Water use efficiency (WUE) refers to the amount of new biomass produced per unit of water transpired by a plant. The less water used, the better a plant is capable of tolerating drought. In addition, if plants are capable of surviving on less water, more water will be retained in the soil profile. Plants that develop extensive, deep root systems and uniform sods, like tall fescue, also enhance water conservation by improving infiltration (Nelson, 1996). This is accomplished by slowing runoff and facilitating water infiltration through root channels.
Tall fescue has been shown to be better adapted to drought conditions than a number of forage and turf species in a variety of water-deficit environments. Out of 12 species tested, Lauriault et al. (2005) found that only tall fescue, Russian wildrye [Psathyrostachys juncea (Fisch.) Nevski], Russian wheatgrass [Thinopyrum junceiforme (A. Löve & D. Löve) A. Löve], tall wheatgrass [Thinopyrum ponticum (Podp.) Barkworth & D. R. Dewey], and western wheatgrass [Pascopyrum smithii (Rydb.) Barkworth & D. R. Dewey] were able to establish and maintain ground cover in New Mexico under three soil-water regimes. The other species succumbed to drought stress and did not develop adequate ground cover. A large portion of the ability of tall fescue to survive under drought stress has been attributed to its deep root system. In a comparison of drought tolerance mechanisms in turfgrasses, tall fescue produced 65% more root mass in the 31- to 90-cm depth of the soil profile than did Kentucky bluegrass (Poa pratensis L.) (Ervin and Koski, 1998). This allowed tall fescue to maintain better cover and turf quality during periods of water deficit by extracting water from deep in the soil profile. There is wide genetic variability in tall fescue for rooting depth that can be exploited to enhance drought tolerance (Bonos et al., 2004).
In addition to genetic variability for drought avoidance characteristics, West et al. (1993) showed that the presence of the fungal endophyte Neotyphodium coenophialum (Morgan-Jones and Gams) Glenn, Bacon, and Hanlin improved the survival of tall fescue in drought conditions. The involvement of the fungal endophyte in improved WUE in tall fescue has been noted in several studies. The endophyte has been shown to increase abscisic acid levels in tall fescue plants resulting in stomatal closure and conservation of leaf water under drought stress (Joost, 1994). Further evidence of the role of the endophyte in water conservation in tall fescue plants was provided by Elmi and West (1995), who observed that stomatal conductance decreased more rapidly under water deficit in endophyte infected (E+) than in endophyte free (E-) plants. The ability of tall fescue to survive and remain productive under low soil water availability is due to a number of characteristics. The deep rooting and optimization of water use through stomatal control are critical to the high WUE of tall fescue. Efficient water use is very important to the conservation of limited water resources. In addition to the conservation of limited water through improved infiltration and low water use, the maintenance of water quality by filtering out nutrients and sediments is essential.
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