Plants have developed enzymatic and nonenzymatic antioxidant mechanisms to prevent oxidation of cellular compartments. Enhancing these mechanisms might help plants cope with encountered stresses. Greenhouse and field studies were conducted to examine the influence of seaweed (Ascophyllum nodosum) extract on antioxidant enzymes activities, forage Growth, and persistence of tall fescue (Festuca arundinacea Schreb.). Furthermore. effects of soil moisture, plant genotype, and infection with the endophyte Neotyphodium coenophialum ([Morgan-Jones and Gams] Glenn, Bacon and Hanlin) were investigated. In a greenhouse experiment, seaweed extract was applied to ‘Martin’ tall fescue at 0, 2, 4, 6, 8, and 10 kg ha in a randomized block design with four replicates. Seaweed extract linearly increased ( P 0.05) glutathione reductase activity. Superoxide dismutase and ascorbate peroxidase were also increased but responses differed by time and treatment rates. In a second greenhouse experiment, seaweed extract was applied at 4 kg ha to endophyte-infected and non-infected ‘Georgia Jessup’ and ‘KY-31’ tall fescue grown with 50-100% and 30-100% field capacity soil moisture in a completely randomized design with four replications. Glutathione reductase activity increased (P 0.05) in both genotypes in response to seaweed extract and moisture stress and tended to increase ( P 0.07) in response to the endophyte. Seaweed extract increased (P 0.05) superoxide.

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