The amount and type of light reaching a plant canopy influences the ability of a plant to acquire water and nutrients, allocate dry matter, and carry on energy-dependent metabolic processes. In contrast with our current knowledge of the influence of endophyte and water-related stress on the host, there is little information on the effect of endophyte infection and the response of the host plant to light quantity and quality. Belesky and Malinowski (1999) considered how light affects host-endophyte responses to abiotic stress. Light influences allocation of photosynthate among plant structures and regulates the expression of tillers. Since E+ plants appear to have greater rates of tillering, interacting effects of light quality and endophyte infection could influence this process and ultimately the plant's competitive ability in a range of canopy environments.

Uncut canopies of Jesup tall fescue infected with a native endophyte produced more pyrrolizidine alkaloids in full sun than in plants grown in shaded (either ~35 or 80% of full sunlight) conditions (Belesky et al., 2009). Shade increased the pyrrolizidine alkaloid concentration when the same host-endophyte association was clipped to either a 5- or a 10-cm residue. When Jesup was infected with AR542, pyrrolizidine alkaloid concentrations were less that those occurring in the Jesup-native endophyte association, but did increase in concentration with decreasing light, irrespective of how the canopy was managed. Jesup without endophyte was devoid of pyrrolizidine alkaloid. Ergot alkaloids were present in Jesup infected by the native endophyte, but were absent from Jesup devoid of endophyte or host to AR542. Shading decreased the concentration of ergot alkaloid when canopies were not cut, but had minimal effect when cut to a 5- or 10-cm residue (Belesky, unpublished data, 2005). The antioxidants represented by total phenols decreased in leaf blades as light availability decreased, and Jesup host to a native endophyte produced greater concentrations of phenols than did either Jesup devoid of endophyte or Jesup host to AR542 (Belesky, unpublished data, 2005). These observations provide important clues for managing silvopasture or home lawns where trees are common features of the landscape. Endophyte infected plants provide a beneficial mechanism of natural herbivore control by eliciting alkaloid production. Shading seems to increase alkaloid production as well, so that home site landscaping that includes both trees and turf might have complementary influences on plant persistence by deterring insects that consume turf plant species.

Herbage production or plant size is linked to competitiveness of plants (Harper, 1977). Dense swards influence light quality by absorbing more of certain light wavelengths within the canopy. The altered light environment leads to different amounts and quality of light, which can influence tiller production, leaf elongation, and phytomass. Changes in allocation affect the amount of photosynthate available for growth, storage, or use in secondary metabolite production, as well as nutrient and water acquisition. High temperatures and high light intensities are likely to occur simultaneously, especially in southern portions of the tall fescue adaptation zone (Fig. 4-1). Ability to tolerate water deficit and maintain photosynthetic capability at high leaf temperature and light intensity is a competitive advantage for cool-temperate origin species in the southeastern United States. This ability contributes to the versatility of tall fescue as a forage resource and provides pasture managers with plant material of a high quality for use in areas dominated by warm-season species.

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