Seasonal variations in the content of phototoxic compounds in giant hogweed.

The Candida inhibition method of Daniels was used for monthly determinations of the relative amounts of phototoxic substances in giant hogweed from April to September. Significant seasonal variations were found in the highest content of active constituents in April and May, with a decrease in the following months. Except in May (where it was highest for the root), the highest content of phototoxic substances was found in the leaves. It was lower in the root and lowest in the stem and stalk.