0971-4693/94 US $ 5.00Allelopathy Journal25 (2): 275-312 (2010)International Allelopathy Foundation2010Tables: 7, Fig : -Role of Allelopathy in vegetables crops productionJACOB JOHN, J. SHIRMILA, S. SARADA and S. ANU*Cropping Systems Research Centre, P.O. KaramanaKerala Agricultural University, Thiruvananthapuram, Kerala 695 002E. Mail: jjkau@rediffmail.com(Received in revised form: January 21, 2010)CONTENTS1. INTRODUCTION2. SOIL SICKNESS2.1 Tomato2.2 Cucumber2.3 Eggplant3. AUTOTOXICITY3.1 Cucumber3.2 Tomato3.3 Pea3.4 Watermelon3.5 Red pepper3.6 Brinjal4. ALLELOPATHIC INTERACTIONS4.1 Vegetables and other crops interactions4.1.1 Vegetables on other crops4.1.2 Other crops on vegetables4.1.3 Vegetables on vegetables4.2 Vegetables and weeds interactions4.2.1 Summer season vegetables4.2.2 Cool season vegetables4.3 Effects of vegetables on weeds4.4 Allelopathic effects of trees on vegetables (Agroforestry)5. PESTICIDAL POTENTIAL OF VEGETABLE CROPS5.1 Fungitoxic/antibiotic effects5.2 Nematicidal effects6. ALLELOCHEMICAL STUDIES IN VEGETABLES6.1. Cucumber6.2. Other vegetables7. FUTURE RESEARCH PROSPECTS8. REFERENCESCorrespondence author* 276John et alABSTRACTVegetables are important worldwide, but their production faces problemsof yield decline due to soil sickness and autotoxicity, when grown continuously forseveral years . Besides, allelopathic effects of other crops, weeds and trees also reducesyields. Thus the allelochemical interactions and their effects on vegetables areimportant in vegetable production. Although, research on various aspects ofallelopathy in vegetable crops has been done but not compiled. Soil sickness iscomplex phenomenon due to several factors involved and autoxicity is major one. Theautotoxic potential of certain vegetables has been discussed. In multi-s torey croppingsystems, where numerous crops and trees are grown together, vegetables are essentialcomponents and allelopathic interactions arise. Several vegetables possessantimicrobial principles and hence, allelopathically inhibit phytopathogenic fungi andbacteria. Certain vegetables possess nematicidal principles and therefore, offerimmense potential for nematode control in their cultivation. Several s tudies have beendone to elucidate the role of allelochemicals in vegetables across the world. Theallelopathic interactions between the vegetables and other crops/weeds/trees and thepotential of vegetables for pathogen and nematode management/control are reviewedin this paper. Future allelopathic research in vegetables should focus on (i) separatingthe allelopathic interference from competition in vegetable fields and vegetables basedcropping systems, (ii) screening the germplasm/varieties of vegetable crops forallelopathic potential and later on do genetic manipulations to breed new varieties, (iii)exploiting the allelopathic potential of vegetable crops for weed control and plantprotection, (iv) determining the critical concentrations of allelochemicals in eachvegetable crop to express their inhibitory/stimulatory influences, (v) identifying thecompatible and beneficial associations of vegetable crops with other crops and treesand (vi) determine the harmful and beneficial effects of allelopathy in vegetable cropsin pot culture and field studies.Keywords:Agroforestry, allelopathy, allelochemicals, autotoxicity, crop, nematode,pathogen, soil sickness, tree, vegetable crops, weeds1. INTRODUCTIONThe term allelopathy includes both harmful and beneficial biochemicalinteractions between all types of plants including microorganisms, through the release ofchemicals from plant parts by leaching, root exudation, volatilization, residuedecomposition and other processes in both natural and agricultural systems (119).Allelopathy can affect many aspects of plant ecology including occurrence, growth, andplant succession, the structure of plant communities, dominance, diversity and plantproductivity (150). Allelopathy is a universal phenomenon and is not the domain of fewselected plants. Some vegetables with strong allelopathic effects have been investigated(201). The allelopathic interactions between the vegetables and other crops/weeds/treesand the potential of vegetables for pathogen and nematode management/control arereviewed in this paper. Allelopathic interactions are complex and can involve differentclasses of chemicals. Allelochemicals are present in all plant tissues, several chemicals canbe released together and may exert additive or synergistic effects (145). The vegetablesproduction faces many problems (i). Yield decline due to autotoxicity and soil sickness(when vegetables are grown continuosly for several years) and (ii). Low yield ofvegetables in monoculture and cropping systems due to allelopathic effects of crops, weeds Role of Allelopathy in vegetables crops production277and trees (72,80,146,201). Researchers study only the competition for nutrients andmoisture and not the allelopathic effects (145).The allelochemical interactions and their effects on vegetables are importantaspects to be considered in vegetables production. Although, research on allelopathy invegetable crops has been done worldwide, however, it has not been compiled. Hence thisreview on allelopathy in vegetables production has been prepared. It deals with (i).problems of soil sickness and autotoxicity, (ii). use of vegetables allelopathy for weedmanagement, pest management and (iii). identify ideal combinations (companion, rotationsand agroforestry systems) of vegetable crops.2. SOIL SICKNESSThe soil sickness has been observed in many vegetables (Table 1) and has beenattributed to several factors, viz. depletion of soil nutrients, accumulation of phytotoxiccompounds, and accumulation of soil-borne pathogens due to improper cultural practices,such as continuous monoculture and improper management of soil-borne diseases (136).Continuous cultivation of vegetables in same field reduces their growth, yield and quality(50,72). However, the magnitude of yield decline varies with crops. Legumes, cucurbitsand solanaceae crops are most vulnerable to soil sickness (91).Table 1. Vegetable crops affected by soil sickness and autotoxicityCrop ReferencesSoil SicknessBean (Phaseolus vulgarisL.) 75Pea (Pisum sativumL.) 58, 122, 183Potato (Solanum tuberosumL.) 13, 64Asparagus (Asparagus officinalisL.) 17, 49, 64, 119, 139, 140, 197, 198, 157Cabbage (Brassica oleraceaL.) 12Celery (Apium graveolensL.) 19Cucurbits (Cucumisspp. andCucurbitaspp.) 196, 200, 210, 207Eggplant (Solanum melongenaL.) 210Tomato (Lycopersicon esculentumMill.) 133, 210AutotoxicityCucumber (Cucumis sativusL.) 107, 109, 201, 205, 207Tomato (Lycopersicon esculentumMill.) 57, 88, 108, 167, 175Pea (Pisum sativumL.) 33, 54, 91, 176, 189Watermelon (Citrullus lanatus(Thunb.) Mansf.) 53, 124Red pepper (Capsicum annuumL.) 184Egg plant(Solanum melongenaL.) 54, 1752.1. Tomato(Lycopersicon esculentumMill.)Tomato (Lycopersicon esculentumMill.), when grown in monoculture for longperiods leads to soil sickness (212). Autotoxicity and soil problems in tomato productionhave been reported long ago (111,213). Continuous cropping reduces the root vitality andfruit quality of tomatoes (196). Zhouet al.(214) reported the presence of di-iso-octylphthalate, di-iso-butyl phthalate, tannic acid and salicylic acids in tomato plants. 278John et alSinghet al.(164) studied the autotoxic effect of tomato on germination, seedlinggrowth, biophysical and biochemical parameters of plants grown in hydroponic culture.The root, stem and leaf leachates of tomato had varying effects on various parameters. Thegermination was stimulated by higher concentration of root and leaf leachates. Themoderate concentration of root leachates inhibited the germination than lower and higherconcentrations. The higher concentration of leachates stimulated the radicle growth, whilelower concentration inhibited it. However, the reverse was true for plumule growth.Chlorophyll a, chlorophyll b and total chlorophyll content decreased with increasingconcentration of leachates. The inhibition of chlorophyll synthesis was concentrationdependent with maximum inhibition in leaf leachate. The root and stem leachatesdecreased the protein content to minimum level. Superoxide dismutase and peroxidaseactivity was inhibited but catalase activity was stimulated. The leachates adversely affectthe biophysical and biochemical parameters resulting in arrested growth and low yield oftomato. The influence of leachates on growth followed the order roots > stem > leaf. Theroot leachates drastically decreased the tomato fruit growth than control, while leafleachates completely prevented fruit formation.Soil amendment with organic matter (56,69), or some compounds (172) controlsthe soil sickness due to soil-borne plant pathogens (Table 3). InBrassicaspp., allyl-isothiocyanate was the main toxin to control the soil-borne pathogens (30,31,115). KauriPaasuke (86) reported that amendment of soil with milled peat, or green manuring ofweeds [50% couchgrass (Agropyron repens(L.) Beauv.)] suppressed the soil sickness inapple tree nursery, but led to the buildup of nematophagous fungi and tardigrade beetles.The soil amendment with formulated organic compounds [S-H mixture (a compostedmixture of bagasse, rice husks, oyster shell powder, urea, potassium nitrate, calciumsuperphosphate and mineral ash (172)], controls the Fusarium wilt of radish (RaphanussativusL.) and watermelon (Citrullus lanatus(Thunb.) Mansf. and clubroot of crucifers(66). Use of Perlka(granulated calcium cyanamide product, SKW TrostbergT MAktiengesellschaft, Germany), as nitrogen fertilizer controls the plant pathogens[Cercosporella herpotrichoidesFron.,Plasmodiophora brassicae(152) andSclerotiniasclerotiorum(Lib.) de Bary (74,97,114,152)]. The mechanism of pathogen suppressionmay be due to the release of toxic ammonia gas from urea fertilizer (56,70,71,73).Table 2. Effects of crucifers biomass added as soil amendments to control soil sicknessSoil amendment Effects on test plant ReferencesCrucifers Reduced severity of damping-off of cabbage caused by30, 31Rhizoctonia solaniKuhnCrucifers Reduced severity of damping-off of watermelon caused by32Pythiumsp.Crucifers Reduced severity of potato dry rot caused byFusarium115sambucinumFuckelCrucifers Reduced severity of root rot of pea caused byAphanomyces120euteichesDrechsler2.2.Cucumber(Cucumis sativusL.)The effects ofFusarium oxysporiumf. sp.cucumerinum[the pathogen causingFusarium wilt in cucumber (Cucumis sativusL.)] and cinnamic acid, a principal autotoxic Role of