Degreening treatment is widely used as a postharvest practice to improve the external color of earlyseason citrus fruit.Nevertheless,the application of ethylene associated with calyx senescence.2,4-dichlorophenoxyacetic acid(2,4-D)has been widely used to retard calyx abscission,drying and browning caused by degreening treatment.However,2,4-D also has some security problems when applied.In the present research,oligochitosan was used as a potential alternative of 2,4-D.The results of scanning electron microscope(SEM)indicated that ethylene degreening treatment could accelerate the formation of pedicel abscission layers.Both 1.5%oligochitosan and 2,4-D treatments could significantly slow the formation of pedicel abscission layers of ethylene chlorotic fruit,and inhibit pedicel browning.The activity of polygalacturonic acid enzyme(PG)and cellulase(CX)in the abscission area increased rapidly in control fruit,while 1.5%oligochitosan and 2,4-D could inhibit the activities of pectinmethylesterase(PME),PG and CX,thus delaying the degradation of protopectin and cellulose.Besides,these two treatments could promote the generation of lignin of ethylene chlorotic fruit and delay the degradation of lignin during the late storage by increasing the activities of cinnamate-4-hydroxylase(C4H)and 4-coumar-ate: coenzyme A ligase(4CL).Inhibition effect on the increase of abscisic acid(ABA)content was also found in these two treatments.Besides,1.5%oligochitosan treatment was more effective in promoting the accumulation of carotenoids in fruit peel,thus improving the external color of the peel.However,compared with ethephon-treated fruit,2,4-D-treated fruit had relatively poor color,basically behaving in uneven coloring of peel,significantly lower color values(a*)and lower accumulation of carotenoid in peel.In conclusion,postharvest application of 1.5%oligochitosan could be a potential alternative to 2,4-D to avoid calyx senescence of mandarin fruit induced by a degreening treatment.