Directory UMM :Data Elmu:jurnal:T:Tree Physiology:Vol16.1996:
Tree Physiology 16, 1023--1029
© 1996 Heron Publishing----Victoria, Canada
Carbon economy of sour orange in response to differentGlomus spp.
J. H. GRAHAM, D. L. DROUILLARD and N. C. HODGE
Citrus Research and Education Center, University of Florida-IFAS, 700 Experiment Station Road, Lake Alfred, FL 33850-2299, USA
Received October 25, 1995
Summary Vesicular-arbuscular mycorrhizal (M) fungal
colonization, growth, and nonstructural carbohydrate status of
sour orange (Citrus aurantium L.) seedlings were compared at
low- and high-phosphorus (P) supply following inoculation
with four Glomus isolates: G. intraradices (Gi, FL208), G.
etunicatum (Ge, UT316), G. claroideum (Gc, SC186), and
Glomus sp. (G329, FL906). Nonmycorrhizal (NM) seedlings
served as controls. At low-P supply, increases in incidence of
M colonization, vesicles and accumulation of fungal fatty acid
16:1ω5C in roots were most rapid for G329-inoculated seedlings, followed closely by Gi- and Gc-inoculated seedlings.
Glomus etunicatum was a less aggressive colonizer and produced lower rates of fungal fatty acid accumulation in seedling
roots than the other Glomus species. Nonmycorrhizal and
Ge-inoculated seedlings had lower P status and growth rates
than seedlings inoculated with Gi or G329. Glomus claroideum
increased seedling P status, but growth rate was lower than for
seedlings colonized by Gi or G329, suggesting higher belowground costs for Gc colonization. In P-sufficient roots colonized by Gi, Gc, or G329, starch and ketone sugar
concentrations were lower than in P-deficient NM and Ge-inoculated plants.
Under conditions of high-P supply where mycorrhizae provided no P benefit to the seedlings, colonization by Gc, Gi, and
G329 was delayed and reduced compared to that at low-P
supply; however, the relative colonization rates among Glomus
spp. were similar. Colonization by Ge was not detected in roots
until 64 days after inoculation. Compared to NM seedlings,
growth rates of mycorrhizal seedlings were reduced by the
three aggressive fungi but not by the less aggressive Ge. After
64 days, starch and ketone sugar concentrations were lower in
fibrous roots colonized by Gc, Gi, and G329 than in NM roots,
indicating greater utilization of nonstructural carbohydrates in
roots colonized by the aggressive fungi. After 49 days, colonization by the aggressive fungi increased root biomass allocation which may have contributed to the lower growth rate of
mycorrhizal seedlings compared to NM seedlings. Thus,
Glomus spp. that were aggressive colonizers of roots at low-P
supply were also aggressive colonizers at high-P supply, resulting in greater belowground C costs and growth depression
compared with the less aggressive colonizer, Ge.
Keywords: aggressiveness of mycorrhizal fungi, colonization
rate, growth depression, phosphorus status, root carbon allocation, root carbohydrate status.
Introduction
In soils with a low phosphorus (P) content, the ability of
vesicular-arbuscular mycorrhizal (M) fungi to increase plant
growth is explained by increased P acquisition. Generally, the
higher the rate of root colonization by M fungi, the greater the
growth response as a result of more rapid establishment of
external hyphae in the soil and greater P uptake from beyond
the P-depletion zones around the root (Abbott and Robson
1977, Sanders et al. 1977). The efficiency of P uptake by M
hyphae is strongly affected by the spatial distribution of hyphae and by M fungal-specific differences in uptake capacity
per unit length of hypha (Jakobsen et al. 1992a, 1992b). Although growth responses of citrus and subterranean clover
appear to be related to the amount of hyphae or the rate of
hyphal spread from the roots (Graham et al. 1982, Jakobsen et
al. 1992a), often no clear relationship exists between the
amount of hyphae and the growth responses of M-colonized
plants (Abbott and Robson 1985).
At low-P supply, Glomus intraradices isolate FL208 aggressively colonized roots of sour orange and increased P uptake
per unit root length over threefold compared with nonmycorrhizal (NM) roots, resulting in a greatly increased plant growth
rate (Eissenstat et al. 1993). However, M colonization also
enhanced belowground C partitioning, soil C, and root/soil
respiration compared with NM plants of similar P status.
Because there was no compensatory increase in shoot C assimilation, the relative growth rate (RGR) of plants inoculated
with G. intraradices FL208 was slightly reduced compared
with that of NM plants of equivalent P status, consistent with
the increase in belowground C expenditure.
High-P supply reduced colonization of Volkamer lemon by
G. intraradices FL208, but there was still considerable fungal
colonization of roots, as revealed by the detection of the fatty
acid 16:1ω5C in fungal vesicles in roots 52 days after inoculation (Peng et al. 1993). This colonization neither increased P
status nor C assimilation of Volkamer lemon. At 52 days after
transplanting, colonization at high-P supply reduced seedling
RGR by 9 to 12% as a result of a substantial effect of G.
intraradices on root allocation (19% higher root dry weight
and 10% higher daily root growth rate). This response partly
accounted for an estimated 37% higher root/soil respiration in
M seedlings than in NM seedlings. Root construction and
respiration estimates identified the building of intraradical
vesicles (lipid-rich roots) as 10% of the cost, with 51% of the
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GRAHAM, DROUILLARD AND HODGE
increase due to greater M root biomass allocation. The remaining 39% was presumably required for maintenance of the
fungus in the root, and growth and maintenance of hyphae in
soil. Even for G. intraradices FL208, which produces a preponderance of intraradical vesicles and chlamydospores, only
a small proportion of the respiratory costs associated with M
colonization was directly attributable to building lipid-rich
roots (Peng et al. 1993).
We predicted that other Glomus species that aggressively
colonize citrus roots at low-P supply are capable of stimulating
belowground allocation at high-P supply but provide no P
benefit or means of C gain aboveground (Graham et al. 1991,
Graham and Eissenstat 1994). Here, we define aggressiveness
in terms of rate of root colonization because this characteristic
is a useful predictor of growth response at low-P supply (Abbott and Robson 1985). We evaluated the rate of M colonization and growth, and the nonstructural carbohydrate status of
roots to measure responses of sour orange (Citrus aurantium L.) at low- and high-P supply to four Glomus species,
including G. intraradices FL208 and three Glomus species
intensively studied by Sylvia et al. (1993). When the responses
of a grass (sorghum) and legume (soybean) host to these three
M fungi were evaluated in ten soils in the eastern United States,
two of the Glomus isolates were effective and the other was
ineffective in stimulating plant growth under a wide range of
edaphic conditions. In sorghum, there was a strong relationship between colonized root length and growth response,
whereas this was not the case in soybean. Thus, the mechanisms underlying fungal effectiveness at low-P supply and host
responses to these fungi at high-P supply require further study.