Allelopathy in vegetables crops production279component in the root exudates of cucumber were studied, on the plant growth,photosynthesis and incidence of Fusarium wilt in cucumber, to elucidate the interaction ofautotoxins and soil borne pathogens in the soil sickness.F. oxysporumf. sp.cucumerinum(FO) and cinnamic acid (CA) at 0.1 or 0.25 mM significantly decreased net photosyntheticrate, stomatal conductance and yield of Photosystem II photochemistry (Phi PS II).Thereafter the plant biomass production was reduced, but did not induce photoinhibition.Pretreatment with CA before inoculation with FO increased the effectiveness of FO,together with a slight photoinhibition. CA pretreatment significantly increased thepercentage of plants affected by Fusarium wilt, browning index of vascular bundle andFusarium population in the nutrient solution. All these results indicate that CA enhancedthe Fusarium wilt by predisposing cucumber roots to infection by FO through a directbiochemical and physiological effect. It is likely that soil sickness results from aninteraction of many factors (198). Introduction of antagonistic bacteria and beneficialmicrobes could overcome the problems caused by pathogenic microbes and autotoxiccompounds (212) that led to soil sickness in tomato, cucumber, melon (Cucumissp.) andeggplant or brinjal (Solanum melongenaL.).2.3.Eggplant(Solanum melongenaL.)The possibility of repeated use of substrate in the growing of eggplant wasstudied (138). It was found that peat repeatedly used showed a negative effect on thegrowth and yield of eggplant due to the decrease in water in substrate and accumulation ofphenolic compounds. On the other hand in the mixture of bark and peat, a positive effectof repeated use of substrate was observed. In this type of substrate, the amount of waterincreased and phenolic compound concentration was low. Response of eggplant dependedon the type of substrate and on the time of its use. The longer the peat substrate was used,the lower was the dry matter of eggplant leaves and the fresh weight of fruits. An inversedependence was found, when eggplant was grown in bark with peat substrate. Incomparison to first eggplant growing, in the third substrate use, the dry weight of leaves ofplants grown in peat was 30% lower and the fresh weight of fruits was 16% lower. On theother hand, in bark with peat substrate, the dry weight of leaves increased by 150% andfresh weight of fruits by 21%. In both types of substrates, the dry weight of stems wassimilar and did not change depending on the time of substrate use. Comparison of theeffects of repeated use of substrates indicates that symptoms of substrate fatigue thatoccurred in peat substrate were not observed in the bark+peat substrate. It is postulated thatthis finding reflects differences in the physico-chemical properties of the substrates(mainly the water content and C/N ratio). These differences were responsible for thedifferences in the speed of release of phenolic compounds from the decaying crop residuesof eggplant and their subsequent degradation (138).3. AUTOTOXICITYSoil sickness is complex phenomenon and of the several factors involved,autoxicity is major one. Autotoxicity has been reported in several plants grown inmonocultures (162,206,208,209) and also in vegetables crops (Table 1). Mostly, theseplants release autotoxins through leachings from the living and dead plant parts, root 280John et alexudates, biomass decomposition and volatilization (18,148). Many autotoxins [simpleorganic acids, straight chain alcohols, aldehydes or ketones, unsaturated lactones, fattyacids, naphthoquinones, complex terpenoids, polypeptides, alkaloids, glucosinolates etc.]have been isolated from plants and they affect many biochemical and physiologicalprocesses (126,162,192). The autotoxic potential of certain vegetables are discussed.3.1. Cucumber(Cucumis sativusL.)The poor growth due to successive cropping of cucumber has been observed.Gaidmark (44) grew several crops of tomato and cucumber in gravel culture and detectedsome unidentified toxins in the nutrient solution. Testa of cucumber var. PI 169391 seedcontains inhibitor (s) that suppressed the seedling growth of cucumber (107). Seedfermentation and/or leaching with distilled water reduced the inhibition, while activatedcharcoal eliminated the toxicity. Inhibitory effects of cucumber seed extract on cucumberdecreased with maturity of the cucumber fruit. Extracts of seeds and fruits of cucumberwere non-toxic, however, the fruit juice extracts from PI 169391 suppressed the growth ofcucumber (107).Root exudates:The cinnamic and hydroxybenzoic acids present in the cucumber rootexudates drastically reduced the activities of dehydrogenase, root ATPase, nitratereductase and superoxide dismutase and inhibited the uptake of K, NOand HPOions+--324(109). Yu and Matsui (204,206) investigated the autotoxic potential of cucumber rootexudates and reported that addition of activated charcoal to nutrient solution for the cultureof cucumber did not affect the inorganic composition in nutrient. But, it significantlyimproved the growth of cucumber. Root exudates at reproductive stage showed higherphytotoxicity than the vegetative stage. Nine compounds including benzoic acids,cinnamic acids andp-thiocyanatephenol were identified as phytotoxic substances (205).These compounds enhancedion leakage from cucumber roots and inhibited ion uptake(112, 205, 207). Compounds such as cinnamic acid showed great activity even at aconcentration less than 0.01 mM, indicating the possibility of autotoxicity under fieldconditions (207). Yu and Matsui (205) examined the effects of root exudates of cucumber,aromatic carboxylic acids in root exudates and their analogues upon the uptake of NO,3HPO, SO, K, Ca, Mg, and Feby intact cucumber seedlings. Root exudates2-+2+2+2+244inhibited the uptake of all ions analyzed except HPO. Inhibition of ion uptake by24cinnamic acid, a main component of root exudates, was both concentration and pHdependent. Inhibitory effect on ion uptake increased with decreasing pH. In continuouscropping, allelochemicals from root exudates of cucumber roots are inhibitory to its owngrowth (201).Cucumber plants were grown hydroponically with or without addition ofactivated charcoal (AC) to the nutrient solution at different temperatures and photoperiods.Significant growth differences were observed between charcoaled and non-charcoaledplants. Plants without AC were severely retarded in growth than those with AC. A numberof simple phenolic and aliphatic acids were identified in the cucumber root exudates alongwith a chlorinated compound, 2,4-dichlorobenzoic acid (2,4-DCBA). All the identifiedcompounds inhibited the growth of test plants in a concentration dependent manner and2,4-DCBA was most inhibitory. To overcome autotoxicity, cucumber plants were grown inthe nutrient solution added with 2,4-DCBA and also with or without addition of a Role of Allelopathy in vegetables crops production281microbial strain or AC. It was revealed that vegetative growth of cucumber plants grownwith 2,4-DCBA and with a microbial strain recovered significantly. Susceptible cucumberplants grafted on tolerant cucumber plants decreases the autotoxicity (143).3.2. Tomato(Lycopersicon esculentumMill.)Hirano (57) and Soil Microbiological Laboratory of Japanese AgricultureTechnology Institute (167) confirmed the release of inhibitors during the decomposition oftomato plant residues. The phytotoxicity of the residual nutrient solution after cultivationof tomato was greater on itself than on other plant species (175). The phytotoxicity wasmainly due to the acidic substances in the solution (176,203). Mizutani (118) isolatedvanillic, p-hydroxybenzoic and gentisic acids as allelochemicals from tomato roots.Rahman and Newton (148) reported that the addition of activated charcoal to the nutrientsolution during the growth of tomato significantly improved its growth. The aqueousextract of tomato leaves inhibited the seedlings growth and biomass accumulation intomato (88). Yuet al. (208) confirmed this effect and reported that only the nutrientsolution at the reproductive stage of tomato showed phytotoxicity. Phthalic acid (PA) andphloroglucinol dihydrate (PD) was isolated and identified in the root exudates of tomato(90). Yu and Matsui (203) identified benzoic, palmitic and sinapic acids from the residualculture solution and benzoic, phenylacetic, 2-hydroxy-3-phenylpropanoic, p-hydroxybenzoic, vanillic, palmitic, ferulic, caffeic and sinapic acids from the usedactivated charcoal.The detrimental effects of autotoxicity decreases the plant population,regeneration failure and subsequent yield decline in tomato (88). Potential autotoxicityalso exists in root exudates of tomato growing in hydroponics (108). The autotoxicsubstances viz., phthalic acid and phloroglucinol dehydrate caused root oxidative damageof tomato seedlings. Potted tomato seedlings were cultured in perlite and treated withphthalic acid (PA) and phloroglucinol dihydrate (PD) as exogenous autotoxins at 1 mM, 5mM and 10 mM concentrations. The application of both PD and PA and especially PDincreased the MDA (malondialdehyde) contents. The activities of SOD (superoxidedismutase), CAT (catalase) and POD (peroxidase) depended on autotoxins (PA or PD),their time of action and concentration. The enzyme activities increased with application ofPA on 5day and decreased on 10day except at 10 mM PA. On 20day, the activities oft ht ht hall enzymes decreased except SOD at 1 mM. Similar trend of enzyme changes was presentin the treatments of PD, except POD activity that kept growing on the 10day. Resultsthindicated the adverse effects of exogenous PA and PD on enzymes of antioxidant defencesystem, resulting in lipid peroxidation in roots of tomato seedlings (211).3.3. Pea(Pisum sativumL.)In pea grown continuously, nearly 50% yield decreases in the subsequent crop(91). Soil sickness of pea could be overcome by leaching soil with water or ethanol,suggesting that phytotoxins played an important role in this problem (33). The addition ofsap of living or decaying root of pea to nutrient solution stunted the growth of pea (54).The residual nutrient solution after culture of pea not only inhibited the growth of pea, butalso showed significant phytotoxicity to other crops such as carrot (Daucus carotaL.),eggplant, bean (Phaseolus vulgarisL.) and Chinese cabbage (Brassica oleraceacv.Chinensis) (59,60,61). Hatsudaet al.