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Summary The ability of cotyledons of spruce seedlings to develop their photosynthetic apparatus when grown in the dark was used to study the effect of the «bleaching» herbicide norflurazon (NF) on the formation of their etiochloroplasts. Ultrastructural analyses showed that in etiochloroplasts of cotyledons of 14-day-old seedlings grown in the dark on NF, the number of thylakoids, in particular of grana-thylakoids, was reduced. In plastids resulting after growth on a 200 μmol/L solution of NF, only single thylakoids developed, while in those developed after growth on 20 μmol/L of NF, incomplete stacking of thylakoids appeared. Lower concentrations of NF were less harmful, so that etiochloroplasts with grana regions developed. NF did not influence the formation of prolamellar bodies in the etiochloroplasts. Very strong reduction of carotenoid content, in particular of carotenes and violaxanthin, was obtained after growth of the seedlings in the dark on 200 and 20 μmol/L of NF. At the same time, the content of chlorophylls was also reduced, although to a much lesser degree than that of carotenoids. Analyses of membrane proteins indicated that in these etiochloroplasts the content of polypeptides of LHC II, the major light-harvesting complexes of PS II, was impaired. Only seedlings with less altered plastids were able to develop some photosynthetic activity. High concentrations of NF (200 and 20 μmol/L) caused bleaching of the cotyledons and damage of their plastids when seedlings germinated in the dark were transferred to the light and illuminated for several days.