PARTITION OFIONIC CONSTITUENTS
BETWEEN ORGANS
W. Dijkshoorn
Inadditiontothephosphateand chloride,the
plant accumulatesunchanged nitrateand sulphate
anions. Thesum ofequivalents (N0^+C1~+H2P0^+S0=)
intheplantrepresents theinorganic anioncontentA.
Inthetissuesofhigherplants thesumofequivalentsofthemetallic cationsC (=K++Na++Mg+++Ca"^1")
isgreater thanA,and theexcess (C-A)accumulatesin
theformof carboxylates,suchasmalates,citrates,
oxalates,succinates,etc.ofthemetalliccations.
Carboxylatesariseintheleaves fromtheconversionofnitratesand sulphates. Withoutnitratein
themedium,bicarbonate isabsorbed alongwiththe
metallic cations,andentersorganic combinationto
formcarboxylates intheroots. Allotherplantparts
receive their carboxylates byredistribution.
Thecontrolof thelevelofcarboxylates in
heterotrophic plant organs involves importviathe
phloem system. Potassium isthemaincationtransferred;itmovesinproportion totheamount oftrans=
locateddrymatter and,beingdominant,there islittle
447
y?
Ci
•*h
448
DIJKSHOORN
change intheconcentration of thesumofmilliequivalentsofcations inthedriedmaterial throughout the
life oftheorgan.
Thelevelofcarboxylates inreserve organsis
controlled byboththe import ofionsvia thexylem,
and theexport ofpotassium carboxylatesviathe
phloem. Theabsorbed calcium andmagnesiumare
retained,with theexportofpotassium and drymatter,
calciumeventuallybecomesdominant,and theconcentrationofcarboxylates inthedriedmaterialis
raised toahighervalue. During thischangethe
citrate content isreduced andmalateeventually
becomes themaincarboxylate.
Young leaveshaveheterotrophic requirementsat
first. Later,whenfully expanded,their synthetic
capacities aredirected towards export ofassimilates
and theleafacquires features typicalofreserve
organs. Inplantswithaterminalgrowth,plant
compositionmaybecome stratified insuchamanner
thatupper leaves arelowincalciumandcarboxylates,
while thematured lower leaveshavehigherconcentrations ofcalciumand carboxylates,withmalateas
themajor carboxylate. Iftherootsarenotprogrammed
toabsorb largerquantities ofcalcium,asincereals
and grasses,potassiummay remaindominant and there
willbe lesschange inthe levelofcarboxylates inthe
wholefoliageduring ageingoftheplant.
Maizeplantsgrownonasupply ofnitrate andthe
othernutrient saltexhibithigh levelsofnitratebut
nocarboxylate inthexylem liquid. If transferred to
a nitrate-freemedium thenitrate levelfallsand
carboxylate,synthesized intherootsfromabsorbed
bicarbonates,substitutesfornitrate inthexylem
liquid. From thisitisconcluded thatallthe
absorbed nitrateistransferred via thexylem tothe
shoot,andentersmetabolism inbalancewithmetallic
cations. The carboxylates replacing theconsumed
nitrates thendistributevia thephloemalongwith
PARTITION OFIONICCONSTITUENTS
449
potassium totheotherplant parts. Xylem transferof
carboxylates occursonlywhennitrate isexhaustedand
therootsbegin toproduce carboxylates fromabsorbed
bicarbonates.
A scheme ispresented toaccount for theability
ofgrassesandcereals toaccumulateorganicnitrogen
inexcessofcarboxylates. Itisbased ontheunique
translocation propertiesofpotassium,and ondecarboxylationof thecarboxylates received by therootswith
exchangeof thereleased bicarbonate forextranitrate
anionsfrom themedium
INTRODUCTION
Theoccurrenceof inorganic saltsand carboxylates
intheplant isduetotheaccumulation and processing
ofnutrient ionsthat itabsorbs from themedium.
Of themajor routesofanionutilization that
leading toorganic phosphatesdoesnotchange theionic
stateofthephosphate.
Inaddition,plantsaccumulateunchanged nitrate
and sulfateanions inexcessofmetabolicrequirements,
andallthechloridethat isabsorbed.
