Phytotoxins from the leaves of Laggera decurrens

Upon biological screening of a series of African medicinal plants, substantial phytotoxic activity was found in the leaves of Laggera decurrens (Vahl.) Hepper & Wood (Asteraceae), using a Lemma minor bioassay. Bioassay-guided fractionation of the leaves led to the isolation of two physiologically active compounds: 3-hydroxythymoquinone and 5-acetoxy-2-hydroxythymol, causing death of Lemna minor in the 25-100 mu M range. Symptoms were a rapidly developing chlorosis, followed by necrosis of fronds. The compounds also inhibited growth and germination of the grass weed Agrostis capillaris down to 250 mu M. The mode of action of both compounds could not be elucidated, but they do not appear to he photosystem II inhibitors.