Overview of Mycorrhiza Symbiosis

(Based on a chapter in Principles and Applications of Soil Microbiology)

Gallery – under construction

SUMMARY

Mycorrhizae are symbiotic associations that form between the roots of most plant species and fungi. These symbioses are characterized by bi-directional movement of nutrients where carbon flows to the fungus and inorganic nutrients move to the plant, thereby providing a critical linkage between the plant root and soil. In infertile soils, nutrients taken up by the mycorrhizal fungi can lead to improved plant growth and reproduction. As a result, mycorrhizal plants are often more competitive and better able to tolerate environmental stresses than are nonmycorrhizal plants.

Mycorrhizal associations vary widely in form and function. Ectomycorrhizal fungi are mostly basidiomycetes that grow between root cortical cells of many tree species, forming a Hartig net. Arbuscular mycorrhizal fungi belong to the order Glomales and form highly branched structures called arbuscules, within root cortical cells of many herbaceous and woody plant species.

Plant responses to colonization by mycorrhizal fungi can range from dramatic growth promotion to growth depression. Factors affecting this response include the mycorrhizal dependency of the host crop, the nutrient status of the soil, and the inoculum potential of the mycorrhizal fungi. Management practices such as tillage, crop rotation, and fallowing may adversely affect populations of mycorrhizal fungi in the field. Where native inoculum potential is low or ineffective, inoculation strategies may be helpful. With the current state of technology, inoculation is most feasible for transplanted crops and in areas where soil disturbance has greatly reduced the native inoculum potential.

INTRODUCTION

Mycorrhiza refers to an association or symbiosis between plants and fungi that colonize the cortical tissue of roots during periods of active plant growth. The association is characterized by the movement of plant-produced carbon to the fungus and fungal-acquired nutrients to the plant

The term mycorrhiza, which literally means fungus-root, was first applied to fungus-tree associations described in 1885 by the German forest pathologist A.B. Frank. Since then we have learned that the vast majority of land plants form symbiotic associations with fungi: an estimated 95% of all plant species belong to genera that characteristically form mycorrhizae. The mycorrhizal condition is the rule among plants, not the exception.

The benefits afforded plants from mycorrhizal symbioses can be characterized either agronomically by increased growth and yield or ecologically by improved fitness (i.e., reproductive ability). In either case, the benefit accrues primarily because mycorrhizal fungi form a critical linkage between plant roots and the soil. Mycorrhizal fungi usually proliferate both in the root and in the soil. The soilborne or extramatrical hyphae take up nutrients from the soil solution and transport them to the root. By this mechanism, mycorrhizae increase the effective absorptive surface area of the plant. In nutrient-poor or moisture-deficient soils, nutrients taken up by the extramatrical hyphae can lead to improved plant growth and reproduction. As a result, mycorrhizal plants are often more competitive and better able to tolerate environmental stresses than are nonmycorrhizal plants.

GLOBAL PERSPECTIVE

Mycorrhizal associations vary widely in structure and function. Despite the many exceptions, it is possible to state broad generalizations about latitude (or altitude), soil properties, and structure and function of the different mycorrhizal types that colonize the dominant vegetation in a gradient of climatic zones (Read, 1884). Ericaceous plants, which dominate the acidic, high-organic heathland soils of subarctic and subalpine regions, are colonized by a group of ascomycetous fungi, giving rise to the ericoid type of mycorrhiza. This mycorrhizal type is characterized by extensive growth within (i.e., intracellular) cortical cells, but little extension into the soil. The fungi produce extracellular enzymes that break down organic matter, enabling the plant to assimilate nutrients mineralized from organic compounds present in the colloidal material surrounding roots. Moving along the environmental gradient, coniferous trees replace ericaceous shrubs as the dominant vegetation. These trees are colonized by a wide range of mostly basidiomycetous fungi that grow between (i.e., intercellular) root cortical cells forming the ectomycorrhizal type of mycorrhiza. Ectomycorrhizal fungi may produce large quantities of hyphae on the root and in soil. These hyphae function in the absorption and translocation of inorganic nutrients and water, but also release nutrients from litter layers by production of enzymes involved in mineralization of organic matter. At the warmer and drier end of the environmental gradient, grasslands often form the dominant vegetation. In these ecosystems nutrient use is high and phosphorus is frequently a limiting element for growth. Grasses and a wide variety of other plants are colonized by fungi belonging to the order Glomales. These fungi form arbuscules or highly branched structures (the term literally means little trees) within (intracellular) root cortical cells, giving rise to the arbuscular type of mycorrhiza. The Glomalean fungi may produce extensive extramatrical hyphae (i.e., hyphae outside the root) and can significantly increase phosphorus-inflow rates of the plants they colonize.