Thelevelofotherdetectable inorganic anionsis
extremely lowsothat thesumofmiliequivalents:
(NO?+ CI - +H2PO4+ S0=)=A
represents theinorganic anioncontent,including the
phosphateanions inorganiccombination.
Theexcessofmiliequivalents ofthecations,obtainedby thesummation:
(K++Na ++Mg+++Ca4-1")=C,
450
DIJKSHOORN
over theinorganic anions appears tobe inbalancewith
carboxylateanions,thecarboxylate content isgivenby
(C- A ) .
Carboxylates areformed asanecessary concomitant
of theconversionofnitratesofthemetallic cations
intonon-ionic organicnitrogenand sulfur (Dijkshoorn,
1962). Therefore,ifpotassiumnitratewere toactas
thesolesourceoforganicnitrogen,itsequivalentof
potassium carboxylatewouldbeformedwithin thetissues.
Another sourceofcarboxylates istheuptakeof
metallic cationswithbicarbonateasthepartnering
anion. Theabsorbed bicarbonates enterorganiccombination toformcarboxylates. Ifinthemedium,the
nitratesarereplaced by chlorides orsulfates,bicarbonate (derived fromthedissociationofwaterand
carbondioxide)isabsorbed,and carboxylates areaccumulated intheroots.
Inryegrassnitrates arereadily transferred to
theshootand themetabolism generates carboxylatesin
excessofthequantity retained by thefoliage. Butin
thecaseofbicarbonate absorption thecarboxylates,
produced intheroots,arelessreadily transferred to
theshoot,and theplantsmaybe showntobecomeprogressively starved ofcarboxylates.
Whencultured onanitratemedium,thelevelof
carboxylates, (C - A ) , ishighintheshootandlowin
theroots,and thissituation isreversedwhenthenitrateisexhausted (Dijkshoornetal., 1968).
Thegreaterpotentialofthenitrate systemin
supplying carboxylates seemsafeatureofmanyhigher
plants. Fig.1refers toyoungpotatoplantsraised
fromwhole tubersplanted insand culture. Whensuppliedwithnutrientsalts containingnitrate,organic
nitrogen inthewholeplant increased tomorethan
PARTITIONOFIONICCONSTITUENTS
451
me.(C-A)
20
A
16
12
?y/
8
0
U
8
12
16 20
me.Norg
FIG. 1 Amountsoforganicnitrogen (N o r g),and
ofcarboxylates (C-A), insmallpotato
plantsgrown insand culturefrom seed tubers
withlow (opensymbols)andhigh (solid symbols)
nitrogen content. Dataexpressed asionequivalentsofcarboxylateandofnitrate,respectively,perplant including theseedtuber.
Squaresdenoteamounts intheseed tuberbefore
planting;triangles:plants suppliedwithnitrate inthenutrient salts;circles:plants
grownwithallthenitratereplaced bysulfate
inthenutrientsalts.
twice theamount initially present intheseedtuber.
Expressed asionequivalent ofnitrate,theincreasein
organicnitrogenwasequal tothatof thecarboxylates,
indicatingthat allthecarboxylatereleased by thenitratemetabolismwasretained by theplant. Withadditional sulfate inplaceofnitrate,theamountof
452
DIJKSHOORN
organicnitrogen remained unchanged and little increase
intheamountofcarboxylates occurred.
Itisapparent thatthecontrolofthelevelof
carboxylates involves ionuptake,translocationand
metabolism. Oneresultof thisprocesswhich isamenabletostudy isthepatternofdistribution over the
different tissuesoftheplant. Thispatternwould
suggest that ionuptake,availability forreleaseto
thexylem,siteofmetabolism,andavailability for
release totheconducting elements ofthephloem,are
allselectiveandmay greatly influencecomposition.
Thepurposeofthepresent communication isto
drawattention tothelinksbetweenmigrationand
metabolism ofabsorbed ionsand thecarboxylatecontent
ofdifferent plantorgans.
SELECTIVEUPTAKEOFCATIONS
Thefirst selectiveprocess isuptakefromthemedium. Comparedwithdicotyledons,thecerealsand
grasses appear toabsorbpreferentially potassium,
whilecalcium islessreadily absorbed.
Theproportionofpotassium tocalciuminthe
bleeding sapofmaizeplantsappeared tobeequalto
thatinthebody of theshootandof theroots (Fig.2).