We hypothesized that Glomus isolates that are aggressive colonizers of sorghum and soybean would colonize citrus more
rapidly and be more beneficial in terms of P acquisition than
less aggressive colonizers. However, at high-P supply, more
aggressive colonizers would produce greater belowground C
allocation, utilization and growth depression than those produced by less aggressive colonizers.
Materials and methods
Plant inoculation and growth conditions
Sour orange (Citrus aurantium) seeds were germinated in a
commercial 1/1/1 (v/v) mix of peat/perlite/bark with starter
nutrients and grown in 150 cm3 plastic containers for 3 months
(two to four true-leaf stage). The medium was thoroughly
washed from roots, and seedlings were transplanted to 150 cm3
of autoclaved (121 °C, 1.1 kg cm −2) Candler fine sand soil
(pH 6.8) with 3.8 mg kg −1 of available P as determined by
double-acid extraction (Mehlich 1953). For M treatments,
0.5 g of air-dried Sudan grass (Sorghum sudanense (Staph.)
Piper) roots containing from 170 to 350 propagules (determined by most-probable number analysis) of Glomus intraradices FL208 (Gi), G. etunicatum UT316 (Ge),
G. claroideum SC186 (Gc), or Glomus sp. FL906 (G329) were
placed below each seedling at transplanting. Soil in the NM
treatment received a 5-ml extract of each inoculum passed
through a 45-µm sieve to establish similar root microflora in
association with the NM plants. The M and NM plants were
grown from May 11 to July 14, 1994 in a shaded greenhouse
having a maximum photon flux density of 1000 µmol m −2 s −1.
Average day/night temperatures were 33/25 °C and relative
humidity was 60 to 100%. Seedlings were watered to excess
every other day with Hoagland’s solution (Hoagland and Arnon 1965) containing 0.25 mM (low-P) or 2.0 mM (high-P) P.
At 21, 36, 49, and 64 days after transplanting, seedlings from
each treatment were harvested. There were five M treatments,
two P regimes, four harvests, and five replicate seedlings per
treatment per harvest. Seedlings were located in racks in the
greenhouse in a split-plot design for fertilizer treatments and
relocated on the bench every 3 weeks until harvest. An experiment of the same design, with three different harvest dates, was
conducted in 1993 with similar results. The results of the 1994
experiment are presented because the 1993 experiment had
only three harvests.
Growth analysis and colonization assessment
At each harvest, roots were gently washed free of soil with tap
water. Roots (fibrous roots < 2 mm diameter and tap roots
> 2 mm), stems, and leaves were dried at 70 °C for 24 h. Rates
of plant growth (g day −1) were calculated over the four harvests
by regression analysis. Biomass allocation to roots was assessed using the ratio of roots (fibrous and tap) to total
biomass. At the final harvest (64 days), roots had completely
explored the soil in the 150 cm3 containers but were not
constricted in the bottoms of the containers.
Total incidence of intraradical (IR) M colonization was
estimated as the percentage of 10 1-cm fibrous root segments
per plant with vesicles, arbuscules, and hyphae present (Graham et al. 1991), and as the percentage incidence of vesicles
only in the same 10 root segments. Fibrous root content of the
fungal fatty acid 16:1ω5C, which is not found in host tissue
(Graham et al. 1995), was determined with a Hewlett-Packard
5890 gas-liquid chromatograph after methanol extraction of
minced, dried roots (to pass through a 2.0-mm screen), fatty
acid saponification, and derivatization to methyl esters. Concentrations of 16:1ω5C were expressed relative to the highest
value based on the integrated areas of the total fatty acid profile
(Peng et al. 1993).
Nonstructural carbohydrate and P analysis
Dried fibrous roots and leaves were ground to pass through a
40-mesh screen (2.0 mm opening size). Root tissue was suspended in 15 ml of water, boiled for 2 min, and centrifuged at
800 g for 2 min as previously described (Eissenstat and Duncan 1992). The supernatant was analyzed for ketone sugars
(including sucrose, fructose, and fructans) using resorcinol
reagent (Roe et al. 1949). Soluble and insoluble starch were
measured using amyloglucosidase after correcting for free
glucose (Haissig and Dickson 1979). Glucose was used as the
standard for analyses of starch and ketone sugars.
Leaf tissue P concentration was determined by inductively
coupled plasma atomic emission spectrometry after the tissue
had been ashed (500 °C, 4 h) and resuspended in 1 mM HCl.
CARBON RESPONSES OF SOUR ORANGE TO GLOMUS SPP.
Data analysis
Rates of M colonization and plant growth were subjected to the
General Linear Model (GLM) procedure (SAS Institute, Cary,
NC) for analysis of variance with time as a repeated measure.
Linear contrasts were made in the univariate mode. Nonlinear
responses of tissue P and fibrous root carbohydrates and root
allocation in the early phase of seedling growth and M colonization were expected (Eissenstat et al. 1993). Thus, data were
subjected to ANOVA at individual harvest times when treatment effects were most discernable (usually 64 days). Comparisons among treatments were made with Duncan’s multiple
range test.
Results
Mycorrhizal colonization responses
At low-P supply, rates of intraradical (IR) colonization of sour
orange by the four Glomus spp. varied significantly (Figure 1A). The rate was highest for G329, intermediate for Gi
and Gc, and significantly lower for Ge by 64 days after inoculation. Similar trends among Glomus spp. occurred for rates of
increase in incidence of IR vesicles (Figure 1B). Differences
in rates of colonization between G329, Gi and Gc and the less
aggressive Ge were corroborated by accumulation of fungal
16:1ω5C fatty acid in plant roots (Figure 1A inset). This fatty
acid represents about 40 to 60% of the total fatty acid content
in the chlamydospores of these fungal species (Graham et al.
1995).
1025
By all measures, colonization by Ge was similar to that of
the other three fungi up to Day 35, after which the rate of IR
colonization by Ge slowed, whereas the other three fungi
continued to colonize at similar rates. However, colonization
by Ge continued to convert from hyphae and arbuscules (IR)
to vesicles at almost the same rate as the other three fungi, as
indicated by the increase in vesicle incidence (Figure 1B) and
the accumulation of 16:1ω5C in the roots (Figure 1A inset).
Increased P supply delayed and reduced colonization by all
Glomus species, but the relative colonization rates among the
Glomus species were similar to those at low-P supply: G329 =
Gi > Gc > Ge (Figures 1A and 1C). Colonization by Ge was
not detected in roots until Day 64 and Ge failed to form IR
vesicles (Figures 1C and 1D). Vesicle incidence in roots colonized by G329, Gi, and Gc was low and variable (Figure 1D).