The diversity of these root-fungal associations provides plants with a range of strategies for efficient functioning in an array of plant-soil systems. The objective of this paper is to provide an overview of this diversity and to evaluate the roles and potential for management, of the mycorrhizal symbioses in native and managed ecosystems. Because ectomycorrhizae and arbuscular mycorrhizae are the most widespread, we will emphasize these types of associations.

TYPES OF MYCORRHIZAE 

Ectomycorrhizae

The diagnostic feature of ectomycorrhizae (EM) is the presence of hyphae between root cortical cells producing a netlike structure called the Hartig net, after Robert Hartig who is considered the father of forest biology. Many EM also have a sheath, or mantle, of fungal tissue that may completely cover the absorbing root (usually the fine feeder roots). The mantle can vary widely in thickness, color, and texture depending on the particular plant-fungus combination. The mantle increases the surface area of absorbing roots and often affects fine-root morphology, resulting in root bifurcation and clustering. Contiguous with the mantle are hyphal strands that extend into the soil. Often the hyphal strands will aggregate to form rhizomorphs that may be visible to the unaided eye. The internal portion of rhizomorphs can differentiate into tubelike structures specialized for long-distance transport of nutrients and water.

Ectomycorrhizae are found on woody plants ranging from shrubs to forest trees. Many of the host plants belong to the families Pinaceae, Fagaceae, Betulaceae and Myrtaceae. Over 4,000 fungal species, belonging primarily to the Basidiomycotina, and fewer to the Ascomycotina, are known to form ectomycorrhizae. Many of these fungi produce mushrooms and puffballs on the forest floor. Some fungi have a narrow host range, such as Boletus betulicola on Betula spp., while others have very broad host range, such as Pisolithus arhizus (also called P. tinctorius) which forms ectomycorrhiza with more than 46 tree species belonging to at least eight genera).

Arbuscular Mycorrhizae

The diagnostic feature of arbuscular mycorrhizae (AM) is the development of a highly branched arbuscule within root cortical cells. The fungus initially grows between cortical cells, but soon penetrates the host cell wall and grows within the cell. The general term for all mycorrhizal types where the fungus grows within cortical cells is endomycorrhiza. In this association neither the fungal cell wall nor the host cell membrane are breached. As the fungus grows, the host cell membrane invaginates and envelops the fungus, creating a new compartment where material of high molecular complexity is deposited. This apoplastic space prevents direct contact between the plant and fungus cytoplasm and allows for efficient transfer of nutrients between the symbionts. The arbuscules are relatively short lived, less than 15 days, and are often difficult to see in field-collected samples.

Other structures produced by some AM fungi include vesicles, auxiliary cells, and asexual spores. Vesicles are thin-walled, lipid-filled structures that usually form in intercellular spaces. Their primary function is thought to be for storage; however, vesicles can also serve as reproductive propagules for the fungus. Auxiliary cells are formed in the soil and can be coiled or knobby. The function of these structures is unknown. Reproductive spores can be formed either in the root or more commonly in the soil. Spores produced by fungi forming AM associations are asexual, forming by the differentiation of vegetative hyphae. For some fungi (e.g., Glomus intraradices), vesicles in the root undergo secondary thickening, and a septum (cross wall) is laid down across the hyphal attachment leading to spore formation, but more often spores develop in the soil from hyphal swellings.

The fungi that form AM are currently all classified in the order Glomales (Morton, 1988). The taxonomy is further divided into suborders based on the presence of: (i) vesicles in the root and formation of chlamydospores (thick wall, asexual spore) borne from subtending hyphae for the suborder Glomineae or (ii) absence of vesicles in the root and formation of auxiliary cells and azygospores (spores resembling a zygospore but developing asexually from a subtending hypha resulting in a distinct bulbous attachment) in the soil for the suborder Gigasporineae.

