Uniconazole-induced tolerance of rape plants to heat stress in relation to changes in hormonal levels, enzyme activities and lipid peroxidation

Winter rape (Brassica napus L. cv. 601) seedlings were treated with 50 mg.l(-1)of foliar-applied uniconazole and then exposed to heat stress with a light/dark temperature regime of 43 degrees C/38 degrees C for 3 days at the stem elongation stage. Heat stressed plants contained lower endogenous GA(3), IAA and zeatin contents than the controls, while ABA content and ethylene level were increased significantly. 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 markedly after plants were subjected to heat stress, and foliar sprays of uniconazole retarded the degradation of chlorophyll and increased respiratory capacity of roots. Following exposure to heat, the activities of superoxide dismutase and peroxidase were significantly reduced. Uniconazole-induced heat tolerance was accompanied by increased activities of various antioxidant enzymes. Foliar applications of uniconazole reduced electrolyte leakage and malondialdehyde accumulation caused by heat stress, suggesting that uniconazole may have decreased heat-induced lipid peroxidation and membrane damage. Foliar sprays of uniconazole increased the tolerance of rape plants to heat stress.