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New Phytol. (1997), 135, 575–585
Functioning of mycorrhizal associations
along the mutualism–parasitism
continuum*
BN. C. JOHNSON"
†, J. H. GRAHAM# F. A. SMITH$
"Biological Sciences,Northern Arizona University,PO Box 5640,Flagstaff,Arizona
86011-5640,USA
#Citrus Research and Education Center,University of Florida,Lake Alfred,Florida
33850,USA
$Department of Botany,University of Adelaide,SA 5005,Australia
(Received 5 November 1996; accepted 6 February 1997)
A great diversity of plants and fungi engage in mycorrhizal associations. In natural habitats, and in an ecologically
meaningful time span, these associations have evolved to improve the fitness of both plant and fungal symbionts.
In systems managed by humans, mycorrhizal associations often improve plant productivity, but this is not always
the case. Mycorrhizal fungi might be considered to be parasitic on plants when net cost of the symbiosis exceeds
net benefits. Parasitism can be developmentally induced, environmentally induced, or possibly genotypically
induced. Morphological, phenological, and physiological characteristics of the symbionts influence the functioning
of mycorrhizas at an individual scale. Biotic and abiotic factors at the rhizosphere, community, and ecosystem
scales further mediate mycorrhizal functioning. Despite the complexity of mycorrhizal associations, it might be
possible to construct predictive models of mycorrhizal functioning. These models will need to incorporate
variables and parameters that account for differences in plant responses to, and control of, mycorrhizal fungi, and
differences in fungal effects on, and responses to, the plant. Developing and testing quantitative models of
mycorrhizal functioning in the real world requires creative experimental manipulations and measurements. This
work will be facilitated by recent advances in molecular and biochemical techniques. A greater understanding of
how mycorrhizas function in complex natural systems is a prerequisite to managing them in agriculture, forestry,
and restoration.
Key words: Mycorrhizal functioning, mutualism, parasitism, cost–benefit analysis, fitness.
Relationships among species are often defined by the
effect of the interaction on each of the species
(Burkholder, 1952; Bronstein, 1994). There is a
continuum of interactions ranging from mutually
beneficial (,) to mutually detrimental (®,®)
(Lewis, 1985; Fig. 1). Mutualisms are relationships
that benefit both species. Commensalism occurs
when one species benefits and the other is not
affected. Parasitism, predation, herbivory, and fung-
ivory are all consumer–resource relationships in
* This paper is based on a discussion}workshop entitled ‘Can
mycorrhizal associations be parasitic ? Re-addressing our defini-
tion of mycorrhiza: structure vs. function’ at the First In-
ternational Conference on Mycorrhizae in Berkeley, California,
USA, 4–9 August 1996.
†To whom correspondence should be addressed.
E-mail: Nancy.Johnson!NAU.edu
which one species (the consumer) benefits at the
expense of the other species (the resource). Amen-
salism occurs when one species is inhibited and the
other is not affected. Finally, competitive inter-
actions are mutually detrimental because both spe-
cies are inhibited by the relationship.
Mycorrhizas can be defined in structural terms as
associations between symbiotic soil fungi and the
absorbing organs of plants (Gerdemann, 1970).
Mycorrhizas are often considered to be classical
mutualisms: many experimental investigations have
shown that both plant and fungal symbionts benefit
from the reciprocal exchange of mineral and organic
resources. However, this is not always the case and
upon closer analysis, there appears to be a continuum
of plant responses to mycorrhizal colonization rang-
ing from positive to neutral to negative. Reports of
neutral or negative plant growth responses are
576 N.C.Johnson,J.H.Graham and F.A.Smith
Gradient of fungal responses
Gradient of plant responses to mycorrhizal associations
–
+
0Neutralism
(0,0)
Mutualism
(+,+) Commensalism
(0,+)
+0
Competition
(–,–)
Amensalism
(–,0)
Parasitism
(–,+)
–
Figure 1. Types of species interactions (modified from
Burkholder, 1952 and Bronstein, 1994, This paper focuses
on the continuum of plant responses to mycorrhizal
formation illustrated by the shaded region.
remarkably common (e.g. Koide, 1985; Fitter, 1991;
Modjo & Hendrix, 1986 ; Bougher, Grove & Mala-
jczuk, 1990; see also references in Smith & Smith,
1996). Does this suggest that mycorrhizal fungi are
sometimes parasitic on plants? The term parasitic is
value-laden, and many mycorrhiza researchers resist
using it to describe mycorrhizal interactions. How-
ever, technically it might be an accurate description
of some mycorrhizal associations where the fungus is
detrimental to the plant or vice versa (Leake, 1994;
Smith & Smith, 1996).
One of the best ways to assess the value of a
paradigm is to investigate and understand the
conditions where it does not ‘appear’ to work. In the
long run, we will have a much better understanding
of mycorrhizal associations if we understand the
conditions in which they appear to be neutral or even
parasitic. The purpose of this paper is to explore the
spectrum of plant responses to colonization by
mycorrhizal fungi and develop a conceptual frame-
work on which to construct testable hypotheses
about mechanisms generating this diversity of re-
sponses. Although also important, the spectrum of
fungal responses to associations with plants is not
analysed in this paper. Because the majority of
mycorrhizal fungi are obligate biotrophs with ob-
vious benefits from the association, we will focus on
the interactions illustrated by the shaded region on
the bottom of Figure 1, in which the fungus always
benefits from the interaction.
Mycorrhizal associations are complex hierarchical
systems (see O’Neill, O’Neill & Norby, 1991). At the
core of every association is a fungus and a plant
living symbiotically. The functioning of this sym-
biosis is mediated by direct and indirect effects of
biotic and abiotic factors of the surrounding rhizo-
sphere, community, and ecosystem (Fig. 2). It is
important to recognize the complexity of mycor-
rhizal systems, and address the appropriate scale
Figure 2. The functioning of mycorrhizal systems is
mediated by a hierarchy of abiotic and biotic factors.
when assessing mycorrhizal function. The larger the
scale of interest, the greater the potential for
moderating mycorrhizal responses through indirect
interactions. For example, a plant and associated
fungus can be grown in sterile media in a growth
chamber with very little mediation from biotic and
abiotic factors in the environment. This study system
is appropriate for many physiological questions, but
it is not appropriate for many ecological questions
because it is lacking properties that emerge from the
actions and interactions between the plant–mycor-
rhiza–fungus and the milieu of biotic and abiotic
factors in the rhizosphere, community, and eco-
system.
Some studies reporting ‘ parasitic effects’ of mycor-
rhizal fungi were conducted in extremely simplified
experimental systems, or were the result of a single
season of research. It is possible that when systems
are more thoroughly studied at larger spatial or
temporal scales, interactions that originally appeared
parasitic might become commensal or mutualistic.
For example, a recent tillage study found maize
grown under adequate soil phosphorus conditions in
no-till fields to have higher levels of mycorrhizal
colonization, and lower yields, compared with maize
grown in fields tilled with a mouldboard plough
(McGonigle & Miller, 1996). If only these two
treatments are considered in the analysis one would
conclude that mycorrhizal colonization is negatively
correlated with crop performance. But if a third
treatment is considered (ridge-till) that uncouples
the effects of residues on soil temperature and the
effects of soil disturbance on mycorrhizal inoculum,
then colonization is positively correlated with early-
season crop performance (McGonigle & Miller,
1993). It is important for investigators to study
systems thoroughly in space and time so that
confounding factors (such as soil temperature and
soil disturbance in the example above) can be
uncoupled from the functioning of mycorrhizas.
At the most basic level, mycorrhizal associations are
beneficial (mutualistic) to plants when net costs are
less than net benefits, and detrimental (parasitic)
Functioning of mycorrhizas 577
Mutualism Parasitism
Cost Benefit Cost Benefit
Unmanaged Soil Fertilized Soil
Cost Benefit Cost Benefit
Cost Benefit Cost Benefit
Normal Light Reduced Light
Effects on plant fitnessEffects on plant fitness Effects on plant fitness
(a)
(b)
(c)
Figure 3. Mycorrhizal associations are mutualistic to
plants when benefits exceed costs and parasitic when costs
exceed benefits (a). Fertilization diminishes benefits and
can generate parasitic associations (b). Factors that limit
production of photosynthate, such as reduced irradiance,
can generate parasitic associations by increasing relative
costs (c).
when costs exceed benefits (Fig. 3 a). Potential fitness
is often the currency used in ecological cost:benefit
analysis (Cushman & Beattie, 1991). In natural
systems, plant fitness is typically measured by
survival and fecundity, and biomass changes might
or might not be a good indicator of reproductive
success. By contrast, biomass is usually a good
variable to measure in many agricultural systems
where seed or biomass yields are the currency of
agricultural success. Fitness is measured at the scale
of an individual. But in the real world, individuals
are not isolated in experimental pots. Interactions at
the scale of populations, communities, and eco-
systems, mediate the actual fitness of individuals.
