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n andprecipitation havebeenusedforthispurpose insomestudies. Thesehaveprovedmore difficult andlesseffective thantheadsorption technique developed by STEINBERG (11)andimproved byARNONandSTOUT(4),inwhichheavy metals arecoprecipitated withcalcium phosphate. Inrecent yearsatten- tionhasbeencalled tothereaction ofheavymetals withcertain organic molecules toformchelate complexes whichcanberemovedfromnutrient solutions byextraction (7,8,12).Thismeansofeliminating trace metals fromnutrient solutions suggested thepossibility thatsuchchelating agents mightbeusedinnutrient solutions indirect contact withtheroots ofgrow- ingplants toprovide acontinuing control oftrace metalcontamination in thenutrient media.Thepresent studywasundertaken todetermine the effect ofchelating agents addedtonutrient solutions onplants growing in these solutions. Testsreported describe theresponses ofseedlings ofsev- eralspecies andofoldertomatoplants tosixsuchchelating compounds present innutrient solutions. A chelating agent(fromtheGreekChele, meaningclaw)usually isa complex organic molecule containing twoelectron-donating groupswhich, acting similar tothetwojawsofa claw,mayremovesimplemetalions fromsolution. Thesechelating reactions areslightly reversible, thede- greeofreversibility, characterized asthe"stability constant," depending ontheagentusedandthemetalioninvolved. Themechanism ofchelation isillustrated withthefollowing example.Oxine(fig. 1,A),8-hydroxy- quinoline (HOC6H3N: CHCH:CH),isa doubleringcompoundformed bythefusion (intheorthoposition) ofapyridine ringtoabenzene ring withthehydroxyl groupontheeighth position ofthedoubleringcom- pound.Oxineionizes toformoxinate andhydrogen ions, thehydrogen comingfromthehydroxyl group.Thechelating effect ofoxineresults whentheoxygen andnitrogen, eachacting asaclaw,jointoametalion (Zn++) thusforminga ring, called a chelate ring.Theoxygen, witha freenegative valence, formsanelectrovalent linkage withthezincion whilenitrogen formsa co-ordinate covalent bond.Thusthezincionis boundsecurely totheoxine molecule. However, thezincionhastwofree valences andforthisreason twomolecules oftheoxinearerequired to 1Contribution fromtheBotanical Laboratory, TheIUniversity ofTennessee, New

CHELATION OF TRACE METALS IN NUTRIENT SOLUTIONS