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Eventheinteriorofweatherablemineralsmight
becomeexploited byectomycorhizalhyphae. Thin
sectionmicrographsoffeldsparsandhornblendes
fromconiferousforestsoilshowopentunnelswith
roundedends, curvedtracksandadiameterof
310 m(Refs 32,34). Thetunnelswithaconstantdiameterdiffermorphologicallyfromthe
crystallographicallyorientedetchpitsandsaw-tooth
cracksthatwouldresultfromchemicalweathering35
(Box 4). Thepresenceofhyphaeinsidethetunnels
indicatesthatsinglehyphaereachtheinteriorofthe
tunneledweatherableminerals32,34,36.Exudationof
organicacidsatthehyphaltipswouldfreeessential
cationssuchasK+, Ca2+ andMg2+ fromthemineral
interior32 andcould beresponsibleforformationof
thetunnels32,34,36. Subsequenttransportofdissolved
cationsthroughtheectomycorrhizalmycelium
towardsthehosttreerootscould benefittree
growth37. Ithas beenshownthatisotopicallylabeled
Mgcan betranslocatedoveradistanceofseveral
centimeterstoectomycorrhizalroots, improvingthe
Mgnutritionofthehostplant37. Somefeldspars
containapatiteinclusions32,38 andectomycorrhizal
hyphaemightaccesstheseenclosedP sources
throughtunnelgrowth, therebyexploitinga
mineralPsourceunavailabletoplantroots.
SelectivedissolutionofCa-richinclusionsinVOLCANIC GLASS has beenascribedtoacidexcretion
byinvadingplantsymbioticfungalhyphae36 and
bacteriaingroundwatersystemsselectively
colonizeandweatherP-richfeldsparswhenP is
inshortsupply38.
Implications of ectomycorrhizalweathe ring
Theimplicationsofectomycorrhizalfungalmineral
weatheringaremanifold.
First, itredefinesourtraditionalviewontherole
ofectomycorrhizalfungiinrootnutrientuptake. The
effectofmineralnutrientMOBILIZATION by
ectomycorrhizalfungiaddstotheestablished
quantitativeeffectsonuptakeandtranslocationof
dissolvedelementsfromthesoilsolutionandthe
TRENDSin Ecology & Evolution Vo l.16 No.5 May 2001
http://tre e.trends.com
251Review
As well as the quantit ati ve effects on p lan t
nutrien t uptake (increase in uptake surface and
exp loited soil vo lume),the ectomycorrh iza l
fungus inf luences the uptake of p lan t nutrientsin two qua lit ative ways:
(F ig. Ia) Via enzyme producti on ,the
ectomy corrhiza l fungus can utilize organic
nitrogen (N) and phosphoro us (P) forms,
which would otherwise remain large ly
unava ilab le to roots. Nutrien t mobiliza tion
from am ino acids , pep tides, proteins , am ino
sugars,chitin and nucleic acids has been
showna,toge ther with transfer of N and P
into the host plan ta,b. Direct hyphal absorption
of am ino acids and simple peptide s can
also occur a.
(F ig. Ib) The ectomycorrh iza l fungus canmobil ize P, potass ium (K),calci um (Ca) and
magnesium (Mg) from so lid m ineral substra tes
through organ ic acid excre tionc,d,e.In addit ion,
tunnels in weatherable m inerals enable
ectomycorrh iza l hyphae to reach the inter ior of
the m inerals and access P from a pa tit e
inclus ions . Essential nutrien ts be come
ava ilab le to the host plant via the
ectomycorrh iza l myceliumc. Ana logous to
the ir organ ic nutrien t mob ili zing capab ilities ,
the abilities of d ifferent ectomycorrh iza l fung i
to mobil ize inorganic nu trien ts m ight be
sp ecies speci fic f.
References
a Chalot, M. and Brun,A. (1998)Physiology of
organicnitrogenacquisition by ectomycorrhizal
fungiandectomycorrhizas. FEMSMicrobiol. Rev.
22, 2144
Box 3.Nutrientmob ilizationbyectomycorrhizalfungi
I
b Antibus, R.K. etal. (1997)Rootsurface
phosphataseactivitiesanduptakeof32P-
labelledinositolphosphateinfield-collected
gray birchandredmapleroots. Mycorrhiza
7, 3946c Wallander, H. (2000)UptakeofP from
apatite byPinussylvestrisseedlings
colonized bydifferentectomycorrhizal
fungi. PlantSoil218, 249256
d Wallander, H.andWickman, T. (1999)Biotite
andmicroclineaspotassiumsourcesin
ectomycorrhizalandnon-mycorrhizalPinus
sylvestrisseedlings. Mycorrhiza9, 2532
e Wallander,H. (2000)Use ofstrontium
isotopesandfoliarKcontentto estimate
weatheringofbiotite induced bypineseedlingscolonised by ectomycorrhizal
fungifromtwodifferentsoils.PlantSoil
222, 215229
f Lapeyrie,F. etal. (1991)Phosphate-
solubilizingactivityofectomycorrhizal
fungiinvitro. Can. J. Bot. 69, 342346
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morerecentlystudied qualitativeeffectsof
enzymaticreleaseofNandP fromorganicsources16
(Box 3). Inadditiontoproducingalargesurfacearea
incontactwiththesoil, ectomycorrhizalhyphaecan
activelydissolvetheirmineralsurroundingsand
releaseessentialplantnutrients(Box 3). The
selectivecolonizationandweathering byectomycorrhizalhyphaeofK-, Mg-orCa-rich
feldsparscouldspecificallyimproveK, MgorCa
nutritionoftrees.