The challenge in analysing plant responses to
formation of mycorrhizas is measuring costs and
benefits at scales that are appropriate for the question
of interest.
From a plant perspective, costs of mycorrhizas are
traditionally expressed in terms of photosynthate
allocated to the fungus and the supporting root tissue
(e.g. Koide & Elliot, 1989 ; Fitter, 1991; Peng et al.,
1993). Estimates of the amount of carbon allocated to
fungal associates can be substantial, ranging from 4
to 20% of a plant’s total C budget (Peng et al., 1993;
Rygiewicz & Andersen, 1994; Tinker, Durall &
Jones, 1994; Watkins et al., 1996). Mycorrhizas of
heterotrophic (achlorophyllous) plants (i.e. orchid
and monotropoid mycorrhizas) are major exceptions
to this pattern, because in these associations, C flow
is reversed so that the fungus supplies the plant
heterotrophically acquired C for all or part of the
plant’s life cycle (Alexander & Hadley, 1985; Leake,
1994). In these cases, the plant appears to be parasitic
on the fungus–what benefits the latter derives from
the association are unclear (Leake, 1994).
Where mycorrhizal plants are linked by a hyphal
network, the issue of costs and benefits, and hence of
parasitism, can become blurred if C flows from a
‘donor’ to a ‘receiver ’ plant (Francis & Read, 1984).
Transfer of C has been primarily demonstrated with
"%C and this has mostly given no indication of the
amount of C transferred, its form, or even whether
there is in fact always net transfer. For example,
complex cycling of organic C (and of organic N) will
be associated with synthesis of amino acids and
amides in a mycorrhizal fungus that connects two
plants: see Smith & Smith (1996) for a critical
discussion of this issue. Recent work with stable C
isotopes has overcome many of the problems and has
demonstrated significant plant to plant transfer of
organic C through a common hyphal network
(Watkins et al., 1996). Furthermore, recent "$C-
labelling experiments with Festuca turves indicate
that C exported to neighboring plants does not enter
the root tissues of the receiver, but remains in the
arbuscular mycorrhizal (AM) fungus (J. D. Graves
et al., unpublished). Consequently, the ecological
and evolutionary significance of mycorrhizal links
between autotrophic plants is still unclear (Janos,
1987, Smith & Smith, 1996).
Benefits from mycorrhizas are traditionally recog-
nized as improved access to limiting soil resources,
most notably immobile nutrients (e.g. P, Cu, Zn, and
ammonium), but also organic C in the case of orchids
and monotropoids. The nutritional benefits of
mycorrhizas can be significant. For example, Mar-
schner & Dell (1994) estimated that external hyphae
of AM fungi can deliver up to 80% of a plant’s P
requirements and 25% of a plant’s nitrogen require-
ments. Nutritional benefits are even greater in most
ectomycorrhizal associations. For example, ecto-
mycorrhizal pine roots can supply up to 3±2 times
more P and 1±8 times more N than a nonmycorrhizal
root system (Bowen, 1973). Resource limitation is a
key component of cost:benefit analysis of mycor-
rhizal effects on plant fitness (see Eissenstat et al.,
1993). Carbon allocated to a fungus is only a cost if
it could otherwise have been allocated to increase
plant fitness,and resources gained through the
578 N.C.Johnson,J.H.Graham and F.A.Smith
activities of a fungal symbiont are only beneficial if
those resources are in limiting supply.
Although reciprocal exchange of limiting re-
sources is the most obvious (and traditional) choice
for cost:benefit analysis, in many natural systems,
other (often subtle) mycorrhiza-induced changes
might ultimately be more important to plant fitness.
For example, plant morphology, allometry, phe-
nology, and chemistry are affected by the presence of
mycorrhizal fungi (e.g. Allen, Moore & Christensen,
1980; Miller, Jarstfer & Pillai, 1986; Pacovsky,
Bethlenfalvay & Paul, 1986; Hetrick, Wilson &
Leslie, 1991; Bethlenfalvay, Mihara & Schreiner,
1994; Lu & Koide, 1994). Some of these changes
will complicate the cost:benefit analysis. Thus, if
formation of a mycorrhiza results in reduction of
root growth compared with that in an equivalent
non-mycorrhizal plant, then the cost to the plant in
terms of loss of C to the fungus might be less than
the cost resulting from ‘ additional’ root formation in
the non-mycorrhizal plant.
Put into a community or ecosystem context,
mycorrhizal symbioses can substantially impact
plant fitness both directly and indirectly through
altered relationships with other components of the
system (e.g. Marx 1972; Grime et al., 1987;
Linderman, 1988; Malajczuk, 1988; Miller & Jast-
row, 1990). These relationships might be difficult to
disentangle. For example, in a winter annual grass
system, Newsham, Fitter & Watkinson (1994) found
that colonization by AM fungi was not directly
related to plant fecundity, but mycorrhizal inter-
ference with asymptomatic root pathogens was
positively correlated with fecundity. Consequently,
mycorrhizas contributed to plant fitness by pro-
tecting plants from pathogens. Depending upon the
system of interest, mycorrhizal effects on plant-
pathogen interactions can be either a benefit (as
shown in the previous example), or a cost to plant
fitness. Although mycorrhizas frequently reduce the
incidence of soil-borne diseases (Graham, 1988) they
might indirectly enhance the incidence of other
diseases, like viruses, that are stimulated by im-
proved plant nutrition, growth, and other physio-
logical factors (Dehne, 1982).
Complexity at community and ecosystem scales
means that mycorrhiza-induced changes in plant
allocation patterns might have unpredictable effects
on plant fitness. Streitwolf-Engel et al. (1997)
showed that colonization by different isolates of AM
fungi differentially affected reproductive allocation
by two Prunella species. Some isolates stimulated
clonal propagation through stolons, while other
isolates stimulated sexual reproduction through
flowering. Whether these fungal isolates increase or
decrease plant fitness is a function of the criteria used
to measure fitness and the scale of observation. If
fitness is measured only as annual seed set, then the
flower-inducing fungal isolates would normally be
considered more beneficial than the stolon-inducing
isolates. However, if the system of interest is
expanded to include hypothetical herbivores that
selectively graze blooming plants, then the flower-
inducing fungi would be less beneficial than the
stolon-inducing fungi because they indirectly attract
herbivores that ultimately decrease the fitness of the
individual.
The hypothetical example linking the functioning
of Prunella mycorrhizas with its herbivores is not
unreasonable. In one of the relatively few analyses of
mycorrhizal functioning at the ecosystem level,
Gehring & Whitham (1994; 1995) linked the in-
cidence of ectomycorrhizas in pinyon pine with
environmental stress and herbivory. Trees growing
in low-fertility cinder-soils had more mycorrhizal
colonization than trees growing in nearby sandy
loam soil. Furthermore, herbivory by stem and
cone-boring moths caused significant reductions of
mycorrhizal colonization in the stressful cinder soil,
but similar levels of herbivory on trees growing in
less stressful sandy loam soil did not reduce
mycorrhizal colonization. These results suggest that
pine trees growing in stressful conditions were more
C limited than those growing in more fertile soils,
and also suggests that herbivores can out compete
mycorrhizal fungi for C in this system.
Given the complexity of mycorrhizal functioning in
the real world, is there any hope of understanding
these systems well enough to manage them con-
sistently and over long periods of time in restoration,
forestry, and agriculture? A starting place might be
to assume mutualism as the normal state of mycor-
rhizal functioning and try to understand causes of
‘parasitic’ exceptions to this norm. Situations in
which net costs of a mycorrhizal association are
expected to exceed net benefits should be identified.
These hypothesized formulas for parasitism can be
experimentally tested, modified, re-tested, and
eventually used to help design management practices
that avoid parasitism by mycorrhizal fungi. Based on
the mycorrhizal system illustrated in Figure 2, fungal
parasitism of a host plant can be hypothesized to
result from (1) developmental factors (temporal
relationships of the plant–mycorrhiza–fungus), (2)
environmental factors (outside the plant–mycor-
rhiza–fungus), or (3) genotypic factors (inside the
plant–mycorrhiza–fungus).
