Effects of brassinosteroid infiltration prior to cold treatment on ion leakage and pigment contents in rape leaves

The effect of 24-epibrassinolide (BR27) on cold resistance of rape seedlings was studied by ion leakage and photosynthetic pigment degradation measurements. Aqueous solutions of BR27 were injected into cotyledons or primary leaves of rape plants and these plants were incubated at 2 degrees C or 20 degrees C. Cold treatment (2 degrees C) without BR27 injection elevated the membrane permeability in both primary leaves and cotyledons significantly. Surprisingly, injection of leaves with water or 0.467 % aqueous ethanol solution led to a massive increase in membrane permeability after cold stress at 2 degrees C. The synergistic effect of leaf infiltration and cold on permeability was abolished by 0.05 and 1.00 mu M of BR27 in primary leaves and by 1.00 mu M of BR27 in cotyledons. On the other hand, BR27 solutions strongly elevated the membrane permeability at 20 degrees C, while water and ethanol solutions brought about only negligible increases. Water or ethanol infiltrations strongly reduced the leaf contents of chlorophyll (Chl) a, Chl b and carotenoids at 2 degrees C but less markedly at 20 degrees C. However, in seedlings exposed to 2 degrees C pigments content was significantly higher in BR27-treated leaves as compared to water/ethanol control. There were no differences between pigment contents of leaves injected with BR27 solutions or only water/ethanol at 20 degrees C. The above data strongly support the stress protecting effect of BR27.