Radermachera sinica L . is an ornamental plant with demonstrated sensitivity to ethylene-induced leaf abscission . In this study, we examine the relationship between abscisic acid (ABA) and ethylene in initiating the abscission response . Treatment with 1 tl L-1 of ethylene, 1 mM 1-aminocyclopropane-l-carboxylic acid (ACC) or 1 mM ABA resulted in complete defoliation of leaf explants . Application of 0 .125 mM silver thiosulfate (STS) inhibited ethylene-and ACC-induced abscission but had no effect on explants treated with ABA . The ABA-induced abscission was unaffected by treatment with aminoethoxyvinylglycine (AVG) or aminooxyacetic acid (AOA) . Treatment of explants with 1 mM cobalt chloride (CoC12) or 2000 µl L -1 of norbornadiene (NBD) completely inhibited abscission in explants treated with 1 ttl L-1 ethylene or 1 mM ACC but they were only marginally effective in blocking ABAinduced abscission despite the lower level of endogenous ethylene . ABA appeared to increase the sensitivity of explants to ethylene . However, the evidence suggests that ABA may also function independent of ethylene to induce leaf abscission in R. sinica .Abbreviations: ABA = abscisic acid ; ACC = 1-aminocyclopropane-l-carboxylic acid ; AOA = aminooxyacetic acid ; AVG = aminoethoxyvinylglycine ; CoC12 = cobalt chloride; NBD = norbornadiene ; STS = silver thiosulfate . 243