(1) Developmental factors
Parasitic mycorrhizal associations can occur at
particular stages in the development of the as-
sociation. For example, formation of arbuscular
mycorrhizas can depress seedling growth in the first
few weeks following germination. At this time
benefits are low because necessary resources are
Functioning of mycorrhizas 579
obtained internally from seed reserves, and costs are
high because C allocated to the developing fungus
decreases allocation to photosynthetic or defense
structures or compounds that would increase a
seedling’s chances for survival. Many reports show
growth depressions in AM plants during the first
several weeks of seedling development which dis-
appear as internal seed reserves become depleted
(e.g. Bethlenfalvay, Brown & Pacovsky, 1982 ; Koide,
1985). In other words, short-term losses are often
compensated by long-term gains. In contrast to
early-stage net C drain in arbuscular mycorrhizas,
some orchid mycorrhizas might potentially generate
late-stage C drain as the endophyte shifts from being
a C source to a C sink (Leake, 1994). The balance
between net costs and net benefits is remarkably
dynamic through the development of a mycorrhizal
association and it is dependent on interactions with
the environment.
(2) Environmental factors
Parasitic mycorrhizal associations can occur when
the chemical, physical, or biotic environment of
mycorrhizal systems cause net costs to exceed net
benefits. Nutrient status of soil is the best studied,
and probably the most relevant environmental
mediator of plant responses to mycorrhizal associ-
ations. Fertilizing a system can potentially eliminate
resource limitations so that mycorrhizas become
superfluous for facultatively mycotrophic plants (e.g.
Mosse, 1973; Bethlenfalvay, Bayne & Pacovsky,
1983; Kiernan, Hendrix, & Maronek, 1983; Koide,
1985; Johnson, 1993; Graham, Drouillard & Hodge,
1996). If colonization does not decrease with fertil-
ization, then net costs will remain intact (Fig. 3 b) but
where colonization and hence total fungal biomass
decreases the net costs to the plant will also decrease.
Effects of increased soil P on colonization vary, in
some cases a large decrease in colonization occurs
and in others there is little if any effect until levels of
added P become very high. Generally speaking,
increasing the availability of a limiting soil resource
can convert balanced mutualistic relationships into
less balanced ones, some of which are clearly parasitic
(Fig. 3b).
Humans might inadvertently be altering the
relationships between plants and mycorrhizal fungi
and so might be affecting the cost:benefit balances.
Use of fertilizers has increased exponentially in the
past several decades (Vitousek, 1994). Also, wet and
dry deposition of N emitted from livestock pro-
duction, agricultural operations and internal com-
bustion engines now exceeds natural N inputs in
many ecosystems (Vitousek & Matson, 1993).
Anthropogenic inputs of N might be linked to the
alarming disappearance of mushroom-forming
fungi, including a disproportionate number of ecto-
mycorrhizal taxa (Arnolds, 1991). Whether the loss
of ectomycorrhizal fungi is a cause or a symptom of
forest decline still needs to be resolved (Cherfas,
1991; Jaenike, 1991). Nitrogen fertilization can
also change the species composition of AM fungal
communities (Johnson, 1993). Experiments suggest
that the fungal taxa that proliferate in fertilized soils
might be less beneficial as mutualists to native prairie
grasses than fungal taxa in unfertilized prairie soil
(Johnson, 1993). Again, whether the shift in mycor-
rhizal fungal communities is a cause or a symptom of
the loss of plant species diversity in eutrophied
grasslands (sensu Wedin & Tilman, 1996) is yet
unresolved.
Just as fertilization can cause mycorrhizal costs to
exceed benefits, so can insufficient light; only here
the benefits of a mycorrhizal association might
remain constant while relative costs increase (Fig.
3c). Low light intensities can restrict photosynthetic
capabilities of plants. Allocation of a limited supply of
photosynthate to a fungal associate might potentially
reduce plant allocation to functions related to its
fitness. This constitutes a relatively greater cost
because C has become relatively more limiting than
soil resources. Mycorrhizal growth reductions
associated with low light intensities have been
commonly observed experimentally (e.g. Bjo
$rkman,
1942; Hayman, 1974; Daft & El Giahmi, 1978; Son
& Smith, 1988). Anthropogenic pollution can alter
radiation levels and photosynthetic potential of host
plants, thus changing the balance between mycor-
rhizal costs and benefits. For example, Duckmanton
and Widden (1994) found that ozone changed the
morphology of arbuscular mycorrhizas in sugar
maple (Acer saccharum) seedlings. The relative
abundance of arbuscules decreased while the abun-
dance of vesicles and hyphal coils increased. These
changes suggest that mycorrhizas are extremely
sensitive to ozone generated plant stress. Again, the
relationship between mycorrhizal morphology and
functioning needs to be considered in light of the
recent decline of maples in many forest ecosystems
(Klironomos, 1995).
The biotic components of the environment in
which a mycorrhizal plant grows are known to
influence mycorrhizal functioning (e.g. Garbaye &
Bowen, 1987; Linderman, 1988; Hetrick & Wilson,
1991; Fitter & Garbaye, 1994), but the mechanisms
involved are often elusive. The complexity of biotic
interactions is great enough to overwhelm probably
even the most advanced super-computer. Yet, simple
hypotheses concerning the effects of functional groups
of organisms on mycorrhizal plants can be con-
structed using a cost-benefit framework. For ex-
ample, herbivores consume photosynthetic organs
and deplete plant reserves of photosynthate. For
facultatively mycotrophic hosts, high levels of herb-
ivory could deplete plant C reserves to the extent
that the cost of maintaining a mycorrhizal association
outweighs its benefits (e.g. Daft & El Giahmi, 1978 ;
580 N.C.Johnson,J.H.Graham and F.A.Smith
Figure 4. Mycorrhizal phenotypes are manifestations of
the interaction between plant and fungal genotypes and
environmental conditions. These factors determine the
functioning of mycorrhizas along the mutualism–
parasitism continuum.
Bayne, Brown & Bethlenfalvay, 1984). By contrast,
for obligately mycotrophic hosts (such as the Pin-
aceae) that cannot survive without mycorrhizae, no
C cost is too great to maintain a mycorrhizal associate
and costs never exceed benefits. Fungivores also
potentially affect mycorrhizal functioning. By con-
suming the network of external mycelium, fungi-
vores might potentially reduce the ability of mycor-
rhizal fungi to supply limiting soil resources to its
host (e.g. McGonigle & Fitter, 1988; Gange &
Brown, 1992; Fitter & Garbaye, 1994).
Positive mycorrhizal growth responses decrease as
the density of plants increase (Sanders, Koide &
Shumway, 1995). Where large numbers of roots and
mycorrhizal fungal hyphae are close together the
mycorrhizal roots appear to compete with each other
(and with any non-mycorrhizal root), so that growth
of all the individual plants is reduced compared with
growth of plants spaced well apart. This might be
one reason why in the field AM associations
sometimes exhibit little benefit in improving growth.
(3) Genotypic factors
Parasitic mycorrhizal associations occur when the
genotypes of plants and fungi involved in the
mycorrhizas do not establish ‘win–win’ symbioses.
Extreme examples of this are Armillaria mellea and
Rhizoctonia solani which are beneficial mutualists in
orchid mycorrhizas, but potentially destructive para-
sites (pathogens) to many woody and herbaceous
plants (Smith & Douglas, 1987 ; Leake, 1994).
Ultimately, it is the genotypes of the plants and fungi
involved that determine the potential functioning of
the symbiosis. However, mycorrhizal phenotypes
are manifestations of interactions between plant and
fungal genotypes and environmental conditions (Fig.
4).
Mycorrhizal dependency varies greatly among
taxa and varieties of plants (e.g. Lambert & Cole,
1980; Burgess, Malajczuk & Grove, 1993; Graham
& Eissenstat, 1994). At one extreme are non-host
plants that do not form functioning mycorrhizas,
and if inoculated with mycorrhizal fungi can
exhibit growth depressions and root necrosis (Allen
& Allen, 1984; Allen, Allen & Friese, 1989; Francis
& Read, 1994). However, truly non-mycorrhizal
plant species and genera are remarkably rare in
nature and the great majority of plants are my-
cotrophic to some degree (Newman & Reddel, 1987).