(52) isolated vanillic acid and p-coumaric acid from 282John et althe water extract of pea root. Takijima and Hayashi (176) fractionated the residual solutionafter hydroponic culture of pea and found that acidic and neutral substances accumulatedin the nutrient solution mainly caused the phytotoxicity. Hirayoshiet al.(61) reported thatphytotoxicity of root exudates was mainly due to acidic substances and at least twophenolic acids were present in the exudates. Vaughan and Ord (189) identified ferulic,vanillic, p-coumaric and p-hydroxybenzoic acids in the root exudates of pea seedlings.3.4. Watermelon(Citrullus lanatus(Thunb.) Mansf.)In commercial production of watermelon, an interval of 5 to 7 years isrecommended, in areas where soil sickness is a serious problem. Hatsudaet al.(53)isolated o-hydroxybenzoic acid from water extracts of watermelon root as a phytotoxicsubstance. Nishimura (124) observed that o-hydroxybenzoic acid was a precursor chemicalcausing the browning of vascular bundles in Fusarium wilt. When seedling was incubatedin o-hydroxybenzoic acid solution, it exhibited symptoms similar to that caused byFusarium (181). Studies indicated that among the tested phenolic acids, o-hydroxybenzoicacid had the strongest ability to inhibit ion uptake by cucumber (207). Confirmatorystudies on the role of o-hydroxybenzoic acid in the soil sickness problem of watermelonare needed.3.5. Red pepper(Capsicum annuumL.)In Korea, the yield of red pepper decreases due to continuous cropping. Althoughthis decline had been mainly attributed to the occurrence of diseases, it was suggested thatallelopathy may partly account for the decline. Tsuchiyaet al. (184) assayed the growthinhibition of red pepper by a germination test using water or organic solvent extracts of itsleaf, stem and root. Methanol extracts of stem and root of red pepper strongly inhibited theradicle growth of red pepper. Methanol extracts of leaf and root and water extract of rootinhibited the hypocotyl growth. The methanol and water extracts of red pepper rootcontained several phenolic acids such as p-hydroxy benzoic acid and vanillic acid. It wasinferred that the red pepper decline under continuous cropping was due to allelopathy.3.6. Brinjal(Solanum melongenaL.)Hayashi and Takijima (54) reported that amending soil with dryroots of brinjalinhibited the growth of brinjal itself. Takijima and Hayashi (175) investigated thephytotoxicity of root exudates and found that the exudates of brinjal showed greatphytotoxicity to brinjal itself and tomato, whereas, it improved the growth of wheat andupland rice. However, no inhibitors have been identified from roots or root exudates.4. ALLELOPATHIC INTERACTIONS4.1. Vegetable crops and their interactionsSeveral workers have investigated the allelopathic interaction of vegetables withother crops and between vegetables. Role of Allelopathy in vegetables crops production2834.1.1. Effects of Vegetable crops on other cropsBhatt and Dhawan (16) reported that addition of crushed seeds of carrots andcoriander (Coriandrum sativumL.) gave significantly higher grain yield of wheat. Carrotat all levels and coriander at 4 % resulted in reduction ofHeterodera avenaecystpopulation.The aqueous extracts of fresh and dried roots of Chinese cabbage reduced its ownradicle growth, fresh and dry weight and the growth of mustard (Brassica campestrisL.).Extracts of dried roots were more inhibitory than fresh roots. Inhibition of growth wasmore in mustard than Chinese cabbage (2).Peterson and Harrison (133) screened yellow squash (Cucurbita pepoL.) and twocucumber (PI 165046 and PI 169391) accessions for the presence of constitutive chemicalfactors which interfere with the germination and growth of proso millet (PanicummilliaceumL.). Hexane, ethylacetate and water soluble components were all inhibitory,especially those obtained from leaves, stems or roots. Extracts obtained from these plantorgans inhibited germination between 61 and 97 % at 50 mg DW equivalent per ml.Extracts from seed parts inhibited germination by 18-64 % at the same extractconcentrations (50 mg DW equivalent per ml). The combined hexane and ethanol extractsof leaves strongly inhibited germination. Aqueous leaf homogenates of the three cucurbits,incorporated into the potting media, inhibited growth of millet. Squash leaf materialinhibited shoot length of millet by 26%. The cucumber accessions had no effect. Shootsdry weight was inhibited by 49, 32 and 35% and root dry weight by 73, 56 and 67% byhomogenates of squash, cucumber 165046 and cucumber 169391 leaves, respectively.Allelopathic effects of potato (Solanum tuberosumL.) tuber shoots ongermination and seedling growth of bean, soyabean (Glycine max(L.) Merr), lentil (LenseulinarisMedik.), maize (Zea maysL.) and wheat were studied (131). Fresh shoots wereallowed to decay for 72, 120, 168, 240 and 336 h in normal water in the ratio of 1:10 w/v.These water extracts were applied to the crop seeds. All extracts reduced seed germinationand seedling growth. For lentil and maize, the lowest negative effect on germination wasobtained with the 120 h extract, for bean and wheat with the 168 h extract and forsoyabean with the 240 h extract. Maize had least negative effect from 72 and 120 hextracts, bean and lentil from the 120 h extract, wheat from the 120 and 168 h extracts andsoyabean from the 240 h extract (132).4.1.2. Effects of other crops on vegetable cropsVolatile emissions from residues of the winter cover legumes, Berseem clover(Trifolium alexandrinumL.), hairy vetch (Vicia hirsute(L.) SF.Gray), and crimson clover(Trifolium incarnatumL.), inhibited germination and seedling development of onion(Allium cepaL.), carrot and tomato. Hydrocarbons, alcohols, aldehydes, ketones, esters,furans, and monoterpenes were identified in these residue emission mixtures (20).Growth chamber tests demonstrated that alfalfa residue is toxic to cucumber seedgermination and seedling growth. Ground alfalfa roots (0.5% w/w, dry weight) inhibitedgermination when added to the growing medium. Alfalfa roots (0.5% w/w, dry weight)were also toxic to pre-germinated cucumber seed. However, cucumber seedlings grewnormally if the medium in which ground roots were added was irrigated so as to leachdown chemicals and kept for >1 day before planting (39). 284John et alWater leachates from guayule (Parthenium argentatum) processing residues, orresidues mixed with peat moss or soil significantly delayed onset of germination anddecreased percentage germination of broccoli (Brassica oleraceaevar.italicacv.Emperor) cantaloupe (Cucumis melocv.Topmark), cauliflower (Brassica oleraceaevar.botrytiscv. Snow crown), lettuce cv. Empire, pepper (Capsicum annuumcv. NM 6-4) andtomato cv.GS 12. These effects can be ascribed to p-anisic acid present in leaf resin and avariety of substituted benzoic acid, cinnamic acids present as carboxylate salts in leavesand bagasse (193).Stoimenova (170) reported that cucumbers grown after soyabean had shorterroots than the controls or those grown afterAmaranthus retroflexusor the mixtures.Cucumber hypocotyl length was slightly greater after soyabean and/orA. retroflexusthanin the control.The effect of sesbanimide (an alkaloid from Sesbania seeds) and extracts from theground powders of wholeSesbania bispinosaseeds, and the isolated embryos, testae andintact seeds ofS. punicea(a noxious weed), on the germination and growth of vegetable(including cucumbers cv. Stono, lettuce cv. Queen and radish Sparkler) was investigated(188). An inhibitory effect was observed for all treatments, being most noticeableregarding seedling growth than germination. In all cases, roots were more sensitive andhence, its growth more severely retarded than shoots. This inhibitory effect was alsodetected if cucumber seeds were soaked in extracts prior to sowing. The embryo and testaextracts ofS. puniceaproduced an inhibitory response in the test species, the former beingthe most inhibitory (188).Seeds of cucumber, green beans (Phaseolus vulgarisL.), tomatoes, redrootpigweed (Amaranthus retroflexus) and annual Italian ryegrass (Lolium multiflorum) wereexposed to 0, 16.7, 33.3, and 66.7 g Lof kenaf (Hibiscus cannabinusL.) extract (154).-1Distilled water and three concentrations of polyethylene glycol (PEG) were included ascontrols. Germination in tomato was reduced by 30% when exposed to the highestconcentration of unweathered kenaf (154).-1Rye seed densities of 50 and 100 kg hainhibited lettuce seedling growth andvigour. The visual symptoms were similar to atrazine and benzoxazolinone (BOA) effects.Leaf yellowing and vigour were dependent on rye density, atrazine and BOAconcentrations, where, leaf yellowing increased while vigour decreased with an increase indensity and concentration (168).Nearly 239 medicinal plants were evaluated for allelopathic activity on lettuce cv.Great Lakes using the sandwich method (43). Around 223 species inhibited, whereas 17species promoted radicle growth in lettuce. Radicle growth inhibition of >80, 60-79, 40-59, 20-39, and 0.30-19.0% was attributed to 19, 16, 43, 72 and 73 species, respectively.Artabotrys odoratissimus (A. hexapetalus) inhibited lettuce growth the most. Stronginhibitory activity was also recorded forAnnona cherimola, Dialium guianense,Tamarindus indica, Emblica pectinata, Hevea brasiliensis, Garcinia oblongifolia,Elaeocarpus serratus, Schleichera oleosa, Paeonia lactiflora and Sandoricum koetjape(43).Aqueous leaf extracts ofAloe veraextracts did not significantly affect seedgermination but promoted root length and seedling height of lettuce, radish and turnip(Brassica rapaL.). The extracts also increased the number of roots in lettuce and turnip,but not in radish (103). Role of Allelopathy in vegetables crops production285Pot culture experiments revealed that aqueous extracts of cotton (Gossypium sp.)significantly inhibited the seed germination and seedling growth of tomato (104).4.1.3. Effects of Vegetable crops on vegetable cropsKim and Kil (89) reported that the aqueous extract of leaves, stem and root oftomato heavily inhibited the seed germination and growth of lettuce and tomato. Ingeneral, the phytotoxicity of the tomato extracts increased in the following order: time forextracting 25 h < 48 h < 72 h. Tomato exhibited auto inhibition. Four phytotoxicphenolics, salicylic acid, gallic acid, protocatechuic acid and vanillic acid were obtainedfrom the aqueous extracts of tomato leaves by paper chromatography. The aqueousleachates of tomato significantly inhibited growth of certain crops in pot culture.Root exudates from cucumber, squash [Cucurbita