At high-P supply, the fungal fatty acid 16:1ω5C was not detected in roots inoculated with any of the fungi in the absence
of substantial development of lipid-rich vesicles in the roots.
Phosphorus and plant growth responses
At low-P supply, leaf P concentrations in NM plants and
Ge-inoculated plants declined to below P sufficiency (<
0.10%) within 35 days after transplanting, whereas P concentrations in G329-, Gc-, and Gi-inoculated plants remained
above 0.12%, which is within the P-sufficiency range (Figure 2A). Plants colonized by G329 and Gi had significantly
higher growth rates than P-deficient NM plants (Figure 2B),
whereas growth rates of Gc and Ge-inoculated plants were not
significantly greater than those of NM plants.
Figure 1. Mycorrhizal colonization of
sour orange roots by four Glomus
spp. as measured by intraradical (IR)
incidence (arbuscules, vesicles, and
hyphae), vesicle incidence and accumulation in fibrous roots of 16:1ω5C
fungal fatty acid expressed in relative
units (Panel A inset). Seedlings were
grown at low-P supply (Panels A and
B) or high-P supply (Panels C and
D). Gc = G. claroideum SC186, Ge =
G. etunicatum UT316, G329 =
Glomus sp. FL906, and Gi = G. intraradices FL208. Curves followed
by different letters have significantly
different (P ≤ 0.05) rates of colonization according to linear contrast analysis.
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GRAHAM, DROUILLARD AND HODGE
Figure 2. Leaf P concentration and
plant growth rate (g day −1) of nonmycorrhizal (NM) and mycorrhizal sour
orange seedlings inoculated with four
Glomus spp. (see Figure 1 for identification). Seedlings were grown at lowP supply (Panels A and B) or high-P
supply (Panels C and D). In panels A
and C, points (n = 5) followed by different letters at 64 days after inoculation indicate differences at P ≤ 0.05
according to Duncan’s multiple range
test. In Panels B and D, growth rates
are derived from the slope of the regression of the total plant dry weight
with time based on four harvests.
Curves followed by different letters
have significantly different (P ≤ 0.05)
rates of growth according to linear contrast analysis.
At high-P supply, leaf P in all plants varied with leaf flushing
activity (data not shown), but concentrations were well within
the range of P sufficiency (Figure 2C). Nonmycorrhizal seedlings had the highest growth rate, followed by seedlings inoculated with Ge (Figure 2D). Colonization by G329, Gi, and Gc
significantly reduced growth rates of sour orange seedlings
compared to NM seedlings.
Carbohydrate and root allocation responses
Starch concentrations in M fibrous roots generally reached a
peak at 49 days after inoculation and then declined (Figures 3A
and 3C), whereas starch concentrations of NM roots either
remained constant (low-P) or increased (high-P) after Day 49.
Ketone sugars in NM roots increased up to Day 49 (low-P) or
Day 36 (high-P) and then declined (Figures 3B and 3D). In
roots colonized by G329, Gi and Gc, ketone sugars steadily
declined in low-P roots, but remained constant in high-P roots
for the first 49 days after inoculation and then declined after
the increased colonization rate between Days 36 and 49 (Figure 1D). Although starch and ketone sugar concentrations
fluctuated in Ge-colonized roots, concentrations of nonstructural carbohydrates were similar to those in NM roots after
Day 64, whereas concentrations of starch and ketone sugars in
roots colonized by the other three Glomus spp. were lower than
in NM roots (Figures 3A--D).
At low-P supply, C allocation to root growth, as measured
by the root biomass/plant biomass ratio, steadily increased
over time as the NM plants became P deficient (Figure 4A). In
contrast, the overall effect of inoculation with G329, Gi or Gc,
all of which enhanced seedling acquisition of P soon after
colonization began, was to maintain a constant root
biomass/plant biomass ratio of about 0.29 ± 0.2 (SD) during
the course of biomass gain. The less aggressive Ge, which
colonized but failed to enhance seedling acquisition of P,
stimulated C allocation to roots but did not sustain plant P
sufficiency. Hence, the root biomass/plant biomass ratio was
higher for Ge-colonized plants than for the other M treatments
and the ratio converged with that of NM plants by Day 64.
At high-P supply, the root biomass/plant biomass ratio of
P-sufficient NM seedlings did not show a net change over the
64-day study, whereas the ratio increased in G329-, Gi- and
Gc-colonized seedlings. In contrast to the three aggressive
fungi, the root biomass/plant biomass ratio of Ge-colonized
seedlings decreased slightly over time.
Discussion
We predicted that, at low-P supply, Glomus spp. would be
aggressive colonizers of sour orange roots and would enhance
P acquisition and growth, whereas at high-P supply the fungi
would not only be aggressive colonizers but would also result
in greater belowground C costs and growth depression than in
plants inoculated with less aggressive colonizers.
At low-P supply, two of the four Glomus spp., G329 and Gi,
enhanced seedling acquisition of P, resulting in significant
increases in growth rate compared to NM seedlings. In contrast
to Ge’s high effectiveness on herbaceous hosts (Sylvia et al.
1993), Ge did not promote P acquisition and growth of sour
orange seedlings. This Glomus species was the least aggressive
colonizer, confirming that colonization rate is a useful predic-
CARBON RESPONSES OF SOUR ORANGE TO GLOMUS SPP.
1027
Figure 3. Starch and ketone sugar concentrations in fibrous roots of nonmycorrhizal (NM) and mycorrhizal sour
orange seedlings inoculated with four
Glomus spp. (see Figure 1 for identification). Seedlings were grown at low-P
supply (Panels A and B) or high-P supply (Panels C and D). Points (n = 5) followed by different letters at 64 days
after inoculation indicate differences at
P ≤ 0.05 according to Duncan’s multiple range test.
Figure 4. Root biomass per total
plant biomass of nonmycorrhizal
(NM) and mycorrhizal sour orange
seedlings inoculated with four
Glomus species (see Figure 1 for
identification) and grown at low-P
supply (Panel A) or high-P supply
(Panel B). Points (n = 5) followed
by different letters at each harvest
time after inoculation indicate differences at P ≤ 0.05 according to
Duncan’s multiple range test.
tor of fungal effectiveness under P-deficient conditions (Abbott and Robson 1977, Sanders et al. 1977). Among the
Glomus spp. studied, Gc was intermediate in aggressiveness;
colonization by Gc increased P acquisition of the seedlings but
did not produce a seedling growth response greater than the
relatively poor colonizer Ge. This may indicate greater carbon
expenditure on colonization by Gc than by the other Glomus
spp. including Gi, which has been previously identified as
capable of increasing belowground allocation and reducing
growth rate of sour orange (Eissenstat et al. 1993). Pearson and
Jakobsen (1993) observed a greater belowground carbon par-
titioning and a lower mycorrhizal efficiency (calculated as 14C
use per unit 32P transported) for Scutellospora calospora than
for two Glomus spp., suggesting that C--P exchange varies
considerably among M fungi.
