生物专业英语.docx

生物专业英语.docx

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生物专业英语

Assessingpotentialdietarytoxicityofheavymetalsinselectedvegetablesandfoodcrops

Abstract:

Heavymetals,suchascadmium,copper,lead,chromiumandmercury,areimportantenvironmentalpollutants,particularlyinareaswithhighanthropogenicpressure.Theirpresenceintheatmosphere,soilandwater,evenintracescancauseseriousproblemstoallorganisms,andheavymetalbioaccumulationinthefoodchainespeciallycanbehighlydangeroustohumanhealth.Heavymetalsenterthehumanbodymainlythroughtworoutesnamely:

inhalationandingestion,ingestionbeingthemainrouteofexposuretotheseelementsinhumanpopulation.Heavymetalsintakebyhumanpopulationsthroughfoodchainhasbeenreportedinmanycountries.Soilthresholdforheavymetaltoxicityisanimportantfactoraffectingsoilenvironmentalcapacityofheavymetalanddeterminesheavymetalcumulativeloadinglimits.Forsoil-plantsystem,heavymetaltoxicitythresholdisthehighestpermissiblecontentinthesoil（totalorbioavailableconcentration）thatdoesnotposeanyphytotoxiceffectsorheavymetalsintheediblepartsofthecropsdoesnotexceedfoodhygienestandards.Factorsaffectingthethresholdsofdietarytoxicityofheavymetalinsoil-cropsysteminclude:

soiltypewhichincludessoilpH,organicmattercontent,claymineralandothersoilchemicalandbiochemicalproperties;andcropspeciesorcultivarsregulatedbygeneticbasisforheavymetaltransportandaccumulationinplants.Inaddition,theinteractionsofsoil-plantroot-microbesplayimportantrolesinregulatingheavymetalmovementfromsoiltotheediblepartsofcrops.Agronomicpracticessuchasfertilizerandwatermanagementsaswellascroprotationsystemcanaffectbioavailabilityandcropaccumulationofheavymetals,thusinfluencingthethresholdsforassessingdietarytoxicityofheavymetalsinthefoodchain.Thispaperreviewsthephytotoxiceffectsandbioaccumulationofheavymetalsinvegetablesandfoodcropsandassessessoilheavymetalthresholdsforpotentialdietarytoxicity.

Keywords:

Heavymetals,Dietarytoxicity,Vegetables,Foodcrops

1.Introduction

Regulation,handlingandbioremediationofhazardousmaterialsrequireanassessmentoftherisktosomelivingspeciesotherthanhumanbeing,orassessmentofhazardtotheentireecosystem.Assessmentendpointsarevaluesoftheecosystemthataretobeprotectedandareidentifiedearlyintheanalysis.Suchendpointsmayincludelifecyclestagesofaspeciesandreproductiveorgrowthpatterns.Ecosystemriskassessmentisatitsdawnwiththisareaofenvironmentsciencesstillrequiringextensiveworkintheindustrializednationsoftheworldforsustainabilityoftheglobalecosystem.

Heavymetals,suchascadmium,copper,lead,chromiumandmercury,areimportantenvironmentalpollutants,particularlyinareaswithhighanthropogenicpressure.Theirpresenceintheatmosphere,soilandwater,evenintraces,cancauseseriousproblemstoallorganisms.Heavymetalaccumulationinsoilsisofconcerninagriculturalproductionduetotheadverseeffectsonfoodquality（safetyandmarketability）,cropgrowth（duetophytotoxicity）（Maetal.,1994;MsakyandCalvert,1990;Fergusson,1990）andenvironmentalhealth（soilflora/faunaandterrestrialanimals）.Themobilizationofheavymetalsintothebiospherebyhumanactivityhasbecomeanimportantprocessinthegeochemicalcyclingofthesemetals.Thisisacutelyevidentinurbanareaswherevariousstationaryandmobilesourcesreleaselargequantitiesofheavymetalsintotheatmosphereandsoil,exceedingthenaturalemissionrates（Nriagu,1989;Bilosetal.,2001）.Heavymetalbioaccumulationinthefoodchaincanbeespeciallyhighlydangeroustohumanhealth.Thesemetalsenterthehumanbodymainlythroughtworoutesnamely:

inhalationandingestion,andwithingestionbeingthemainrouteofexposuretotheseelementsinhumanpopulation.Heavymetalsintakebyhumanpopulationsthroughthefoodchainhasbeenreportedinmanycountrieswiththisproblemreceivingincreasingattentionfromthepublicaswellasgovernmentalagencies,particularlyindevelopingcountries.

Vegetablesconstituteessentialdietcomponentsbycontributingprotein,vitamins,iron,calciumandothernutrients,whichareusuallyinshortsupply（ThompsonandKelly,1990）.Theyalsoactasbufferingagentsforacidicsubstancesproducedduringthedigestionprocess.However,theycontainbothessentialandtoxicelementsoverawiderangeofconcentrations.Metalaccumulationinvegetablesmayposeadirectthreattohumanhealth（Türkdoganetal.,2003;Damek-PoprawaandSawicka-Kapusta,2003）.Chinesecabbage（BrassicachinensisL.cv.Zao-Shu5）,wintergreens（B.rosularisvar.TsenetLeecv.Shang-Hai-Qing）,pakchoi（BrassicachinensisL.）andcelery（ApiumggraveolensL.var.dulceDC）aresomecrops,whichwereassessedforheavymetaltoxicity.Vegetablestakeupmetalsbyabsorbingthemfromcontaminatedsoils,aswellasfromdepositsondifferentpartsofthevegetablesexposedtotheairfrompollutedenvironments（Zurera-Cosanoetal.,1989）.Ithasbeenreportedthatnearlyhalfofthemeaningestionoflead,cadmiumandmercurythroughfoodisduetoplantorigin（fruit,vegetablesandcereals）.Moreover,somepopulationgroupsseemtobemoreexposed,especiallyvegetarians,sincetheyabsorbmorefrequently‘tolerabledailydoses’.

