Effects of exogenous ABA on antioxidant enzymes in detached citrus leaves treated by rapid freezing

Two-year-old seedlings of trifoliate orange (Poncirus trifoliata (L.) Raf.) and lemon (Citrus limonia Osbeck.) were sprayed with 0.1 mmol times L super(-1) abscisic acid (ABA) and then their leaves were collected randomly to expose to freezing stress. Specifically, these detached leaves were treated at 0, -3, -6, -9 and -12 degree C for 1 h respectively. The concentration of malondialdehyde (MDA), percentage of electrolyte leakage (EL) and the activities of several antioxidant enzymes (superoxide dismutase, catalase and peroxidase) were determined spectrophotometrically. The results show that the percentage of electrolyte leakage in trifoliate orange was lower than that in lemon, while the MDA concentration and the antioxidant activities of antioxidant enzymes in trifoliate orange were higher than those in lemon. Pretreatment with the abscisic acid can significantly reduce the membrane damage caused by freezing stress. However, the lipid peroxidation damage caused by low temperature and the activities of antioxidant enzymes were irregularly influenced by ABA pretreatment. We were thus able to infer that exogenous ABA can enhance the cold tolerance of citrus by a process rather than by activating antioxidant enzymes.