moschata(Duch.) Poir] andmelon seedlings inhibited tomato seed germination. Squash exudates had the greatesteffect, reducing germination from 91.4% (control) to 76.8%. Squash root exudates reducedthe germination rate index from 0.46 (controls) to 0.43, whereas root exudates fromcucumbers and melons increased it to 0.50 and 0.48, respectively. The effects of rootexudates on 30 or 45 day old seedlings were mixed. All the exudates reduced seedling DWwhen applied to 30 day old plants but had no significant effect on seedling DW whenapplied to 45 day old plants. Squash and melon root exudates significantly increased rootand seedling FW when applied to 30 and 45 day old plants (129).Aqueous extracts of asparagus roots inhibited seed germination in tomato andlettuce (Lactuca sativaL.), but not in cucumber. The extracts reduced hypocotyl growth inlettuce, shoot growth in asparagus, and inhibited radicle elongation in lettuce andasparagus. Seedling growth in tomato was not affected. Inhibition was concentrationdependent. Radicle growth in 'Grand Rapids' lettuce was sensitive to an extractconcentration as low as 0.05 g dry root tissue/100 ml HO. Asparagus radicles were more2sensitive than asparagus shoots. Aqueous root extracts ofA. racemosisalso inhibitedgermination and radicle growth in 'Grand Rapids' lettuce (55).Water extracts of soils from pea and taro fields with continuous croppinginhibited the radicle growth of lettuce. Since the same phenomenon was observed evenwhen the extracts were autoclaved, it is considered that the inhibition may be caused bysome allelochemicals rather than by the effect of harmful soil microorganisms (182). Leafextracts of vegetables such as asparagus, watermelon, pea, tomato and cucumber inhibitedthe radicle growth of lettuce, regardless of the low EC (electrical conductivity) value of theextracts. In contrast, root extracts of these vegetables enhanced the inhibition of the rootgrowth of lettuce as the EC value and the contents of total polyphenols increased. The rootextract of asparagus strongly inhibited the germination of lettuce in spite of low EC value.It is considered that the root of asparagus may contain some allelochemicals (182).Hydrophobic root exudates of cucumber collected at different growth stages withAmberlite XAD 4 resin were bioassayed with lettuce seedlings (205). The exudates at thereproductive stage were more phytotoxic than those at the vegetative stage. The exudatescontained organic acids such as benzoic, p-hydroxybenzoic, 2,5 dihydroxybenzoic, 3phenylpropionic, cinnamic, p-hydroxycinnamic, myristic, palmitic and stearic acids, aswell as p-thiocyanatophenol and 2 hydroxybenzothiazole. All chemicals, except 2hydroxybenzothiazole, were toxic to the growth of lettuce (205). 286John et alPot trials were carried out to investigate the effects of plant incorporation intosand and of plant extracts and isolated chemicals on the growth of several vegetables andon lettuce seed germination (183). Pea growth was inhibited up to 30% by theincorporation of pea tops (shoots) into sand (4 g/1.7 kg sand). The growth of pea tops wasinhibited by incorporation of taro, especially the roots. The incorporation of taro roots alsoinhibited taro root growth. Cucumber growth was inhibited by incorporation of taro rootsor tops or cucumber tops. Pea growth was inhibited 10-15% by water extracts of pea ortaro roots. Ethyl acetate extracts of pea roots inhibited pea growth and lettuce seedgermination. Several phenolic acids, considered to act as allelochemicals in the roots ofvegetables, inhibited lettuce seed germination in the order salicylic acid > p-hydroxybenzoic acid > vanillic acid > p-coumaric acid > gentisic acid. Pea growth wasinhibited 20% by vanillic acid and p-coumaric acid at 300 ppm. Tomato growth wasinhibited by vanillic acid, gentisic acid and p-hydroxybenzoic acid (in descending order).Watermelon growth was inhibited by 10 and 70% with 10 and 500 ppm salicylic acid,respectively. Concentrations >500 ppm induced stem rot like symptoms, leading to theconclusion that salicylic acid is an indirect cause of soil sickness in watermelons (183).Allelopathy properties of solvent extracts from broccoli harvested before the headformation were evaluated through biological assays on germination and root elongation ofseedlings of broccoli, lettuce and tomato. Chloroform extracts of broccoli drymatter aswell as its chromotographic fractions showed high allelopathic activity on both seedgermination and root elongation of broccoli, lettuce and tomato. The allelopathic activitywas exhibited mainly as inhibition in root elongation rather than in the seed germination.Among the five chromatographic fractions, three of them (I, II, III) were very effective,while subfractions obtained from the fraction II of chloroform extract showed non-significant allelopathic effects. As the allelopathic activity decreased with fractionation, itmay be attributed to a complex of non-polar compounds (35).The aqueous extract of tomato significantly inhibited the growth of cucumber,radish, lettuce, Chinese cabbage (Brassica pekinensis(Lour.) Rupr.) and broccoliseedlings. But, tomato volatiles had no significant effect on growth of green gram (Vignaradiata(L.) Welczek), chinese cabbage, lettuce or tomato seedlings (213). Forty days aftertransplanting, tomato seedling root exudates significantly inhibited cucumber, but notlettuce, growth. It is suggested that tomato should be avoided as an intercrop withcucumbers in solution culture or in solar energy greenhouse (213).4.2. Vegetables and weeds interactions4.2.1. Summer season vegetable cropsWeeds, besides competing with vegetables for nutrients and water, also exertallelopathic effects on vegetables. Tripati and Srivastava (180) studied that effect ofaqueous extracts of certain weeds on the seed germination of brinjal.The plant extractsexerted varied effects on seed germination. The extracts ofHolarrhena antidysentrica,Cannabis sativa, Crinum asiaticum, Nelumbo nucifera, Sansevieria roxburghianaandSalvinia natans(leaf extract) inhibited germination. Extracts ofSpirodela polyrrhiza,Marsilea minuta, Salvinia natans(shoot and root extracts),Ceratoplyllum demersumandEichhornia crassipes(extracts of shoot, root and flowers) had stimulatory effects. Role of Allelopathy in vegetables crops production287Almodovaret al.(7, 8)determined the allelopathic effects of root exudates ofseven weed speciesviz.,Parthenium hysterophorus,Echinochloa colonum,Amaranthusdubius,Sorghum halepense,Rottoelia exalata,andTrianthema portulacastrumonpumpkin (Cucurbita maxima(Duch.) Poir) and egg plant. The treatments included (0):root exudates not added; (3): root exudates applied in 3 alternate days for 6 weeks and (5):root exudates applied during 5 consecutive days for 6 weeks. Root exudates of all weedspecies evaluated caused adverse allelopathic effects on egg plant seedlings throughdecreased stem length and dryweight. Besides, the effect on vine length and dry weight, adecrease of more than 50 % in pumpkin yields was recorded when weeds grew freely inassociation with the crop.Laboratory and polyhouse experiments were conducted to determine theallelopathic effects of aqueous extracts of leaves of common weeds(Cynodon dactylon,Cyperus rotundus, Eupatorium odoratum, Imperata cylindrica, Ipomoea sepiaria, MikaniacordataandParthenium hysterophorus)on tomato (157). The results revealed that allweeds inhibited germination of tomato seeds than control (distilled water). The aqueousleaf extracts ofI.sepiariaandE. odoratumwere most inhibitory to germination of tomatoseeds, even 15 days after sowing. Leaf extracts of these weeds, obtained 240 h afterdecaying, recorded more adverse allelopathic effect in seed germination upto 7 DAS (daysafter sowing) and upto 15 DAS in pots than weed extracts of 120 h after decaying. Shootlength of tomato was shorter in presence ofC. dactylon, I. sepiariaandE. odoratumextracts in both decaying periods of 120 and 240 h, while plants were taller in extracts ofother weeds than control. These extracts also decreased the root length of tomato over thecontrol andC. dactylon, I. sepiariaandE. odoratumcaused maximum reduction. Watersoluble allelochemicals extracted fromDittrichia viscosainhibited the germination rate,final germination (%) and radicle growth ofLycopersicon esculentum(169).Chauhanet al. (24) investigated the effect of pollen of weed species onin-vitropollen germination and pollen tube growth of okra/ladys finger (Abelmoschus esculentusL.) and brinjal. The pollen ofCannabis sativaandRicinus communiscompletely inhibitedthe pollen germination (100% inhibition) in brinjal.The inhibition in pollen germinationand tube length of this crop in the presence of the pollens of weeds was very drastic. Inokra, neitherCannabisnorRicinusweed pollen imposed any significant inhibitoryallelopathic effect on the pollen germination or pollen tube growth and time taken forgermination.The results ofin-vivostudies are similar toin-vitroresults (24). The pollens ofbothCannabis sativaandRicinus communiscompletely inhibited the germination ofbrinjal. The pollen grains of brinjal failed to germinate on their own stigma, when theirstigma surface was artificially dusted with pollens and the pollen of any of the two weeds,Cannabis sativaorRicinuscommunis.The flowers of brinjal pollinated with the mixtureof the pollens of the weeds and its own pollen withered and fell-off within a week andthus, no fruit-set was observed. NeitherCannabis sativanorRicinus communisshowedany pollen allelopathic effect on the pollen germination and tube length of the okra.Alsaadawi and Salih (9) reported that root exudates ofCyperus rotundussignificantly reduced the root and shoot growth of tomato and cucumber plants, while, itsresidues incorporated at 3 and 6 g per kg soil inhibited the seedling growth of cowpea(Vigna unguiculata(L.) Walp.). The reduction increased with the increased rates ofresidues. In soil, the toxicity ofC. rotundusresidues started 2 weeks after incorporation, 288John et alcontinued up to 8 weeks and then declined. Allelopathy was the causative factor for thereduction in growth of test crops.4.2.2. Cool season vegetable cropsIn laboratory experiments, tomato, onion and cucumber were treated with 0.1, 1and 10% solutions of ethanolic root extracts ofParthenium hysterophorusbeforeflowering (101). No effect was evident on germination, but radicle length was generallyreduced in all plants by the 10% extract. The 1% extract increased the radicle length oftomato.The effects of aqueous