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The term allelopathy was introduced by Molisch (22) in 1937, and refers to all biochemical interactions (stimulatory and inhibitory) among plants, including microorganisms. The fact that the term is literally translated as “mutual harm or suffering” has probably led to other interpretations and confusion. Several scientists have suggested narrowing the definition to include only higher plants and harmful interactions. Rice (38) defined allelopathy in his first edition to include only harmful effects, but he recently opted to restate Molisch's premise in his 2nd edition (39). Molisch's broad definition of allelopathy is probably appropriate because considerable research has indicated that involvement of microorganisms and lower plants in phytotoxin production. Also, natural compounds that inhibit growth at certain concentrations often enhance growth at lower concentrations.

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Cyperus kyllingia and Cyperus brevifolius are problematic turfgrass weeds in Hawaii. Both are closely related weed species with similar morphology and growth characteristics. C. kyllingia appears to be a more successful weed with regards to interference than C. brevifolius. Greenhouse experiments were conducted to compare the levels of interference exerted by C. kyllingia and C. brevifolius upon Cynodon dactylon turfgrass. C. kyllingia reduced the growth of C. dactylon by about 50 %, while C. brevifolius did not significantly reduce C. dactylon growth. These results correspond with the chemical profiles of C. kyllingia and C. brevifolius. Analysis has shown that C. kyllingia contains two sesquiterpenes which have been identified as potentially allelopathic components of Cyperus rotundus. C. brevifolius contains waxes and the two sesquiterpenes found in C. kyllingia are absent. This suggests that allelopathy may be the mechanism responsible for the different levels of interference exhibited by C. kyllingia and C. brevifolius, and these species may provide an important model for the study of allelopathy.

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Allelopathy is broadly defined as the “biochemical interactions between all types of plants, including microorganisms” ( Rice 1984 ). Although the term is used most often in a negative context to describe inhibitory effects, the general concept of

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ability (e.g., pathogenic bacteria, fungi, and nematodes), and the number of beneficial microbes decreases, thus affecting the normal growth of crops and causing yield reductions. The use of reasonable cropping systems and the employment of allelopathy

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by competing with weeds for available resources and by promoting conditions that are unfavorable for weed germination and establishment ( Teasdale, 1998 ). The latter mechanism includes allelopathy, which is the inhibitory or stimulatory effect of a

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Observations that tomato transplants died or were severely stunted when set into unincorporated sorghum-sudan hybrid surface mulch led us to further investigate the potential allelopathic impacts of this warm-season cover crop in a series of field experiments. Survival and dry weights of tomato, lettuce, and broccoli transplants were determined in fallow, incorporated sorghum-sudan-, and unincorporated sorghum-sudan-mulched soils. All three species transplanted into plots in which the sorghum-sudan had been cut and left on the soil surface had a significantly lower dry weight than plants transplanted into fallow soil or into soil where the sorghum-sudan had been incorporated. Additionally, fewer transplants survived in the mulch treatment. The surface mulch plots also significantly reduced weed biomass nearly 10-fold. We believe that a water-soluble compound that is leached out of the sorghum-sudan hybrid is toxic to all three of the plants tested. Further laboratory and greenhouse tests are under way to determine the exact nature of the toxic substance.

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Little research has been conducted to quantify allelopathic suppression of weeds in the field. The objectives of this study were to develop an adequate control for separating physical from allelochemical effects, use the control to quantify allelochemical suppression in the field, and determine whether a mixture of cover crops would provide a broader spectrum of weed control than single species. Hairy vetch, rye, crimson clover, and barley were cut into 5-cm pieces, shaken in distilled water (pH 6) to leach allelochemicals, and redried. A seed germination bioassay confirmed that leached cover crops were nontoxic to germinating seeds. Physical suppression of Eastern black nightshade by the four cover crop species occurred in the field study, as did allelochemical suppression by crimson clover. Only rye physically suppressed yellow foxtail, and none of the cover crops suppressed yellow foxtail allelochemically.

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). The herbicidal activity of MSM is due to allelopathy induced by hydrolysis products of glucosinolates (GSLs) ( Hoagland et al., 2008 ). Certain plants in the Brassicaceae family are known to possess allelopathic properties, and are used as cover

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Peters, H.C. Luu, K.T. 1985 Allelopathy in tall fescue 273 283 Thompson A.C. Chemistry of allelopathy: biochemical interactions among plants ACS Washington, D.C Smith, M.W. 1991 Pecan

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growth observed with cypress mulch, but the potential role of allelopathy warrants further investigation. Literature Cited Abouziena, H.F. Hafez, O.M. El-Metwally, I.M. Sharma, S.D. Singh, M

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