A Survey of California Plants for Water-Extractable and Volatile Inhibitors

We tested 55 plant species in northern California for water-extractable and volatile inhibitors of plant growth with seed bioassays: 69% significantly inhibited, and none significantly stimulated, radicle growth of Hordeum vulgare when tested as 1:25 or 1:50 (g tissue:ml water) extracts, whereas 38% inhibited and 15% stimulated Bromus mollis. Extracts of species in the Compositae and Labiatae, as well as Aesculus californica, Ailanthus altissima, Brassica nigra, Ceanothus integerrimus, Lupinus arboreus, Nicotiana glauca, Ribes cereum, and Scrophularia californica, were the most inhibitory. Only Artemisia tridentata, Heteromeles arbutifolia, Salvia sonomensis, and Trichostema lanceolatum released strongly inhibitory volatiles; T. lanceolatum was clearly the most toxic species tested, its extracts and volatiles greatly suppressing growth of the indicators. Five of the species most inhibitory in the initial screening were selected for further investigation Their extracts reduced germination and growth of a number of northern California plants, but when the extracts were inoculated with 0.5 g soil and incubated several months at room temperature, most gradually diminished in toxicity