Presence of the endophytic; fungus Acremonium coenophialum Morgan-Jones and Gams in tall fescue (Festuca artuidinacea- Sehreb.) enhances host persistence in drought-prone environments. However, the physiological mechanism is not well understood. We investigated the influence of endophyte infection and water deficit on osmotic adjustment, stomatal conductance, tiller survival and leaf elongation rate of genotype GA87-122 and cv. Kentucky-31 (KY31) of tall fescue. Plants were grown in a greenhouse in pots containing fine-silty topsoil (experiment 1) or sand (experiment 2). In expt 1, endophyte-infected (EI) and endophyte-free (EF) isolines of GA87-122 were exposed to two drought preconditioning cycles, after which all pots were re-watered and osmotic adjustment determined. Osmotic adjustment was −0.46 and −0.51 MPa in El leaf blades and tiller bases and −0.29 and −0.13 MPa in EF leaf blades and tiller bases, respectively. In expt 2, EI and EF GA87-122 and KY31 Mere exposed to severe drought after two preconditioning cycles, then re-watered. During the second preconditioning drying cycle of KY31, stomatal conductance tended to be lower in EI than in EF plants, but the difference was significant (P 0–05) only at two sampling times. Shoot tissues osmotically adjusted 0.17–0.31 MPa more in EI than in EF plants. Tiller survival and leaf elongation rates were higher (P 0.05) in preconditioned EI than in EF plants. Basal-zone osmotic adjustment was correlated with tiller survival rate in GA87-122 and KY31 (r = 0.87, P 0.01 for both). Enhanced osmotic adjustment in the meristematic and growing zone might account far improved survival of tillers by facilitating protection of the apical meristem.
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