Eric

Figure 3.6 Ecological distribution of mycorrhizal types in relation to plant leaf litter resource quanty (represented as C:N ratios adjacent to tree name) of a selection of tree species and heather (Calluna vulgaris), the rate of decomposition of that plant litter and the pH of soil. (Redrawn from Read, 1991.)

at high altitudes or at high latitudes in which the annual heat sum is insufficient to maintain biological activity for considerable lengths of time during the year. The concept of a direct cycling system, whereby the mycorrhizal fungal community effects the decomposition of recalcitrant organic components, mineralization of nutrients, and direct uptake of those mineralized nutrients into the host plant, was proposed by Went and Stark (1968). It has been shown that these ecosystems tend to be most limited by nitrogen, and the production of mycorrhizal-generated enzymes affords the plant community with greater access to organic forms of nitrogen (Stribley and Read, 1980; Bajwa and Read, 1985; Leake and Read, 1989; 1990a,b). Indeed, Read and Kerley (1995) show that ericoid mycorrhizal plants derive most of their nitrogen from organic sources in highly organic soils (Table 3.10). Evidence for the use of organic nitrogen and phosphorus by ericoid mycorrhizae comes from a number of studies. Mitchell and Read (1981), Myers and Leake (1996), and Leake and Miles (1996) showed that Vaccinum macrocarpon could access phosphate from inositol hexaphosphate (a commonly u

Figure 3.7 Relationship between the dominance of mycorrhizal type in ecosystems (above the line) and to forest development (below the line) to the changes in plant litter resources quality, its rate of decompositon and the enzxyme competence of the mycorrhizal community. (Modified from Read, 1991 and Dighton and Mason, 1985.)

Figure 3.7 Relationship between the dominance of mycorrhizal type in ecosystems (above the line) and to forest development (below the line) to the changes in plant litter resources quality, its rate of decompositon and the enzxyme competence of the mycorrhizal community. (Modified from Read, 1991 and Dighton and Mason, 1985.)

occurring phosphorus compound in organic soils) and both P and N from phosphodiesters from nuclei (Fig. 3.8). Kerley and Read (1995) demonstrated the ability of the ericoid mycorrhizal fungus Hymenoscyphus ericae to decompose chitin and the ability of this fungus to effect transfer of some 40% of the nitrogen contained in N-acetylglucosamine to its host plants, Vaccinium macrocarpon and Calluna vulgaris. More recently, Xiao and Berch (1999) have shown that

Table 3.10 Proportion of Nitrogen Forms in the Soil Supporting the Growth of the Ericoid Mycorrhizal Plant Calluna vulgaris, Indicating the Central Role That the Mycorrhizal Fungi Play in the Acquisition of Nitrogen from Organic Sources

Nitrogen source Proportion of sources of N in soil

Hydrolysable organic N 70

Humin and other recalcitrant N 26

Extractable NH4-N < 1 Free amino acid N 1-4

Source: Data from Read and Kerley (1995).

Figure 3.8 Shoot phosphorus content (open bars) and nitrogen concentration (solid bars) of the ericaceous plant Vaccimium macrocarpon in the presence (M) or absence (NM) of ericoid mycorrhizal inoculum when provided with orthophosphate (Ortho-P) or nutrients supplied in the form of nuclei. Data from Myers and Leake (1996).

Figure 3.8 Shoot phosphorus content (open bars) and nitrogen concentration (solid bars) of the ericaceous plant Vaccimium macrocarpon in the presence (M) or absence (NM) of ericoid mycorrhizal inoculum when provided with orthophosphate (Ortho-P) or nutrients supplied in the form of nuclei. Data from Myers and Leake (1996).

the ericoid mycorrhizae (Oidiodendron maius and Acremonium strictum) of salal (Gautheria shallon) are able to utilize the amino acid, glutamine, the peptide, glutathione, and the protein, bovine serum albumin, as nitrogen sources. In the southern hemisphere, the Epicridaceae occupy a similar ecological niche to the Ericaceae of the northern hemisphere. Members of this family are also able to access organic forms of nutrients, as shown by the mycorrhizal endophytes of Woollsia pungens, which are able to degrade glutamine, argenine, and bovine serum albumin (Chen et al. 2000).

In soils in which most of the nutrients are in the form of organic compounds, nitrogen is not the only nutrient that becomes scarcely available for plant growth. In these soils, Phosphorus is also complexed within organic compounds and can be released through the action of a variety of phosphatase enzymes. Ericoid mycorrhizae are capable of producing phosphatase enzymes (Pearson and Read, 1975; Mitchell and Read, 1981; Straker and Mitchell 1985). In these low-pH soils, heavy metals are often more available than in other soils. Concentrations of iron and aluminum greater than 100 mgl"1 were shown to be inhibitory to phosphatase production by the ericoid mycorrhizal fungus Hymenoscyphus ericae (Shaw and Read, 1989). In low-pH soils, however, ericoid mycorrhizal associations have been said to "detoxify" the ecosystem by assimilation of phenolic and aliphatic acids (Leake and Read, 1991) and complexing toxic metal ions (Bradley et al., 1982). This ability allows the establishment of the host plant in extreme environmental conditions. (See Chap. 6.) The importance of ericoid mycorrhizae, their role in the acquisition of nutrients, and their tolerance of heavy metals may be of great importance to those ericaceous plant species that have been brought into cultivation. There is little

documented evidence of the role of ericoid mycorrhizae in these cultivated forms (Goulart et al., 1993), in which the extent of root colonization is much higher than expected, based on their survey of native and cultivated blueberry (Vaccinium corymbosum) in the United States.

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