Foodcontaminationbyheavymetalsdependsbothontheirmobilityinthesoilandtheirbioavailability.Thoughsomeofthemobilityandbioavailabilityfactorsareeasytomeasure,determinationofthefoodriskcontaminationistricky.Theaimofthepresentpaperistoreviewconciselythephytotoxiceffectsandbioaccumulationofheavymetalsinvegetablesandfoodcropsandassessmentofsoilheavymetalthresholdsforpotentialdietarytoxicity.

2.HazardousEffectsofHeavyMetalsonHumanHealth

Chroniclow-levelintakesofheavymetalshavedamagingeffectsonhumanbeingsandotheranimals,sincethereisnogoodmechanismfortheirelimination.Metalssuchaslead,mercury,cadmiumandcopperarecumulativepoisons.Thesemetalscauseenvironmentalhazardsandarereportedtobeexceptionallytoxic（Ellenetal.,1990）.Vegetablestakeupmetalsbyabsorbingthemfromcontaminatedsoils,aswellasfromdepositsonpartsofthevegetablesexposedtotheairfrompollutedenvironments（Zurera-Cosanoetal.,1989）.

Metalcontaminationofgardensoilsmaybewidespreadinurbanareasduetopastindustrialactivityandtheuseoffossilfuels（Chronopoulosetal.,1997;Sánchez-Camazanoetal.,1994;Sterrettetal.,1996;vanLune,1987;Wong,1996）.Heavymetalsmayenterthehumanbodythroughinhalationofdust,directingestionofsoil,andconsumptionoffoodplantsgrowninmetal-contaminatedsoil（Cambraetal.,1999;DudkaandMiller,1999;Hawley,1985）.Potentiallytoxicmetalsarealsopresentincommerciallyproducedfoodstuffs（DEFRA,1999）.Exposuretopotentiallytoxicmetalsfromdustinhalationorsoilingestionisusuallymodelledsimplyastheconcentrationofacontaminantmeasuredinthesoilmultipliedbythequantityofdustinhaledorsoilingested（Konzetal.,1989）.Thisisaconservativeapproachtoestimatedose,becausethebioaccessibilityofheavymetalsadsorbedoningestedsoilisnot100%（Rubyetal.,1999）.However,predictingexposuretopotentiallytoxicmetalsfromconsumptionoffoodcropsismorecomplicatedbecauseuptakeofmetalsbyplantsdependsonsoilpropertiesandplantphysiologicfactors.Thisleadstomuchlargeruncertaintiesassociatedwithestimatingpotentialdosesthroughfoodchainscomparedtotheuncertaintiesassociatedwithotherexposurepathwayssuchassoilingestionanddustinhalation（McKone,1994）.

Leadisatoxicelementthatcanbeharmfultoplants,althoughplantsusuallyshowabilitytoaccumulatelargeamountsofleadwithoutvisiblechangesintheirappearanceoryield.Inmanyplants,Pbaccumulationcanexceedseveralhundredtimesthethresholdofmaximumlevelpermissibleforhuman（Wierzbicka,1995）.TheintroductionofPbintothefoodchainmayaffecthumanhealth,andthus,studiesconcerningPbaccumulationinvegetableshaveincreasingimportance（Coutate,1992）.AlthoughamaximumPblimitforhumanhealthhasbeenestablishedforediblepartsofcrops（0.2mg/kg）（ChineseDepartmentofPreventiveMedicine,1994）,soilPbthresholdsforproducingsafevegetablesarenotavailable.

KnowledgeofZntoxicityinhumansisminimal.ThemostimportantinformationreportedisitsinterferencewithCumetabolism（Baroneetal.,1998;GyorffyandChan,1992）.ThesymptomsthatanacuteoralZndosemayprovokeinclude:

tachycardia,vascularshock,dyspepticnausea,vomiting,diarrhea,pancreatictisanddamageofhepaticparenchyma（Salgueiroetal.,2000）.AlthoughmaximumZntoleranceforhumanhealthhasbeenestablishedforediblepartsofcrops（20mg/kg）（ChineseDepartmentofPreventiveMedicine,1995）,soilZnthresholdforproducingsafevegetablesisnotavailable.

AccordingtoHoughetal.（2004）underPartIIAoftheEnvironmentalProtectionAct1990,theUKgovernmentfavorsa“suitableforuse”approachtoredevelopment（DETR,2000）:

Landiscontaminatedonlyifthecurrentorintendeduseofasitehasthepotentialtocauseanunacceptablehealthrisktohumanoccupantsortotheenvironment.UndertheUKTownandCountryPlanningAct1990（DETR,2000）,thisapproachrequiresthatlandbeassessedforredevelopmentonasite-specificbasis.Atpresent,concentrationsofmetalsinthesoilarecomparedtometal-specific“triggervalues”（termed“maximumcontaminantlevels”or“maximumcontaminantconcentrations”inNorthAmerica）.Inthepastthese