One might hypothesize that the greater the mycor-
rhizal dependency of a plant, the smaller the
probability of a parasitic effect of mycorrhizas
because obligate mycotrophs have a greater
cost:benefit differential than facultative mycotrophs.
Alternatively, because plant genotypes that are
highly dependent on mycorrhizas for nutrient uptake
appear to have a greater propensity to allocate C to
support of higher rates of colonization than do less
dependent plant genotypes (Graham, Duncan &
Eissenstat, 1997), in highly fertilized systems, more
dependent plants might have a greater risk of
mycorrhizal parasitism where they do not closely
control ‘unnecessary’ root growth or rate of colon-
ization to reduce cost of colonization when benefit is
not forthcoming.
Just as plant taxa vary in mycorrhizal dependency,
fungal taxa and isolates vary in mycorrhizal effective-
ness. When tested on a single host species, mycor-
rhizal fungal isolates can increase, decrease, or
have little effect on plant growth (e.g. Molina, 1979;
Miller, Domoto & Walker, 1985 ; Bethlenfalvay et
al., 1989; Dosskey, Linderman & Boersma, 1990;
Burgess, Dell & Malajczuk, 1994 ; see also references
in Smith & Smith, 1996). An extreme example of a
mycorrhizal fungal association that decreases plant
performance is observed in an intensively managed
agricultural system in Kentucky, USA . A series of
field and glasshouse studies has shown that colon-
ization of tobacco by an endemic isolate of Glomus
macrocarpum significantly reduces root length,
aboveground biomass, and flowering (Modjo &
Hendrix 1986; Jones & Hendrix, 1987; Modjo,
Hendrix & Nesmith, 1987; Hendrix, Jones &
Nesmith, 1992). Although this isolate of G.macro-
carpum is strongly parasitic (pathogenic) on tobacco
in field soils with extremely high P, it does not
appear to have parasitic effects on other crops.
Furthermore, it appears to have some specificity for
tobacco because, in field studies, G.macrocarpum
populations (and stunt-disease) can be controlled by
rotation with Festuca (containing an endophyte that
inhibits G.macrocarpum) as effectively as by fu-
migation (Hendrix et al., 1992 ; An, Guo & Hendrix,
1993; Hendrix, Guo & An, 1995).
As we have seen, environmental factors mediate
mycorrhizal effects. Fungal isolates have been shown
Functioning of mycorrhizas 581
to increase plant growth in one experimental system
and not in another experimental system (e.g. Ander-
son, Hetrick & Wilson, 1994). Alternatively, some
isolates of AM fungi have been found that are
equally effective in promoting plant growth over a
wide range of edaphic and host conditions (Sylvia et
al., 1993). The question remains, are there genotypes
of mycorrhizal fungi that are universally bad for
plants i.e. are there constitutive fungal parasites ? If
so, there remains the unresolved evolutionary issue
of why plants have not evolved mechanisms to
prevent their colonization. In this context, it is
dangerous to assume that the selective advantages of
mycorrhizal associations lie only in the mutual
exchange of nutrients or other simple physiological
benefits, as emphasized by Fitter (1985, 1991).
The potential for parasitism by mycorrhizal fungi
has been discussed in the context of ‘cheating’ by
Janos (1985, 1987, 1996; see also Smith & Smith,
1996). A ‘cheater’ is an individual of a partner
species that receives the benefits of mutualism but
does not reciprocate (Soberon & Martinez del Rio,
1985). From an evolutionary context, the appearance
of cheaters within mutualistic associations is highly
probable (Boucher, James & Keeler, 1982). Based on
an analysis of cheating in plant-pollinator mutual-
isms, Soberon & Martinez del Rio (1985) proposed
that factors that impede the discrimination of
cheaters and non-cheaters by the cheated partner, or
decrease the interdependency of the partners, favour
the evolution of cheating. Native plant populations
have diverse gene pools that are continually modified
through selection pressures exerted by their en-
vironment, including the endemic mycorrhizal
fungal community. One would expect that over time,
plant genotypes that maximize mycorrhizal benefits
would be at a selective advantage, and come to
predominate in the population. By contrast, crop
plants and plantation stock have no mechanism to
link their gene pool with mycorrhizal functioning.
Farmers, fruit producers, and foresters generally do
not choose plant genotypes on the basis of their
functioning with local mycorrhizal fungi, and fertil-
ization and tillage might further uncouple the
interdependency of plants and mycorrhizal fungi.
Consequently, management practices might actually
select for mycorrhizal fungi that are cheaters (John-
son & Pfleger, 1992).
}
The hypothesized formulas for parasitism describe
types of situations in which parasitic effects might be
expected to occur, but mechanistic details of the
causes of these effects remain largely unexplored.
Morphological, phenological, and physiological
characteristics of the symbionts (both plants and
fungi) influence the functioning of mycorrhizas (see
Smith & Smith, 1996). Can certain combinations of
characteristics signal the mutualistic or parasitic
qualities of mycorrhizal associations ? Imbalances
between transfer of inorganic nutrients and organic
C might be the physiological events that determine
whether the symbiosis is mutualistic or parasitic, but
there are several possible causes of this imbalance,
ranging from synthesis, operation and regulation of
transport proteins to structural features of the
plant–fungus interface. An example of the latter in
arbuscular mycorrhizas is the number of arbuscules
and their rate of formation and turnover (Dickson &
Smith, 1991; Duckmanton & Widden, 1994). Rec-
ognition of appropriate indicators, and development
of a predictive model of mycorrhizal functioning
would be extremely useful for selecting appropriate
fungi for management or inoculation efforts to
maximize benefits of the symbiosis. A helpful start
would be to construct a model that uses as its basis
the approach of Barber and associates (Silberbush &
Barber, 1983).
A predictive model of mycorrhizal functioning
needs to incorporate variables and parameters that
account for plant responses to, and control of,
mycorrhizas. Plant genotypes differ measurably in
their dependency on mycorrhizas. In general, plants
with coarse root systems benefit more from mycor-
rhizal associations than the genotypes with fibrous
root systems (e.g. Baylis, 1975 ; Pope et al., 1983;
Graham & Syvertsen, 1985 ; Hetrick, Kitt & Wilson,
1988). Similarly, plant species and cultivars with
high shoot:root ratios can have a higher mycor-
rhizal dependency than those with low shoot:root
ratios (Jakobsen, 1991). As noted above, the potential
for plants to vary C allocation to roots and maintain
control of mycorrhizal colonization appears to be
important. Among citrus genotypes colonization rate
varies in relation to non-structural carbohydrate
allocation to roots. The more dependent the citrus
genotype on arbuscular mycorrhizas for P acqui-
sition, the greater the tendency to allocate carbo-
hydrate to roots to support colonization (Graham et
al., 1997). Plasticity in shoot :root ratios can be
another mechanism by which plants control C
allocation to mycorrhizal fungi (Koide, 1991).
A predictive model of mycorrhizal functioning
also needs to account for fungal effects on, and
responses to, the plant. As already noted, genotypes
of mycorrhizal fungi differ in their ability to acquire
and deliver limiting resources to their hosts (e.g.
Sanders et al., 1977 ; Graham, Linderman & Menge
1982; Jakobsen, Abbott & Robson, 1992). The rate
and extent of colonization is likely to be important.
Fungi that are rapid colonists appear to generate the
greatest growth benefits in low-P-fertility soils, and
also the greatest growth depressions in high fertility
soils (Abbott & Robson, 1985; Graham et al., 1996).
This relationship between colonization rate and
mutualistic or parasitic response is presently recog-
582 N.C.Johnson,J.H.Graham and F.A.Smith
nized for a narrow range of Glomus spp., and needs
to be investigated over a wider range of Glomalean
and ectomycorrhizal fungi. Functional differences at
the host–fungus interface is also likely to be im-
portant. For example, isolates of AM fungi that are
rapid colonizers have higher rates of fungal fatty acid
accumulation in roots than slower colonizers; and, a
greater effect on allocation of nonstructural carbo-
hydrates to roots in support of fungal and host
construction and maintenance of the mycorrhizas
(Peng et al., 1993; Graham et al., 1996). In heavily
fertilized soils, fungal genotypes with the potential to
overcome plant control of the rate and extent of
colonization and C allocation would be at a selective
advantage over genotypes that are more sensitive to
plant regulation (Fig. 4).
