Germination Regulation by Amaranthus palmeri and Ambrosia artemisiifolia

An apparent allelopathic interference having been observed in which residues of Palmer amaranth, Amaranthus palmeri S. Wats., inhibited the growth of onion, carrot and Palmer amaranth itself, aqueous and organic solvent extracts of amaranth plant parts were made and used to investigate the growth regulatory activity of the chemical constituents of this amaranth species. The resultant crude extracts, as well as isolated and identified chemical compounds, were assayed in vitro as germination regulators, by using both crop and weed seeds. Some crude organic solvent extracts proved inhibitory, but inhibitions of onion, Palmer amaranth, and carrot germination did not result from the same extracts or compounds. Four compounds have been isolated from Palmer amaranth: 2,6-dimethoxy- benzoquinone, vanillin, phytol, and chondrillasterol. All showed biological activity, to varying degrees, in seed germination bioassays. Parallel studies of extracts and mixtures of compounds isolated from ragweed, Ambrosia artemisiifolia L., were also made. Water extracts of ragweed proved highly inhibitory of seed germination. Mixed sesquiterpenes isolated from ragweed were very inhibitory of onion, oat, ryegrass, and Palmer amaranth germination when applied in a dichloromethane solution pretreatment.