. Removal of the inflorescences from the ethylene treatment prevented subsequent flower abscission. This implies that ethylene treatment did not induce an autocatalytic production of ethylene. Compounds that are known to compete for the ethylene receptor (100 and 500 ppb 1-methylcyclopropene or 100 and 500 ppm 2,5-norbornadiene) did not reduce abscission in this system. Also, application of the ethylene biosynthesis inhibitor, aminooxyacetic acid at 1 mM, was ineffective at preventing ethylene-induced flower abscission. In contrast, one compound known to block protein production (100 μM cycloheximide) and a non-competitive inhibitor of ethylene action (2 mM silver thiosulfate) did prevent ethylene-induced abscission. We conclude that flower abscission in cut inflorescences of Plectranthus is very likely mediated by endogenous ethylene production, but that control of ethylene-induced flower abscission in this genus can not be readily obtained by most ethylene antagonists that are known to be effective in other systems.