What are the best approaches to study the dynamics
of mycorrhizal functioning? Because of the ubiquity
of mycorrhizas in most ecosystems, their manipu-
lation in field trials is challenging. Fungicides and
fumigation have been utilized experimentally with
varying success to reduce the incidence of mycor-
rhizas and manipulate field function and cost benefit
outcomes (Fitter & Nichols 1988 ; Gange & Brown,
1992; Graham & Eissenstat, 1996; Wilson & Hart-
nett, 1996). The nonselectivity of some biocides
raises concerns that growth responses might be
attributable to control of pathogens or altered
nutrient dynamics rather than reducing colonization
of mycorrhizas. Jakobsen (1994) suggests making
comparisons between uninoculated and inoculated
plants grown in fumigated field plots to reduce the
confounding effects of enhanced nutrient availability
following fumigation. Several agronomic and for-
estry practices can be used instead of biocides to
reduce mycorrhizal colonization rate. Tillage as a
form of soil disturbance is well known to disrupt
hyphal networks and reduce colonization by arbus-
cular mycorrhizas (Jasper, Abbott & Robson, 1991;
McGonigle & Miller, 1996). Forest clear-cutting
and intensive site preparation is also known to
reduce ectomycorrhizal colonization rates (Perry,
Molina & Amaranthus, 1987). Pre-planting with
non-host plants or maintaining fallow might also
reduce mycorrhizal fungal populations moderately
to drastically, depending on their duration, and
reduce colonization rate in subsequent experimental
plants (Black & Tinker, 1979; Thompson, 1987;
Perry et al., 1989).
For mycorrhizal systems in which enhanced P
uptake is the dominant benefit and C drain is the
dominant cost, the ideal experimental condition to
study field functioning is in low-P soil that is
experimentally treated to: (1) reduce mycorrhizal
colonization using one of the methods described
above; and, (2) enrich P supply to provide for plant
sufficiency without mycorrhiza-mediated uptake.
Furthermore, to the extent possible, no resource
other than C assimilation (e.g. nutrient or water)
should be limiting plant growth. These experi-
mental preconditions are not atypical of intensively
fertilized and irrigated systems and are most easily
interpreted with AM plants. The predicted outcomes
of mycorrhizal reduction at low and high P supply
are designed to demonstrate the extremes of mutual-
istic to parasitic effects of indigenous mycorrhizal
fungi or those introduced through preinoculation of
the plants or planting site. At low P supply, reduced
colonization will reveal mycorrhizal benefits and
result in early P limitation and reduced growth rate
depending on the mycorrhizal dependency of the
host, and the effectiveness of the fungi. Phosphorus
fertilization eliminates mycorrhizal benefits due to
enhanced P uptake, yet maintains the costs if the
fungi are actively colonizing roots in spite of high P
availability, as discussed above. In these circum-
stances, reduced colonization often reduces mycor-
rhizal cost and might increase plant growth rate and
yield response. A similar approach can be used to
test costs and benefits when increased uptake of
another nutrient (e.g. Zn or NH%
+) is suspected or
known to be the dominant benefit. However, this
simplistic ‘functional’ approach has its dangers if the
assumption that increased uptake of a single limiting
nutrient is the only mycorrhizal benefit turns out to
be incorrect. Thus, a positive mycorrhizal response
in soil with non-limiting P would indicate another
mycorrhizal benefit.
Once differences in mycorrhizal functioning are
identified it is then desirable to identify the fungi and
study their interactions (Friese & Allen, 1991).
Morphological characters of colonization cannot
always be used to identify mycorrhizal fungi to the
species level in the field where usually several
morphological species coexist in the same root
system. Strict identification and quantitative meas-
urements of individual species require molecular
probes and biochemical analyses. Molecular appro-
aches involving amplification of nuclear DNA en-
coding the small rRNA and attendant ITS regions
yield diagnostic PCR profiles for ectomycorrhizal
basidiomycetes (Gardes & Bruns, 1993) but profiles
for AM fungi from field soils are highly variable
(Sanders et al., 1995a; Sanders, Clapp & Wiemken,
1996). Until the basis for the genetic heterogeneity is
understood, PCR techniques will be of limited use
for study of AM fungi. Biochemical techniques,
including isozyme polymorphisms and activities
(Rosendahl, 1992), fatty acid profile analysis of
fungal lipids (Graham, Hodge & Morton, 1995), and
immunofluorescence (Friese & Allen, 1991; Hahn,
Gianinazzi-Pearson & Hock, 1994) seem to hold
some immediate promise for both identification and
quantification of AM fungi in the roots.
Functioning of mycorrhizas 583
Inability to monitor individual isolates of mycor-
rhizal fungi in roots imposes severe limitations on
study of their competitive behavior. Necessarily, the
ability of individual isolates to colonize roots and
their effectiveness in providing net benefits must
first be characterized under controlled environments
with a minimum of competition from other fungi.
However, competitive ability of the isolate becomes
an important consideration in the field where many
fungi co-occur in the same root system. Whether
isolates that are highly successful colonists indi-
vidually are more competitive in mixed populations,
especially in the long term, is unknown.
Mutualism and parasitism are extremes of a dynamic
continuum of species interactions. Mycorrhizal
associations are generally at the mutualistic end of
the continuum, but they can be parasitic when the
stage of plant development or environmental condi-
tions make costs greater than benefits, or possibly
when the genotypes of the symbionts do not form
‘win–win’ associations. In natural systems, plant
genotypes exist because they successfully propagate
more plants, and fungal genotypes exist because they
successfully propagate more fungi. Usually, by living
together, plants and mycorrhizal fungi improve each
other’s probability for survival and reproductive
success. But sometimes, and being anthropomorphic,
plant ‘interests’ are in conflict with those of fungi. In
the words of Dawkins (1978) ‘ … we must expect lies
and deceit, and selfish exploitation of communication
to arise whenever the interests of the genes of
different individuals diverge.’ The ‘interests’ of
plants and mycorrhizal fungi are likely to diverge in
highly managed agricultural systems, where ferti-
lization eliminates shortages of soil nutrients, and
plant genotypes are selected by humans and not by
millennia of natural selection.
Based on earlier definitions proposed by Gerde-
mann (1970) and Harley (1992), Trappe (1994)
defined mycorrhizas in functional and structural
terms as ‘dual organs of absorption formed when
symbiotic fungi inhabit healthy absorbing organs
(roots, rhizomes, or thalli) of most terrestrial plants
and many aquatics and epiphytes’ and suggested
that mutualistic functioning of these associations
should be a defining criteria of the term mycorrhiza.
However, defining mycorrhizas as healthy absorbing
organs does not address their effects on whole-plant
fitness, and the mutualistic criterion becomes prob-
lematic whenever the effects of these associations are
neutral or negative to either the plant or the fungal
partner. For example, a mycorrhizal root might be
healthy, yet its presence might reduce the plant’s
fitness in its environment when compared with a
non-mycorrhizal root. We suggest that the structural
and functional accuracy of Trappe’s excellent defin-
ition are not compromised if the dynamic nature of
plant responses to mycorrhizal associations is ac-
cepted and they are considered to be generally
mutualistic, with occasional commensal and parasitic
excursions from this norm.
Twenty years ago many mycorrhizal researchers
optimistically predicted that mycorrhizal associ-
ations could be successfully managed to reduce
reliance on chemical fertilizers, but, ‘promises
concerning the applied value of mycorrhizal fungi in
agriculture, forestry and horticulture have been more
rhetorical than deliverable’ (Miller & Jastrow, 1992).
Perhaps one reason for this is that we often
underestimate the ecological complexity of mycor-
rhizal systems. If our long term goal is to understand
and manage mycorrhizal functioning in the real
world, then we must study the full spectrum of plant
responses to formation of mycorrhizas, and better
understand the factors generating these responses in
complex systems.
We wish to thank Edie Allen, Michael Allen, Christopher
Blackwood, James Hendrix, David Janos, Terry McGon-
igle, Randy Molina, Joe Morton, Mike Miller, Ian
Sanders, Jim Trappe, Paul Widden, Tom Whitham, and
other participants in the ICOM workshop for contributing
to the discussion that led to the writing of this manuscript.
Also thanks to Geoff Barnard and Rick Johnson for their
technical assistance. Financial Support was provided to
N.C.J. by the National Science Foundation (DEB
9527317), to J.H.G. by the United States Department of
Agriculture (USDA-NRICGP 94–37107–1024) and to
F.A.S. by the Australian Research Council.
