Upon biological screening of a series of African medicinal plants, substant
ial 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 ac
tive compounds: 3-hydroxythymoquinone and 5-acetoxy-2-hydroxythymol, causin
g death of Lemna minor in the 25-100 mu M range. Symptoms were a rapidly de
veloping chlorosis, followed by necrosis of fronds. The compounds also inhi
bited 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.