extracts fromMimosa bimucronataspring, summer andautumn leaves and from green and mature fruits on seed germination and radicle growthwere tested using lettuce, carrot, cucumber, cabbage (Brassica oleraceaL. var. capitata)and tomato (81). Extracts from green or mature fruits did not inhibit germination but thosefrom green fruits inhibited radicle growth. Extracts from autumn (dry) leaves inhibitedgermination in lettuce, carrots and tomatoes, and radicle growth was inhibited in allspecies. The effects increased proportionally with the extract concentration.Laboratory, greenhouse and field studies were conducted to determine theallelopathic potential ofLantana camaraandCromolaena odorataon the germination andgrowth of spinach (Spinacia oleraceaL.), chinese cabbage, cucumber and chilly(Capsicum frutescens) (155). Emergence and the dry weight were affected whenL.camaraorC. odoratadebris was present on the soil surface or incorporated into the soil.Emergence and the dry weight of crops, except the emergence of spinach, were notaffected when crops were grown in soil samples collected from beneath field grownL.camara. The dry weight of Chinese cabbage andCapsicum frutescenswas reduced whenthese crops were grown inC. odoratacontaminated soil. The germination of Chinesecabbage andCapsicum frutescensdecreased progressively when exposed to increasingconcentration of aqueousL. camaraextract. However, theL. camaraextract at full-1strength (66.7 g L) did not reduce the germination of spinach and cucumber seeds.C.odorataextract, when applied at full strength to seeds of spinach, Chinese cabbages andCapsicum frutescens, reduced germination by 10, 12, 21 and 19% of controls, respectively.Full strength extracts ofL. camaraandC. odoratadecreased seedling length and the FWof all crops.Dried shoot extracts of redroot pigweed significantly reduced germination ofcabbage seeds grown in Petri dishes. Extracts of nettle leaved goosefoot (Chenopodiummurale) inhibited germination of cabbage, carrot and pepper seeds (146). Shoot dryweights of all crops were reduced with extracts of both weed species compared with thecontrol. The inhibitory effect was more pronounced at higher extract concentrations andthe crops differed in their sensitivity to weed extracts. Incorporation of dried shootresidues of either weed species in soil mixtures severely reduced germination of cabbage,carrot and cauliflower and aubergine (brinjal) seeds, and delayed germination of pepperand tomato. Residues ofC. muraledelayed, while those ofAmaranthus retroflexusstrongly inhibited germination of the cucumber. Cauliflower and tomato were the mostsensitive crop species to weed residues. Plants from direct sown seeds or transplants weregreatly affected by weed residues. The inhibitory effect was residue rate dependent.Decayed residues ofC. muraledelayed germination of cauliflower and tomato butincreased their shoot dry weights, while those ofA. retroflexusretained their toxicity on Role of Allelopathy in vegetables crops production289tomato and significantly lowered shoot dry weight and leaf surface area of this crop.Results showed that the inhibitory effects of weed residues on tomato could be overcomeby supplying more nutrients to the growth medium.Ismail and Kumar (79) conducted laboratory and greenhouse experiments todetermine the effects of aqueous extracts and residues ofMikania micranthadecomposedat different periods on the germination and growth of cucumber and Chinese cabbage.They found that germination, radicle length and fresh weight of the 2 crops decreasedprogressively when plants were exposed to increasing concentrations (12.5, 25 and 50g/litre) of aqueous leaf or root extract ofM. micrantha. The phytotoxic effect of rootextract on the germination and growth of both test crops was greater than that of the leafextract. The fresh weight and the rate of emergence of the 2 bioassay species decreasedwith an increase ofM. micrantharesidues (leaf or root residues). The emergence of bothspecies was greatly reduced whenM. micranthadebris (root or leaf) was incorporated intothe soil after 2 weeks of decomposition. The fresh weight of cucumber seedlings wasconsiderably reduced by exposure to root debris at both 1 and 2 weeks of decomposition,whereas that of Chinese cabbage was reduced only when exposed to debris after 2 weeksof decomposition.Root exudates ofCymbopogon citratus,Ageratum conyzoidesandBidens pilosawith or without treatment with XAD 4 resin were applied to seeds of radish and cucumber(210). The exudate decreased germination rate, root length of seedlings and seedlingheight of both the crops. Treatment with resin reduced the effect of the exudates. ExudatesofB. pilosahad the greatest effect.The allelopathic potential of volatiles, foliage leachates, root exudates, and shootdried residues of white top (Cardaria draba) and Syrian sage (Salvia syriaca) on cabbagecv. Pronzwik, carrot cv. Natus, cucumber cv. Beithalpha, squash cv. Byrouti, onion cv.Texas Early Grana, pepper cv. Red Common and tomato cv. Special Back) was assessedthrough different laboratory and glasshouse experiments (145). Volatiles from Syrian sagefresh shoots reduced germination and inhibited seedling growth of most crops. Foliageleachates or root exudates of both weeds were toxic to different crops under laboratoryconditions, with tomato and cabbage being most affected. In pot experiments, surfaceplaced shoot residues of both weeds significantly delayed seed germination and reducedseedling growth of all crops with carrot, onion, and tomato being the most affected.Decayed residues of white top were also toxic, but lesser than when fresh materials wereused. Foliage leachates or root exudates of both weed species added or released into thesoil mixture reduced seedling growth of cabbage and tomato.The effect of different concentrations of underground organ and foliage extractsofCyperus rotunduswere tested on germination of cucumber seeds (38). The undergroundorgan and foliage extracts (5%) ofC. rotundusinhibited cucumber germination, reducedradicle and plumule lengths, fresh and dry weights, contents of endogenous hormones(auxins, gibberellins and cytokinins), altered the protein pattern of cucumber seedlings,and increased the contents of endogenous inhibitors (ABA and phenols). The foliageextract tended to be more potent. Chromatography fractionations and bioassays of growthregulating substances lead to the isolation of many phenolic compounds (38). 290John et alTable 3. Inhibitory allelopathic effects ofAgeratum conyzoideson vegetable crops in bioassays andpot cultureAllelochemical Recipient plant spp. Inhibitory effects on recipientspp. ReferencesLaboratory BioassayVolatile oilCucumis sativusL. Fresh weight, root length and93, 94shoot heightLycopersicon esculentumMill.Fresh weight, root length and93, 94shoot heightRaphanus sativusL. Fresh weight, root length, shoot68, 93, 94,height and chlorophyll content95AqueousAllium sativumL. Germination, fresh weight, root67extractslength and shoot heightCucumis sativusL. Germination, fresh weight, root67length and shoot heightLactuca sativaL. Germination, root length and67shoot heightPurified allelo-Cucumis sativusL.Germination, fresh weight, root93, 95chemicalsLycopersicon esculentumMill.length and shoot heightRaphanus sativusL. Root length and shoot height of93, 95,seedling110Pot cultureVolatile oilCucumis sativusL. Fresh weight, content of92, 96chlorophyllAllelopathic activity of weeds (Amaranthus gracilisDesf.,Convolvulus arvensisL.,Lactuca serriolaL. andPortulaca oleraceaL.) was studied on cabbage, carrot,cucumber, onion, pepper, squash and tomato under glasshouse conditions (124). The rootexudates of weeds released into the soil had variable effects on different test crops.L.serriolaroot exudates were most toxic to growth of different crops. Soil-incorporated driedshoot residues, reduced the seedlings growth of the test crops and the degree of inhibitionwas residue rate dependent. Soil-surface placed weed residues delayed seedlingemergence, but their effects on seedling growth was lower than soil-incorporated residues.Decayed residues of different weeds inhibited the seedlings growth but their effect wasless as compared to un-decayed residues. In all experiments, roots were more sensitivethan shoots to allelopathic effects and both positive and negative effects were observed.C.arvensisproved most harmful to all test crops, while cabbage, onion and tomato were mostaffected than other crops.The leaf leachates ofCassia unifloraweed stimulated the seed germination ofradish at low concentrations (2.5-5%), but higher concentrations (15-20%) inhibited bothseed germination and seedling growth (47). Application of either fresh or dry material ofSicyos deppeiweed decreased the relative growth rate, leaf area and dry weight of tomatoseedlings (130). Variation in growth was caused by the decreased net assimilation rate andthe physiological traits. These results linked with an imbalance of antioxidant enzymeactivity, increase in hydrogen peroxide and NADPH oxidase activity and an increase inlipid peroxidation in leaves of tomato plants. Role of Allelopathy in vegetables crops production291Laboratory and greenhouse assays revealed that aqueous whole plant extract fromConyza canadesisinhibited germination, germination speed and seedling growth ofcucumber and radish (45). Radish was more sensitive to the extracts. The aqueous extractdecreased the photosynthesis activity but increased the malondialdehyde contents ofcucumber and radish.4.3. Effects of vegetables on weedsSome vegetables allelopathically inhibits the weeds. Cucumber accessions (256nos.) from 41 countries were grown in pots withBrassica hirtaand proso millet.Cucumber variety 'PI211728' inhibited growth of the former species by 59 % and the latterby 87 %, while 25 accessions inhibited growth by 50 % or more. The effect of leachatescollected from separate pots containing a nontoxic accession and two toxic accessions onthe growth of seedlings the weed species suggested that inhibition was due to allelopathy.The possibility of incorporation of the allelopathic trait into plants can be considered toenhance their competitive ability against weed species (144).Lockerman and Putnam (105, 106) investigated the allelopathic potential of anumber of cucumber accessions on weeds. Cucumber wild accession PI 169391suppressed adjacent weeds much more effectively than Pioneer in the field. PI 169391suppressed fresh weight and population of proso millet (Panicum miliaceum) two timesmore