Abbott LK, Robson AD. 1985. Formation of external hyphae in
soil by four species of vesicular–arbuscular mycorrhizal fungi.
New Phytologist 99: 245–255.
Alexander C, Hadley G. 1985. Carbon movement between host
and mycorrhizal endophyte during the development of the
orchid Goodyera repens Br. New Phytologist 101: 657–665.
Allen EB, Allen MF. 1984. Competition between plants of
different successional stages: mycorrhizae as regulators.
Canadian Journal of Botany 62: 2625–2629.
Allen MF, Moore TS, Christensen M. 1980. Phytohormone
changes in Bouteloua gracilis infected by vesicular–arbuscular
mycorrhizae. I. Cytokinin increases in the host plant. Canadian
Journal of Botany 58: 371–374.
Allen MF, Allen EB, Friese CF. 1989. Responses of the non-
mycotrophic plant Salsola kali to invasion by vesicular–
arbuscular mycorrhizal fungi. New Phytologist 111 : 45–49.
An ZQ, Guo BZ, Hendrix JW. 1993. Mycorrhizal pathogen of
tobacco: cropping history and current crop effects on the
mycorrhizal fungal community. Crop Protection 12 : 527–531.
Anderson RC, Hetrick BAD, Wilson GWT. 1994. Mycorrhizal
dependence of Andropogon gerardii and Schizachyrium sco-
parium in two prairie soils. American Midland Naturalist 132 :
366–376.
Arnolds E. 1991. Decline of ectomycorrhizal fungi in Europe.
Agriculture,Ecosystems and Environment 35 : 209–244.
Baylis GTS 1975. The magnolioid mycorrhiza and mycotrophy in
root systems derived from it. In: Sanders FE, Mosse B, Tinker
PB, eds. Endomycorrhizas. London : Academic Press, 373–389.
Bayne HG, Brown MS, Bethlenfalvay GJ. 1984. Defoliation
effects on mycorrhizal colonization, N fixation and photo-
synthesis in the Glycine–Glomus–Rhizobium symbiosis. Physio-
logia Plantarum 62: 576–580.
584 N.C.Johnson,J.H.Graham and F.A.Smith
Bethlenfalvay GJ, Bayne HG, Pacovsky RS. 1983. Parasitic
and mutualistic associations between a mycorrhizal fungus and
soybean: the effect of phosphorus on host plant–endophyte
interactions. Physiologia Plantarum 57: 543–548.
Bethlenfalvay GJ, Brown MS, Pacovsky RS. 1982. Parasitic
and mutualistic associations between a mycorrhizal fungus and
soybean: development of the host plant. Phytopathology 72:
889–893.
Bethlenfalvay GJ, Franson RL, Brown MS, Mihara KL. 1989.
The Glycine–Glomus–Bradyrhizobium symbiosis. IX. Nutri-
tional, morphological and physiological responses of nodulated
soybean to geographic isolates of the mycorrhizal fungus Glomus
mosseae.Physiologia Plantarum 76: 226–232.
Bethlenfalvay GJ, Mihara KL, Schreiner RP. 1994. Mycor-
rhizae alter protein and lipid contents and yield of pea seeds.
Crop Science 34: 998–1003.
Bjo$rkman E. 1942. U
$
ber die Bedingungen der Mykorrhiz-
abildung bei Kiefer und Fichte. Symbolae Botanicae Upsaliensis
6: 1–191.
Black R, Tinker PB. 1979. The development of endomycorrhizal
root systems. II. Effect of agronomic factors and soil conditions
on the development of VAM infection in barley and on the
endophyte spore density. New Phytologist 83 : 401–413.
Boucher DH, James S, Keeler KH. 1982. The ecology of
mutualism. Annual Review of Ecology and Systematics 13 :
315–347.
Bougher NL, Grove TS, Malajczuk N. 1990. Growth and
phosphorus acquisition of karri (Eucalyptus diversicolor F.
Muell.) seedlings inoculated with ectomycorrhizal fungi in
relation to phosphorus supply. New Phytologist 114: 77–85.
Bowen GD. 1973. Mineral nutrition of mycorrhizas. In: Marks,
G. C., Kozlowski T. T. eds. Ectomycorrhizas. New York :
Academic Press, 151–201.
Bronstein JL. 1994. Our current understanding of mutualism.
The Quarterly Review of Biology 69 : 31–51.
Burgess TI, Malajczuk N, Grove TS. 1993. The ability of 16
ectomycorrhizal fungi to increase growth and phosphorus
uptake of Eucalyptus globulus Labill. and E diversicolor F.
Muell. Plant and Soil 153 : 155–164.
Burgess T, Dell B, Malajczuk N. 1994. Variation in mycorrhizal
development and growth stimulation by 20 Pisolithus isolates
inoculated on to Eucalyptus grandis W. Hill ex Maiden. New
Phytologist 127: 731–739.
Burkholder PR. 1952. Cooperation and conflict among primitive
organisms. American Scientist 40: 601–631.
Cherfas J. 1991. Disappearing mushrooms : another mass ex-
tinction? Science 254: 1458.
Cushman JH, Beattie AJ. 1991. Mutualisms : assessing the
benefits to hosts and visitors. Trends in Ecology and Evolution 6:
193–195.
Daft MJ, El-Giahmi AA. 1978. Effect of arbuscular mycorrhiza
on plant growth. VIII. Effects of defoliation and light on
selected hosts. New Phytologist 80: 365–372.
Dawkins R. 1978. The selfish gene. New York : Granada
Publishing Company.
Dehne H. W. 1982. Interaction between vesicular–arbuscular
mycorrhizal fungi and plant pathogens. Phytopathology 72 :
115–1119.
Dickson S, Smith SE. 1991. Quantification of active vesicular–
arbuscular mycorrhizal infection using image analysis and other
techniques. Australian Journal of Plant Physiology 18 : 637–648.
Dosskey MG, Linderman RG, Boersma L. 1990. Carbon-sink
stimulation of photosynthesis in Douglas fir seedlings by some
ectomycorrhizae. New Phytologist 115: 269–274.
Duckmanton L, Widden P. 1994. Effect of ozone on the
development of vesicular–arbuscular mycorrhizae in sugar
maple saplings. Mycologia 86: 181–186.
Eissenstat DM, Graham JH, Syvertsen JP, Drouillard DL.
1993. Carbon economy of sour orange in relation to mycorrhizal
colonization and phosphorus status. Annals of Botany 71: 1–10.
Fitter AH. 1985. Functioning of vesicular–arbuscular mycor-
rhizas under field conditions. New Phytologist 99: 257–265.
Fitter AH. 1991. Costs and benefits of mycorrhizas: implications
for functioning under natural conditions. Experientia 47:
350–355.
Fitter AH, Garbaye J. 1994. Interactions between mycorrhizal
fungi and other soil organisms. Plant and Soil 159: 123–132.
Fitter AH, Nichols R. 1988. The use of benomyl to control
infection by vesicular–arbuscular mycorrhizal fungi. New
Phytologist 110: 201–206.
Francis R, Read DJ. 1984. Direct transfer of C between plants
connected by vesicular–arbuscular mycorrhizal mycelium.
Nature 307: 53–56.
Francis R, Read DJ. 1994. The contributions of mycorrhizal
fungi to the determination of plant community structure. Plant
and Soil 159: 11–25.
Friese CF, Allen MF. 1991. Tracking the fates of exotic and local
VA mycorrhizal fungi: methods and patterns. Agriculture,
Ecosystems and Environment 34 : 87–96.
Gange AC, Brown VK. 1992. Interactions between soil-dwelling
insects and mycorrhizas during early plant succession. In: Read
DJ, Lewis DH, Fitter AH, Alexander IJ, eds. Mycorrhizas in
Ecosystems. Oxford: C. A. B. International, 177–182.
Garbaye J, Bowen GD. 1987. Effect of different microflora on
the success of ectomycorrhizal inoculation of Pinus radiata.
Canadian Journal of Forest Research 17 : 941–943.
Gardes M, Bruns T. 1993. ITS primers with enhanced specificity
for basidiomycetes: application to identification of mycorrhizae
and rusts. Molecular Ecology 2: 113–118.
