Drought is one of the most detrimental abiotic stresses for turfgrass growth across a wide range of geographic locations. Most cool-season grass species are not well adapted to extended periods of drought, particularly during the summer months. Decline in turf quality caused by drought stress is a major concern in turfgrass culture. Therefore, developing management practices for improving drought resistance of turfgrasses has become imperative in arid and semiarid regions, especially during periods of water use restriction. One strategy to improve plant drought resistance is to promote drought avoidance by reducing water loss during drought, which may be achieved by slowing the growth rate of shoots and lowering the canopy leaf area to reduce the demand for water (Nilsen and Orcutt, 1996). Another mechanism serving to increase drought tolerance is through osmotic adjustment, which allows plants to maintain leaf cellular hydration and sustain metabolic activities during drought (Nilsen and Orcutt, 1996).
Previous studies have reported that plants with slow-growing shoots may survive more extended periods of drought than faster-growing plants (Kondoh et al., 2006; O'Reagan et al., 1993; Simane et al., 1993). Slow growth may reduce the adverse impact of drought by conserving water and carbon energy, such that plants can use limited water to survive drought for an extended period of time (Kang, 2002). Plant growth regulators such as trinexapac-ethyl (TE) are traditionally used to suppress vertical shoot growth for reducing mowing frequency of turfgrasses (Turgeon, 1999). TE blocks the final step in the biosynthesis pathway of the biologically active forms of gibberellins, results in slower vertical shoot growth (King et al., 1997), enhances superoxide dismutase and photochemical activity (Zhang and Schmidt, 2000), and has no negative impact on root growth (Fagerness and Yelverton, 2001). Reduction in vertical shoot growth may reduce the demand for water, and thus, may reduce the water requirement for plant survival in water-limiting conditions for a prolonged period of time. Jiang and Fry (1998) have shown that foliar TE treatments increased turf quality of perennial ryegrass (Lolium perenne L.) during soil dry-down. In our previous work, exogenous application of TE before plant exposure to stress significantly improved growth and physiological activities of creeping bentgrass subjected to combined heat and drought for 21 d (McCann and Huang, 2007).
Abscisic acid (ABA) is a plant hormone and growth regulator known to be involved in plant adaptation to drought stress. Exogenous application of ABA has been reported to improve drought tolerance in various plant species such as maize (Zea mays L.; Bochicchio et al., 1991), pepper (Capsicum annuum L.; Leskovar and Cantliffe, 1992), old jack pine (Pinus banksiana L.; Rajasekaran and Blake, 1999), and Tradescantia virginiana L. (Franks and Farquhar, 2001). Foliar application of ABA improved the growth of Kentucky bluegrass (Wang et al., 2003) and tall fescue (Festuca arundinacea Schreb.; Huang and Jiang, 2002) under drought stress. ABA-induced plant tolerance to water deficit has been associated with changes in various physiological processes, including inhibition of leaf growth or transpirational area for water loss (Alves and Setter, 2000; Bacon et al., 1998), induction of stomatal closure (Kirkham, 1983; Wilkinson and Davies, 2002), and enhancement of osmotic adjustment (Kirkham, 1983; LaRosa et al., 1987).
Although there is evidence that TE or ABA application may promote drought tolerance of turfgrass plants (Huang and Jiang, 2002; Jiang and Fry, 1998; McCann and Huang, 2007; Wang et al., 2003), the information on how TE and ABA may regulate turfgrass responses to drought stress is still limited. In addition, the relative effects of TE and ABA on drought tolerance for different turfgrass species are not well documented. We have hypothesized that treatment of turfgrass plants with TE or ABA may allow plants to survive a prolonged period of drought stress with greater tolerance than controls by regulating shoot growth and water relations. Therefore, the objectives of this study were to investigate the effects of exogenous application of TE and ABA on the responses of two cool-season turfgrass species, Kentucky bluegrass and creeping bentgrass, to drought stress, and to examine changes in water relations associated with improved drought tolerance from TE or ABA treatment.
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