Ecophysiology and Application of Arbuscular Mycorrhizal Fungi in Arid Soils
Arbuscular mycorrhizas (AM) are the most widely spread type of fungal plant-root symbiosis, and the zygomycetes involved in these associations are often the most abundant fungi known in soil from all the continents (Harley 1991). Fungi closely resembling
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1 Introduction Arbuscular mycorrhizas (AM) are the most widely spread type of fungal plant-root symbiosis, and the zygomycetes involved in these associations are often the most abundant fungi known in soil from all the continents (Harley 1991). Fungi closely resembling mycorrhizal fungi have also been described in the underground organs of fossils of early land plants, suggesting that roots and the mycorrhizal fungi probably have evolved together and are likely to have produced combinations suitably adapted for survival within the biotic communities of their origin (Simon et al. 1993). Although research on the ecology and applied biology of mycorrhizas has continued over the last 100 years, many workers have not yet grasped the immense variety of types and roles that mycorrhizas play in both native and man-made environmental systems. These fungi are found in almost every terrestrial ecosystem, may be present not only in pioneer zones or very saline environments, and are reputed to represent the second largest biomass component of many terrestrial ecosystems. Mycorrhizal associations are found in a broad Tange of habitats. These include ecosystems ranging from aquatic (Bagyaraj et al. 1979) to deserts (Williams and Aldon 1976; Singh and Varma 1980, 1981; Neeraj et al. 1991), from lowland tropical rain forests (Hogberg 1982; Janos 1987) to high altitudes (Christie and Nicolson 1983; Laursen 1985; Allen et al. 1987), and in the canopy epiphytes (Nadkarni 1985). AM fungi are found in nearly all soils where plants grow, including environments that are considered stressful to plant growth. In fact, the author expects mycorrhizas to have their greatest impact when plants are exposed to growth-limiting environmental stress (see review, Sylvia and Williams 1992). There remains a great potential for increasing the efficiency of growth of agricultural, forestry, arboriculture, and horticultural crops by either inoculation of effective isolates of AM fungi or the management of indigenous populations of mycorrhizal fungi by agronomic practices, particularly in the tropics. Although the role of mycorrhizas is principally the same in all 1 School
of Life Sciences, lawaharlal Nehru University, New Dehli 110067, India
A. Varma/B. Hock (Eds.) Mycorrhiza
© Springer-Verlag Berlin Heidelberg 1995
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geographic zones, maximum benefit results from associations in arid soils. The aim of this chapter is to provide a comprehensive review of how mycorrhizas change the supply of mineral nutrients that a given soil can supply thereby modifying soil fertility, mycorrhizosphere, and aggregates of soil particles, an important consideration in arid-stressed environments.
2 What Characterizes an Arbuscular Mycorrhiza? The mutualistic nature of the interaction is a critical character that differentiates a mycorrhiza from other plant-fungus associations. The structural nature of the relationship in a general sense also distinguishes a mycorrhiza, despite a wide variety of types in this relationship. The fungus extends into b
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