Figure 5. The difference in modes of action between ethephon and 2, 4-D. 2, 4-D induces the production of ACC synthase, therefore catalyzes the synthesis of ACC.

ACC is the precursor of ethylene and can be converted into ethylene under catalysis by ACC oxidase. 2, 4-D also induces the production of NCED, which catalyzes the pyrolysis of xanthophylls to form xanthoxin, and finally turned to ABA. Ethylene and ABA cause the damage of cell membrane and the increase of RWC and H₂O₂ in plant tissues. In contrast with 2, 4-D, ethephon does not require ACC to produce ethylene and releases abundant ethylene directly after entering tissues. ACC: 1-aminocyclopropane-1-carboxylate; ABA: abscisic acid; NCED: 9-cis-epoxycarotenoid dioxygenase; SAM: S-adenosylmetionine.