than Pioneer. They suggested that introducing the allelopathic characters from wildaccessions to cultivated crops may be a useful approach to control weeds.The volatiles from the youngest fully expanded leaves of 35 days old, field-grownplants of turnip and kale (Brassica oleraceavar. acephalaL.) slightly inhibited thegermination of barnyard grass [Echinochloa crusgalli(L.) Beuv.] (128).Radish has the potential to control summer weeds likeSorghum halepense,Cynodon dactylon,Amaranthus retroflexus,Portulaca oleraceaeandXanthium.strumariumthat cause yield losses in maize and cotton fields (187). Radish successfullycontrolsSorghum halepensein cotton fields, when grown in rotation with cotton (186).When watermelon seeds were cultured in a Petri dish together with amaranthus(Amaranthus retroflexus), barnyard grass, cockscomb (Amaranthus caudatus), lettuce ortomato seeds, the shoot growth of amaranth and cockscomb was markedly promoted,whereas the shoot growth of lettuces and tomatoes was inhibited (100). The shoot growthof barnyard grass was not affected. The results suggest that plant selective allelopathicsubstance(s) affecting the shoot growth of other plant seedlings were exuded fromwatermelon seeds. An allelopathic substance was isolated from the exudates ofgerminating watermelon seeds and identified as vanillic acid. Vanillic acid was found topromote the shoot growth of cockscomb at concentrations of 10 to 300 ppm and that ofamaranth at concentrations of 3-30 ppm, although the shoot growth of amaranth wasinhibited by 300 ppm of vanillic acid. The shoot growth of lettuces and tomatoes wasinhibited at concentrations higher than 30 mg/litre by vanillic acid. However, the shootgrowth of barnyard grass was not affected at the concentrations used. All these resultssuggest that vanillic may be the allelopathic substance in exudates of germinatingwatermelon seeds (100).Chilliroot, shoot and rhizosphere soil (fresh)leachates (10, 20 and 30 %)decreased the germination ofAmaranths viridisL. andTrianthema portulacastrumL(161). As the concentration of the leachate increased, the magnitude of inhibition of 292John et algermination also increased. The lower concentration (10%) did not affect germination. Theleachates also reduced the seedling growth of the weeds.The inhibition was morepronounced at higher concentrations.Silvaet al.(160) reported a concentration dependent inhibition ofPortulacaoleraceagermination by the juice of ground roots of radish. The aqueous extracts of radish(33, 50, 60, 100 %) reduced the germination and rhizome development ofSorghumhalepense,Cynodon dactylon,Amaranthus retroflexus,Portulaca oleraceae,Xanthiumstrumarium(36).An allelopathic vegetable can potentially be used to control weeds by planting avariety with allelopathic qualities, either as a smother crop, in a rotational sequence, orwhen left as a residue or mulch, especially in low-till systems, to control subsequent weedgrowth. Alternatively, application of allelopathic vegetable residues before, along with, orafter synthetic herbicides could increase the overall effect of both materials, therebyreducing application rates of synthetic herbicides.4.4. Allelopathic effects of trees on vegetable crops (Agroforestry)In multi-storey cropping systems, where numerous crops and trees are growntogether, vegetables are essential components. The multipurpose trees comprise a virtuallyuntapped reservoir of allelochemicals. The allelopathic effects of several trees on crops hasbeen reported (Table 4).Jacobet al. (80) conducted laboratory bioassays and pot culture experiments toassess the allelopathic influence of leaf leachates of multipurpose trees (Artocarpusheterophyllus,Mangifera indica,Ailanthus triphysa,Anacardium occidentale,Tamarindusindica,Tectona grandis,Thespesia populnea,Casuarina equisetifolia,Gliricidia sepium,Strychnos nux-vomica) planted in the multi-storey home gardens of Kerala, on cowpea,bitter gourd (Momordica charantiaL.) and brinjal. The nature and degree of allelopathiceffects of trees varied with crop species (Table 5). Tamarind, teak, casuarina and strychnoscaused severe allelopathic inhibition of cowpea, hence were incompatible. Gliricidia,portia, tamarind, teak and cashew were incompatible with brinjal. Ailanthus, cashew,strychnos, mango, portia, tamarind and teak were detrimental to bitter gourd. In general,the inhibitory effects were most prominent in brinjal than cowpea and bitter gourd andcannot be overcome with dilution. The findings will help in differential selection ofvegetables, for combining with trees in agroforestry systems.A green house pot experiment assessed the allelopathic effects ofAcacia niloticaleaves on the growth and metabolic activities of 45-day-old pea plants (190). The lowerdoses ofAcacialeaf residue (0.25 and 0.5% w/w) stimulated the growth of pea shoot androot, but the higher doses (0.75, 1.0, 1.5 and 2%, w/w) inhibited seedling growth and theeffect was concentration dependent. The total phenolic content of pea shoots (particularlyphenolic glycosides), increased at lower doses ofAcaciaresidue and decreased with higherones. Chlorophylls a, b and carotenoids accumulated in pea shoot at lower doses ofAcacialeaf residues, accompanied by accumulation of total sugar, mainly the insoluble fraction.On the other hand, the inhibition in the contents of photosynthetic pigments at higher dosesofAcaciaresidues was paralleled by significant reduction in all sugar fractions. Role of Allelopathy in vegetables crops production293 294John et al Role of Allelopathy in vegetables crops production295Table 5. Effect of fresh leaf leachate (1:10 w/v) of trees on vegetable crops (Pot culture)Treatment PlantNo. ofDW per plant (g).heightleavesPod Seed Root(cm) MAPMAPCOWPEA(Vigna unguiculataL.)Ailanthustriphysa(Dennst)Alston 65.97 21.93 12.33 3.78 1.28AnacardiumoccidentaleL. 60.17 23.67 11.75 3.11 1.38CasuarinaequisettifoliaJ.R.35.53 16.00 9.30 2.11 1.30Gliricidia sepiumJacq Walp.57.60 21.67 7.71 3.11 0.84ArtocarpusheterophyllusLamk.57.53 20.27 8.66 2.56 1.64Strychnos nuxvomicaL. 36.27 23.00 8.01 2.33 1.51MangiferaindicaL.58.00 20.33 11.25 2.89 1.56Thespesiapopulnea(L.) Soland52.50 22.93 8.56 2.56 1.63TamarindusindicusL.0.00 0 0 0 0TectonagrandisL.f.0.00 0 0 0 0Control 66.97 24.00 12.29 3.56 1.70CD (0.05) 19.445 4.238 2.959 1.003 0.313TreatmentPlant height (cm) No. of leaves Weight (g) perplant1MAP 2 MAP 1MAP 2 MAP FruitRootFWDWBITTER GOURD(Momordica charantiaL.)Ailanthustriphysa(Dennst)Alston 40.10 148.00 11.20 30.67 453.67 3.78AnacardiumoccidentaleL. 39.63 168.67 12.83 30.00 522.10 3.71CasuarinaequisettifoliaJ.R.30.30 200.33 10.17 36.00 624.33 4.10Gliricidia sepiumJacq Walp.40.40 194.67 9.87 37.00 628.4 3.93ArtocarpusheterophyllusLamk.39.43 186.67 11.63 31.00 613.97 3.40Strychnos nuxvomicaL. 32.53 138.33 8.53 32.33 432.57 1.79MangiferaindicaL.43.73 186.33 11.53 36.00 563.33 2.73Thespesiapopulnea(L.) Soland48.50 216.00 12.43 40.67 543.70 1.41TamarindusindicusL.25.83 157.33 7.97 38.67 472.63 3.10TectonagrandisL.f.31.07 150.33 10.70 31.67 453.67 3.37Control 49.83 226.33 11.87 43.33 625.33 3.81CD (0.05) 11.160 43.889 2.517 6.831 41.036 0.487EGG PLANT(Solanum melongenaL.)Ailanthustriphysa(Dennst)Alston 15.00 41.33 7.00 21.33 317.64 3.03AnacardiumoccidentaleL. 11.00 38.33 7.00 15.33 260.39 1.94CasuarinaequisettifoliaJ.R.13.67 42.00 8.33 21.33 329.98 3.72Gliricidia sepiumJacq Walp.0 0 0 0 0 0ArtocarpusheterophyllusLamk.13.00 35.67 6.33 23.33 303.77 3.41Strychnos nuxvomicaL. 15.67 35.33 6.33 24.00 291.37 2.10MangiferaindicaL.14.67 32.33 6.67 23.00 314.05 3.89Thespesiapopulnea(L.) Soland0 0 0 0 0 0TamarindusindicusL.10.33 27.00 6.33 16.67 248.43 1.93TectonagrandisL.f.10.67 37.00 6.67 19.00 255.90 1.67Control 15.07 47.67 7.33 24.33 308.29 4.82CD (0.05) 2.498 8.005 NS 4.715 46.433 1.193MAP: Months after planting; DW: Dry weight; FW Fresh weight. 296John et alEucalyptus:Eucalyptus tereticornisleaf extracts significantly decreased the germination,shoot and root length of okra (Abelmoschus esculentusL), than bark and root extracts. Theinhibitory effects of extracts were concentration dependent. Leaf extract ofEucalyptusproved most detrimental to seedling vigour of okra than the bark and root (6). The aqueousextract of dry leaves ofEucalyptus camaldulensisDehn. inhibited the germination, vigourand growth of seedlings of cucumber and radishthan green leaves (1). Pinaet al.(135)studied the influence of leaf extracts ofEucalyptusdysentricaon sesame (SesamumindicumL.) and radish. Their study also evaluated the effects of fresh and dry leaf extractsand the extract pH (4.7-7.0) on target species. The extract did not influence thegermination but drastically reduced the seedling growth. The roots were more affected bythe extracts than shoots. Dry-leaf extracts were more inhibitory to sesame and radishseedlings than fresh leaf extracts. The extract pH did not influence the allelopathicresponse of target species. The extracts resulted in the abnormal growth of seedlingsviz.,shorter roots, root tissue darkening, early lateral root development, less number of roothairs and lateral roots and altered gravitropic response. The inhibitory effects of leavesextracts on seedling growth were more intense, when soil was substrate. The aqueous leafextracts ofE. dysentericaat concentration as low as 1% inhibited the initial growth, roothair and lateral root differentiation and impaired gravitropic responses of sesame andradish seedlings.Gattiet al.