The less aggressive Ge differed from the other Glomus spp.
in its effect on C allocation to roots at low-P supply. In the
absence of increased seedling acquisition of P during early
colonization by Ge, the biomass of the root fraction increased
compared to that of the M-colonized plants in which enhanced
P uptake promoted C allocation to shoot growth. Similarly,
Eissenstat et al. (1993) concluded that, in P-deficient NM
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GRAHAM, DROUILLARD AND HODGE
plants, C allocation for root growth increased with time,
thereby increasing P acquisition.
At low-P supply, NM and Ge-colonized plants had higher
concentrations of starch and ketone sugars than plants colonized by the other Glomus spp., possibly as a result of greater
C allocation to P-deficient root tissues. Alternatively, because
M fungi are obligate biotrophs that derive all of their C from
the host (Harley and Smith 1983), they probably readily utilize
available pools of sugars transported to roots, such as ketone
sugars and sucrose (Patrick 1989). Greater utilization of nonstructural carbohydrates in roots colonized by aggressive fungi
is likely a result of higher belowground costs associated with
additional construction costs and maintenance respiration of
roots and intra- and extraradical fungal structures (Peng et al.
1993).
Although the confounding effects of P nutrition on C production and allocation are minimized under conditions of
high-P supply (Eissenstat et al. 1993), this condition delays
and reduces the frequency of colonization in sour orange
(Menge et al. 1978). We found that the aggressive fungi produced appreciable colonization under high-P supply, whereas
the less aggressive Ge did not. At high-P supply, concentrations of starch and ketone sugars in roots of P-sufficient NM
plants increased and decreased, respectively. In contrast, pools
of nonstructural carbohydrates fluctuated in roots of Glomuscolonized plants, presumably in response to M-induced
changes in C allocation to roots with time. After 64 days, pools
of starch and ketone sugars in roots colonized by the three
aggressive Glomus spp. declined compared with pools in NM
roots, suggesting greater utilization of nonstructural carbohydrates by these fungi. The less aggressive Ge had the least
effect on these pools after 64 days.
The aggressive Glomus spp., G329, Gi, and Gc, reduced
growth rate of sour orange, whereas the less aggressive fungus
Ge had little impact on growth. At high-P supply, the reduction
in growth rate by the aggressive fungi was indicative of high
belowground costs of colonization, as previously demonstrated for Gi in Volkamer lemon seedlings (Peng et al. 1993).
Because Gi is highly vesicular and directly converts from
intraradical vesicles to chlamydospores in the root, the high
belowground costs of colonization incurred by this species
may not be a general phenomenon of Glomus spp. However,
both G329 and Gc, which are capable of aggressive colonization at low-P supply, colonized roots at high-P supply and
reduced seedling growth and affected C allocation to roots and
C utilization in the same way as did Gi. In contrast, colonization by the less aggressive Ge at high-P was insufficient to
affect C costs and growth rate of the sour orange seedlings.
Of the four Glomus species examined, G329 was the most
effective growth promoter at low-P supply and the most effective growth inhibitor at high-P supply. Sylvia et al. (1993)
reported similar findings of the growth promotory effects of
this fungus based on studies with sorghum and soybean in
low-P soils. Reduction of woody plant growth by M fungal
inoculation at high-P supply has been reported for sweetgum
(Kiernan et al. 1983) and apple (Miller et al. 1985). In both
cases, several species of Glomus were screened and those
isolates that colonized aggressively and increased growth most
at low-P supply were the fungi that colonized most extensively
at high-P supply. Similarly, isolates of G. etunicatum and
G. claroideum were the most aggressive colonizers of sweetgum in the high fertility treatment and both species produced
growth depression. Another species, G. fasciculatum, colonized at a moderate to low rate at both high and low fertility
and produced a smaller reduction in growth of sweetgum at
high fertility than the aggressive colonizers.
The results for sweetgum and citrus indicate that, for woody
tree species, there is a relationship between aggressiveness of
Glomus spp. at low- and high-P supply and the potential for
growth depression at high-P. Although fungi with an aggressive colonization phenotype commonly occur in M fungal
populations in the field (Abbott and Robson 1991, Brundrett
1991), the functional significance of their effects on plant
growth in the field remains to be determined (Abbott and
Gazey 1994). Tobacco stunt disease is associated with the
occurrence of G. macrocarpum in heavily fertilized Kentucky
field soils (Modjo and Hendrix 1986). Citrus soils in Florida
commonly contain G. intraradices and other intraradically
sporulating Glomus spp. (Graham and Fardelmann 1986). Despite high-P fertility (160 µg g −1 soil), these M fungi colonize
citrus roots of mature trees to a high level of incidence (60 to
80%) (Graham et al. 1991). We have evidence (Graham and
Eissenstat, unpublished observations) that benomyl treatment
controls M colonization and depresses P acquisition of young
citrus trees grown with high-P supply under orchard conditions, indicating a fungitoxic effect of benomyl (Fitter and
Nichols 1988). No other soil-borne fungi pathogenic to citrus
roots are present in this field site. Control of the M fungal
colonization apparently relieves young citrus trees of belowground expenditure of C for use in aboveground processes and,
as a result, the growth rate of young Valencia orange trees on
four different rootstocks treated with benomyl increased by
approximately 10%.
We conclude that if aggressive fungi are present in populations of indigenous M fungi, they have the potential to colonize
at high-P supply and affect crop productivity. A strategy for
management of crops under high-P fertility conditions would
be to lower soil P availability to concentrations at which M
fungi function as mutualists rather than as parasites. For example, the Glomus spp. G329, Gc and Ge promoted growth of
sorghum and soybeans in six soils with less than 10 µg extractable P kg −1 soil, but not in four soils with greater than
20 µg kg −1 (Sylvia et al. 1993). This strategy assumes that
symbiotically effective fungi exist within each diverse field
population. Unfortunately, the relevance of species (or isolate)
diversity to the functioning of mycorrhizae in the field is not
yet known (Abbott and Gazey 1994).
Acknowledgments
This is Florida Agricultural Experiment Station Journal Series No.
R-04811. This research was supported in part by USDA-NRICGP
(94--37107--1024).
