Tall fescue is grown on millions of hectares in the warmer portions of the humid eastern United States. It is used on soils where low pH and limited nutrient supply restrict plant productivity, caused by interactions among prior land use practices, relatively high rainfall amounts, and soil geochemistry. Plants that are adapted to acidic soils possess a variety of mechanisms that enable them to tolerate or overcome adverse soil chemical conditions. For example, soil acidity affects plant growth through a complex of chemical changes in the rhizosphere involving increased H+, Al3+, and Mn2+. These include inhibition of metal cation (Ca2+, Mg2+) uptake, a decrease in P and Mo solubility and increased efflux of nutrients and metabolites from roots. Root morphology and function also change when soil chemical conditions are less than ideal for plant growth. For example, when soil water is scarce, some species can explore large areas of soil, and, in other instances, mineral challenges can elicit or suppress the production of fine root structure (Huang, 2001). The net effect modifies nutrient activity in the root zone, expressed as physiological and morphological adaptations to stress in root tissue.
Host-endophyte responses to management and environment can vary in unpredictable ways that complicate our understanding of responses to abiotic stresses. Endophyte infected tall fescue produced less root mass than E- plants under simulated acid rain, although the response depended on plant age (Cheplick, 1993). Soil acidity influenced dry matter production and allocation differently in clones of five tall fescue-endophyte associations (Belesky and Fedders, 1995). In general, E+ plants tended to tolerate soil acidity, but not all associations did so to the same extent, attesting to the role of genetic interactions between host and endophyte.
Fine-leafed fescues (e.g., Festuca rubra L., F. ovina L., F. longifolia Thuill.) tolerate low to intermediate soil fertility. When grown under Al-induced stress, E+ fine-leafed fescues had longer and heavier roots and heavier shoots than E- lines (Liu et al., 1996), suggesting greater Al tolerance in E+ than in E- plants. Larger root systems could be a function of plant age, rooting volume, or an expression of fine-leafed fescue morphology. Response to Al-induced stress varied among specific clonal lines and associated endophytes of fine-leafed fescues (Zaurov et al., 2001), with growth influenced by plant genotype-endophyte interaction. In some cases, shoot mass was greater in E+ than in E- plants grown on soils with high exchangeable Al, but in most instances infection did not influence shoot mass. High Al levels suppressed growth of some E+ clones. Consequently, broad generalization about endophyte infection and tolerance to Al stress could not be made.
Effects of Roots
A major advance in understanding endophyte involvement in mineral nutrition was the discovery of apparent chemical modifications in the rhizosphere and the regulatory effects that nonspecific root exudates had on uptake of certain minerals (Malinowski et al., 1998a; Malinowski and Belesky, 1999a,b; Malinowski et al., 2000, 2005b). Release of phenolic-like compounds with the ability to reduce iron (Fe3+) and chelate Al and Cu seemed to be associated with enhanced P acquisition by E+ tall fescue plants (Malinowski and Belesky, 2000). Roots of E+ (N. lolii) perennial ryegrass plants also released phenolic compounds (Zhou et al., 2003), but not much is known about involvement of these exudates in mineral uptake (Malinowski et al., 2005b).
Tolerance by Plants
Mechanisms of soil acidity tolerance in grasses are complex, particularly in E+ grasses. Low soil pH and high soluble Al concentrations are associated with restricted root growth and inhibit N uptake and N supply to growing leaves. Phenolic-like compounds can chelate Al and may help plants transport Al in nontoxic forms. Tall fescue accumulated much less Al in shoots than roots, suggesting a means to sequester Al in the roots (Foy and Murray, 1998). Phenolic-like compounds exuded from roots of E+ tall fescue appeared to be involved in Al tolerance. Aluminum was excluded from E+ plants in hydroponic conditions (Malinowski and Belesky, 1999a). More Al (47%) and P (49%) were desorbed from root surfaces of E+ than E- plants, depending on root dry matter. More Al (35%) but less P (10%) was found in root tissues of E+ plants, suggesting that Al is sequestered in roots and P translocated to shoots. Changes in nutrient solution pH occurred for hydroponically grown E+ tall fescue, especially under limited P and high Al (Malinowski and Belesky, 1999a; Malinowski et al., 1999). In control (nonfertilized) soil and soil supplied with a low amount (13 mg P kg-1) of commercial P fertilizer, soil pH increased more rapidly when associated with E+ than with E- plants (Malinowski and Belesky, 1999a).
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