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Upon illumination of etiolated barley seedlings, vitamin K₁ is synthesized at the same time as the chlorophylls, thylakoids and carotenoids. The natural plant hormone abscisic acid depresses the light-induced formation of vitamin K₁, chlorophylls and carotenoids. The low isoprenoid synthesis in the dark (K₁, carotenoids) is not inhibited by abscisic acid. There is no specific inhibition in the synthesis of a single isoprenoid lipid. Thus the functional concentrations of the pigments and K₁ in chloroplasts are formed, even though the total amount synthesized is below normal. It is suggested that abscisic acid interferes with the induction of thylakoid formation which then results in a reduced isoprenoid synthesis.