The term vesicular-arbuscular mycorrhiza (VAM) was originally applied to symbiotic associations formed by all fungi in the Glomales, but because a major suborder lacks the ability to form vesicles in roots, AM is now the preferred acronym. The order Glomales is further divided into families and genera according to the method of spore formation. The spores of AM fungi are very distinctive. They range in diameter from 10 mm for Glomus tenue to more than 1,000 mm for some Scutellospora spp. The spores can vary in color from hyaline (clear) to black and in surface texture from smooth to highly ornamented. Glomus forms spores on the ends of hyphae, Acaulospora forms spores laterally from the neck of a swollen hyphal terminus, and Entrophospora forms spores within the neck of the hyphal terminus. The Gigasporineae are divided into two genera based upon the presence of inner membranous walls and a germination shield (wall structure from which the germ tube can arise) for Scutellospora or the absence of these structures for Gigaspora.

The AM type of symbiosis is very common as the fungi involved can colonize a vast taxonomic range of both herbaceous and woody plants, indicating a general lack of host specificity among this type. However, it is important to distinguish between specificity, innate ability to colonize, infectiveness, amount of colonization, and effectiveness, plant response to colonization. AM fungi differ widely in the level of colonization they produce in a root system and in their impact on nutrient uptake and plant growth.

Ericaceous Mycorrhizae

The term ericaeous is applied to mycorrhizal associations found on plants in the order Ericales. The hyphae in the root can penetrate cortical cells (endomycorrhizal habit); however, no arbuscules are formed. Three major forms of ericaceous mycorrhiza have been described:

(i) Ericoid — cells of the inner cortex become packed with fungal hyphae. A loose welt of hyphae grows over the root surface, but a true mantle is not formed. The ericoid mycorrhizae are found on plants such as Calluna (heather), Rhododendron (azaleas and rhododendrons) and Vaccinium (blueberries) that have very fine root systems and typically grow in acid, peaty soils. The fungi involved are ascomycetes of the genus Hymenoscyphus.

(ii) Arbutoid — characteristics of both EM and endomycorrhizae are found. Intracellular penetration can occur, a mantle forms, and a Hartig net is present. These associations are found on Arbutus (e.g., Pacific madrone), Arctostaphylos (e.g., bearberry), and several species of the Pyrolaceae. The fungi involved in the association are basidiomycetes and may be the same fungi that colonize EM tree hosts in the same region.

(iii) Monotropoid — the fungi colonize achlorophyllous (lacking chlorophyll) plants in Monotropaceae (e.g., Indian pipe), producing the Hartig net and mantle. The same fungi also form EM associations with trees and thereby form a link through which carbon and other nutrients can flow from the autotrophic host plant to the heterotrophic, parasitic plant.

Orchidaceous Mycorrhizae

Mycorrhizal fungi have a unique role in the life cycle of plants in the Orchidaceae. Orchids typically have very small seeds with little nutrient reserve. The plant becomes colonized shortly after germination, and the mycorrhizal fungus supplies carbon and vitamins to the developing embryo. For achlorophyllous species, the plant depends on the fungal partner to supply carbon throughout its life. The fungus grows into the plant cell, invaginating the cell membrane and forming hyphal coils within the cell. These coils are active for only a few days, after which they lose turgor and degenerate and the nutrient contents are absorbed by the developing orchid. The fungi participating in the symbiosis are basidiomycetes similar to those involved in decaying wood (e.g., Coriolus, Fomes, Marasmius) and pathogenesis (e.g., Armillaria and Rhizoctonia). In mature orchids, mycorrhizae also have roles in nutrient uptake and translocation.

Mixed Infections

Several fungi can colonize the roots of a single plant, but the type of mycorrhiza formed is usually uniform for a host. In some cases, however, a host can support more than one type of mycorrhizal association. Alnus (alders), Salix (willows), Populus (poplars), and Eucalyuptus can have both AM and EM associations on the same plant. Some ericoid plants have occasional EM and AM colonization.