(46) reported the allelopathic potential of aqueous extracts of the leaf,stem and root ofOcotea odoriferaon the germination and growth of lettuce and radish.The leaf extract ofOcotea odoriferadrastically reduced the seed germination in lettuceand radish, whereas, the stem and root extracts delayed their germination. The roots ofvegetables were the most sensitive to allelochemicals.Aqueous extracts of fresh leaves of five mangrove tree spp. (Avicennia marina(Forsk.) Vierh.,Aegiceras corniculata(L.) Blanco, Kandelia candel(L.) Druce,Rhizophora stylosaGriff. andBruguiera gymnorrhiza(L.) Lam.) reduced the germinationrate and seedling growth of cabbage (25). The allelopathic intensity of aqueous extracts oncabbage increased with the development of mangrove succession.5. PESTICIDAL POTENTIAL OF VEGETABLE CROPS5.1. Fungitoxic/antibiotic effectsSeveral vegetables possess antimicrobial principles and hence, allelopathicallyinhibit phytopathogenic fungi and bacteria. Extraneous application of plants or itsconstituents controls several fungal pathogens (Table 6).The phytochemical a-tomatine is a steroidal glycoalkaloid found in tissues ofmembers of the genusLycopersiconand has been shown to exhibit antibiotic activityagainst a wide range of organisms. Studies have revealed that this secondary plantcompound is toxic to many microorganisms (14,78,158). Tomatine and tomatidine (0.3mM) when incorporated into growth media (agar), inhibited the growth of three fungalbioherbicidal phytopathogens.Alternaria cassiae, the most sensitive pathogen wasinhibited 70% by both compounds (42). Tomatine inhibited the growth ofColletotrichumtruncatumandFusarium subglutinasns(produces fumonisin, phytotoxic to many plants) Role of Allelopathy in vegetables crops production297Table 6. Vegetable crops with fungicidal activityVegetable Pathogens affected ProductReferencetestedDaucus carotaL. Aspergillus sp, Curvularia sp., Penicillium sp. Oil 51MomordicaDreschlera oryzae,Pyricularia oryzae,SAE, EE 177charantiaL.Rhizoctonia solaniAllium sativumL.Dreschlera oryzae,Pyricularia oryzae,SAE, EE 177Rhizoctonia solaniAllium cepaL.Alternaria alternata,Asperigillus niger,BotrytisOil 82, 113,allii,Chrysoporium tropicum,Claviceps161purpurea,Kerratinophyton terreum,Malbrancheapulchella,Microsporum gypseum,Phytophthorainfestans,Verticillium tenuipesCapsicum annumL.Aspergillus nigerOil 22SAE: Steamed aqueous extract, EE: Ethanolic extractby 63% and 50%, respectively; while tomatidine inhibited the growth of these latter twopathogens by 50% and 15%, respectively. These natural plant products have broad rangephytotoxicity and fungitoxicity, which may be important in plant defence mechanisms. a-Tomatine is a saponin (steroidal glycol-alkaloid) produced by tomato and some otherSolanum species (153). a-Tomatine consists of a branched tetrasaccharide (-D-glycopyranosyl-(1 2)- (-D- xylopyranosyl- (1 3)-D-glucopyranosyl- (1 4)-D-galactose), attached to 0-3 of the steroidal aglycone, tomatidine. This tetrasaccharide,called lycotetraose, occurs in solanaceous species: tomatoes and potatoes etc.(42).Tomatine accumulates in plant stems, leaves, and roots and is fungitoxic to some plantpathogens (63).Ajoene:Ajoene, a compound derived from the garlic (Allium sativumL.) and produced bychemical synthesis, was investigatedin vitroandin vivofor its activity against 20phytopathogenic and epiphytic fungi and bacteria (149). The main interest was focussed onthe fungi which belonged to the group of obligate biotrophic parasites (powdery mildews),other leaf pathogens, soil-borne pathogens, vascular wilt fungi, and yeasts. The minimum- 1inhibitory concentrations (in vitrotests), ranged from 2 to 200 mg Ldepending on theorganism, method, and nutrient medium used. The phytopathogenic fungiCladosporiumfulvumandVerticillium dabliaeand the phytopathogenic bacteriumErwinia amylovorawere the most sensitive species. In greenhouse experiments, the inhibiting action of ajoeneagainstCladosporium fulvumcould be confirmed with tomato plants after protectivetreatment. However, only powdery mildew of tomatoes (Oidium lycopersicum) and roses(Sphaerotheca pannosavar.rosae) could be inhibited completely.Aqueous and ethanolic extracts of seeds of radish, cauliflower and cress weretested for allelopathic effects onRhizoctonia solaniin vitro. The tested extracts wereinhibitory toR. solani. The application of powdered seeds of radish to the soil infestedwithR. solanireduced the damping off in cotton seedlings (40).Two compounds, separated chromatographically from extracts of potato peel andfrom extracts of pulp tissue, were found fungistatic toHelminthosporium carbonumandreduced its growth (98). These compounds were identical to chlorogenic and caffeic acid 298John et alin physical and chemical properties. It appears these acids are associated with theimmunity of white potatoes to attack ofH.carbonum.5.2. Nematicidal effectsCertain vegetables possess nematicidal principles and therefore, offer immensepotential for nematode control in vegetables cultivation (Table 7). Tomato contains a-tomatine effective againstPanagrellus redivivus(4). Potato contains -chaconine effectiveagainstMeloidogyne incognita(5). Asparagus contains glucoside asparagusic acideffective againstTrichodorus christie(131).Table 7. Vegetables with nematicidal activityName of plant NematicidalNematode test species Referenceplant partAllium fistulosumL. BulbMeloidogyne incognita172A. sativumL. BulbMeloidogyne incognita169Amaranthus gracilisDesf. expoir LeafMeloidogyne javanica121Asparagus sp.RootTrichodorus christie130Capsicum annumL. PodMeloidogyne incognita169Momordica charantiaL. SeedMeloidogyne incognita77Raphanus sativusL. Cover cropPratylenchus62TylenchorhynchusTrichosanthes anguinaL.SeedMeloidogyne incognita77a-tomatine and a-solanine are major glycoalkaloids biosynthesized in plants ofSolanaceae family, for plant defence against phytopathogens. They are also plasmacholinesterase inhibitors and teratogens. Using liquid chromatography, their levels in 15tomato cultivars were estimated in response to root-knot nematode (Meloidogyneincognita)attack at an infestation level of 2 juveniles. a-tomatine content was found muchhigher in roots and shoots of resistant over the susceptible cultivars (116). In roots, the-1concentration of a-tomatine was maximum (1773 mg kg) in PAU-8 among the resistantcultivars versus 356 mg kgin susceptible cultivar Hybrid-2. In shoots, the a-tomatine-1level ranged from 710 mg kgin Hybrid -1 (susceptible) to 5530 mg kgin Rutgers- 1-1(resistant) cultivars. The levels of both a-tomatine and a-solanine decreased with time.However at fruiting, the level increased in green fruit to maximum of 877 mg kgin-1-1cultivar PAU-9 as against 276 mg kgin Hybrid -2 and declined rapidly in yellow and redfruits. a-tomatine content in roots could be correlated to reduction in population ofM.incognitain soil and the hazard indices measured in terms of galling of tomato roots. Thestudies revealed that a-tomatine contents in roots of tomatoes may prove an excellentmarker for resistance againstM. incognita.The application of wheat flour at 5% rhizospheric soil drench to tomato cv. PusaRuby and okra cv. Purbani Kranti plants (24 h after inoculation of roots withMeloidogyneincognitajuveniles), reduced the infestation (99). Wheat flour, a-amylase and a-amylaseinhibitor did not kill the nematodes inin vitrotest, but decreased the a-amylase activity ininoculated roots of tomato and okra. a-amylase inhibitor applied on tomato and okra leaves(inoculated withM. incognita), ameliorated the root-knot. Both a- amylase and a-amylase Role of Allelopathy in vegetables crops production299inhibitors enhanced the enzyme activity in inoculated roots than in non-inoculated roots oftomato and okra plants.6. ALLELOCHEMICAL STUDIES IN VEGETABLES6.1. CucumberHolappa and Blum (65) assessed the relative sensitivity of cucumber toexogenously applied concentrations of ferulic acid. Ferulic acid inhibited leaf growth andwater utilization of cucumber. Increased endogenous abscisic levels were found incucumber following ferulic acid treatment.p-Thiocyanatophenol was extracted from the root exudates of hydroponicallygrown cucumber in a column filled with Amberlite XAD-4 resin, through which thenutrient solution was constantly recycled. In bioassays, p-thiocyanatophenol markedlyretarded the root elongation of lettuce seedlings and the nutrient ion uptake of cucumberseedlings (204).Lehmanet al.(102) reported that ferulic acid was more inhibitory to cucumberleaf expansion than p-coumaric acid. If present together, the effects of ferulic and p-coumaric acids on leaf expansion are additive.Activated charcoal added to the nutrient solution of growing cucumber cv. PI169319 seedlings when desorbed was found to contain four compoundsviz., benzoic, p-hydroxybenzoic, 2,4 dichlorobenzoic and phthallic acids (15). These compounds wereused in a cucumber bioassay at 0, 2, 10 and 20 l L. Only 2,4 dichlorobenzoic acid at 20-1 l Linhibited seedling growth, while phthallic acid stimulated its growth at 2 l L.-1-1Hence, 2,4 dichlorobenzoic acid may be responsible for growth reduction of cucumbers inhydroponic solution. In another bioassay, 2,4 dichlorobenzoic acid at 20 l Lwas added-1-1in mixtures with all other compounds at 10 l L, and the growth inhibition of cucumberseedlings was found to be ameliorated.Terziet al.(178) investigated the effects of juglone on the growth of cucumbercv. Beith Alpha) seedlings with respect to physiological and anatomical parameters.Growth parameters (seedling elongation, and fresh and dry weights) were reduced by 1mM juglone. Juglone also reduced chlorophyll a and b contents, and some anatomicaltissues (xylum vessel and bundle radius of stem, stomatal length, and number of stomataon cotyledons). The anatomical changes in stem and cotyledon were related to the growth-inhibiting effect of juglone. On the other hand, catecholase and tyrosinase (catecholoxidase) activities also increased due to juglone application.The effects of exogenous cinnamic acids were studied on cucumber growth andphysiological characteristics at seedling stage (195). The soils used were from agreenhouse, where cucumbers were continuously planted for 18 years and an open fieldwhere no cucumbers were planted, respectively. Seedling growth, contents ofphotosynthetic pigment, root activities and H-ATPase activities of root membrane were+inhibited by cinnamic acids. Inhibitions on growth and root activities of seedlings weresignificantly different between the greenhouse soil treated with 100 mg kgand open field-1-1soil treated with 200 mg kg.The dynamics of cinnamic acid uptake in cucumber plants and its residue in soilwere studied. The cucumber Changchun Mici was grown in pots and cinnamic acid was 300John et alapplied at 0, 25, 50, 100 and 200 mg kgsoil. The application of cinnamic acid increased-1its content in cucumber plants and in soil with its increasing dose. The retention rate (incucumber and in soil) decreased with high doses. The rate of allelochemicals addition andsoil retention probably controls the allelochemicals content in soil. The allelochemicalscontent in soil was the result of interactions among the plant and soil microbes (197).Cucumber seedlings were subjected to allelochemical stress by treating their rootswith 0.5 mM solutions of ferulic andp-coumaric acids (139). Production of ethylene bycucumber seedlings, the increase in phenylalanine ammonia-lyase activity and ligninsynthesis as well as the reduction in growth indices of cucumber roots were observed. Theresults indicated that ethylene participates in the retardation of cucumber root growth bythese phenolic compounds.Cucumber seeds were treated with solutions of ferulic andp-coumaric acids. Thephenolic acids changed the activity of hydrolytic enzymes and impeded the seedsgermination. Their effects depended on the plant species, enzyme type, concentration ofapplied phenolic acid and stage of germination (137).6.2. Other vegetable cropsAlsaadawiet al.