Drought and heat stress are the two most detrimental environmental stresses for cool-season turfgrass growth in arid and warm climatic regions. Turfgrass plants develop various mechanisms in their adaptation to drought or heat stress, including changes in photosynthetic activities and regulation of water use and water uptake capacity (Beard, 1973; Fry and Huang, 2004). Generally, drought-resistant turfgrasses are able to maintain cellular hydration by closing stomata that limits transpirational water loss, increasing water uptake, and/or adjusting osmotically to maintain photosynthesis and other metabolic functions (Bonos and Murphy, 1999; Qian and Fry, 1997; White et al., 1992; Zhang and Schmidt, 1999). Heat tolerance in turfgrasses has been associated with the maintenance of transpiration and photosynthesis among various other mechanisms (Jiang and Huang, 2000).
Kentucky bluegrass (Poa pratensis L.) (KBG) is a commonly used cool-season turfgrass species for home lawns, athletic fields, and golf courses. It forms an attractive turf when temperatures are optimal (18 to 24 °C) and when supplied with adequate water, but quickly loses color, becomes thin, and goes dormant during extended heat and drought conditions (Meyer and Funk, 1989). Texas bluegrass (Poa arachnifera Torr.) (TBG) is a cool-season forage that is native to the southern United States and has good drought and heat resistance but is characterized by low turf quality and poor seed production (Gould, 1975). Interspecific hybridization between KBG and TBG has been used to develop hybrid bluegrass with improved turf quality and stress tolerance (Bonos et al., 2000; Read et al., 1999; Vinall and Hein, 1937). Some newly developed hybrids exhibited superior turf quality compared with Kentucky bluegrass genotypes under drought stress alone (Abraham et al., 2004) and under heat stress alone or in combination with drought stress compared with Kentucky bluegrass (‘Apollo’) and tall fescue (Festuca arundincea Schreb.) (‘Dynasty’) (Su et al., 2007). The mechanisms underlying the improvement in hybrid bluegrass over KBG are not well understood. Understanding differential physiological responses of hybrid bluegrass and KBG genotypes to heat and drought stress is important for the identification of stress tolerance mechanisms and developing breeding strategies to select more stress-tolerant cool-season grasses.
Therefore, the objectives of this study were to determine the relative improvement in hybrid bluegrass for more drought or heat tolerance and examine differential mechanisms of hybrid bluegrass and KBG tolerance to drought and heat stress.
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