ABSCISIC-ACID-INDUCED AND ETHYLENE-INDUCED DEFOLIATION OF RADERMACHERA-SINICA L

Radermachera sinica L. is an ornamental plant with demonstrated sensit ivity to ethylene-induced leaf abscission. In this study, we examine t he relationship between abscisic acid (ABA) and ethylene in initiating the abscission response. Treatment with 1 mu l L(-1) of ethylene, 1 m M 1-aminocycropropane-1-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 h ad no effect on explants treated with ABA. The ABA-induced abscission was unaffected by treatment with aminoethoxyvinylglycine (AVG) or amin ooxyacetic acid (AOA). Treatment of explants with 1 mM cobalt chloride (CoCl2) or 2000 mu l L(-1) of norbornadiene (NBD) completely inhibite d abscission in explants treated with 1 mu l L(-1) ethylene or I mM AC C but they were only marginally effective in blocking ABA-induced absc ission despite the lower level of endogenous ethylene. ABA appeared to increase the sensitivity of explants to ethylene. However, the eviden ce suggests that ABA may also function independent of ethylene to indu ce leaf abscission in R. sinica.