CARBON RESPONSES OF SOUR ORANGE TO GLOMUS SPP.
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© 1996 Heron Publishing----Victoria, Canada
Carbon economy of sour orange in response to differentGlomus spp.
J. H. GRAHAM, D. L. DROUILLARD and N. C. HODGE
Citrus Research and Education Center, University of Florida-IFAS, 700 Experiment Station Road, Lake Alfred, FL 33850-2299, USA
Received October 25, 1995
Summary Vesicular-arbuscular mycorrhizal (M) fungal
colonization, growth, and nonstructural carbohydrate status of
sour orange (Citrus aurantium L.) seedlings were compared at
low- and high-phosphorus (P) supply following inoculation
with four Glomus isolates: G. intraradices (Gi, FL208), G.
etunicatum (Ge, UT316), G. claroideum (Gc, SC186), and
Glomus sp. (G329, FL906). Nonmycorrhizal (NM) seedlings
served as controls. At low-P supply, increases in incidence of
M colonization, vesicles and accumulation of fungal fatty acid
16:1ω5C in roots were most rapid for G329-inoculated seedlings, followed closely by Gi- and Gc-inoculated seedlings.
Glomus etunicatum was a less aggressive colonizer and produced lower rates of fungal fatty acid accumulation in seedling
roots than the other Glomus species. Nonmycorrhizal and
Ge-inoculated seedlings had lower P status and growth rates
than seedlings inoculated with Gi or G329. Glomus claroideum
increased seedling P status, but growth rate was lower than for
seedlings colonized by Gi or G329, suggesting higher belowground costs for Gc colonization. In P-sufficient roots colonized by Gi, Gc, or G329, starch and ketone sugar
concentrations were lower than in P-deficient NM and Ge-inoculated plants.
Under conditions of high-P supply where mycorrhizae provided no P benefit to the seedlings, colonization by Gc, Gi, and
G329 was delayed and reduced compared to that at low-P
supply; however, the relative colonization rates among Glomus
spp. were similar. Colonization by Ge was not detected in roots
until 64 days after inoculation. Compared to NM seedlings,
growth rates of mycorrhizal seedlings were reduced by the
three aggressive fungi but not by the less aggressive Ge. After
64 days, starch and ketone sugar concentrations were lower in
fibrous roots colonized by Gc, Gi, and G329 than in NM roots,
indicating greater utilization of nonstructural carbohydrates in
roots colonized by the aggressive fungi. After 49 days, colonization by the aggressive fungi increased root biomass allocation which may have contributed to the lower growth rate of
mycorrhizal seedlings compared to NM seedlings. Thus,
Glomus spp. that were aggressive colonizers of roots at low-P
supply were also aggressive colonizers at high-P supply, resulting in greater belowground C costs and growth depression
compared with the less aggressive colonizer, Ge.
Keywords: aggressiveness of mycorrhizal fungi, colonization
rate, growth depression, phosphorus status, root carbon allocation, root carbohydrate status.
Introduction
In soils with a low phosphorus (P) content, the ability of
vesicular-arbuscular mycorrhizal (M) fungi to increase plant
growth is explained by increased P acquisition. Generally, the
higher the rate of root colonization by M fungi, the greater the
growth response as a result of more rapid establishment of
external hyphae in the soil and greater P uptake from beyond
the P-depletion zones around the root (Abbott and Robson
1977, Sanders et al. 1977). The efficiency of P uptake by M
hyphae is strongly affected by the spatial distribution of hyphae and by M fungal-specific differences in uptake capacity
per unit length of hypha (Jakobsen et al. 1992a, 1992b). Although growth responses of citrus and subterranean clover
appear to be related to the amount of hyphae or the rate of
hyphal spread from the roots (Graham et al. 1982, Jakobsen et
al. 1992a), often no clear relationship exists between the
amount of hyphae and the growth responses of M-colonized
plants (Abbott and Robson 1985).
At low-P supply, Glomus intraradices isolate FL208 aggressively colonized roots of sour orange and increased P uptake
per unit root length over threefold compared with nonmycorrhizal (NM) roots, resulting in a greatly increased plant growth
rate (Eissenstat et al. 1993). However, M colonization also
enhanced belowground C partitioning, soil C, and root/soil
respiration compared with NM plants of similar P status.
Because there was no compensatory increase in shoot C assimilation, the relative growth rate (RGR) of plants inoculated
with G. intraradices FL208 was slightly reduced compared
with that of NM plants of equivalent P status, consistent with
the increase in belowground C expenditure.
High-P supply reduced colonization of Volkamer lemon by
G. intraradices FL208, but there was still considerable fungal
colonization of roots, as revealed by the detection of the fatty
acid 16:1ω5C in fungal vesicles in roots 52 days after inoculation (Peng et al. 1993). This colonization neither increased P
status nor C assimilation of Volkamer lemon. At 52 days after
transplanting, colonization at high-P supply reduced seedling
RGR by 9 to 12% as a result of a substantial effect of G.
intraradices on root allocation (19% higher root dry weight
and 10% higher daily root growth rate). This response partly
accounted for an estimated 37% higher root/soil respiration in
M seedlings than in NM seedlings. Root construction and
respiration estimates identified the building of intraradical
vesicles (lipid-rich roots) as 10% of the cost, with 51% of the
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GRAHAM, DROUILLARD AND HODGE
increase due to greater M root biomass allocation. The remaining 39% was presumably required for maintenance of the
fungus in the root, and growth and maintenance of hyphae in
soil. Even for G. intraradices FL208, which produces a preponderance of intraradical vesicles and chlamydospores, only
a small proportion of the respiratory costs associated with M
colonization was directly attributable to building lipid-rich
roots (Peng et al. 1993).
We predicted that other Glomus species that aggressively
colonize citrus roots at low-P supply are capable of stimulating
belowground allocation at high-P supply but provide no P
benefit or means of C gain aboveground (Graham et al. 1991,
Graham and Eissenstat 1994). Here, we define aggressiveness
in terms of rate of root colonization because this characteristic
is a useful predictor of growth response at low-P supply (Abbott and Robson 1985). We evaluated the rate of M colonization and growth, and the nonstructural carbohydrate status of
roots to measure responses of sour orange (Citrus aurantium L.) at low- and high-P supply to four Glomus species,
including G. intraradices FL208 and three Glomus species
intensively studied by Sylvia et al. (1993). When the responses
of a grass (sorghum) and legume (soybean) host to these three
M fungi were evaluated in ten soils in the eastern United States,
two of the Glomus isolates were effective and the other was
ineffective in stimulating plant growth under a wide range of
edaphic conditions. In sorghum, there was a strong relationship between colonized root length and growth response,
whereas this was not the case in soybean. Thus, the mechanisms underlying fungal effectiveness at low-P supply and host
responses to these fungi at high-P supply require further study.
