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Source: Data from Rousseau et al. (1994).

Source: Data from Rousseau et al. (1994).

the plant in which nutrient (P) demand is greater than readily available supplies of the nutrient in soil; otherwise the cost of maintenance of the mycorrhizal symbiont is equivalent to the cost of root maintenance (Table 3.7).

The structural adaptations, physiology, and efficiencies of nutrient uptake by mycorrhizae are reviewed by S. E. Smith et al. (1994). The ability of mycorrhizal plants to access a larger pool of nutrients than nonmycorrhizal root systems was elegantly demonstrated by Nye and Tinker (1977) and Owusu-Bennoah and Wild (1979) using radiotracer phosphate to measure the depletion of phosphate in the soil around arbuscular mycorrhizal root systems. The distance that the depletion zone extended from the mycorrhizal root was shown to be greater than that from the nonmycorrhizal plant (Fig. 3.4), indicating that

Table 3.7 Cost of Plants of Maintenance of Arbuscular Mycorrhizal Infection

Biomass of mycorrhizal fungus

Cost of growth and maintenance of the fungus

Root maintenance cost

10-20% of root biomass

1-10% of fungal biomass d-1; i.e., 0.1-1% of root biomass d-1

Note: The cost of maintaining mycorrhizae ; root maintenance cost. Source: Data from Fitter (1991).

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Figure 3.4 A model of increasing P depletion zones from a root surface created by the addition of root hairs and arbuscular mycorrhizae as protrusions from the root surface into the soil. Model derived from the data of Nye and Tinker (1977) and Owusu-Bennoah and Wild (1979).

the fungal hyphae were responsible for exploiting a larger soil volume than root hairs alone. Clark and Zeto (2000) have recently reviewed the literature on nutrient uptake by arbuscular mycorrhizae. They cite information from Li et al. (1991a, b), Jakobsen (1995), and Jakobsen et al.(1992a) that show that the depletion zone around the roots of clover are extended from 10 to 20 mm because of the presence of arbuscular mycorrhizae and that this distance can be extended up to 110 mm in some cases. The actual effect of the mycorrhizal association depends on the rate of growth of the extraradical hyphae of the fungal species, with Acaulospora laevis having hyphal extension rates of approximately 20 mm week"1, but that of Glomus spp. less than 10mm week"1. In some cases in the ectomycorrhizal condition, the fungal partner has evolved not only individual extraradical hyphae, but may also develop mycelial structures called strands or rhizomorphs that have a distinct structure with conductive elements analogous to the vascular tissue of plants. These strands have been shown to be important in long-distance transport of nutrients and water (Duddridge et al., 1980), thus it is probable that in the ectomycorrhizal symbiosis the influence of the fungal partner can extend to great distances from the root surface. Indeed, we shall see in the next section that the distal parts of extraradical hyphal structures are capable of producing the enzymes that are usually associated with saprotrophic decomposer fungi. In addition to the development of adventitious hyphal structures to exploit soil for nutrients, arbuscular mycorrhizal fungi have been shown to alter the architecture of root systems. Berta et al. (1993) showed that the number of lateral roots produced by mycorrhizal plants was significantly greater than nonmycorrhizal plants, suggesting that there could be dual benefits of the mycorrhizal habit, one of increased root branching and the other of the fungal exploitation of soil for nutrients.

Table 3.8 Regression Analysis of the Variables Associated with Leek Response to Mycorrhizal Association with the Arbuscular Mycorrhizal Fungi Glomus intraradices and G. versiforme at Low Soil Phosphorus Availability (< 200 |xgg_1 soil)

Mycorrhizal species

Variable

Model R2

P value

G. intraradicies

1-2mm diam. aggregates

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