As a definition of mycorrhizae, Smith and Read (1997) proposed "a symbiosis in which an external mycelium of a fungus supplies soil derived nutrients to a plant root." Mycorrhizae are further divided into six types based on anatomical characteristics, which are: (a) arbuscular mycor-rhizae (AM), (b) ectomycorrhizae, (c) orchid mycorrhizae, (d) ericoid mycorrhizae, (e) monotropoid mycorrhizae, and (f) arbutoid mycorrhizae. Some plants have requirement for mycorrhizae in order to complete their life cycle. Mycor-rhizae may influence host plant survival in regeneration niches and mycorrhizae can also increase seed production, seed quality, and host and offspring vigor. Some types of mycorrhizae enhance host plant resistance against severe environmental conditions. The most widely studied and most-commonly encountered mycorrhizal systems are the ectomycorrhizae and arbuscular mycorrhizae.

2.3.1 Ectomycorrhizae

About 5000 fungi, Asco- and Basidiomycetes, are known to form ectomycorrhizal association with about 2000 species of woody plants (Kendrick and Berch 1985). Roughly 5% of the vascular plants are known to develop ectomycorrhizae and these associations are typically seen by the intercellular development of Hartig nets. Ectomycorrhizal fungi are known to enhance the uptake of water and nutrients by the host plant, and to promote plant growth. Growth effects have been observed in a broad range of forest trees, such as Douglas fir, pine, and eucalyptus, with ectomycorrhiza associations forming in nurseries and in the field. Laccaria laccota, Pisolithus tinctorius, Suillus plorans, Hebeloma cylindro-sporum, and H. crustuliniforme have been used as soil inoculations and their growth promoting performances were dependent on the host plant. Large increases in the growth of pines have been recorded in field experiments. For instance, 25-100% increases in growth have been reported for three pine species inoculated with Pisolithus tinctorius on five reforestration sites in the southern United States (Marx et al. 1977). Inoculation with Paxillus involutus has been associated with a marked increase in stem diameter and volume, especially with sessile oak at Bouxieres where the volume almost doubled over 7 years (Garbaye and Churin 1997).

The increase in growth resulting from inoculation with mycorrhizal fungi has been attributed to improved nutrition of the host plant in most cases. Ectomycorrhizal fungi are able to absorb and accumulate phosphorus, nitrogen, potassium, and calcium in the fungal mantles more rapidly and for longer periods of time than nonmycorrhizal feeder roots. Ecto-mycorrhizal fungi improve the efficiency of phosphorus uptake principally through the development of extramatrical hyphae, which increase the absorptive surface and effective rooting density of the plant. Ectomycorrhiza are likely to enhance N uptake where the fungus and host plant differ in their capacity to absorb and assimilate NO3-N. Mycorrhizal fungi generally have a preference for NH4-N, although a number of species can also utilize NO3-N (Plassard et al. 1991).

2.3.2 Arbuscular Mycorrhiza

Arbuscular mycorrhizal (AM) associations are due to Glomales, an order of Zygomycetes (Morton and Benny 1990). The order consists of 7 genera, Glomus, Entro-phospora, Acaulospora, Archaeospora, Paraglomus, Gigaspora, and Scutellospora. Arbuscular mycorrhizal fungi develop arbuscules or hyphal coils within host plant cortical cells, and have a wide host range including many agricultural and horticultural crops worldwide. Growth promotion has been seen in many AM-associated plants including maize (Baltruchat 1987), tomato (Mohandas 1987), asparagus (Pedersen et al. 1991), Boston fern (Ponton et al. 1990), and gerbera (Wang et al. 1993). Despite the morphological differences between ecto- and arbuscular mycorrhizae, there appear to be many common features in their growth-promoting effects. Arbuscular mycorrhizal fungi develop extraradical hyphae that grow into the surrounding soil, increasing the potential of the root system for nutrient and water absorption, and improving the soil structure for better aeration and water penetration. One of the mechanisms of growth promotion by AM fungi involves the transport of phosphorus by AM fungi from the soil to the plant. Direct measurements of phosphorus transfer by AM fungal hyphae have been made by Jakobsen (1994) and Schweiger et al. (1999). Colonization of roots by AM fungi modifies the growth response of the plant and increases supplies of phosphorus (Abbott et al. 1995), however, some studies have shown that effectiveness, in terms of plant growth promotion, is not related to the extent of host root colonization (Jensen

1982; Sanders and Fitter 1992). Efficient phosphorus uptake has been found to be more closely related to the quantity of mycelium partitioned into the extraradical phase of the fungi (Abbott and Robson 1985; Morin et al. 1994). Jakobsen et al. (2001) reported that the phosphorus transport capacity of AM fungi is related not only to colonization rate, but also to the transport character of AM fungi themselves. The AM fungi cause few changes to root morphology, but the physiology of the host plant may change significantly. Tissue concentrations of growth-regulating compounds and other chemical constituents change, photosynthetis rates increase, and the partitioning of photosynthate to shoots and roots changes (Bethlenfalvay 1992). Allen et al. (1980) demonstrated differences in cytokinin content between Bouteloua gracilis plants with and without associated Glomus fasciculatus. They also reported quantitative and qualitative changes in GA-like substances in the leaves and roots of AM-associated plants (Allen et al. 1982). Incleases in auxin, cytokinin, GA and B-vitamin production have also been reported in plants associated with ectomycorrhizal fungi (Crafts and Miller 1974; Slankis 1973; Strzelczyk et al. 1977).

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