PHOTOSYNTHESIS AND STOMATAL CONDUCTANCE OF SYMBIOTIC AND NONSYMBIOTIC TALL FESCUE

Desirable and undesirable agronomic characteristics of tall fescue (Festuca arundinacea Schreb.) have been attributed to infection by a nonpathogenic fungal endophyte (Acremonium coenophialum Morgan-Jones & Gams, section Alba-lanosa). It has been suggested that stomatal control of water loss is more closely regulated in symbiotic than nonsymbiotic tall fescue, a feature that may improve drought resistance. A greenhouse experiment was designed to study apparent photosynthesis, stomatal conductance, transpiration, and leaf water potential in two genotypes of symbiotic and nonsymbiotic tall fescue grown in polyethylene glycol solutions of -0.1, -0.3, -0.7, and -1.0 MPa water potential. Symbiotic plants of one genotype (CB1) had slightly higher photosynthesis, stomatal conductance, and transpiration at intermediate levels of water stress than nonsymbiotic plants. Gas exchange of the second genotype (DN7) may have been affected by endophyte infection, as photosynthetic rates of symbiotic plants grown at -1.0 MPa were slightly higher than nonsymbiotic plants. The increased photosynthesis and stomatal conductance in genotype CB1 were apparently the result of turgor maintenance in mature leaves of symbiotic plants. At moderate and low water potential, leaves of symbiotic plants had a greater turgor potential than nonsymbiotic plants. These data suggest that the endophyte of genotype CB1 does not enhance stomatal closure under water deficit stress, but induces a more favorable turgor pressure in mature leaves, which may lead to greater photosynthesis at lower water potentials.