Theplantsweregrownonnutrient solutionswitha
ratioofequivalents ofpotassium tocalciumof0.5.
Sincethisratiowasabout 4inboth thexylem exudate
and intheplanttissues,thereseemsnodoubtthat,
here,therelativeexclusionofcalcium iscontrolled
by theroots,prior toiontransfer tothexylem.
Inmanydicotyledons there isno suchgreatpreferenceforpotassiumabsorption,and calciummayaccumulatetohighconcentrations inmatured plantparts.
PARTITION OF IONIC CONSTITUENTS
453
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DIJKSHOORN
Judging fromratesofredistributionoverdifferent
plantorgansitseemsthat,here,calciumtransferis
restricted by itsslowrelease totheconductingelementsof thephloem,potassium eventually assuming
thedominant roleinthesecond stageoftransferwith
thestreamofassimilates.
SELECTIVEDISTRIBUTIONOFCATIONS
Throughout thelifeoftheplantassuccessive
organsdevelop,each inturnimportsfrom,orexports
to,therestof theplant. Transfer ofassimilates
occursvia thephloem,andastudy of iondistribution
betweenorgansofallagesandclassesoffersameans
ofassessing theselectivity involved inphloemtransport.
Thenutritional systemdisplayedby thedeveloping
potato tubermayberegarded asbeing typicalofaheterotrophic organ. Assimilates and ionicsubstancesare
supplied by theautotrophic shootand this transportis
mediated by thephloem. Transport continues throughout
thelife-timeofthedeveloping tubers,and potassium
contributes for85percent,and calcium forlessthan
5 percent,tothetotalinfluxofcations (Fig.3).
Theamountofpotassium entering thetubersincreases fairlywell inproportion tothedrymatter.
Theslopeof theline indicatesaconstantconcentrationof0.5me.ofpotassiumpergramdrymatter.
The influxoftotalcations (C),being delegatedmainly
topotassium,alsoproceedsverynearly inproportion
to thedrymaterial.
Theextremely lowrateof transferofcalciumsuggests that,here,exclusionofcalcium takesplaceat
thesiteswherein theionsarereleased totheconductingelementsofthephloem. Further circumstantial
PARTITIONOFIONIC CONSTITUENTS
455
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support forthisviewcomesfromdataonchangeincompositionofreserveorgansduring exportofmaterialto
theotherpartsoftheplants.
Fig.4showstheprogressivelossindrymatter
andcationsfromthemother tuberofpotatoplantsduringexporttothegrowingplant. Again,thequantity
ofpotassiumvariesfairlywellinproportion tothe
changeindryweightof thetuber,anddecreasesata
constantrateof0.4me.pergramlossindrymatter.
Inthemother tubertheothercationsalsomove
inwardsviatheuptakesystem,onlyasmallproportion
ofthesemoving outwardsagainbecausetheyareless
readily available forreleasetothephloem thanpotassium. Asaresult,thefallinpotassium isclosely
mirroredbyanincreaseinthequantity ofcalciumretainedbythetuber. Theprogressivegainincalcium
andlossindrymaterialraisesthecalciumlevelfrom,
initially,0.02me.per gramdrymatter,toafinal
levelof1.3me.pergramdrymatter. Formagnesium,
theexportisbalancedbyuptake,and itsconcentration
increases inproportiontothereciprocaldryweight.
Inthedaughter tuberwithpotassiumdominantin
theimportofcationsviathephloem,the concentration
oftotalcations,C,inthedriedmaterialremains
nearlyconstant.
Butinthemother tuber,wherecalcium eventually
becomesdominant,thequantity oftotalcations,C,
fallsatarelatively smallerratethanthedryweight.
Here,theconcentrationoftotalcations,C,isincreased throughthecombined effectsofcationretentionandlossindrymaterial,fromtheinitialvalue
of0.5,toafinalvalueof2me.pergramdrymatter.
