Tall fescue persists under a wide range of soil conditions, including those of high exchangeable Al, low pH, low P availability, and where distinct physical challenges associated with shallow and eroded soils occur (see Chapter 3). Apparently, tall fescue can avoid or tolerate chemical and physical stresses by making physiological and morphological adaptations that in some instances seem to be associated with the presence of endophyte. For example, plant-available P supply often is limited in acidic soils. Endophyte, although localized in the aboveground portion of the plant, elicits some of the same responses in tall fescue as mycorrhizal fungi do in other plant species, particularly improved P and water acquisition by the host (Wittenmayer and Merbach, 2005). Grasses commonly form symbiotic associations with mycorrhizal fungi, but Neotyphodium spp. endophytes appear to suppress mycorrhizal colonization in tall fescue (Baker, 1987; Chu-Chou et al., 1992; Guo et al., 1992), perennial ryegrass (Müller, 2003), and annual ryegrass (L. multiflorum Lam.) (Omacini et al., 2005), suggesting alternative strategies to ensure survival under suboptimal soil conditions.
Cheplick et al. (1989) suggested that the host plant incurred a metabolic cost to support the endophyte. The cost, expressed in terms of smaller plants, would not be realized unless stressful environmental conditions occurred, with differences between E+ and E- associations greater in young than in well-established plants. Responses varied among cultivars and clones within cultivars in later experiments on mineral stress (Belesky and Fedders, 1995; Zaurov et al., 2001). Endophyte infected plants either grew better, similar to, or were impaired relative to E- plants when nutrients were limited. Growth of very young E+ plants was depressed relative to E- plants at low nutrient status, and no differences in growth were apparent under high nutrient conditions. Early seedling development and resource allocation in host plants of similar genotype but with different endophyte strains suggest that early seedling growth, up to 6 wk of age, and tiller production of certain E+ plants is slower than those in E- plants when grown with an ample nutrient supply (Belesky et al., 2008).
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