ABIOTIC STRESS TOLERANCES (MOISTURE, NUTRIENTS) AND PHOTOSYNTHESIS IN ENDOPHYTE-INFECTED TALL FESCUE

The ecological significance resulting from the association of each tall fescue (Festuca arundinacea Schreb.) genotype and its companion fungal endophyte (Acremonium coenophialum Morgan-Jones and Gams) is probably inherent in the nutritional interactions, and the resulting physiological and biochemical requirements of each. The colonization of this grass by the fungus follows the natural sequence of fescue seed germination, seedling and tiller growth. Infected grasses are natural and extensive, therefore tall fescue should be considered a symbiotic plant. The nature of this relationship at the population level is more appropriately described as an obligately biotrophic conjunctive mutualism. The use of this terminology at the population level describes the overall ecological effect; however, allowances must be made for infected tall fescue genotypes within the population that may not show any positive adaptive strategies. Genotypes of this later category may be categorized, possibly only transiently, as obligately neutral symbiotic. Infected genotypes of the earlier category offer adaptations to environmental stresses and may be exploited for these characteristics. Evidence from research is reviewed to indicate that selected genotypes within the population of tall fescue are more tolerant of environmental abiotic stresses than uninfected grasses. Infected tall fescue seed require more moisture to germinate than uninfected seed. Endophyte-infected seedlings require more nutrients than uninfected seedlings. Although infected tall fescue contains less soluble nitrogen which would encourage more predation, strains resulting from the stresses of insect herbivory are prevented because of an accumulation of an insect deterrent, toxins and their synergists. At the morphological level infected grasses show tolerance to water stress by early shedding of older leaves and rolling of younger leaves. Tolerance to water stress is further evidenced by low stomatal conductances and by development of an enhanced osmoregulatory system that produces increased cellular turgor pressure. One genotype of tall fescue contains polyols, some of which are absent in the uninfected clone. Infected tall fescue also shows increased efficiency to low soil nitrogen, possibly due to an increased level of glutamine synthetase which would enhance its competitive ability under low soil nitrogen. There is no endophyte effect of consequence on photosynthesis and associated processes in several genotypes of infected grasses.