Chilling-induced leaf abscission of Ixora coccinea plants. I. Induction byoxidative stress via increased sensitivity to ethylene

Exposing ixora (Ixora coccinea) plants to chilling temperatures (3-9 degree s C for 3 days) resulted in increased leaf abscission, initiated 3 days aft er transfer to 20 degrees C. Exposure to chilling also induced a 7-fold inc rease in ethylene production rates of abscission zone (AZ) tissue during th e initial 5 h after chilling. The ethylene burst resulted from the high lev els of 1-aminocyclopropane-1-carboxylic acid (ACC) accumulated in the AZ du ring the chilling period. ACC levels following chilling decreased also due to enhanced conjugation to 1-(malonyl-amino)cyclopropane-1-carboxylic acid (MACC), Treating plants prior to chilling with antioxidants, such as butyla ted hydroxyanisole (BHA), n-propyl gallate (PG), and vitamin E, significant ly reduced chilling-induced leaf abscission, This effect was obtained despi te the fact that ethylene production in the treated plants resembled that o f chilled plants receiving no BHA. In addition, exposure of plants to ethyl ene (0.5-10 mu l 1(-1)) for 1-3 days significantly enhanced leaf abscission only when they had been pre-chilled, These data imply that chilling-induce d leaf abscission was closely correlated with increased sensitivity of the AZ to ethylene rather than with the chilling-induced ethylene burst, Eased on the findings that the ethylene action inhibitor, 1-methylcyclopropene (1 -MCP), and the antioxidant BHA inhibited both the chilling-induced and the ethylene-enhanced leaf abscission, it is concluded that: (1) although ethyl ene is essential for chilling-induced abscission, it is not the triggering factor; (2) oxidative processes derived from the chilling stress seem to be the trigger of chilling-induced leaf abscission, operating via increased s ensitivity to ethylene.