(10) conducted a study to test whether interference ofchlorophyll metabolism and ion uptake may be mechanisms through which some phenolicacids inhibit the growth of cowpea seedlings. Three concentrations (10M, 5 x 10M and-4-410M) of each of syringic, caffeic, and protocatechuic acids were used in sand-culture-3medium. It was found that seedling growth, chlorophyll a, total chlorophyll, chlorophylla/b ratio, and the uptake of N, P, K, Fe and Mo were significantly reduced by most of thetest concentrations of the phenolic acids. However, chlorophyll b content and the Mguptake were not significantly affected by all the phenolic acid concentrations. Calciumuptake was significantly inhibited by 5 x 10M and 10M of caffeic acid and 5 x 10M-4-3-4of protocatechuic acid. In most cases, the reduction in dry weight was parallel to thereduction in chlorophyll content and ion uptake, and the reduction in chlorophyll was alsoparallel to the reduction in ion uptake.Phenolic compounds such as gallic acid, ferulic acid, p-hydroxybenzoic acid,vanillic acid, salicylic acid, tannic acid and hydroquinone were identified from the aqueousextracts and volatile substances of tomato plant (90). The seed germination and seedlinggrowth of the lettuce and egg plant was severely inhibited by 5 x 10Molar of phenolic-3reagents identical to those identified from tomato plant. Germination and growth rate of 5-4-5-3x 10M and 5 x10M were higher than that of 5 x 10M of phenolic compounds wouldbe assumed to be threshold concentration for allelopathic effects.The inhibitory effect of the glycoalkaloids chaconine, solanine, tomatine,solasonine and solamargine and of the aglycones solanidine, solasodine and tomatidine-1(200 and 400g ml) on lettuce seed radicle elongation were determined. Solasodine andtomatidine had negligible effects on the elongation, while solanidine produced over 50 %of the total measured inhibition (48).A methanol:water extract of tomato apices increased the growth of the algaChlamydomonas reinhardtii. The active substance from the dried shoot apices was purifiedby Cflash column and high performance liquid chromatography. The purified extract18enhanced the growth of tomato, corn and rice seedlings at concentrations less than 1.0ppm. Nuclear magnetic resonance and mass spectroscopy indicated that the purified Role of Allelopathy in vegetables crops production301fraction was a mixture of compounds having sugar moieties. Analysis by thin layerchromatography showed that the fraction was ninhydrin positive and more polar than theknown plant hormone studied (191).Holappa and Blum (65) assessed the sensitivities of tomato and bean toexogenously applied concentrations of ferulic acid. Ferulic acid inhibited leaf growth andwater utilization of wild type tomato and flacca tomato, but not of bean. Increasedendogenous abscisic levels were found in wild type tomato and flacca tomato subsequentto ferulic acid treatment.Allelochemicals alpha-pinene, borneol, chlorogenic-acid, coumarin, scopoletin,limonene, terpinolene in roots of carrot; a-pinene, chlorogenic-acid, cinnamaldehyde, o-cresol, quercetin, rutin, salicylaldehyde in fruit of tomato; and alpha-pinene, chlorogenic-acid, limonene, scopoletin, terpinolene (fruit)inchillyhave been identified (37).Thirteen natural and synthetic phenylpropanoids as well as coumarin were testedfor their biological activity on radish germination and subsequent root growth in light anddarkness (3). Coumarin was the most potent inhibitor. Coumarin was formedspontaneously by photooxidation of 2-hydroxycinnamic acid. Microscopic observation ofroot treated with coumarin suggest that this substance inhibits the elongation of cells of thedifferentiating zone of the root.Sterile root exudates from carrot seedlings stimulate the hyphal development ofGigaspora margarita, a vesicular-arbuscular mycorrhizal fungus, in the presence ofoptimal COenrichment (142). Three flavonols (quercetin, kaempferol, rutin or quercetin23-rutinoside) and two flavones (apigenin, luteolin) were identified in carrot root exudates.Flavonols like quercetin and kaempferol are known to have stimulatory effects on hyphalgrowth ofG.margarita.The influence of carrot seed oil and its major components (caryophyllene andcarotol) on the growth and germination of cress and carrot was studied in two independenttests,viz., (i) the action of vapours of volatile substances and (ii) the action if applieduniformly dispersed in water (83). The oil and its components exhibited phytotoxicproperties regardless of the mode of application and inhibited the germination of test plantsat the highest concentrations. Their post emergence influence on plant growth was evenmore pronounced than that observed in seed germination.Capsaicin (from chilli) inhibited germination, roots and shoots of alfalfa,cress,lettuce, crabgrass(Digitaria sanguinalis),timothy(Phleum pratense)and ryegrass (85).Increasing the dose of capsaicin increased the inhibition. The concentrations for 50 %inhibition of the root growth were 2.7, 0.32, 2.1, 0.27, 0.29 and 0.57 mM for alfalfa, cress,lettuce, crabgrass, timothy and ryegrass, respectively. The concentrations for 50 %inhibition of shoot growth were 17, 0.87, 6.7, 2.3, 1.4 and 6.2 mM for alfalfa, cress,lettuce, crabgrass, timothy and ryegrass, respectively. Germination was inhibited by 50 %at 82, 88, 68. 4.8, 22 and 11 mM concentrations of alfalfa, cress, lettuce, crabgrass,timothy and ryegrass, respectively. Thus, effectiveness of capsaicin on the plant growthdiffered with species and targets, and suggests that capsaicin may act as an allelochemicalto other plants.The volatile extracts of essential oils of cinnamon (Cinnamomum zeylanicumBlume), alecrim-pimenta, capim-citronella and alfavaca-cravo revealed allelopathicpotentialities on lettuce seed germination and radicle growth. The effect varied according 302John et alto the oil concentration (11). The volatile extract of essential oil stimulated radicle growthand did not inhibit lettuce seeds germination.The phytotoxic activity of the compounds isolated from sugarcane straw wasevaluated on seedling growth of lettuce (156). Three compounds were identified: ferulic(FA), syringic (SA) and vanillic (VA) acids. VA drastically inhibited the root elongation,followed by FA and SA. These phytotoxins increased the root membrane permeability anddepressed the root metabolic activity in lettuce. VA and FA inhibited the mitotic index,while, FA and SA stimulated the proliferation of secondary root.Pea seeds were treated with solutions of ferulic andp-coumaric acids. Thephenolic acids changed the activity of hydrolytic enzymes and impeded the seedsgermination. Their effects depended on the plant species, enzyme type, concentration ofapplied phenolic acid and stage of germination (137).Hoagland (63) conducted studies to examine the phytotoxic and antibiotic effectsof tomatine and tomatidine in a variety of plant species (including weeds). a-tomatine wasnot highly phytotoxic and slightly inhibited the stem elongation (7 to 13%) when appliedas a spray to etiolated 4-d-old seedlings of sesbania (Sesbania exaltata(Raf) Rybd),sicklepod (Senna obtusifoliaL.), mungbean, wheat and sorghum(Sorghum vulgareL.).Tomatidine had less effect on stem elongation than tomatine and reduced the elongation (5to 10%). Tomatine was more effective than tomatidine in reducing chlorophyll content inexcised etiolated tissues of sesbania, sicklepod, mungbean, wheat and sorghum. Inhibitionof chlorophyll accumulation by tomatine ranged from 16 to 89% of control values,whereas inhibition by tomatidine ranged from 0 to 30 % of control in these species. Bothtomatine and tomatidine increased electrolyte leakage of corn,kudzu (Pueraria lobata(Willd.) Ohwi), palmleaf morning glory (Ipomea wrightiiGray) and wild senna (CassiamarilandicaL) leaf disks at 24 to 72 h after exposure to the compounds (0.5 mM) andlight (100 E ms). Tomatidine caused greater electrolyte leakage than tomatine in2-1tissues of these species.7. FUTURE RESEARCH PROSPECTSThe thrust areas for future allelopathic research are as under:i. Separate the allelopathic interference from competition in vegetable fields andvegetables based cropping systems.ii. Screen the germplasm/varieties of vegetables for allelopathic potential and later on dogenetic manipulations to breed new varities.iii. Exploit the vegetables allelopathy for weed control and plant protection.iv. Determine the critical concentrations of allelochemicals of each vegetable needed toexpress their inhibitory/ stimulatory influences.v. Identify the compatible and beneficial associations of vegetables with other crops,vegetables and trees.vi. Determine the harmful and beneficial effects of allelopathy through detailed potculture and field studies. Role of Allelopathy in vegetables crops production3038. REFERENCES1. Ahmed, R., Uddin, M.B. and Hossain, M.K. (2004). Allelopathic effects of leaf extracts ofEucalyptuscamaidulensisDehn. on agricultural crops.Bangladesh Journal of Botany33: 79-84.2. Akram, M. and Hussain, F. (1987). The possible role of allelopathy exhibited by root extracts and exudatesof Chinese cabbage in hydroponics.Pakistan Journal of Scientific and Industrial Research30:918-21.3. Aliotta, G., Cafiero, G., Fiorentino, A. and Strumia. S. (1993). Inhibition of radish germination and rootgrowth by coumarin and phenyl proponoids.Journal of ChemicalEcology19: 175-183.4. Allen, E.H. and Feldmesser, J. (1970). Nematicidal effect of alpha tomatine onPanagrellus redivivus.Phytopathology60: 1013.5. Allen, E.H. and Feldmesser, J. (1971). Nematicidal effect of alpha chaconine: effect of hydrogen ionconcentration.Journal of Nematology3: 58-61.6. Allolli, T.B., Reddy. M. and Patil, M.G. (2004). Allelopathic effects of Eucalyptus(E. tereticornis)plantextracts on the germination and seedling growth of okra(Abelmoschus esculentus):Implicationfor Agroforestry.Abstr acts, IV International Conference Allelopathy in Sustainable Terrestrialand Aquatic Ecosystems(Eds., S.S. Narwal and Barbara Politycka). pp. 54. InternationalAllelopathy Foundation, Rohtak, Haryana, India.7. Almodovar-Vega, L., Guzman-Perez, C.D. and Semi