Investigationsoncotyledonsofsoyabeanhaverevealed asituation similar tothatofthemother tuber
ofpotato. Fig.5showsthequantitiesofcationsin
457
PARTITIONOFIONICCONSTITUENTS
me/200 cotyledons
i
12
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8 12 16
gramsdm/200cotyledons
FIG.5 Changeindryweightand inmilliequivalentsofcationsduring theprogressive
exhaustionofsoyabeancotyledonsby
thegrowing plant. Cdenotes thesumof
cations. FromdataofMCALISTERand
KROBER (1951).
thecotyledonsforaseriesofstages,inrelationto
thefallindryweightduetoexportviathephloemto
thegrowingplant. Potassium isfreelyreleased,and
varies inproportion tothedryweightataconstant
concentrationof0.5me.pergramdrymaterial inthe
residualcotyledon. Calciumandmagnesium eventually
accumulategreatly inexcessoftheirinitialquantity.
Withcalciumassuming thedominantrole,theconcentrationoftotalcations,C,israised from,initially,
0.8me.pergramdrymatter,toafinallevelof3me.
DIJKSHOORN
458
pergramdrymatter,duetoretentionofcalciumand
magnesiumand exportofdrymaterial.
TRANSFERANDRETENTIONOF CARBOXYLATES
meinsprouts
2
I |
•
A
N-
X
1.0
x'
A
/
i
/
1/
/
'
(C-A)
/ /
1
2
grams dry matter
FIG.6 Transfer ofendogenousnitrogen (triangles)andcarboxylate (crosses)from
unplanted potato tubers todeveloping
sprouts inrelation todryweightof
thesprouts.
Exportofcarboxylates from thepotato tuberto
thesproutswasdemonstrated inthefollowingway.
Tuberswere leftunplanted inthelightor inthedark,
anddeveloped smallgreensprouts,orwhitelarge
sprouts,respectively. Thesproutswere excised,and
analyzed fornitrogenandash-alkalinity. Sincethere
wasnonitrate inthetissue,thesevaluesrepresent
organicnitrogenandcarboxylate content,respectively
PARTITION OFIONICCONSTITUENTS
459
(vanTuiletal.1964). Thetubershadnoexternal
supplyofnutrient ions. Fig.6showsthattheendogenously-supplied carboxylatesmoved into thesprouts
atanetrateof0.9 me.pergramdrymatter storedby
thesprouts.
Accumulation ofcarboxylates inthetuber through
thecombined effectofuptakeofionsfromthemedium,
and export tothegrowing plant,wasshownbymeansof
thefollowing experiment. Uniformly-sized seed tubers
(50gramsfreshweight)wereplanted insandculture,
and replicatesanalyzed forinitialcomposition. Nutrient salts,with sufficientpotassiumnitratetosupport thegrowth,wereadded tothesand. Theplanted
tuberswereremovedafter 3or4weeks ofgrowth,separated fromtheplants,andanalyzed. Totalnitrogen
and ash-alkalinitywerecorrected fornitratesoasto
obtainnitrogenand carboxylate content (vanTuilet
al. 1964). Nitratewaslowcomparedwith thatinthe
plantsremoved fromthetubers.
Thedryweightsandmilliequivalentspertuberare
shown inFig.7. Twosetsoftuberswereused,one
high innitrogen (solid triangles),theother lowin
nitrogen (opentriangles). Therewasnodifferencein
carboxylate content (crosses)ofthetwo.
Fig. 7shows thatnitrogenwasexported atarelatively fasterratethanthedrymaterialand,consequently,theconcentrationofnitrogen intheresidual
drymatter felltoaround onehalf theinitiallevel.
But thenet loss incarboxylateswassmallcompared
with thatofthedrymatter. Thepointsforcarboxylatefitadecrement of0.3me.pergramlossindry
weightwhich isthesameasthatoftotalcationsin
Fig. 4. Theretentionofcarboxylates relativetothe
drymaterialraised thecarboxylate concentrationfrom
itsinitialvalueof0.5 me.pergramdrymatter,toa
levelof0.8me.pergramdrymatter.
460
DIJKSHOORN
me. in mother tuber
U
me. in mother tuber
li,
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16
20
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12
16
20
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FIG.7 Milliequivalents oforganicnitrogen
(triangles)and carboxylates (crosses)
inrelation todrymatter inthemother tuber
ofpotatoplants,during exhaustionbythe
growing plant. Notetherapid releaseofN,
andretentionofcarboxylates,relativetodry
matter. Thesolid trianglesrefer totubers
ofhighernitrogen content. Resultsoftwo
experiments.
