Allelochemicals from palmer amaranth,Amaranthus palmeri S. Wats

The presence of Palmer amaranth (AMAPA) residues in the soil reduced fresh weight accumulation in onions and carrots and markedly decreased seedling field establishment of carrots. Solid-phase separation techniques were used to isolate fractions containing water-soluble organic compounds from AMAPA residues and soil amended with such residues. At concentrations of 20–100 mg/liter most of the organic solids thus extracted were inactive in seed germination assays using onion, carrot, AMAPA, and tomato seeds. Extracts from the roots of AMAPA increased 72-hr germination percentages in carrot, AMAPA, and tomato. A time-study of AMAPA residue decomposition in soil showed an increase in extractable inhibitors of onion germination after 62 days but no other significant changes in the activity. The most active allelochemicals from AMAPA proved to be volatile compounds. Volatiles emitted by soil containing AMAPA residues and by the dried and partially rehydrated leaf and flower residues themselves reduced carrot and tomato seed germination to less than 7%. Freshly harvested aerial AMAPA inhibited only carrot seed. Germination of AMAPA and carrot seeds was retarded by exposure to volatiles from dried AMAPA residues. Residues from AMAPA grown in Texas and Louisiana exhibited comparable inhibitory activity after air-drying two weeks. Onion seeds were also inhibited by volatiles from AMAPA residues.Names of companies or commercial pruducts are given solely for the purpose of providing specific information; their mention does not imply recommendation or endorsement by the U.S. Departemnt of Agriculture over others not mentioned.     

Identification of volatile allelochemicals fromAmaranthus palmeri S. Wats

Allelopathic volatiles associated with the weed Palmer amaranth (Amaranthus palmeri S. Wats.; AMAPA) were trapped on Tenax GC, thermally desorbed, and identified by gas chromatography-mass spectroscopy. Methyl ketones and alcohols (C₄–C₁₁) were the principal components of the volatiles mixture. Seedheads, stems, or roots were placed in a glass container and incubated at 31 °C (10 hr)/21 °C (14 hr) for three days prior to trapping the volatiles. Seedheads were rich in 2-heptanone which was consistently found, together with 2-heptanol, in all AMAPA tissues. Vapors of authentic 2-heptanone and (±)-2-heptanol at concentrations of 1 ppm or higher strongly inhibited the germination of onion and carrot and almost completely suppressed the germination of tomato and AMAPA seeds.Names of companies or commercial products are given solely for the purpose of providing specific information; their mention does not imply recommendation or endorsement by the U.S. Department of Agriculture over others not mentioned.     

Relationships between chemical structure and inhibitory activity of C6 through C9 volatiles emitted by plant residues

Leaf, stem, flower, fruit, and root residues of a wide variety of plants have been reported to emit bioactiven-alkanes, 2-alkanols,n-alkanals, 2-alkenals, 2-alkanones, andn-alkanoic acids containing from six to nine carbon atoms. During a 72-hr exposure to the vapor phase of these compounds (6.9, 20.8 or 34.4 M/liter), germination of onion, carrot, and tomato seeds was inhibited to varying degrees. The extent of inhibition caused by these plant residue volatiles depended upon the compound type and concentration, carbon-chain length, and the seed species tested. Tomato seeds recovered more fully from exposure to these volatile inhibitors than did those of carrot and onion. Degree of recovery in all three species depended on the type and concentration of volatile present. The C₇ and C₈ compounds were the most inhibitory among these 24 volatiles. Of the six classes of chemicals examined, the 2-alkenals were the most inhibitory, followed by the 2-alkanols,n-alkanals, and 2-alkanones, which were equally effective as seed germination inhibitors. The straight-chain alkanes and alkanoic acids were relatively noninhibitory. Tests of a C₇ and C₉ alkadienal indicated that the C₇ compound was the more inhibitory.Mention of a commercial or proprietary product does not constitute an endorsement by the U.S. Department of Agriculture.     

Volatile seed germination inhibitors from plant residues

Volatile emissions from residues of the winter cover legumes, Berseem clover (Trifolium alexandrinum L.), hairy vetch [Vicia hirsuta (L.) S.F. Gray], and crimson clover (Trifolium incarnatum L.), inhibited germination and seedling development of onion, carrot, and tomato. Using GC-MS, 31 C₂-C₁₀ hydrocarbons, alcohols, aldehydes, ketones, esters, furans, and monoterpenes were identified in these residue emission mixtures. Mixtures of similar compounds were found in the volatiles released by herbicide-treated aerial and root residues of purple nutsedge (Cyperus rotundus L.) and the late-season woody stems and roots of cotton (Gossypium hirsutum L.). Vapor-phase onion, carrot, and tomato seed germination bioassays were used to determine the time- and concentration-dependent inhibition potential of 33 compounds that were either identified in the plant residue emissions or were structurally similar to identified compounds. Cumulative results of the bioassays showed that (E)-2-hexenal was the most inhibitory volatile tested, followed by nonanal, 3-methylbutanal, and ethyl 2-methylbutyrate. All the volatile mixtures examined contained at least one compound that greatly inhibited seed germination.Mention of a commercial or proprietary product does not constitute an endorsement by the U.S. Department of Agriculture.