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 stress with a light/dark temperature regime of 2 degrees C/-3 degrees C for 5 days at the seedling stage. Stressed plants contained lower endogenous GA(3) and IAA contents than the controls, while zeatin and ABA contents and ethylene levels were significantly increased. Uniconazole-treated plants had lower endogenous GA(3) and IAA contents, and higher zeatin and ABA contents and ethylene levels. Leaf chlorophyll content and respiratory capacity of roots were reduced significantly after plants were subjected to freezing stress, and foliar sprays of uniconazole retarded the degradation of chlorophyll and increased respiratory capacity of roots. Uniconazole-induced freezing tolerance was accompanied by increased activities of various antioxidant enzymes, including superoxide dismutase, catalase and peroxidase. Foliar applications of uniconazole reduced electrolyte leakage and malondialdehyde accumulation caused by freezing stress, suggesting that uniconazole may have decreased freezing-induced lipid peroxidation and membrane damage.