An intermediate mycorrhizal type can be found on coniferous and deciduous hosts in nurseries and burned forest sites. The ectendomycorrhiza type forms a typical EM structure, except the mantle is thin or lacking and hyphae in the Hartig net may penetrate root cortical cells. The ectendomycorrhiza is replaced by EM as the seedling matures. The fungi involved in the association were initially designated “E-strain” but were later shown to be ascomycetes and placed in the genus Wilcoxina.

UPTAKE AND TRANSFER OF SOIL NUTRIENTS

When a nutrient is deficient in soil solution, the critical root parameter controlling its uptake is surface area. Hyphae of mycorrhizal fungi have the potential to greatly increase the absorbing surface area of the root. For example, Rousseau et al. (1994) found that while extramatrical mycelia (aggregates of hyphae) accounted for less than 20% of the total nutrient absorbing surface mass, they contributed nearly 80% of the absorbing surface area of pine seedlings. It is also important to consider the distribution and function of the extramatrical hyphae. If the mycorrhiza is to be effective in nutrient uptake, the hyphae must be distributed beyond the nutrient depletion zone that develops around the root. A nutrient depletion zone develops when nutrients are removed from the soil solution more rapidly than they can be replaced by diffusion. For a poorly-mobile ion such as phosphate, a sharp and narrow depletion zone develops close to the root. Hyphae can readily bridge this depletion zone and grow into soil with an adequate supply of phosphorus. Uptake of micronutrients such as zinc and copper is also improved by mycorrhizae because these elements are also diffusion-limited in many soils. For more mobile nutrients such as nitrate, the depletion zone is wide and it is less likely that hyphae grow extensively into the zone that is not influenced by the root alone. Another factor contributing to the effective absorption of nutrients by mycorrhizae is their narrow diameter relative to roots. The steepness of the diffusion gradient for a nutrient is inversely related to the radius of the absorbing unit; therefore, the soil solution should be less depleted at the surface of a narrow absorbing unit such as a hypha. Furthermore, narrow hyphae can grow into small soil pores inaccessible to roots or even root hairs.

Another advantage attributed to mycorrhizal fungi is access to pools of phosphorus not readily available to the plant. One mechanism for this access is the physiochemical release of inorganic and organic phosphorus by organic acids through the action of low-molecular-weight organic anions such as oxalate which can (Fox et al. 1990): (i) replace phosphorus sorbed at metal-hydroxide surfaces through ligand-exchange reactions, (ii) dissolve metal-oxide surfaces that sorb phosphorus, and (iii) complex metals in solution and thus prevent precipitation of metal phosphates.

Some EM fungi produce large quantities of oxalic acid, and this may partially explain enhanced nutrient uptake by EM roots. Another mechanism by which mycorrhizal fungi release inorganic phosphorus is through mineralization of organic matter. This occurs by phosphatase-mediated hydrolysis of organic phosphate (C-O-P) ester bonds. Significant phosphatase activity has been documented for mycorrhizal fungi grown in pure cultures and for excised and intact EM short roots. In the field, a positive correlation has been reported between phosphatase activity and the length of fungal hyphae associated with EM mantles (Haussling and Marschner, 1989). Care must be exercised in interpreting these data because plant roots and the associated microflora also produce organic acids and phosphatases; however, mycorrhizal fungi certainly intensify this activity.

Ericoid and EM have a special role in the mineralization of nitrogen (Read et al. 1989). Most plant litter entering the soil has a high C:N ratio and is rich in lignin and tannins. Only a few mycorrhizal fungi can mobilize nutrients from these primary sources. However, a wide range of ericoid and EM fungi can obtain nitrogen and other nutrients from secondary sources of organic matter such as dead microbial biomass. A wide range of hydrolytic and oxidative enzymes capable of depolymerizing organic nitrogen have been demonstrated. These types of mycorrhizae may have an important role in nitrogen cycling in the acidic and highly organic soils where they predominate.

CARBON FLUXES IN MYCORRHIZAL PLANTS

Mycorrhizal fungi range from obligate symbionts, which can only obtain carbon from the plant host as in the case of AM fungi to facultative symbionts, which can also mineralize organic carbon from nonliving sources as in the case of some EM species. In nature the heterotrophic mycorrhizal fungi obtain all or most of their carbon from the autotrophic host plant. Ectomycorrhizae and ericoid mycorrhizae transform host carbohydrates into fungal-specific storage carbohydrates, such as mannitol and trehalose, which may produce a sink for photosynthate that favors transport of carbohydrate to the fungal partner. In AM, lipids accumulate in vesicles and other fungal structures and provide an analogous sink for host photosynthate.

