Roots of fine-leaf fescue help them outcompete other plants because they release growth-inhibiting chemicals that mimic nutrients.

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Fescue species release a "fake" amino acid into the surrounding soil. Weeds take it up thinking it is a nutrient when it is really empty (i.e., it's a mimic), and thus the fescue can outcompete the weeds and reduce germination of weed seeds.

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"Because of their stress tolerance and disease resistance, fescue (Festuca spp.) grasses are commonly used in landscape, roadside, and pasture settings, as well as for conservation purposes (4, 5). The unusual ability of many fine leaf fescue species to outcompete other plants is well known, and previous investigations suggested that fescue root exudates have phytotoxic properties (4). Here, we report the isolation, identification, and biological activity of m-tyrosine, a potent, structurally unusual broadspectrum phytotoxin exuded by the roots of some fine leaf fescue grasses." (Bertin et al. 2007:16964)

"We have shown in recent studies that m-tyrosine, an analogue non-protein amino acid, is exuded by the living root system of certain fine leaf fescue species. Interestingly, it possesses potent weed suppressive activity and appears to impact plant growth mainly from initial inhibition of plant root growth or radicle elongation at very low concentrations." (Bertin et al. 2009:1289)

Journal article
Grass roots chemistry: meta-Tyrosine, an herbicidal nonprotein amino acidProceedings of the National Academy of SciencesOctober 12, 2007
C. Bertin, L. A. Weston, T. Huang, G. Jander, T. Owens, J. Meinwald, F. C. Schroeder

Journal article
Assessment of the Phytotoxic Potential of m-Tyrosine in Laboratory Soil BioassaysJ Chem EcolNovember 18, 2009
Cecile Bertin, Roselee Harmon, Mia Akaogi, Jeffrey D. Weidenhamer, Leslie A. Weston

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