We hypothesized that Glomus isolates that are aggressive colonizers of sorghum and soybean would colonize citrus more
rapidly and be more beneficial in terms of P acquisition than
less aggressive colonizers. However, at high-P supply, more
aggressive colonizers would produce greater belowground C
allocation, utilization and growth depression than those produced by less aggressive colonizers.
Materials and methods
Plant inoculation and growth conditions
Sour orange (Citrus aurantium) seeds were germinated in a
commercial 1/1/1 (v/v) mix of peat/perlite/bark with starter
nutrients and grown in 150 cm3 plastic containers for 3 months
(two to four true-leaf stage). The medium was thoroughly
washed from roots, and seedlings were transplanted to 150 cm3
of autoclaved (121 °C, 1.1 kg cm −2) Candler fine sand soil
(pH 6.8) with 3.8 mg kg −1 of available P as determined by
double-acid extraction (Mehlich 1953). For M treatments,
0.5 g of air-dried Sudan grass (Sorghum sudanense (Staph.)
Piper) roots containing from 170 to 350 propagules (determined by most-probable number analysis) of Glomus intraradices FL208 (Gi), G. etunicatum UT316 (Ge),
G. claroideum SC186 (Gc), or Glomus sp. FL906 (G329) were
placed below each seedling at transplanting. Soil in the NM
treatment received a 5-ml extract of each inoculum passed
through a 45-µm sieve to establish similar root microflora in
association with the NM plants. The M and NM plants were
grown from May 11 to July 14, 1994 in a shaded greenhouse
having a maximum photon flux density of 1000 µmol m −2 s −1.
Average day/night temperatures were 33/25 °C and relative
humidity was 60 to 100%. Seedlings were watered to excess
every other day with Hoagland’s solution (Hoagland and Arnon 1965) containing 0.25 mM (low-P) or 2.0 mM (high-P) P.
At 21, 36, 49, and 64 days after transplanting, seedlings from
each treatment were harvested. There were five M treatments,
two P regimes, four harvests, and five replicate seedlings per
treatment per harvest. Seedlings were located in racks in the
greenhouse in a split-plot design for fertilizer treatments and
relocated on the bench every 3 weeks until harvest. An experiment of the same design, with three different harvest dates, was
conducted in 1993 with similar results. The results of the 1994
experiment are presented because the 1993 experiment had
only three harvests.
Growth analysis and colonization assessment
At each harvest, roots were gently washed free of soil with tap
water. Roots (fibrous roots < 2 mm diameter and tap roots
> 2 mm), stems, and leaves were dried at 70 °C for 24 h. Rates
of plant growth (g day −1) were calculated over the four harvests
by regression analysis. Biomass allocation to roots was assessed using the ratio of roots (fibrous and tap) to total
biomass. At the final harvest (64 days), roots had completely
explored the soil in the 150 cm3 containers but were not
constricted in the bottoms of the containers.
Total incidence of intraradical (IR) M colonization was
estimated as the percentage of 10 1-cm fibrous root segments
per plant with vesicles, arbuscules, and hyphae present (Graham et al. 1991), and as the percentage incidence of vesicles
only in the same 10 root segments. Fibrous root content of the
fungal fatty acid 16:1ω5C, which is not found in host tissue
(Graham et al. 1995), was determined with a Hewlett-Packard
5890 gas-liquid chromatograph after methanol extraction of
minced, dried roots (to pass through a 2.0-mm screen), fatty
acid saponification, and derivatization to methyl esters. Concentrations of 16:1ω5C were expressed relative to the highest
value based on the integrated areas of the total fatty acid profile
(Peng et al. 1993).
Nonstructural carbohydrate and P analysis
Dried fibrous roots and leaves were ground to pass through a
40-mesh screen (2.0 mm opening size). Root tissue was suspended in 15 ml of water, boiled for 2 min, and centrifuged at
800 g for 2 min as previously described (Eissenstat and Duncan 1992). The supernatant was analyzed for ketone sugars
(including sucrose, fructose, and fructans) using resorcinol
reagent (Roe et al. 1949). Soluble and insoluble starch were
measured using amyloglucosidase after correcting for free
glucose (Haissig and Dickson 1979). Glucose was used as the
standard for analyses of starch and ketone sugars.
Leaf tissue P concentration was determined by inductively
coupled plasma atomic emission spectrometry after the tissue
had been ashed (500 °C, 4 h) and resuspended in 1 mM HCl.
CARBON RESPONSES OF SOUR ORANGE TO GLOMUS SPP.
Data analysis
Rates of M colonization and plant growth were subjected to the
General Linear Model (GLM) procedure (SAS Institute, Cary,
NC) for analysis of variance with time as a repeated measure.
Linear contrasts were made in the univariate mode. Nonlinear
responses of tissue P and fibrous root carbohydrates and root
allocation in the early phase of seedling growth and M colonization were expected (Eissenstat et al. 1993). Thus, data were
subjected to ANOVA at individual harvest times when treatment effects were most discernable (usually 64 days). Comparisons among treatments were made with Duncan’s multiple
range test.
Results
Mycorrhizal colonization responses
At low-P supply, rates of intraradical (IR) colonization of sour
orange by the four Glomus spp. varied significantly (Figure 1A). The rate was highest for G329, intermediate for Gi
and Gc, and significantly lower for Ge by 64 days after inoculation. Similar trends among Glomus spp. occurred for rates of
increase in incidence of IR vesicles (Figure 1B). Differences
in rates of colonization between G329, Gi and Gc and the less
aggressive Ge were corroborated by accumulation of fungal
16:1ω5C fatty acid in plant roots (Figure 1A inset). This fatty
acid represents about 40 to 60% of the total fatty acid content
in the chlamydospores of these fungal species (Graham et al.
1995).
1025
By all measures, colonization by Ge was similar to that of
the other three fungi up to Day 35, after which the rate of IR
colonization by Ge slowed, whereas the other three fungi
continued to colonize at similar rates. However, colonization
by Ge continued to convert from hyphae and arbuscules (IR)
to vesicles at almost the same rate as the other three fungi, as
indicated by the increase in vesicle incidence (Figure 1B) and
the accumulation of 16:1ω5C in the roots (Figure 1A inset).
Increased P supply delayed and reduced colonization by all
Glomus species, but the relative colonization rates among the
Glomus species were similar to those at low-P supply: G329 =
Gi > Gc > Ge (Figures 1A and 1C). Colonization by Ge was
not detected in roots until Day 64 and Ge failed to form IR
vesicles (Figures 1C and 1D). Vesicle incidence in roots colonized by G329, Gi, and Gc was low and variable (Figure 1D).
