Drought stress is a major factor limiting turfgrass growth in semiarid and arid regions across the world. It becomes more important during the summer months when evapotranspiration demand is increased. Radiation-induced mutations have been used to create genetic diversity and to improve plant tolerance to biotic and abiotic stresses in various major seed-propagated crops (Ahloowalia and Maluszynski, 2001). Radiation-induced mutations have also been used in generating forage grasses and turfgrasses with alterations in various morphological traits. The most frequent mutant character from the irradiated plants is dwarfism. Gamma irradiation has been used to develop the dwarf-type bermudagrass cultivars TifEagle, Tift 94, and Tifway II (Burton, 1985; Hanna and Elsner, 1999; Hanna et al., 1997). Gamma-ray irradiation has also been used in the breeding of centipedegrass [Eremochloa ophiuroides (Munro) Hack.] and st. augustinegrass [Stenotaphrum secundatum S. (Walt.) Kuntze] (Busey, 1980; Dickens et al., 1981). Mutants of st. augustinegrass induced by gamma irradiation exhibited considerable growth retardation (Busey, 1980).
Recent studies with velvet bentgrass (Agrostis canina L.), colonial bentgrass (Agrostis capillaris L.), and creeping bentgrass (Agrostis stolonifera L.) indicated that slow-growing turfgrass may be able to survive drought stress for prolonged periods of time. This stress was associated with reduced water use and increased osmotic adjustment and antioxidant activity (DaCosta and Huang, 2006, 2007; McCann and Huang, 2008). However, studies with tall fescue (Festuca arundinacea Schreb) found that dwarf-type tall fescue had lower drought resistance than the forage-type (‘Kentucky-31’) (Carrow, 1996; Huang and Gao, 1999; White et al., 1993). These studies have revealed interspecific and intraspecific variation in drought resistance in relation to turfgrass growth.
Bermudagrass is a warm-season grass widely used for turfgrass on home lawns, sports fields, and golf courses in warm climatic regions. Bermudagrass grows most rapidly during the summer months. Rapid shoot growth increases demand on water, and therefore, frequent irrigation may be needed to maintain high-quality bermudagrass in areas with limited rainfall. In addition, bermudagrass requires frequent mowing during its active growing season. Due to limited water resources and increased energy costs, there is increasing demand for slow-growing turf with low irrigation requirements.
We developed several dwarf mutants lines of bermudagrass using gamma-ray irradiation. These are excellent germplasms for the investigation of whether dwarf turfgrass may exhibit improved drought resistance and for the analysis of physiological traits associated with drought resistance in dwarf bermudagrass. Previous studies in bermudagrass have examined cultivar variations in their overall turf performance in responses to irrigation schedule and drought stress, but differences in drought resistance between dwarf-type cultivars and common bermudagrass were not specifically evaluated (Baldwin et al., 2006; Jiang and Carrow, 2007; Lu et al., 2006). However, physiological mechanisms of bermudagrass tolerance to drought stress are not well understood. Drought tolerance involves various physiological responses, and antioxidative protection plays an important role in enhanced drought resistance in various plant species (Zhang and Kirkham, 1994; Noctor and Foyer, 1998). Reactive oxygen species (ROS) are induced during drought, which can cause cellular damage; however, plants develop enzymatic and nonenzymatic antioxidant activities to scavenge the ROS and protect cells from oxidative stress (Noctor and Foyer, 1998). The antioxidant defense system consists of several antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX), and some nonenzyme antioxidants such as ascorbic acid (AsA) and glutathione (GSH), which protect plants from the deleterious effects of ROS that are produced when plants are exposed to abiotic stress (Noctor and Foyer, 1998).
The objectives of this study were to examine the morphological traits of radiation-induced dwarf mutants of bermudagrass and to determine water relations and antioxidant responses to drought stress associated with drought resistance for the dwarf bermudagrass mutants.
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