As much as 20% of the total carbon assimilated by plants may be transferred to the fungal partner. This transfer of carbon to the fungus has sometimes been considered a drain on the host. However, the host plant may increase photosynthetic activity following mycorrhizal colonization, thereby compensating for carbon “lost” to the soil. Occasionally plant growth suppression has been attributed to mycorrhizal colonization, but usually this occurs only under low-light (photosynthate limiting) or high-phosphorus conditions.

In an ecosystem, the flow of carbon to the soil mediated by mycorrhizae serves several important functions. For some mycorrhizae, the extramatrical hyphae produce hydrolytic enzymes, such as proteases and phosphatases that can have an important impact on organic matter mineralization and nutrient availability. Extramatrical hyphae of mycorrhizae also bind soil particles together and thereby improve soil aggregation. Typically there are between 1 to 20 m of AM hyphae g^-1 of soil (Sylvia, 1990). Another important consequence of carbon flow to the fungal partner is the development of a unique rhizosphere microbial community called the mycorrhizosphere, which we will discuss shortly. Soil scientists now realize that carbon flow to the soil is critical for the development of soil aggregation and the maintenance of a healthy plant-soil system. Enhanced carbon flow to the soil should be considered an important benefit of mycorrhizal colonization.

INTERACTIONS WITH OTHER SOIL ORGANISMS

Mycorrhizal fungi interact with a wide assortment of organisms in the rhizosphere. The result can be either positive, neutral, or negative on the mycorrhizal association or a particular component of the rhizosphere. For example, specific bacteria stimulate EM formation in conifer nurseries and are called mycorrhization helper bacteria. In certain cases these bacteria eliminate the need for soil fumigation (Garbaye, 1994).

The interaction between rhizobia and AM fungi has received considerable attention because of the relatively high phosphorus demand of N₂ fixation. The two symbioses typically act synergistically, resulting in greater nitrogen and phosphorus content in combination than when each is inoculated onto the legume alone. Legumes are typically coarse-rooted and therefore inefficient in extracting phosphorus from the soil. The AM fungi associated with legumes are an essential link for adequate phosphorus nutrition, leading to enhanced nitrogenase activity that in turn promotes root and mycorrhizal growth.

Mycorrhizal fungi colonize feeder roots and thereby interact with root pathogens that parasitize this same tissue. In a natural ecosystem where the uptake of phosphorus is low, a major role of mycorrhizal fungi may be protection of the root system from endemic pathogens such as Fusarium spp. Mycorrhizae may stimulate root colonization by selected biocontrol agents, but our understanding of these interactions is meager. Much more research has been conducted on the potential effects of mycorrhizal colonization on root pathogens. Mycorrhizal fungi may reduce the incidence and severity of root diseases. The mechanisms proposed to explain this protective effect include: (i) development of a mechanical barrier-especially the mantle of the EM-to infection by pathogens, (ii) production of antibiotic compounds that suppress the pathogen, (iii) competition for nutrients with the pathogen, including production of siderophores, and (iv) induction of generalized host defense mechanisms.

MANAGEMENT OF MYCORRHIZAE

The dramatic plant growth response achieved in pot studies following inoculation with mycorrhizal fungi in low-fertility soils led to a flurry of activity in the 1980s aimed at using these organisms as biofertilizers. Field responses were often disappointing, especially in high-input agricultural systems, and many concluded that mycorrhizae had little practical importance in agriculture. Further studies, however, have confirmed that most agricultural plants are colonized by mycorrhizal fungi and that they can have a substantial impact, both positive and negative, on crop productivity (Johnson, 1993). Certainly, agriculturists should appreciate the distribution of mycorrhizae within their systems and understand the impact of their management decisions on mycorrhizal functioning.

Factors that should be considered when assessing the potential role of mycorrhizae in an agroecosystem include:

Mycorrhizal dependency (MD) of the host crop. This is usually defined as the growth response of mycorrhizal (M) versus nonmycorrhizal (NM) plants at a given phosphorus level; MD = ((M – NM) / NM) x 100. Although most agricultural crops have mycorrhizae, not all benefit equally from the symbiosis. Generally, coarse-rooted plants benefit more than fine-rooted plants.