Gehring CA, Whitham TG. 1994. Comparisons of ectomycor-
rhizae on pinyon pines (Pinus edulus ; Pinaceae) across extremes
of soil type and herbivory. American Journal of Botany 81 :
1509–1516.
Gehring CA, Whitham TG. 1995. Duration of herbivore
removal and environmental stress affect the ectomycorrhizae of
pinyon pines. Ecology 76: 2118–2123.
Gerdemann JW. 1970. The significance of vesicular–arbuscular
mycorrhizae in plant nutrition. In : Toussoun TA, Bega RV,
Nelson PE, eds. Root diseases and Soil-borne Plant Pathogens.
Berkeley, USA: University of California Press, 125–129.
Graham JH. 1988. Interactions of mycorrhizal fungi with soil-
borne plant pathogens and other organisms : an introduction.
Phytopathology 78: 365–366.
Graham JH, Drouillard DL, Hodge NC. 1996. Carbon
economy of sour orange in response to different Glomus spp.
Tree Physiology 16 : 1023–1029.
Graham JH, Duncan LW, Eissenstat DM. 1997. Carbohydrate
allocation patterns in citrus genotypes as affected by phosphorus
nutrition, mycorrhizal colonization and mycorrhizal depen-
dency. New Phytologist 135: 335–343.
Graham JH, Eissenstat DM. 1994. Host genotype and the
formation and function of VA mycorrhizae. Plant and Soil 159 :
179–185.
Graham JH, Eissenstat DM. 1996. Field demonstration of
carbon cost of mycorrhizae on young Valencia orange trees at
high P supply. Program and Abstracts of the First International
Conference on Mycorrhizae. August 4–9, 1996. Berkeley,
California, USA, 54.
Graham JH, Hodge NC, Morton JB. 1995. Fatty acid methyl
ester profiles for characterization of Glomalean fungi and their
endomycorrhizae. Applied Environmental Microbiology 61 :
58–64.
Graham JH, Linderman RG, Menge JA. 1982. Development
of external hyphae by different isolates of mycorrhizal Glomus
spp. in relation to root colonization and growth of Troyer
citrange. New Phytologist 91: 183–189.
Graham JH, Syvertsen JP. 1985. Host determinants of mycor-
rhizal dependency of citrus rootstock seedlings. New Phytologist
101: 667–676.
Grime JP, Mackey JML, Hillier SH, Read DJ. 1987. Floristic
diversity in a model system using experimental microcosms.
Nature 328: 420–422.
Hahn A, Gianinizzi-Pearson V, Hock B. 1994. Character-
ization of arbuscular fungi by immunochemical techniques. In :
Gianiazzi S, Schuepp H, eds. Impact of Arbuscular Fungi on
Sustainable Agriculture and Natural Ecosystems. Basel, Switzer-
land: Birkhauser, 25–39.
Harley JL. 1992. Mycorrhizae. McGraw-Hill Encyclopedia of
Science and Technology. Volume 11, 7th edition, 545–546.
Hayman DS. 1974. Plant growth responses to VA mycorrhiza
VI. Effect of light and temperature. New Phytologist 73: 71–80.
Hendrix JW, Jones KJ, Nesmith WC. 1992. Control of
pathogenic mycorrhizal fungi in maintenance of soil pro-
ductivity by crop rotation. Journal of Agriculture Production 5:
383–386.
Hendrix JW, Guo BZ, An Z-Q. 1995. Divergence of mycorrhizal
Functioning of mycorrhizas 585
fungal communities in crop production systems. Plant and Soil
170: 131–140.
Hetrick BAD, Kitt DG, Wilson GWT. 1988. Mycorrhizal
dependence and growth habit of warm-season and cool-season
tallgrass prairie plants. Canadian Journal of Botany 66 :
1376–1380.
Hetrick BAD, Wilson GWT. 1991. Effects of mycorrhizal
fungus species and metalaxyl application on microbial supres-
sion of mycorrhizal symbiosis. Mycologia 83 : 97–102.
Hetrick BAD, Wilson GWT, Leslie JF. 1991. Root architecture
of warm- and cool-season grasses: relationship to mycorrhizal
dependence. Canadian Journal of Botany 69: 112–118.
Jaenike J. 1991. Mass extinction of European fungi. Trends in
Ecology and Evolution 6: 174–175.
Jakobsen I. 1991. Carbon metabolism in mycorrhiza. In : Norris
JR, Read DJ, Varma AK, eds. Methods in Microbiology,vol. 23.
London: Academic Press: 149–180.
Jakobsen I. 1994. Research approaches to study the functioning
of vesicular–arbuscular mycorrhizas in the field. Plant and Soil
159: 141–147.
Jakobsen I, Abbott LK, Robson AD. 1992. External hyphae of
vesicular–arbuscular mycorrhizal fungi associated with Tri-
folium subterraneum L. 1. Spread of hyphae and phosphorus
inflow into roots. New Phytologist 120 : 371–380.
Janos DP. 1985. Mycorrhizal fungi : agents or symptoms of
tropical community succession? In: Molina R, ed. Proceedings
of the 6th NACOM., Corvallis, OR, USA : Oregon State
University, 98–106.
Janos D. P. 1987. VA mycorrhizas in humid tropical ecosystems.
In: Safir G, ed. VA Mycorrhizae:an Ecophysiological Ap-
proach., Boca Raton, FL, USA: CRC Press, 107–134.
Janos DP. 1996. Mycorrhizas, succession, and the rehabilitation
of deforested lands in the humid tropics. In : Frankland JC,
Magan N, Gadd GM, eds. Fungi and Environmental Change.
Cambridge, UK : Cambridge University Press, 129–162.
Jasper DA, Abbott LK, Robson AD. 1991. The effect of soil
disturbance on vesicular–arbuscular mycorrhizal fungi in soils
from different vegetation types. New Phytologist 118 : 471–476.
Johnson, NC. 1993. Can fertilization of soil select less mutualistic
mycorrhizae? Ecological Applications 3: 749–757.
Johnson NC, Pfleger FL. 1992. Vesicular–arbuscular mycor-
rhizae and cultural stresses. In: Bethlenfalvay GJ, Linderman
RG, eds. Mycorrhizae in Sustainable Agriculture. Special
Publication no. 54 American Society of Agonomy, Madison,
Wisconsin, USA, 71–99.
Jones K, Hendrix JW. 1987. Inhibition of root extension in
tobacco by the mycorrhizal fungus Glomus macrocarpum and its
prevention by benomyl. Soil Biology and Biochemistry 19 :
297–300.
Kiernan JM, Hendrix JW, Maronek DM. 1983. Fertilizer-
induced pathogenicity of mycorrhizal fungi to sweetgum
seedlings. Soil Biology and Biochemistry 15: 257–262.
Klironomos JN. 1995. Arbuscular mycorrhizae of Acer sac-
charum in different soil types. Canadian Journal of Botany 73:
1824–1830.
Koide R. 1985. The nature of growth depressions in sunflower
caused by vesicular–arbuscular mycorrhizal infection. New
Phytologist 99: 449–462.
Koide RT. 1991. Nutrient supply, nutrient demand and plant
response to mycorrhizal infection. New Phytologist 117 :
365–386.
Koide R, Elliot G. 1989. Cost, benefit and efficiency of the
vesicular–arbuscular mycorrhizal symbiosis. Functional Ecology
3: 252–255.
Lambert DH, Cole H. 1980. Effects of mycorrhizae on
establishment and performance of forage species in mine spoil.
Agronomy Journal 72: 257–260.
Leake JR. 1994. The biology of myco-heterotrophic (‘ sap-
rophytic’) plants. New Phytologist 127: 171–216.
Lewis DH. 1985. Symbiosis and mutualism : crisp concepts and
soggy semantics. In : Boucher DH, ed. The Biology of Mutualism
Ecology and Evolution, London: Croom Helm, 29–39.
Linderman RG. 1988. Mycorrhizal interactions with the rhizo-
sphere microflora: the mycorrhizosphere effect. Phytopathology
78: 366–371.
Lu X, Koide RT. 1994. The effects of mycorrhizal infection on
components of plant growth and reproduction. New Phytologist
128: 211–218.
Malajczuk N. 1988. Interaction between Phytophthora cinn-
amomi zoospores and microorganisms on nonmycorrhizal and
ectomycorrhizal roots of Eucalyptus marginata.Transactions of
the British Mycological Society 90 : 375–382.
Marschner H, Dell B. 1994. Nutrient uptake in mycorrhizal
symbiosis. Plant and Soil 159 : 89–102.
