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

Pre-treatment of citrus leaves and leaf explants ( Citrus sinensis [L.] Osbeck cv. Shamouti), with 1-methylcyclopropene (1-MCP), induced endogenous ethylene production when leaves were further incubated in air. The induction of ethylene production was 1-MCP concentration-dependent. Abscission was concomitantly delayed. In leaves pre-treated with 1-MCP followed by exposure to ethylene, abscission was significantly delayed in comparison with those without 1-MCP pre-treatment. When leaf explants were co-treated for 24 h with ethylene and 1-MCP, abscission was delayed quite efficiently. The Lineweaver-Burke plot yielded a half-maximal value of 0.234 μl l⁻¹ for the effect of ethylene on abscission. 1-MCP⁻¹ competed kinetically with ethylene with a K_i value of approximately 1.4−5.5 nl l⁻¹ 1-MCP. Under these experimental conditions there was some competition between 1-MCP and ethylene. However, ethylene was not able to completely counteract the inhibitory effect of 1-MCP. Pre-treatment with 1-MCP, followed by exogenous ethylene treatment, suppressed the induction of endo- β -glucanase (EG) activity at the laminar abscission zone. The ethylene-dependent accumulation of the hydrolyse gene was demonstrated by blocking the accumulation of CsCel a1 mRNA by 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 of CsCel a1 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 abscission zone.