Pieces of callus obtained from seedlings ofDigitalis purpureawere grown on solid Murashige-Skoog's medium supplemented with 1 mg liter−1BA and 0.1 mg liter−1IAA or NAA, with or without phenobarbital (40 mg liter−1). The replacement of the natural auxin IAA by the synthetic auxin NAA increased callus growth and inhibited organogenesis, whereas the addition of phenobarbital had the opposite effect. Morphometric measurements revealed a high ratio of vacuole to cytoplasm (v/v) in callus cells. This ratio was affected by the different treatments in the same way as the fresh weight. The activity of mitochondrial cytochrome P450scc (the enzyme that provides the precursor, pregnenolone, for the biosynthesis of cardenolide in foxglove plants) was detected in the relevant fraction of callus grown under all experimental conditions, and its activity was increased by the addition of phenobarbital. The different treatments tested increased the cardenolide content and quantifiable amounts of digitoxin were detected in all callus tissues. It is of special interest that phenobarbital added to the culture medium increased the accumulation of digitoxin. The mechanism affecting the development and production of cardenolide in callus tissues ofD. purpureaby phenobarbital and the replacement of IAA by NAA is disc
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