Afurtherpoint tobeconsidered istheprogressivesubstitutionofmalateforcitrateinthemother
tuberwhen itsreservesareexhaustedby thegrowing
plant. Citrateisthemainconstituent of thecarboxylatesbut,afterplanting,malateaccumulates steadily
and citratesteadily disappears (Fig.8 ) .
461
PARTITION OF IONIC CONSTITUENTS
d.t.
me.intuber
3
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J-I
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8
weeks
FIG. 8 Milliequivalents of citrate and malate
in themother tuber of potato. The
tubers were planted at zero time,and replicates harvested 4, 6 and 8weeks after planting.
Column at the right of the figure gives citrate
and malate in the daughter tubers of theplants.
Data of JOLIVOT (1959).
To test the findings some of the tubers were also
analyzed for carboxylates. Their free acids were liberated by decationization, extracted with water, isolated by passing the extract through an anion exchange
column, and elution of the carboxylate anions with formic acid. After evaporation to dryness invacuo, the
residue was transferred to a silica gel column and submitted to quantitative partition chromatography. The
procedure has been described elsewhere (Dijkshoorn and
Lampe, 1962).
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PARTITION OF IONICCONSTITUENTS
463
TheresultsareshowninFig.9. With thereduction indryweight duetopartialexhaustionbythe
growingplant,thequantity ofpotassium andofcitrate
inthemother tuberdecreased. Thereisatpresentno
evidenceforpotassiumbeing exported aspotassiumcitrate,and thesignificanceoftheirdecrementsbeing
equalcannotyetbeassessed. Thepointsforpotassium
areclosetotheproportionality linethroughtheorigin,and suggest constancy ofconcentration inthe
driedmaterial. Thedisproportional slopeforcarboxylates, (C-A), showsthatalthough thequantityfalls,
theconcentration inthedriedmaterialisincreased
withexhaustion ofthetuber. Theincrease incarboxylateconcentration isbrought aboutbyanincreasein
calcium,andmalatesubstitutes forcitrateinthe
stored carboxylates.
Sofar,theresults showthatdepletion ofreserves fromplant organsexogenously suppliedwith
nutrient ionstendstoincrease theconcentrationof
calciumandmalate,tomaintain theconcentrationof
potassium,and todiminish thecitrateconcentrationin
thedriedmaterialoftheorgan,and thatthischange
isamanifestation ofselectivity iniontransferby
thephloem.
TRANSFERANDLEAF COMPOSITION
Topleaveshaveatfirstanabsoluterequirement
forassimilatesdonatedby theolder leaves. Theyare
mainly subtainedbyphloemtransferofmaterials,and
willreceiveaproportional amountofpotassiumand
littlecalcium.
Iftheleavesarefullyexpanded theywillprovide
assimilates and ionic substances totheyounger leaves
and otherorgansoftheplant. Theexportpresumably
takesplaceinthephloem,whereas exogenously-supplied
464
DIJKSHOORN
me./g dm
10
me/g dm
10
(a)
org
^
^oK
(C-A)
X
®Ca
\
»(C-A)
mal.
citr.
0.1
I
I
I
L _
12
(a)
(b)
FIG.10 Compositionofleavesofdifferent positiononthestem. Leavesarenumbered
frombase toapexof theshoot. Organic nitrogen
(triangles),sumofcationsC (largecircles),
potassium (smallcircles),carboxylates (crosses),
malate (solid squares),calcium (encircled dots),
citrate (soliddots)inionmilliequivalentsper
gramofdrymaterial,
tobacco,dataofVICKERY (1961).
Brussels sprouts,dataofKIRKBY and DEKOCK
(1965).
PARTITION OF IONICCONSTITUENTS
inorganic saltscontinue tomoveintotheleafviathe
xylem. Potassiumbeing readily availableforexport
thendistributes ratheruniformlywith thedrymatter,
whereascalcium isnot re-exported andaccumulatesto
higherconcentrations.
Muchevidencehasaccumulated tosuggest thatthis
patternofredistributiondetermines leaf composition
inrelation topositiononthestem. Iftheabsorption
ofcalciumisnotmuchrestricted by theselectivityof
theuptakesystem intheroots,calciummay accumulate
tohighconcentrations intheolderleaves.