Nutrient status of the soil. Assuming that the major benefit of the mycorrhizal symbiosis is improved phosphorus uptake, the management of mycorrhizal fungi will be most critical when soil phosphorus is limiting. Many tropical soils fix phosphorus and proper mycorrhization of plants is essential to obtain adequate phosphorus nutrition. In temperate zones, phosphorus is sometimes applied in excess of crop demand. However, with increased concerns about environmental quality, phosphorus use in developed countries may be reduced, resulting in increased dependence on native mycorrhizae for nutrient uptake. Another factor to consider is the interaction of water stress with nutrient availability. As soils dry, phosphorus may become limiting even in soils that test high in available phosphorus.

Inoculum potential of the indigenous mycorrhizal fungi. Inoculum potential is a product of the abundance and vigor of the propagules in the soil and can be quantified by determining the rate of colonization of a susceptible host under a standard set of conditions. Inoculum potential can be adversely affected by management practices such as fertilizer and lime application, pesticide use, crop rotation, fallowing, tillage, and topsoil removal.

Examples of how management practices affect mycorrhizal populations in soil and subsequent growth of the host crop:

Soil disturbance such as tillage can dramatically affect the function of mycorrhizae in an agricultural system. M.H. Miller and coworkers from the University of Guelph, Canada, documented an interesting case where disturbance of an arable, no-till soil resulted in reduced AM development and subsequently less absorption of phosphorus by seedlings of maize in the field (Miller et al., 1995). They hypothesized that soil disturbance reduced the effectiveness of the mycorrhizal symbiosis. To confirm this, they conducted a series of growth chamber studies with nondisturbed and disturbed soil cores collected from long-term field plots. Disturbance reduced both mycorrhizal colonization and phosphorus absorption by maize and wheat roots, but did not reduce phosphorus absorption by two nonmycorrhizal crops, spinach and canola. The authors concluded that under nutrient-limited conditions, the ability of mycorrhizal seedlings to associate with intact hyphal networks in soil may be highly advantageous for crop establishment.

Crop rotation and fallow systems can affect the diversity and function of mycorrhizal fungi. J.P. Thompson described the role of AM fungi in a long-fallow (more than 12 months) disorder of field crops in the state of Queensland, Australia (Thompson, 1987). In semiarid cropping systems, clean fallows conserve soil moisture and nitrate for the subsequent crop. Since the 1940s, some crops sown immediately after long fallow grew poorly and had phosphorus and zinc deficiencies. The Australian researchers found that the fallow resulted in a decline in propagules of AM fungi in the soil and reduced colonization of the crop plants in the field. Furthermore, they conducted inoculation trials and found that increasing inoculum abundance in the soil overcame the deleterious impact of fallow. They recommended that farmers avoid planting mycorrhizal-dependent crops, such as linseed, sunflower, and soybean, following periods of fallow or after a nonhost plant such as canola that lead to reduction in AM propagules.

PROBLEMS AND POTENTIAL FOR INOCULUM PRODUCTION AND USE

In situations where native mycorrhizal inoculum potential is low or ineffective, providing the appropriate fungi for the plant production system is worth considering. With the current state of technology, inoculation is best for transplanted crops and in areas where soil disturbance has reduced native inoculum potential.

The first step in any inoculation program is to obtain an isolate that is both infective, or able to penetrate and spread in the root, and effective, or able to enhance growth or stress tolerance of the host. Individual isolates of mycorrhizal fungi vary widely in these properties, so screening trials are important to select isolates that will perform successfully. Screening under actual cropping conditions is best because indigenous mycorrhizal fungi, pathogens, and soil chemical and physical properties will influence the result.

