Allelopathic influence of nodding thistle (<i>Carduus nutans</i>L.) seeds on germination and radicle growth of pasture plants

Bioassay experiments were used to assess the allelopathic potential of nodding thistle seeds on cumulative germination, speed of germination, and radicle elongation of five test species i.e., cocksfoot, perennial ryegrass, white clover, subterranean clover, and nodding thistle. Nodding thistle seeds inhibited germination of all species except subterranean clover in at least one of the bioassays used. Radicle elongation of all species was substantially inhibited in the presence of nodding thistle seeds both on filter paper and on soil. In soil systems, nodding thistle seeds inhibited radicle elongation of seeds over distances of up to 4-7 mm. A probability analysis demonstrated that a high density of nodding thistle seeds has the potential to exert a substantial negative effect on establishment of newly sown pasture. Nodding thistle seeds were either stimulated or largely unaffected by other seeds of their own species (except when seeds were touching) suggesting that the allelopathic potential of nodding thistle seeds may be an evolved mechanism to encourage establishment of thistle seedlings.