Fig.10showsdataonthecomposition ofsuccessiveleavesof twoplant species. Fromtheapex'ofthe
shoottothebase,thelevelofcalcium,carboxylates
(C-A),andmalate intheleavesincreases steadily,
withcalciumandmalateassuming thedominant rolein
themake-up of thecarboxylates inthematuredbottom
leaves. Thereis,indeed,agradientakintothat
foundbetween theexpanding daughter tubersand the
depletedmother tuberofpotatoplants.
COMPOSITION OFTRANSPORTLIQUIDS
Withnutrient saltssupplied asthenitrates,
chlorides,primary phosphates,and sulfates ofpotassium,sodium,magnesium and calcium insolutionculture,
themedium isamixtureof inorganic,neutral salts
with (C-A)zero. But thewaterdissociates and,with
carbondioxide,itsuppliesH+andHCO3atlowconcentrationbut inunlimited quantity.
Wholeplantanalysis,including theorganic forms
ofNand Sand expressed inequivalentsof thesupplied
ions,reveals theionicbalanceofuptake. Withnitrateasnitrogen source,thecerealsand grassesabsorb theanionsofthenutrient saltsgreatly inexcess
465
466
DIJKSHOORN
oftheircations,anduptakeofH alongwith theexcessofanionswould beconsistentwithelectroneutrality.
If,inthemedium,allthenitrate isreplacedby
sulfateorchloride,theplantabsorbsmetallic cations
inexcessoftheanionsofthenutrient salts,andthis
excess isbalancedbyuptakeofHCO3.
Ingeneraltheassumption seemsvalid forbicarbonateuptakeintheabsenceofnitrate. However,
thereisevidence tosuggest caution intheapplication
oftheassumptionofIF"uptake inthepresenceofnitrate.
Theform theabsorbed nutrient ionsmoveviathe
xylemtotheshootwasstudied inmaize.
Plantsweregrownoncompletenutrient solution
withnitrateandnochloride. Atanappropriate time
anumberofplantsweredecapitated andexudatewas
collected fromthecutstems. Themedium of theother
plantswasreplaced byonecontaining additional sulfateinplaceofthenitrate. During thesubsequent
period ofnitratestarvationwherein theplantsconsumed theirpoolofnitrate thathadaccumulatedunchanged inthetissues,anumberofplantsweredecapitated every fewdays,theexudate collected fora
period oftwohourscentered onnoon,and theplants
discarded.
Measured quantitiesof theliquidsweretransferred toplatincrucibles,mixedwithash-freecellulosepowder (1.5gramsper10mlof exudate),dried on
awaterbath,and ashed at500°C. The ash-alkalinity
wasdetermined byadding standard acid and titrating
theexcesstopH5.
Testswithmixturesofauthentic saltsrevealed
thattheash-alkalinity thusobtained agrees fairly
PARTITION OFIONICCONSTITUENTS
467
wellwith thesumofmilliequivalents ofnitrate,bicarbonateand carboxylateanionsbalancedbymetallic
cationsintheexudationliquid.
Another subsampleoftheexudatewasanalyzed for
nitrateanion,and subtractionof thisvaluefromthe
ash-alkalinity gavethevalue formilliequivalentsof
bicarbonate orcarboxylateanioninbalancewiththe
metallic cations,which is (C-A)of thexylemliquid.
If,withhighnitrateand lowchloridelevels,
(C-A)isnegative,thereisnobicarbonate (carboxylate),andpartof theinorganic anionequivalentsA
isbalancedby H+,thexylem liquid containing free
mineralacid.
With (C-A)zero,thetotalofmetallic cations
equalsthetotalof inorganicanionsderived fromthe
nutrient salts,thexylemliquid isfreefromIT*",HCO3
and carboxylateanions. Of course,withineachgroup
theproportionoftheionsmaydiffer fromthatinthe
mediumowing toselectiveuptake (KT1"entersfaster than
Ca++andNOJfaster thanCa*"1"andNOJ faster than S0=),
but thexylem liquidhas incommonwiththemediumthat
itrepresentsasolutionofneutral,inorganic salts
withnobicarbonates.
If (C-A)ispositive,thexylemtransfersbicarbonatesorcarboxylates alongwith theinorganicsalts,
theirpresencebeingdueeither touptakeofHCO3,or
tometabolism ofnitratesofthemetallic cationsin
therootsand subsequent releaseof theformed carboxylatestotheconducting elementsofthexylem.
