A potential allelochemical was isolated and identified from methanol extracts of asparagus (Asparagus officinalis L.) fresh root tissue. Fractions were collected by cellulose column chromatography and tested for inhibition by an asparagus seed germination bioassay. The fraction showing the greatest inhibition contained caffeic acid, as identified by melting point, thin-layer chromatography, and infrared spectrum analysis. Seed germination bioassays and greenhouse pot tests showed depression of seedling emergence when asparagus seeds were exposed to various dosages of crude filtrate, a methanol extract from crude filtrate, and caffeic acid.
The allelopathic potential of Citrus junos Tanaka waste from food processing industry after juice extraction was investigated under laboratory conditions. C. junos waste powder inhibited the growth of roots and shoots of alfalfa (Medicago sativa L.), cress (Lepidium sativum L.), lettuce (Lactuca sativa L.), crabgrass (Digitaria sanguinalis L.), timothy (Phleum pratense L.) and ryegrass (Lolium multiflorum Lam.). Significant reductions in the growth of roots and shoots were observed as the powder concentration increased. The concentration of abscisic acid-β-d-glucopyranosyl ester (ABA-GE) in C. junos waste was determined to be 17.9 mg · kg–1 dry weight. Its concentration in C. junos waste appears to account mostly for the observed inhibition of tested plant seedlings. These results indicate that C. junos waste is allelopathic with potential for use in agriculture to suppress weed emergence, which should be investigated further in the field.
Little evidence is available concerning possible allelopathic effects of the thiocyanate ion (SCN-), a known toxin commonly found in plants of the Brassicaceae (crucifers). Seeds and seedlings of 39 species of crop plants were exposed to ionic thiocyanate (0.5 mM SCN- supplied as KSCN) to determine their relative sensitivity. Percent germination was unaffected by SCN- for all the species tested. Thiocyanate ion inhibited the total seedling (shoot and root) extension of 46% of the species tested, with 38% of the species showing inhibition of shoot and 49% showing inhibition of root extension. This demonstrates that SCN- has the capacity to harm a range of species. However, the toxicity was selective in that 44% of the test species were unaffected by exposure to SCN-. Wide differences in seed weight among species, in incubation time, and also systematics were major sources of variation for species sensitivity to SCN-.
Possible allelopathic effects of decaying sweet potato plant residue on sweet potato [Ipomoea batatas (L.) Lam.] and cowpea [Vigna unquiculata (L.) Walp.] growth were assessed. Residue treatments consisted of factorial combinations of 2 sweet potato cultivars (‘Jewel’ or ‘Centennial’), 2 plant parts (vines or storage roots), and 2 methods of tissue preparation–dried, or frozen and then dried. Ground sweet potato residues were mixed with sand [2.7% residue (w/w)] and placed in pots. Dry weights of ‘Jewel’ sweet potato plant shoots 57 days after planting were reduced 32% and 74% by vine and storage root residues, respectively, while dry weights of ‘Centennial’ sweet potato plant shoots were reduced 18% and 73%, respectively. Dried vine tissue had no apparent inhibitory effect, but vines frozen prior to drying reduced fresh and dry weights of ‘Centennial’ shoots. Dry weights of ‘Brown crowder’ cowpea plant shoots were reduced 79% and 91% by sweet potato vine and storage root residues, respectively. Nodulation of cowpeas grown in residue-amended pots was negligible compared to plants grown in pure sand. Leachate pH from pots containing sweet potato root residue was 1.4 and 2 pH units lower than that from nonamended pots with both sweet potato cultivars and cowpeas, respectively.
Asparagus (Asparagus officinalis L.) root filtrate (RF) depressed asparagus seedling emergence in a sterile peat–vermiculite medium. In a medium inoculated with Fusarium oxysporum f. sp. asparagi (FO), the effect was magnified. The response to RF dose, regardless of level of FO infestation, was quadratic. Comparisons with a sucrose solution of the same percentage of soluble solids as the RF suggested that reduced emergence may have been due in part to enhancement of FO growth by the energy source added to the medium. However, after germinating seeds in RF until radicle emergence, then rinsing and transferring them to FO-inoculated medium, emergence was reduced relative to controls. Therefore, depression of emergence apparently related both to an autotoxin somehow predisposing young radicles and/or hypocotyls to increased FO infection and to stimulation of FO in the rhizosphere by the soluble solids content of the root exudate. Infection was confirmed to be the only role of FO: sterilization of the spore suspension by Millipore (0.2 µm) filtration eliminated pathogen toxicity.
To determine whether grape (Vitis sp.) replant problem might be an example of autotoxicity, soil samples from fields that had or had not been replanted with grape cuttings were extracted with a neutral solution of ethylenediaminetetraacetic acid (EDTA) and by leaching with half-strength Hoagland's solution. Rooted grape cuttings growing in sand also were leached. Fractions of the leachates and extracts were assayed for toxicity using lettuce seedlings. Grape roots and replant soil yielded a toxic substance not present, or possibly present only at low levels, in non-replant soil. The replant soil extract inhibited phenylalanine ammonia-lyase activity in grape roots more than did the non-replant soil extract. The toxic substance in the replant soil extract was partially purified and a 1H nuclear magnetic resonance (NMR) spectrum taken, but the substance has not been identified. Thus, grape roots appear to be the source of at least one compound that is toxic to plants and accumulates in the soil in which grapes are grown.
