Allelopathic Effects of Trichostema Lanceolatum (Labiatae) in the California Annual Grassland

Abstract

(1) Trichostema lanceolatum, a common herb in the California annual grassland, contains 2-9% volatile essential oil in its fresh foliage. Vapours and aqueous extracts from its leaves have been shown to inhibit germination and growth of other plants in vitro. The present study evaluated the potential of T. lanceolatum to restrict plant growth in the field. (2) Dry soil charged with T. lanceolatum volatiles in the laboratory reduced radicle elongation of seeds, but dry seeds and dry soil exposed to volatiles under T. lanceolatum plants in the field did not show appreciable inhibition. (3) Trichostema lanceolatum throughfall from early-season rains was, at most, only weakly toxic. Soil at the base of T. lanceolatum plants was not rendered toxic by throughfall and stemflow from the first autumn rain. (4) Water and ether extracts of dead, brown T. lanceolatum leaves were very toxic in bioassays in Petri dishes, but similar leaves mixed with soil had little allelopathic effect. (5) Trichostema lanceolatum seedlings were no more resistant to volatiles of T. lanceolatum essential oil than seedlings of other annual grassland species. (6) The allelopathic potential of T. lanceolatum under natural conditions appears to be low. This conclusion is surprising since both green and dead, brown leaves from it were much more inhibitory in laboratory bioassays than were leaves and litter from Salvia leucophylla, a species previously reported to be strongly allelopathic.