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Arbuscular Mycorrhizal Fungi,
a Possible Tool for Restoration of Giant
Sacaton Grasslands
Linda Kennedy Elliott1 , Jean C. Stutz1 ,
Ronald L. Tiller2 and Juliet C. Stromberg2
Sporobolus wrightii Munro ex Scribn, giant sacaton, is a perennial bunchgrass found only in
southwestern North America. Once a dominant species on upper terraces of riparian zones, giant
sacaton is now restricted to only 5% of its original range as upper floodplain terraces were
converted to agricultural use (Stromberg 1993). Recent changes in land management goals have
generated interest in returning these sites to grasslands. However, many restoration attempts
have been unsuccessful, and natural succession has been slow.
Inoculation with arbuscular mycorrhizal (AM) fungi has been an effective tool of restoration
projects in many ecosystems. Many plants and AM fungi form mutualistic relationships within
roots that provide benefits to host plants including enhanced nutrient status, increased drought
stress resistance and heightened resistance to soil borne pathogens. Mycorrhizae form when plant
roots are colonized by AM fungal hyphae from germinating spores or an existing hypha I network in
the soil. This hyphal network exploits the soil matrix and serves as conduit for nutrient flow into
roots of plants that are facultatively or obligately dependent upon mycorrhizae. Erosion or a history
of tillage and pesticide use, conditions common in former agricultural fields, may disrupt this hypha I
network (Johnson and Pfleger 1992). Some species of plant (i.e. Brassicaceae, Chenopodiaceae)
are not dependent on mycorrhizal associations for nutrient uptake, thus are able to establish quickly
in disturbed areas. Revegetation of later seral species of plant that are dependent upon
mycorrhizae may not be successful until an effective AM fungal network is present.
It is important to determine the role of mycorrhizae in giant sacaton before inoculation with AM
fungi is considered as a tool in restoration or revegetation projects, because effectiveness of AM
fungi varies with species of plant and because populations of AM fungi from separate geographic
locations have different impacts on plant hosts (Koide and Schreiner 1992). Our study of the
mycorrhizal ecology of Sporobolus wrightii has three objectives: (1) determine levels of AM fungal
colonization within giant sacaton roots from different habitats, and at different phenological stages
of the host plant, (2) identify AM fungal species present in the rhizosphere of giant sacaton and
(3) quantify the effectiveness of AM fungal inoculation on emergence, survivorship and growth of S.
wrightii.
Objective 1: Giant sacaton plants in the riparian zone of the San Pedro River in Arizona were
sampled six times in 1995 to determine differences in levels of AM colonization between habitats
and between phenological stages of the host plants. Samples were collected from two sites on
upper floodplain terraces where remnants of sacaton grasslands still exist, and from two sites on
1
2
Dept. of Botany, Arizona State University, Tempe AZ
Center for Environmental Studies, Arizona State University, Tempe AZ.
319
Feb
Mar
May
Lower Terrace
90.8±14.4
110.9±13.
2
91.0±10.3
66.3± 7.9
61.5±11.3
65.3+ 9.9
Upper Terrace
58.7 ± 9.6
68.8±10.2
59.6±8.2
63.5±11.7
26.1+ 4.1
32.9+ 5.9
Lower Terrace
42.6
46.6
27.7
21.8
29.9
29.5
Upper Terrace
41.1
37.9
21.2
20.3
17.7
18.7
Jul
Sep
Dec
Total Colonization•
% Relative Moistureb
Figure 1. Levels of AM fungal colonization within the roots of Sporobolus wrightii from upper and
lower floodplain terraces of the San Pedro River. Phenological stages of the plants were assessed as:
Dormant (D), Intermediate (1), Vegetative (V), Panicle (P), or Mature (M). Colonization varied significantly
between upper and lower terraces (Pr>F 0.001) and between sampling periods (Pr>F 0.0004). n=24
lower floodplain terraces where S. wrightii grows in scattered clumps. Roots and soil were
removed from the base of each plant with a soil auger and transported to laboratory facilities.
Phenological stage of each plant was assessed and percent relative moisture of each soil sample
was determined. Sections of root were cleared and stained (Kormanik and McGraw 1982) and
levels of colonization were evaluated (McGonigle eta/., 1990).
Levels of AM fungi colonizing roots of Sporobolus wrightii varied significantly over an annual
period (Table 1). Variation in total colonization of giant sacaton between sampling periods (Figure
1) may reflect changes in the phenology of S. wrightii, a perennial bunchgrass which exhibits a
period of dormancy or near dormancy during winter. Colonization of roots during this stage of
phenology may indicate parasitism by AM fungi. Early in spring, giant sacaton becomes
photosynthetically active, evidenced by growth of new leaves, and an increase in total AM fungal
colonization was observed. Elevated levels of AM fungal colonization may indicate increased
demand on the fungi by the host plant to supply nutrients for actively growing tissue. The prolonged
decline in total colonization during anthesis and seed maturation may indicate carbon allocation to
the plant reproductive effort at the expense of the fungal symbiont (Koide and Schreiner 1992).