Resultsof theexperimentsareshowninFig.11.
Withnitrate inthemedium thelevelofnitratein
thezylemliquidwasabout 16me/liter,and thevalue
for (C-A)waspractically zero. Whentheplantswere
starved onthemedium thatcontainednonitrate,the
DIJKSHOORN
468
N0 3 me./liter
20
16
12
I
\
\
X
X
8
X
12
16
20
(C-A) me./liter
FIG.11 Changes inthelevelofnitrateand
ofcarboxylates, (C-A), inxylemexudateofmaizefollowing thetransfer
tonitrate-freemedium.
levelofnitrate inthexylemliquid felleventually to
zero,and thevaluefor (C-A)increased fromzeroto
about10me./liter.
Afurtherinvestigation intothenatureofthe
(C-A)fractionof thexylemliquidwasmadebyisolatingtheanionsoffreshlycollected exudatesviaion
exchangecolumns,and submitting themtoquantitative
partition chromatography onsilicagel. Resultsare
showninFig.12.
Thetotalamountofcarboxylate anionsdeveloped
offthesilicagelcolumnagreed fairlywellwiththe
valuefor (C-A)calculated fromash-alkalinity and
nitrate.
469
froetionno.
FIG. 12 Integrated chromatograms of organic acids from
xylem exudate of maize plants. Ordinates:
quantities that have passed the column, expressed as
m e . per liter exudate; abscissae: fraction number on
a log scale.
m = malate; c = citrate; p = phosphate, indicated by
the broken terminal sections of the curves and given as
trivalent. The full lines integrate the carboxylate
anions of (C-A) in the exudate, when the corresponding
solvents blanks (lower broken lines) are subtracted.
The upper curve high in (C-A) refers to nitrate-starved
plants, the lower curve low in (C-A) to plants with a
continuous full supply of nitrate in the medium.
470
DIJKSHOORN
Dataof thistypehave shownthelikelihood that
intactplants,raised innitrate-containing medium,
transferallnutrient anionsinbalancewithmetallic
cationsvia theconducting elementsof thexylemtothe
shoot,and thattheshootalonemakesorganicNand
receivesnocarboxylates from theroots.
During itsconversion allthenitratesupplied to
metabolism inbalancewith themetallic cations isreplacedbycarboxylates:
K ++NOJ+8H->K++OH -+ 2H20+ (NH3)
C0 2 +0H~+RH•>RCOO-+H 2 0
where (NH3)denotesorganicN,andRHametabolite
transformed intocarboxylate (RCOO-)bycarboxylation.
Sulfatemetabolismoperatesalongasimilarformulation,but itscontribution isvery small (Dijkshoorn,
1962)and canbeneglected forthepresentconsiderations.
Sinceallthenitrateentersmetabolism inbalance
withmetallic cations,thecarboxylates (C-A)wouldbe
expected toaccumulate inanamountequivalent toorganicN,and thiswasvalid for thepotatoplant. But
maizeplantsaccumulateorganicNatmuchhigherlevels
than (C-A). Onlywhen thenitratesupply isdiscontinued,organicNisdiluted toasimilar levelbythe
growth (Fig.13). Butthenthecarboxylate ismade
fromabsorbedbicarbonatesbycarboxylation inthe
roots:
K ++HC07+RH-»•K ++ RC00~+H 2 0
and isnotderived fromnitratemetabolism.
Formally,itispossible toassumethat,inadditiontothenitratesofthemetallic cationsreleased
PARTITION OFIONICCONSTITUENTS
471
tothexylemvessels,therootsabsorb and assimilate
nitrateanionsalongwith IT1". Thiscombination yields
organicNbutnocarboxylate:
H ++N07+8H -y(NH3)+ 3H20
and therootscould supply theplantwith extraorganic
Nthatmovesvia thexylem totheshoot.
Withnitrate inthemedium,thexylemexudatewas
found tocontainsome10me.perliteroforganicN.
However,this isnoevidencefornitratemetabolismin
theroots. Theamountreleased byexudation isonly
1percentofthetotalpooloforganicNintheroots,
and itspresence intheexudatecouldmerelybedueto
mobility oforganicN. Moreover,theorganicNlevel
ismore than twicethelevelof (C-A),and itseems
unlikely thatsomuchnitrate isabsorbed alongwithH"1"
andmetabolized intheroots.