Germination bioassays were conducted to assess if water-soluble extracts of broccoli (Brassica oleracea L. var. italica L.) affect germination of broccoli, cabbage (Brassica oleracea L. var. capitata L.), and cauliflower (Brassica oleracea L. var. botrytis L.). Greenhouse experiments also examined the phytotoxic potential of soil previously cropped with broccoli and broccoli plant parts on seedling growth of those species. The first bioassay used nonsterile extracts (NSEs) and filter-sterilized extracts (FSEs) of broccoli leaves. The second bioassay used nonsterile and filter-sterilized leaf extracts (LEs), stem and root extracts (SREs), and whole broccoli plant (leaves, stems, and roots) extracts (WPEs). Broccoli and cabbage germination were not affected by NSEs or FSEs, but the latter reduced cauliflower germination by 22%. LEs and SREs decreased germination speed for broccoli, cabbage and cauliflower. Greenhouse seedlings were grown in soil previously cropped with broccoli or fallow soil at three fertilizer levels. Broccoli soil was phytotoxic to cauliflower but enhanced broccoli and cabbage seedling growth. The differential sensitivity to broccoli plant residue was in the order of cauliflower > broccoli = cabbage, with SR residue having the highest phytotoxic potential.
Poor emergence of commercially grown lettuce has been observed when planted immediately after the removal of a celery crop. Greenhouse experiments were conducted to evaluate the possible allelopathic effects of celery residue on the emergence and growth of lettuce. The influence of amount and type of celery tissue, growth medium and fertility, incubation time in soil, and amendment of growth medium containing celery residue with activated charcoal was evaluated with respect to the allelopathic potential of celery. Celery root tissue was 1.8 and 1.6 times more toxic to lettuce seedling growth than was celery petiole or lamina tissue, respectively. Lettuce shoot growth was inhibited to a greater extent when grown in sand amended with celery residue rather than either amended vermiculite or potting soil. Incubation of celery root residue in soil for 4 weeks increased phytotoxicity at 1% (v/v) and decreased it at 4% (v/v). Increasing the fertility of pure sand with varying amounts of Hoagland's solution did not reverse the allelopathic effects of celery residue. The addition of activated carbon to the medium increased the growth of lettuce exposed to celery residues. Celery residues possess allelopathic potential to developing lettuce seedlings. Celery tissue type and concentration, soil type, incubation of celery root residue in soil, and addition of activated carbon to the growing medium influenced the magnitude of the observed phytotoxicity.
`Georgia Red' peanut (Arachis hypogaea L.) and TU-82-155 sweetpotato [Ipomoea batatas (L.) Lam] were grown in monocultured or intercropped recirculating hydroponic systems in a greenhouse using the nutrient film technique (NFT). The objective was to determine whether growth and subsequent yield would be affected by intercropping. Treatments were sweetpotato monoculture (SP), peanut monoculture (PN), and sweetpotato and peanut grown in separate NFT channels but sharing a common nutrient solution (SP-PN). Greenhouse conditions ranged from 24 to 33 °C, 60% to 90% relative humidity (RH), and photosynthetic photon flux (PPF) of 200 to 1700 μmol·m-2·s-1. Sweetpotato cuttings (15 cm long) and 14-day-old seedlings of peanuts were planted into growth channels (0.15 × 0.15 × 1.2 m). Plants were spaced 25 cm apart within and 25 cm apart between growing channels. A modified half-Hoagland solution with a 1 N : 2.4 K ratio was used. Solution pH was maintained between 5.5 and 6.0 for treatments involving SP and 6.4 and 6.7 for PN. Electrical conductivity (EC) ranged between 1100 and 1200 μS·cm-1. The number of storage roots per sweetpotato plant was similar for both SP and SP-PN. Storage root fresh and dry mass were 29% and 36% greater, respectively, for plants in the SP-PN treatment than for plants in the SP treatment. The percent dry mass of the storage roots, dry mass of fibrous and pencil roots, and the length-to-diameter ratio of storage roots were similar for SP and SP-PN sweetpotato plants. Likewise, foliage fresh and dry mass and harvest index were not significantly influenced by treatment. Total dry mass was 37% greater for PN than for SP-PN peanut plants, and pod dry mass was 82% higher. Mature and total seed dry mass and fibrous root dry mass were significantly greater for PN than for SP-PN plants. Harvest index (HI) was similar for both treatments. Root length tended to be lower for seedlings grown in the nutrient solution from the SP-PN treatment.
Field and laboratory studies were conducted to investigate the mechanisms of weed suppression by cover crops. High-performance liquid chromatograph analysis and a seed germination bioassay demonstrated that rye (Secale cereale L.) can be leached of its allelochemicals, redried, and used as an inert control for separating physical suppression from other types of interference. In a field study, rye, crimson clover (Trifolium incarnatum L.), hairy vetch (Vicia villosa Roth.), barley (Hordeum vulgare L.), and a mixture of the four species suppressed the emergence of eastern black nightshade (Solanum ptycanthum Dun.). Crimson clover inhibited the emergence of eastern black nightshade beyond what could be attributed to physical suppression alone. The emergence of yellow foxtail [Setaria glauca (L.) Beauv.] was inhibited by rye and barley but not by the other cover crops or the cover crop mixture.