Effect of 1-methyleyclopropene on ethylene-induced abscission in citrus

Pre-treatment of citrus leaves and leaf explants (Citrus sinensis [L.] Osbe ck cv. Shamouti), with 1-methylcyclopropene (1-MCP), induced endogenous eth ylene production when leaves were further incubated in air. The induction o f ethylene production was 1-MCP concentration-dependent. Abscission was con comitantly delayed. In leaves pre-treated with I-MCP followed by exposure t o ethylene, abscission was significantly delayed in comparison with those w ithout I-MCP pre-treatment. When leaf explants were co-treated for 24 h wit h ethylene and I-MCP, abscission was delayed quite efficiently. The Linewea ver-Burke plot yielded a half-maximal value of 0.234 mul l(-1) for the effe ct of ethylene on abscission. 1-MCP (-1) competed kinetically with ethylene with a K-1 value of approximately 1.4 - 5.5 nl l(-1) 1-MCP. Under these ex perimental conditions there was some competition between 1-MCP and ethylene . However, ethylene was not able to completely counteract the inhibitory ef fect of 1-MCP. Pre-treatment with 1-MCP, followed by exogenous ethylene tre atment, suppressed the induction of endo-beta -glucanase (EG) activity at t he laminar abscission zone. The ethylene-dependent accumulation of the hydr olyse gene was demonstrated by blocking the accumulation of CsCel a1 mRNA b y 1-MCP. Six hours of exposure of leaves to 1-MCP at various times during a total of 24 h ethylene treatment efficiently reversed ethylene induction o f CsCel al gene at mRNA level up to 18 h. The results demonstrate that the induction of abscission by ethylene is controlled at mRNA level at the absc ission zone.