At high-P supply, the fungal fatty acid 16:1ω5C was not detected in roots inoculated with any of the fungi in the absence
of substantial development of lipid-rich vesicles in the roots.
Phosphorus and plant growth responses
At low-P supply, leaf P concentrations in NM plants and
Ge-inoculated plants declined to below P sufficiency (<
0.10%) within 35 days after transplanting, whereas P concentrations in G329-, Gc-, and Gi-inoculated plants remained
above 0.12%, which is within the P-sufficiency range (Figure 2A). Plants colonized by G329 and Gi had significantly
higher growth rates than P-deficient NM plants (Figure 2B),
whereas growth rates of Gc and Ge-inoculated plants were not
significantly greater than those of NM plants.
Figure 1. Mycorrhizal colonization of
sour orange roots by four Glomus
spp. as measured by intraradical (IR)
incidence (arbuscules, vesicles, and
hyphae), vesicle incidence and accumulation in fibrous roots of 16:1ω5C
fungal fatty acid expressed in relative
units (Panel A inset). Seedlings were
grown at low-P supply (Panels A and
B) or high-P supply (Panels C and
D). Gc = G. claroideum SC186, Ge =
G. etunicatum UT316, G329 =
Glomus sp. FL906, and Gi = G. intraradices FL208. Curves followed
by different letters have significantly
different (P ≤ 0.05) rates of colonization according to linear contrast analysis.
1026
GRAHAM, DROUILLARD AND HODGE
Figure 2. Leaf P concentration and
plant growth rate (g day −1) of nonmycorrhizal (NM) and mycorrhizal sour
orange seedlings inoculated with four
Glomus spp. (see Figure 1 for identification). Seedlings were grown at lowP supply (Panels A and B) or high-P
supply (Panels C and D). In panels A
and C, points (n = 5) followed by different letters at 64 days after inoculation indicate differences at P ≤ 0.05
according to Duncan’s multiple range
test. In Panels B and D, growth rates
are derived from the slope of the regression of the total plant dry weight
with time based on four harvests.
Curves followed by different letters
have significantly different (P ≤ 0.05)
rates of growth according to linear contrast analysis.
At high-P supply, leaf P in all plants varied with leaf flushing
activity (data not shown), but concentrations were well within
the range of P sufficiency (Figure 2C). Nonmycorrhizal seedlings had the highest growth rate, followed by seedlings inoculated with Ge (Figure 2D). Colonization by G329, Gi, and Gc
significantly reduced growth rates of sour orange seedlings
compared to NM seedlings.
Carbohydrate and root allocation responses
Starch concentrations in M fibrous roots generally reached a
peak at 49 days after inoculation and then declined (Figures 3A
and 3C), whereas starch concentrations of NM roots either
remained constant (low-P) or increased (high-P) after Day 49.
Ketone sugars in NM roots increased up to Day 49 (low-P) or
Day 36 (high-P) and then declined (Figures 3B and 3D). In
roots colonized by G329, Gi and Gc, ketone sugars steadily
declined in low-P roots, but remained constant in high-P roots
for the first 49 days after inoculation and then declined after
the increased colonization rate between Days 36 and 49 (Figure 1D). Although starch and ketone sugar concentrations
fluctuated in Ge-colonized roots, concentrations of nonstructural carbohydrates were similar to those in NM roots after
Day 64, whereas concentrations of starch and ketone sugars in
roots colonized by the other three Glomus spp. were lower than
in NM roots (Figures 3A--D).
At low-P supply, C allocation to root growth, as measured
by the root biomass/plant biomass ratio, steadily increased
over time as the NM plants became P deficient (Figure 4A). In
contrast, the overall effect of inoculation with G329, Gi or Gc,
all of which enhanced seedling acquisition of P soon after
colonization began, was to maintain a constant root
biomass/plant biomass ratio of about 0.29 ± 0.2 (SD) during
the course of biomass gain. The less aggressive Ge, which
colonized but failed to enhance seedling acquisition of P,
stimulated C allocation to roots but did not sustain plant P
sufficiency. Hence, the root biomass/plant biomass ratio was
higher for Ge-colonized plants than for the other M treatments
and the ratio converged with that of NM plants by Day 64.
At high-P supply, the root biomass/plant biomass ratio of
P-sufficient NM seedlings did not show a net change over the
64-day study, whereas the ratio increased in G329-, Gi- and
Gc-colonized seedlings. In contrast to the three aggressive
fungi, the root biomass/plant biomass ratio of Ge-colonized
seedlings decreased slightly over time.
Discussion
We predicted that, at low-P supply, Glomus spp. would be
aggressive colonizers of sour orange roots and would enhance
P acquisition and growth, whereas at high-P supply the fungi
would not only be aggressive colonizers but would also result
in greater belowground C costs and growth depression than in
plants inoculated with less aggressive colonizers.
At low-P supply, two of the four Glomus spp., G329 and Gi,
enhanced seedling acquisition of P, resulting in significant
increases in growth rate compared to NM seedlings. In contrast
to Ge’s high effectiveness on herbaceous hosts (Sylvia et al.
1993), Ge did not promote P acquisition and growth of sour
orange seedlings. This Glomus species was the least aggressive
colonizer, confirming that colonization rate is a useful predic-
CARBON RESPONSES OF SOUR ORANGE TO GLOMUS SPP.
1027
Figure 3. Starch and ketone sugar concentrations in fibrous roots of nonmycorrhizal (NM) and mycorrhizal sour
orange seedlings inoculated with four
Glomus spp. (see Figure 1 for identification). Seedlings were grown at low-P
supply (Panels A and B) or high-P supply (Panels C and D). Points (n = 5) followed by different letters at 64 days
after inoculation indicate differences at
P ≤ 0.05 according to Duncan’s multiple range test.
Figure 4. Root biomass per total
plant biomass of nonmycorrhizal
(NM) and mycorrhizal sour orange
seedlings inoculated with four
Glomus species (see Figure 1 for
identification) and grown at low-P
supply (Panel A) or high-P supply
(Panel B). Points (n = 5) followed
by different letters at each harvest
time after inoculation indicate differences at P ≤ 0.05 according to
Duncan’s multiple range test.
tor of fungal effectiveness under P-deficient conditions (Abbott and Robson 1977, Sanders et al. 1977). Among the
Glomus spp. studied, Gc was intermediate in aggressiveness;
colonization by Gc increased P acquisition of the seedlings but
did not produce a seedling growth response greater than the
relatively poor colonizer Ge. This may indicate greater carbon
expenditure on colonization by Gc than by the other Glomus
spp. including Gi, which has been previously identified as
capable of increasing belowground allocation and reducing
growth rate of sour orange (Eissenstat et al. 1993). Pearson and
Jakobsen (1993) observed a greater belowground carbon par-
titioning and a lower mycorrhizal efficiency (calculated as 14C
use per unit 32P transported) for Scutellospora calospora than
for two Glomus spp., suggesting that C--P exchange varies
considerably among M fungi.