Isolation and inoculum production of EM and AM fungi present very different problems. Many EM fungi can be cultured on artificial media. Therefore, isolates of EM fungi can be obtained by placing surface-disinfested portions of sporocarps or mycorrhizal short roots on an agar growth medium. The resulting fungal biomass can be used directly as inoculum but, for ease of use, inoculum often consists of the fungal material mixed with a carrier or bulking material such as peat. Obtaining isolates of AM fungi is more difficult because they will not grow apart from their hosts. Spores can be sieved from soil, surface disinfested, and used to initiate “pot cultures” on a susceptible host plant in sterile soil or an artificial plant-growth medium. Inoculum is typically produced in scaled-up pot cultures. Alternatively, hydroponic or aeroponic culture systems are possible; a benefit of these systems is that plants can be grown without a supporting substratum, allowing colonized roots to be sheared into an inoculum of high propagule number. Sylvia (1994) summarized methods for working with AM inoculum.

Examples where inoculating with either EM or AM fungi is beneficial when planting a mycorrhizal-dependent crop in an area where native inoculum potential is low:

Pines were not native to Puerto Rico, and their fungal symbionts were absent from the soil (Vozzo and Hacskaylo, 1971). As far back as the 1930s ,attempts to establish pine on the island were unsuccessful. Typically, the pines germinated well and grew to heights of 8 to 10 cm in a relatively short time, but then rapidly declined. Phosphorus fertilizers did not substantially improve plant vigor. In 1955, soil from under pine stands in North Carolina was transported to Puerto Rico where it was incorporated as inoculum into soil around 1-year-old “scrawny” pine seedlings growing at Maricao in the western mountains. Thirty-two seedlings were inoculated, and an equal number were monitored as noninoculated control plants. Within one year, inoculated plants had abundant mycorrhizal colonization and had achieved heights of up to 1.5 m, while most of the noninoculated plants had died. Further trials with mixtures of surface soil containing mycorrhizal fungi and with pure inocula, consisting of fungi growing in a peat-based medium, confirmed that inoculated seedlings were consistently more vigorous and larger than nonmycorrhizal ones. Subsequent surveys more than 15 years after inoculation indicated that the inoculated fungi continued to grow and sporulate in the pine plantations.

Beach erosion is a problem in many coastal areas and replenishing the beaches with sand dredged from offshore is often the method of choice for restoring them. Native grasses are planted in the back beach to reduce further erosion and to initiate the dune-building process. In native dunes, beach grasses are colonized by a wide array of AM fungi. However, when these grasses are propagated in nurseries, they do not have mycorrhizae. Furthermore, the replenishment sand is typically devoid of AM propagules. In a series of studies (Sylvia, 1989) AM fungi were isolated from grasses growing in native dunes. The fungi were screened for effectiveness with the given host/soil combination and for compatibility with the nursery production system, and the effect of inoculation was documented on transplants placed on newly restored beaches. In the nursery, moderate amounts of colonization were achieved, even with high levels of pesticide and fertilizer use. After transfer of these plants to a low-nutrient beach environment, AM colonization spread rapidly and enhanced plant growth significantly compared to noninoculated control plants even though plants were equal size when they left the intensively managed nursery. Compared to noninoculated plants after 20 months on the beach, AM-colonized plants had 219, 81, 64 and 53% more shoot dry mass, root length, plant height, and number of tillers, respectively. In most cases the objective of nursery inoculation is not to achieve a growth response, but rather to establish the symbiosis with the plant so that it can be effectively transferred to the field.

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GENERAL REFERENCES

• Allen, M.F. 1992. Mycorrhizal functioning: An integrative plant-fungal process. Chapman and Hall, New York.
• Bethlenfalvay, G.J. and R.G. Linderman. 1992. Mycorrhizae in sustainable agriculture. ASA Special Publication No. 54. Agronomy Society of America, Madison, WI.
• Harley, J.L. and S.E. Smith. 1983. Mycorrhizal symbiosis. Academic Press, New York.
• Norris, J.R., D.J. Read, and A.K. Varma. 1991. Methods in microbiology: Techniques for the study of mycorrhiza. Vol. 23. Academic Press, London.
• Norris, J.R., D.J. Read, and A.K. Varma. 1992. Methods in microbiology: Techniques for the study of mycorrhiza. Vol. 24. Academic Press, Ltd. London.
• Read, D.J., D.H. Lewis, A.H. Fitter, and I.J. Alexander. 1992. Mycorrhizas in ecosystems. CAB International, Wallingford, England.
• Robson, A.D., L.K. Abbott, and N. Malajczuk. 1994. Management of mycorrhizas in agriculture, horticulture and forestry. Kluwer Academic Publishers, Boston