Instead,thereareconstant indications thatpracticallyallthenitrate ismetabolized intheleaves.
Thiswouldmeanthatmetabolism provides alargeexcess
ofcarboxylates over thepoolnormally retainedbythe
plantas (C-A),and thatthisexcess iscontinuously
removed during growthand ionutilization.
Themainpathwaybywhich carboxylates canescape
istheconducting systemofthephloemwhichtranslocatespotassium carboxylate alongwith thecarbohydrates
andaminoacids totheroots (PeelandWeatherley,
1959). Oncereceived bytheroots,thecarboxylates
canfunctionastheendogenous sourceofHCOJbydecarboxylation,and ofexogenousNOJbyanionexchange,nitrateinthemediumbeingreplaced bybicarbonateand
potassium carboxylate intherootsbypotassiumnitrate,
thelatterbeing released tothexylemand transferred
totheshoot formetabolism.
472
DIJKSHOORN
g d.m. per plant
FIG.13 Accumulationoforganicnitrogen (No)and
carboxylates (C-A)inrelation tothedry
matterduringthegrowthofmaizeplants.
(a) plantsstarved ofnitrate inanearlystage
ofgrowth. At1.3 gramsdrymatterthe
mediumwasagainreplacedbyonecontaining
nitrate.
PARTITION OFIONIC CONSTITUENTS
473
gdm.perplant
7
8
9 10
me.perplant
FIG.13 (cont.)
(b) plantssuppliedwithnitrateduring earlygrowth.
Atalevelof1.3 gramsdrymatter,thenitrate
inthemediumwasreplaced byadditionalsulfate.
Itwasnotuntil thenitrateinthetissueswas
exhausted that synthesisoforganicNstopped.
Values inmilliequivalents and ingramsdry
matterperwholeplant. Thestraightbroken
lines indicate concentrations at0.5,1.0and
2.0me./g d.m., respectively. Filledsymbols:
nonitrate inthesupply;opensymbols:medium
containingnitrateasnitrogensource.
474
DIJKSHOORN
Insuchasituation theshootexportspotassium
carboxylate totheroots,and thelatter exportspotassiumnitrate inturntotheshoot,withalimited
poolofK"*"thatcirculatesdownward inthephloemas
carboxylate,andupward inthexylemasnitrate.
Thispicture forthecompleteplant thataccumulatesorganicNinexcessover (C-A)givesthebest
measureofagreementwiththeevidence.
Attempts tocollectphloemsapwereunsuccessful.
Instead,testswerecarriedoutonthecompositionof
honeydewfromaphidsfeedingontheleavesofmaize
plants. TheratioofKtoCawasabout40. Ash-alkalinity and chromatograms showed thatavariety ofcarboxylatesoccurred asthemajor saltatalevelof
around 1.5me.pergramdrymaterial. Inmaizethe
carboxylatecontentofhoneydewwaslittle influenced
bysubstitutionofsulfatefornitrate inthemedium,
and thelevelof (C-A)intheplantmaterialalsoremained unchanged (Fig.13). Inwheat plants,thevalue
for (C-A)inthetissuesand inthehoneydewwasreduced toonehalf thevalue fortheplants receiving
nitrate.
Fromdataofthetypereported inthepresent communicationonlyqualitativeevidence isobtained. Unfortunately,there isasyetnodatafrommoredirect
approaches thatcouldbeinterpreted intermsofrates
oftranslocation.
ACKNOWLEDGEMENTS
Theauthorgratefully acknowledges financialassistance fromtheCommissiesvoor Bijstand inzakehet
Stikstof-enKali-onderzoek. Healsowishestothank
Mrs. Drs.J.Jonker-Smid andMr.J.E.M.Lampe forhelpfulcooperation.
PARTITIONOFIONICCONSTITUENTS
475
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1.
DIJKSHOORN,W. (1962)Metabolic regulationofthe
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2.
DIJKSHOORN,W.andLAMPE,J.E.M. (1962)Theseparationoforganic acidsfrom Lolium perenne L.
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DIJKSHOORN,W.andLAMPE,J.E.M. (1962)Partition
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