Second, ectomycorrhizalhyphaecanaccess
enclosednutrientsourcesinmineralsthatare
unavailabletoroots.Apatiteinclusionsinfeldspars31
areunavailabletoroots butcould beexploited by
hyphae. Organicacidexcretioninthetunnelswill
acceleratefurtherdisintegrationofthemineralsand
resultinenlargedweatherablesurfaceareas,
accessibletorootsaswellasfungi. Rootswillpromote
mineralweathering, butjustastheireffectivityin
nutrientuptakeisrestricted bytheirlimiteddistributioncomparedtofungalmycelium, their
effectivityasweatheringagentswillalso be
restricted.
Third, excretionofoxalate byectomycorrhizal
fungicouldreduceeffectsofsoilacidificationonforest
productivity.Atpresent, acidificationofconiferous
forestsoils, becauseofanthropogenicatmosphericinputs, iswidespread39. Thisacidificationhas been
accompanied bychangesinthe basecations:Alratio,
resultinginhighconcentrationsofAl3+ inthe bulksoil
solution, whichcouldhamperrootuptakeofCa2+ and
Mg2+ andinhibitrootelongation. InresponsetoAl
stress, plantrootsmightsecreteoxalicacidand
detoxifyAl3+ byformationofAloxalatecomplexes40.
WhenexposedtoelevatedAl, mycorrhizalpine
seedlings, aswellasnonmycorrhizalpineseedlings,
showenhancedoxalicacidproduction29. Inadditionto
formationofAloxalatecomplexes, ectomycorrhizal
fungicouldalleviateAlstresstothetrees by
mobilizing basecations(Ca2+ andMg2+)frommicrositesinaccessibletothetreeroots32.
Afourthimplicationinvolvessoil-formingprocesses
andtheirbiogeochemicalconsequences. Takinginto
accountthatectomycorrhizalassociationsare
widespreadandwereestablishedatleast50 Mya41,
mobilizationofmineralelementsbyectomycorrhizal
hyphaeandectomycorrhizalroottipshasprobably
contributedtosoilformationandglobalelementcycling
viaeffectsonmineralweatheringandPODZOLIZATION32.
Prospects
Inmost borealforestsystems, Nisthegrowth-
limitingnutrientforthestandingtrees39,42,43.Nitrogenlimitationimpairsthesynthesisofamino
TRENDSin Ecology & Evolution Vo l.16 No.5 May 2001
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252 Review
Fig. 1. Hyphae linking
plan ts to m inerals. A
thin-section of (a) a
cross-sectioned
ectom ycorrh iza l ro ottip
illus tra tes how (h)
ectom ycorrh iza l hyphae,
emanating from (fm) the
fungal ma ntle aroun d (r)
a root, en close (m)
m ineral pa rticles from the
ad jacen t so il . The direct
con tact be tween the
hyphae and the m inera l
surface is revealed on (b)
a SEM p icture of
branching hyphae tha t
cover and penetra te a
m ineral pa rticle. Scale
bars = 50 m and 10 m
respecti ve ly.
Box 4. Formationofetchpits
Chem ical wea ther ing of m inera ls leads to formation
of regular ly arranged, and regular ly shaped, angu lar
cav ities , so-called etch p its. Etch pits form because
m inera ls disso lve preferentially at and along
crystallographically de ter m ined dislocations and
planes. Saw-tooth cracks eventua lly form when side-
by-side aligned etch pits coalescea (F ig. Ia, from lefttorigh t).
Th in-secti on m icrographs in cross-polar ized
ligh t show ing (Fig. Ib) a che m ically weathered
fe ldspar w ith partly coa lesced e tch pits. By con trast,
a thin-section m icrograph (Fig.Ic) of a tunne led
fe ldspar shows a more or less irregular pattern of
open,tub ular pores, 3-10 m in w idth tha tcriss-cross
the inter ior of the m ineral. Scale bars = 100 m for
Fig.Ib and Ic.
Reference
a Berner, R.A. andHoldren, G.R. (1979)Mechanismof
feldsparsweathering. II. Observationsoffeldsparsfromsoils.
Geochim. Cosmochim. Acta43, 11731185
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