; Kumar et al., 2009 ) while also improving yield of the successive vegetable crop ( Sainju et al., 2002 ). Observed mechanisms of weed suppression reported are weed seed reduction ( Kumar et al., 2009 ), allelopathy ( Rueda-Ayala et al., 2015 ), and
Two studies were conducted to determine if selected grass and dicot species had an allelopathic interaction with pecan (Carya illinoinensis Wangenh. C. Koch). Leachate from pots with established grasses or dicots was used to irrigate container-grown pecan trees. Leachates from bermudagrass [Cynodon dactylon (L.) Pers.], tall fescue (Festuca arundinacea Shreb. cv. Kentucky 31), redroot pigweed (Amaranthus retroflexus L.), and cutleaf evening primrose (Oenothera laciniata Hill) reduced leaf area and leaf dry weight about 20% compared to the controls. Bermudagrass, tall fescue, and primrose leachate decreased pecan root weight 17%, trunk weight 22%, and total tree dry weight 19% compared to the control. In a second study, trees were 10% shorter than the control when irrigated with bermudagrass or pigweed leachate.
Allelopathy in the Cucurbitaceae was first reported by Putnam and Duke (1974) who screened the U.S. PI collection of cucumber ( Cucumis sativus ) germplasm and identified accessions with varying allelopathic potentials. Lockerman and Putnam (1979
and oxazolidinethione ( Brown and Morra, 1996 ). Allelopathy is not the only mechanism by which brassica cover crops could impact germination. Cohen and Mazzola (2006) and Hoagland et al. (2008) have demonstrated that some low-glucosinolate canola
seedling growth bioassays to assess their possible contribution to resistance to root rotting and allelopathy. Materials and Methods Sweetpotato culture and tissue preparation. Sweetpotatoes were grown using standard cultural practices
Crude aqueous extracts from dead stems, crowns, and roots from both field-grown and tissue-cultured asparagus plants delayed, but did not prevent, germination of asparagus seed. Root extract inhibited root and shoot development of asparagus seedlings grown in growth pouches. Stem and crown extracts reduced root growth but not shoot growth. The extracts of all 3 tissues caused more secondary root formation and root branching. The highest concentration of extract from crown-plus-root tissues, 5 g of tissue/100 ml water, inhibited radicle growth and killed seedlings. Toxicity of the crown-root extract was not reduced by adding activated charcoal to the extract or by autoclaving the extract. These results suggest that toxic substances in dead asparagus tissue are water-soluble and stable and may persist in old asparagus fields.
Root exudates of guava (Psidium Guajava L. cv. Beaumont) grown in sand culture were collected on columns of XAD-4 resin attached to the nutrient solution circulation system of sand-cultured plants. The compounds were eluted from the resin columns with methanol and the eluates were concentrated. The root exudates were inhibitory to the radicle growth of both lettuce (Lactuca sativa L. cv. Anuenue) and bristly foxtail (Setaria verticillata L. Beauvois) and lettuce seed germination was inhibited. Fractionating the root exudates resulted in the neutral and acidic fractions being inhibitory, the basic fraction having no effect. Methanolic extracts of oven-dried guava roots were also inhibitory.
`Yecora Rojo' Wheat (Triticum Aestivum L.) And `Florida Petite' Tomato (Lycoper-Sicon Esculentum Mill.) Plants Were Grown In Monocultured Or Intercropped Recirculating Hydroponic Systems To Determine Whether Plant Growth Or Yield Would Be Affected By Intercropping. Mean Fruit Weight Was Slightly Lower (12%) For Intercropped Than For Monocultured Tomato Plants. The Number Of Tillers Per Plant Was Slightly Lower (7%) For Wheat, And Grain Dry Weight Per Plant And Mean Seed Dry Weight Were Slightly Higher (14% And 15%, Respectively) For Intercropped Than For Monocultured Plants. A Lettuce Seedling Bioassay Showed No Evidence Of Allelopathic Compound Accumulation In Monocultured Or Intercropped Hydroponic Systems.
Asparagus (Asparagus officinalis L.) root tissue and root extracts were used to investigate the previously reported release of toxic chemicals from senescing root tissue. Greenhouse studies showed that the severity of crown or root rot of asparagus seedlings increased in direct proportion to increased amounts of dried root tissue incorporated into soil with either F. oxysporum f. sp. asparagi, F. moniliforme, or a combination of these two pathogens. When excised asparagus roots were treated with increasing concentrations of a water extract of dried asparagus root tissues, electrolyte efflux increased, peroxidase activity decreased linearly, and respiration decreased. Active components in the extracts were heat-stable. Our data suggest allelochemicals of asparagus may have direct physiological and biochemical effects on asparagus plants that predisposes them to fusarium diseases.
Several cinnamic acids have been identified as principal toxic components of asparagus (Asparagus officinalis L.) root autotoxin and have been shown to synergize Fusarium infection of asparagus. The basis for this synergism was studied by exposing asparagus seeds and radicles from pregerminated seeds to ferulic (FA), caffeic (CA), or methylenedioxycinnamic (MDA) acids alone and in combinations of two or three of these acids. After treatment, seeds were placed in pots of peat-lite mix, and, depending on the experiment, all or half were inoculated with F. oxysporum (Schlecht) f. sp. asparagi (Cohen). Seedling emergence from each pot was used as a measure of toxicity. All cinnamic acids at 1% suppressed emergence compared with the control. Solutions combining FA and CA (0.5%/0.5%, v/v) were substantially more toxic than 1% solutions of either alone. Exposure of radicles (early postgermination) for 10 minutes to combined FA/CA before planting decreased emergence from pots, whereas emergence following a 10-minute exposure to 1% CA or FA alone did not differ from the controls. The 2-hour exposure to FA or to FA/CA and the 24-hour exposure to CA, FA, or FA/CA decreased emergence, with toxicity progressing as follows: CA < FA < FA/CA. Root tip squashes showed fewer mitotic figures in treated than in untreated radicles, and scanning electron microscopic (SEM) examination of the radicle epidermis revealed damage to the surface of epidermal cells and precocious root hair development, the extent of which paralleled treatment toxicity.