Sporobolus wrightii on lower floodplain terraces exhibited significantly greater total AM fungal
colonization compared to plants on upper terraces (Table 1, Figure 1). This does not indicate
mycorrhizal associations are less important to giant sacaton in drier habitats. Contributions of AM
fungi in semi-arid conditions may be critical to the fitness of the host plant, and geographic isolates
differentially affect water relations of the same species of plant.
Objective 2: AM fungal species are identified using characteristics of spores. However, not all
AM fungal species sporulate under field conditions, or a species may sporulate at such low levels
that detection in field samples is difficult. Several generations of trap cultures (Stutz and Morton
1996) from field soil cores are necessary to determine species composition. Trap cultures have
been established using field soil cores from the above sites, and from riparian zones of other rivers
in Arizona. Each trap culture is maintained in a glasshouse on the Arizona State University campus.
Preliminary results indicate spores from several species of Glomus and Acau/ospora are present in
the rhizosphere of giant sacaton. Living cultures of AM fungal populations from each site have
320
been established which may be used as inoculum for additional glasshouse and field restoration
research.
Objective 3: S. wrightii seed has been collected and will be planted in soil from an existing
giant sacaton grassland that has been autoclaved to destroy any fungal propagules. Twenty pots
will receive AM fungal inoculum from the rhizosphere of giant sacaton on an upper floodplain
terrace, 20 pots will receive inoculum from a lower terrace and 20 pots will serve as control.
Effectiveness of AM fungal inoculation will be quantified as the difference in emergence,
survivorship, and growth between plants grown with and without mycorrhizae and will be determined
experimentally along a gradient of phosphorus availability. This experiment will be concluded in
February, 1997, when all plants will be harvested.
If populations of AM fungi that promote growth or survivorship of giant sacaton are
determined, land use managers may enhance revegetation projects by introduction of these fungal
populations into abandoned agricultural fields. Mycorrhizal associations may serve as a biotic
fertilizer, reducing the amount of time, seed and fertilizer needed to return upper floodplain terraces
to sacaton grasslands. This study will increase our understanding of relationships between AM
fungi and Sporobolus wrightii, and provide baseline data for further studies that may aid restoration
and recovery efforts of this valued and fragile ecosystem.
Feb
Mar
May
Jul
Sep
Dec
Total
Colon izationa
Lower Terrace
90.8.±14.4
110.9.±13.2
91.0.±10.3
66.3 .± 7.9
61.5.±11.3
65.3.± 9.9
Upper Terrace
58.7 .± 9.6
68.8.±10.2
59.6 .± 8.2
63.5.±11.7
26.1.± 4.1
32.9.± 5.9
Lower Terrace
14.9 .± 4.0
18.4 .± 5.0
17.5 .± 5.9
14.0.± 3.8
13.3.± 3.7
5.4.± 1.3
Upper Terrace
9.6 .±2.6
8.2 .±2.3
7.0 .± 1.5
9.3.± 1.8
4.5.± 1.1
3.4.± 0.9
Vesiclesa
%Relative Moistureb
Lower
Terrace
42.6
46.6
27.7
21.8
29.9
29.5
Upper
Terrace
41.1
37.9
21.2
20.3
17.7
18.7
Units are mean number of intersections exhibiting AM fungi per 200 intersections of root (McGonigle
eta/. 1990) :!:. SE
b
Calculated as (net dry weight of soil - net dry weight I net saturated -net dry) X 100
a
Table 1. Arbuscular mycorrhizal colonization of Sporobo/us wrightii and the mean % relative moisture
of soil samples from floodplain terraces of the San Pedro River, AZ. Significant differences between
upper and lower terraces were noted in total colonization, presence of vesicles, and% relative moisture
of soil (Pr>F 0.0001, 0.0039, 0.0029 respectively). Levels of total colonization and % relative moisture
were significantly different between sampling periods (Pr>F 0.0004, 0.0001 ). n 24 for colonization.
=
321
ACKNOWLEDGMENTS
This study is being supported in part by grants from the Arizona Water Protection Fund and the
Graduate Research Support Program from the Office of the Vice President for Research of Arizona
State University, and a fellowship from the Environmental Protection Agency.
REFERENCES
Johnson, N. C. and F. L. Pfleger. 1992. Vesicular-arbuscular mycorrhizae and cultural stresses.
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Koide, R.T. and R.P. Schreiner. 1992. Regulation of the vesicular- arbuscular mycorrhizal
symbiosis. Annu. Rev. Plant Physiol. Plant Mol. Bioi. 43:557-581.
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