UNICONAZOLE-INDUCED ALLEVIATION OF FREEZING-INJURY IN RELATION TO CHANGES IN HORMONAL BALANCE, ENZYME-ACTIVITIES AND LIPID-PEROXIDATION IN WINTER RAPE

Winter rape (Brassica napus L. cv. 601) seedlings were treated with 50 mg.l(-1) of foliar-applied uniconazole and then exposed to freezing s tress with a light/dark temperature regime of 2 degrees C/-3 degrees C for 5 days at the seedling stage. Stressed plants contained lower end ogenous GA(3) and IAA contents than the controls, while zeatin and ABA contents and ethylene levels were significantly increased. Uniconazol e-treated plants had lower endogenous GA(3) and IAA contents, and high er zeatin and ABA contents and ethylene levels. Leaf chlorophyll conte nt and respiratory capacity of roots were reduced significantly after plants were subjected to freezing stress, and foliar sprays of unicona zole retarded the degradation of chlorophyll and increased respiratory capacity of roots. Uniconazole-induced freezing tolerance was accompa nied by increased activities of various antioxidant enzymes, including superoxide dismutase, catalase and peroxidase. Foliar applications of uniconazole reduced electrolyte leakage and malondialdehyde accumulat ion caused by freezing stress, suggesting that uniconazole may have de creased freezing-induced lipid peroxidation and membrane damage.