Tall fescue often grows where pH and factors related to soil acidity diminish P availability and plant growth. Agronomic practices and growing conditions influence P availability and are important when managing E+ tall fescue. For example, P uptake and herbage production were greater in E+ than in E- plants when soil P availability was low; however, mass was less in E+ than in E- plants at high soil P availability (Malinowski et al., 1998b). Low P led to greater specific root length (i.e., finer roots) in E+ plants and greater concentrations of P, Mg, and Ca in roots and shoots of E+ than E- plants (Malinowski et al., 2000). Infected plants had more root dry weight (10%) and greater relative growth rate (16%) than E- plants when P was supplied as phosphate rock, but endophyte mediated responses were minimal when conventional P fertilizer was applied (Malinowski and Belesky, 1999a). This trend was verified in a separate investigation of endophyte influence on mineral concentrations in tall fescue (Vazquez-de-Aldana et al., 1999). Endophytes accumulate P and apparently place some demand on P absorbed by the host (Azevedo and Welty, 1995).

Effect of Soil Type

Rahman and Saiga (2005) grew E+ and E- tall fescue clones selected from native populations cultivated on Black Andisol (low P content, high contents of other nutrients) and Red Andisol (high P content, low contents of other nutrients) soils under controlled conditions. When grown in Black Andisol soil, E+ plants had greater cumulative shoot dry weight and regrowth rates than E- plants, while in Red Andisol the opposite was the case. In P deficient Black Andisol, endophyte infection increased P, K, Ca, and Mg uptake and transport within plants. Although root exudates were not evaluated, results suggested that an internal signaling system between the endophyte and the grass host leads to faster adjustments to environmental factors, when compared to responses occurring in E- plants.

Effect of the Endophyte

Two endophyte related mechanisms for P uptake appear in tall fescue grown in P deficient conditions: (i) altered root morphology and (ii) increased activity of root exudates. Endophyte infected tall fescue grown in nutrient solution produced roots with smaller diameters and longer root hairs than E- plants, regardless of the amount of P available for plant uptake (Malinowski et al., 1998a). The authors also showed that E+ tall fescue produced greater amounts of phenolic-like compounds in roots and shoots when P was limited. Zhou et al. (2003) reported similar findings for perennial ryegrass. These compounds could leak out of roots to change the availability of mineral nutrients or influence microbial or biological activity in the rhizosphere near the root surface. Since phenolic-like compounds were detected in E- plants as well, they might not be endophyte specific but regulated by endophyte when the host plant encounters additional stresses.

Garner et al. (1993) proposed that P was involved in ergot alkaloid biosynthesis for tall fescue endophytes, linking P nutrition with ergot alkaloid production in E+ grasses. Ergot alkaloid concentrations increased as P availability increased in endophyte-tall fescue associations, especially those with low ergot alkaloid production capability (Malinowski et al., 1998b).

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