Abstract: In the present study，effects and underlying mechanisms of 2,4-D，applied at different concentrations and at various time points，on fruit dropping rate and fruit quality of Jincheng（Citrus sinensis Osbeck）were investigated. The results showed that exogenous application of 2,4-D at 20 mg • L-1 and at 200 days after full blooming could prominently reduce the fruit shedding rate，without influencing the fruit quality. It suggests that 2,4-D treatment could keep fruit fresh on tree for approximately 50 days. Moreover，2,4-D treatment was found to delay formation of abscission zone and down-regulated expression of genes encoding enzymes involved in cell wall degradation，such as CX16，PG2，PME1 and PME2. Our finding revealed that the potential mechanism responsible for keeping fruit fresh on tree by 2,4-D is ascribed to inhibition of cell wall degradation and abscission zone formation.

Key words: Citrus sinensis, 2,4-D, keeping fruit fresh on tree, abscission zone, cell wall degradation, ethylene, fruit quality