The less aggressive Ge differed from the other Glomus spp.
in its effect on C allocation to roots at low-P supply. In the
absence of increased seedling acquisition of P during early
colonization by Ge, the biomass of the root fraction increased
compared to that of the M-colonized plants in which enhanced
P uptake promoted C allocation to shoot growth. Similarly,
Eissenstat et al. (1993) concluded that, in P-deficient NM
1028
GRAHAM, DROUILLARD AND HODGE
plants, C allocation for root growth increased with time,
thereby increasing P acquisition.
At low-P supply, NM and Ge-colonized plants had higher
concentrations of starch and ketone sugars than plants colonized by the other Glomus spp., possibly as a result of greater
C allocation to P-deficient root tissues. Alternatively, because
M fungi are obligate biotrophs that derive all of their C from
the host (Harley and Smith 1983), they probably readily utilize
available pools of sugars transported to roots, such as ketone
sugars and sucrose (Patrick 1989). Greater utilization of nonstructural carbohydrates in roots colonized by aggressive fungi
is likely a result of higher belowground costs associated with
additional construction costs and maintenance respiration of
roots and intra- and extraradical fungal structures (Peng et al.
1993).
Although the confounding effects of P nutrition on C production and allocation are minimized under conditions of
high-P supply (Eissenstat et al. 1993), this condition delays
and reduces the frequency of colonization in sour orange
(Menge et al. 1978). We found that the aggressive fungi produced appreciable colonization under high-P supply, whereas
the less aggressive Ge did not. At high-P supply, concentrations of starch and ketone sugars in roots of P-sufficient NM
plants increased and decreased, respectively. In contrast, pools
of nonstructural carbohydrates fluctuated in roots of Glomuscolonized plants, presumably in response to M-induced
changes in C allocation to roots with time. After 64 days, pools
of starch and ketone sugars in roots colonized by the three
aggressive Glomus spp. declined compared with pools in NM
roots, suggesting greater utilization of nonstructural carbohydrates by these fungi. The less aggressive Ge had the least
effect on these pools after 64 days.
The aggressive Glomus spp., G329, Gi, and Gc, reduced
growth rate of sour orange, whereas the less aggressive fungus
Ge had little impact on growth. At high-P supply, the reduction
in growth rate by the aggressive fungi was indicative of high
belowground costs of colonization, as previously demonstrated for Gi in Volkamer lemon seedlings (Peng et al. 1993).
Because Gi is highly vesicular and directly converts from
intraradical vesicles to chlamydospores in the root, the high
belowground costs of colonization incurred by this species
may not be a general phenomenon of Glomus spp. However,
both G329 and Gc, which are capable of aggressive colonization at low-P supply, colonized roots at high-P supply and
reduced seedling growth and affected C allocation to roots and
C utilization in the same way as did Gi. In contrast, colonization by the less aggressive Ge at high-P was insufficient to
affect C costs and growth rate of the sour orange seedlings.
Of the four Glomus species examined, G329 was the most
effective growth promoter at low-P supply and the most effective growth inhibitor at high-P supply. Sylvia et al. (1993)
reported similar findings of the growth promotory effects of
this fungus based on studies with sorghum and soybean in
low-P soils. Reduction of woody plant growth by M fungal
inoculation at high-P supply has been reported for sweetgum
(Kiernan et al. 1983) and apple (Miller et al. 1985). In both
cases, several species of Glomus were screened and those
isolates that colonized aggressively and increased growth most
at low-P supply were the fungi that colonized most extensively
at high-P supply. Similarly, isolates of G. etunicatum and
G. claroideum were the most aggressive colonizers of sweetgum in the high fertility treatment and both species produced
growth depression. Another species, G. fasciculatum, colonized at a moderate to low rate at both high and low fertility
and produced a smaller reduction in growth of sweetgum at
high fertility than the aggressive colonizers.
The results for sweetgum and citrus indicate that, for woody
tree species, there is a relationship between aggressiveness of
Glomus spp. at low- and high-P supply and the potential for
growth depression at high-P. Although fungi with an aggressive colonization phenotype commonly occur in M fungal
populations in the field (Abbott and Robson 1991, Brundrett
1991), the functional significance of their effects on plant
growth in the field remains to be determined (Abbott and
Gazey 1994). Tobacco stunt disease is associated with the
occurrence of G. macrocarpum in heavily fertilized Kentucky
field soils (Modjo and Hendrix 1986). Citrus soils in Florida
commonly contain G. intraradices and other intraradically
sporulating Glomus spp. (Graham and Fardelmann 1986). Despite high-P fertility (160 µg g −1 soil), these M fungi colonize
citrus roots of mature trees to a high level of incidence (60 to
80%) (Graham et al. 1991). We have evidence (Graham and
Eissenstat, unpublished observations) that benomyl treatment
controls M colonization and depresses P acquisition of young
citrus trees grown with high-P supply under orchard conditions, indicating a fungitoxic effect of benomyl (Fitter and
Nichols 1988). No other soil-borne fungi pathogenic to citrus
roots are present in this field site. Control of the M fungal
colonization apparently relieves young citrus trees of belowground expenditure of C for use in aboveground processes and,
as a result, the growth rate of young Valencia orange trees on
four different rootstocks treated with benomyl increased by
approximately 10%.
We conclude that if aggressive fungi are present in populations of indigenous M fungi, they have the potential to colonize
at high-P supply and affect crop productivity. A strategy for
management of crops under high-P fertility conditions would
be to lower soil P availability to concentrations at which M
fungi function as mutualists rather than as parasites. For example, the Glomus spp. G329, Gc and Ge promoted growth of
sorghum and soybeans in six soils with less than 10 µg extractable P kg −1 soil, but not in four soils with greater than
20 µg kg −1 (Sylvia et al. 1993). This strategy assumes that
symbiotically effective fungi exist within each diverse field
population. Unfortunately, the relevance of species (or isolate)
diversity to the functioning of mycorrhizae in the field is not
yet known (Abbott and Gazey 1994).
Acknowledgments
This is Florida Agricultural Experiment Station Journal Series No.
R-04811. This research was supported in part by USDA-NRICGP
(94--37107--1024).
CARBON RESPONSES OF SOUR ORANGE TO GLOMUS SPP.
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