Marx DH. 1972. Ectomycorrhizae as biological deterents to
pathogenic root infections. Annual Review of Phytopathology
10: 429–454.
McGonigle TP, Fitter AH. 1988. Ecological consequences of
arthropod grazing on VA mycorrhizal fungi. Proceedings of the
Royal Society of Edinburgh 94 b: 25–32.
McGonigle TP, Miller MH. 1993. Mycorrhizal development
and phosphorus absorption in maize under conventional and
reduced tillage. Soil Science Society of America Journal 57 :
1002–1006.
McGonigle TP, Miller MH. 1996. Mycorrhizae, phosphorus
absorption, and yield of maize in response to tillage. Soil Science
Society of America Journal 60: 1856–1861.
Miller DD, Domoto PA, Walker C. 1985. Colonization and
efficacy of different endomycorrhizal fungi with apple seedlings
at two phosphorus levels. New Phytologist 100: 393–402.
Miller RM, Jarstfer AG, Pillai JK. 1986. Biomass allocation in
an Agropyron smithii and Glomus symbiosis. American Journal
of Botany 74: 114–122.
Miller RM, Jastrow JD. 1990. Hierarchy of root and mycorrhizal
fungal interactions with soil aggregation. Soil Biology and
Biochemistry 22: 579–584.
Miller RM, Jastrow JD. 1992. The application of VA mycor-
rhizae to ecosystem restoration and reclamation. In: Allen MF,
ed. Mycorrhizal Functioning. New York : Chapman & Hall,
438–467.
Modjo HS, Hendrix JW. 1986. The mycorrhizal fungus Glomus
macrocarpum as a cause of tobacco stunt disease. Phytopathology
76: 688–691.
Modjo HS, Hendrix JW, Nesmith WC. 1987. Mycorrhizal
fungi in relation to control of tobacco stunt disease with soil
fumigants. Soil Biology and Biochemistry 19: 289–295.
Molina R. 1979. Ectomycorrhizal inoculation of containerized
Douglas-fir and Lodgepole pine seedlings with six isolates of
Pisolithus tinctorius.Forest Science 25: 585–590.
Mosse B. 1973. Plant growth responses to vesicular–arbuscular
mycorrhizas. IV. In soil given additional phosphorus. New
Phytologist 72:127–136.
Newman EI, Reddell P. 1987. The distribution of mycorrhizas
among families of vascular plants. New Phytologist 106 :
745–751.
Newsham KK, Fitter AH, Watkinson AR. 1994. Root
pathogenic and arbscular mycorrhizal fungi determine fec-
undity of asymptomatic plants in the field. Journal of Ecology
82: 805–814.
O’Neill EG, O’Neill RV, Norby RJ. 1991. Hierarchy theory as
a guide to mycorrhizal research on large-scale problems.
Environmental Pollution 73: 271–284.
Pacovsky RS, Bethlenfalvay GJ, Paul EA. 1986. Comparisons
between P-fertilized and mycorrhizal plants. Crop Science 26 :
151–156.
Peng S, Eissenstat DM, Graham JH, Williams K, Hodge NC.
1993. Growth depression in mycorrhizal citrus at high-
phosphorus supply: analysis of carbon costs. Plant Physiology
101: 1063–1071.
Perry DA, Molina R, Amaranthus MP. 1987. Mycorrhizae,
mycorrhizospheres, and reforestation: current knowledge and
research needs. Canadian Journal of Forest Research 17:
929–940.
Perry DA, Amaranthus MP, Borchers JG, Borchers SL,
Brainerd RE. 1989. Bootstrapping in ecosystems. Bioscience
39: 230–237.
Pope PE, Chaney WR, Rhodes JD, Woodhead SH. 1983. The
mycorrhizal dependency of four hardwood tree species.
Canadian Journal of Botany 61: 412–417.
Rygiewicz PT, Andersen CP. 1994. Mycorrhizae alter quality
and quantity of carbon allocated below ground. Nature 369 :
58–60.
Rosendahl S. 1992. Influence of 3 vesicular–arbuscular mycor-
rhizal fungi (Glomaceae) on the activity of specific enzymes in
the root system of Cucumis sativus L. Plant and Soil 144 :
219–226.
586 N.C.Johnson,J.H.Graham and F.A.Smith
Sanders FE, Tinker PB, Black RLB, Palmerley SM. 1977. The
development of endomycorrhizal root systems. I. Spread of
infection and growth-promoting effects with four species of
vesicular–arbuscular mycorrhizas. New Phytologist 78: 257–
268.
Sanders IR, Alt M, Groupe K, Boller T, Wiemken A. 1995a.
Identification of ribosomal DNA polymorphisms in and among
spores of the Glomales–application to studies on the genetic
diversity of arbuscular mycorrhizal fungal communities. New
Phytologist 130: 419–427.
Sanders IR, Clapp JP, Wiemken A. 1996. The genetic diversity
of arbuscular mycorrhizal fungi in natural ecosystems –a key to
understanding the ecology and functioning of the mycorrhizal
symbiosis. New Phytologist 133: 123–134.
Sanders IR, Koide RT, Shumway DL. 1995b.Community-
level interactions between plants and vesicular–arbuscular
mycorrhizal fungi In: Varma A, Hock B, eds.Mycorrhiza:
Structure,Function,Molecular Biology and Biotechnology,
Berlin: Springer-Verlag, 607–625.
Silberbush M, Barber SA. 1983. Sensitivity of simulated
phosphorus uptake to parameters used by a mechanistic-
mathematical model. Plant and Soil 74 : 93–100.
Smith DC, Douglas AE. 1987. The biology of symbiosis. London:
Edward Arnold.
Smith FA, Smith SE. 1996. Mutualism and parasitism : diversity
in function and structure in the ‘arbuscular ’(VA) mycorrhizal
symbiosis. Advances in Botanical Research 22: 1–43.
Soberon JM, Martinez del Rio C. 1985. Cheating and taking
advantage in mutualistic associations. In: Boucher DH, ed. The
Biology of Mutualism Ecology and Evolution. London : Croom
Helm, 192–216.
Son CL, Smith SE. 1988. Mycorrhizal growth responses:
interactions between photon irradiance and phosphorus nu-
trition. New Phytologist 108: 305–314.
Streitwolf-Engel R, Boller T, Wiemken A, Sanders I. 1997.
Clonal growth traits of two Prunella species are determined by
co-occurring arbuscular mycorrhizal fungi from calcareous
grasslands. Journal of Ecology,85: 181–191.
Sylvia DM, Wilson DO, Gaham JH, Maddox JJ, Millner P,
Morton JB, Skipper HD, Wright SF, Jarstfer AG. 1993.
Evaluation of vesicular–arbuscular mycorrhizal fungi in diverse
plants and soils. Soil Biology and Biochemistry 25: 705–713.
Thompson JP. 1987. Decline of vesicular–arbuscular mycor-
rhizae in long fallow disorder of field crops and its expression in
phosphorus deficiency of sunflower. Australian Journal of
Agricultural Research 38: 847–867.
Tinker PB, Durall DM, Jones MD. 1994. Carbon use efficiency
in mycorrhizas: theory and sample calculations. New Phytologist
128: 115–122.
Trappe JM 1994. What is a mycorrhiza ? Proceedings of the fourth
European Symposium on Mycorrhizae, Granada, Spain July,
1994.
Vitousek PM. 1994. Beyond global warming: ecology and global
change. Ecology 75: 1861–1876.
Vitousek PM, Matson PA. 1993. Agriculture, the global nitrogen
cycle, and trace gas flux. In: Oremland R. S. ed. The
Biogeochemistry of Global Change :Radiative Trace Gases. New
York: Chapman & Hall 193–208.
Watkins NK, Fitter AH, Graves JD, Robinson D. 1996.
Quantification using stable carbon isotopes of carbon transfer
between C$and C%plants linked by a common mycorrhizal
network. Soil Biology and Biochemistry 28: 471–477.
Wedin DA, Tilman DG. 1996. Influence of nitrogen loading and
species composition on the carbon balance of grasslands. Science
274: 1720–1723.
Wilson G, Hartnett D. 1996. Mycorrhizal influence on floristic
composition and diversity in tallgrass prairie. Program and
abstracts of the First International Conference on Mycorrhizae.
4–9 August 1996. Berkeley, California, USA, 127.
