Common bermudagrass is widely distributed throughout the world between latitudes 45°N and 45°S (Anderson et al., 1993; Harlan and de Wet, 1969), and both common and triploid hybrid [Cynodon × magennisii Hurcombe (= C. dactylon × C. transvaalensis Burtt-Davy)] are used in turf systems (McCarty and Miller, 2002). Bermudagrasses grown in the transition zone of the United States are subject to freeze damage (Anderson et al., 2003; Fry, 1990; Taliaferro et al., 2004) and periodic severe winterkill (Anderson et al., 1997; Hiscock, 1996; Munshaw, 2004; Zhang et al., 2006).
Bermudagrasses undergo cold acclimation (CA), which is induced by a combination of reduced photoperiod and temperatures of less than 10 °C in the autumn. Cold-acclimated cultivars can develop metabolic defenses rapidly to cope with freezing stress (Zhang et al., 2006). Increases in certain sugars or amino acids, synthesis of novel proteins, and increases in the degree of unsaturation of membrane lipid fatty acids, antioxidant capacity, and certain hormones are some of the most important metabolic defenses against freezing stress (Cyril et al., 2002; Hughes and Dunn, 1996; Kalberer et al., 2006; Karpinski et al., 2002; Lee and Chen, 1993; Munshaw et al., 2006; Perras and Sarhan, 1989; Zhang et al., 2006; Zhang and Ervin, 2008).
Abscisic acid (ABA) plays an important role in low temperature response and is correlated with enhanced freezing tolerance in some plant species (Lee and Chen, 1993). It has been reported that endogenous ABA increases during cold acclimation and that application of ABA may induce freezing tolerance in several plant species (Lee and Chen, 1993). A mutant of arabidopsis [Arabidopsis thaliana (L.) Heynh.] deficient in ABA was unable to cold-acclimate unless treated with exogenous ABA (Heino et al., 1990). Exogenous ABA has been shown to induce dehydrin gene expression (Han and Kermode, 1996).
Dehydrin proteins (late embryogenesis abundant D11 family) are members of a family of proteins that range in size from 9 to 200 kDa. They have been shown to be expressed after plant exposure to environmental stimuli with a dehydrative component, including low temperature, drought, salinity, and developmental stages such as seed and pollen maturation (Close, 1997). Dehydrins have been postulated to stabilize cell structures against dehydration (Close, 1997; Danyluk et al., 1998). In several plant species, dehydrins have been shown to possess in vitro cryoprotective activity and in vivo antifreeze activity (Wisniewski et al., 1999). Dehydrins have also been suggested to function as possible osmoregulators (Nylander et al., 2001) or as radical scavengers (Hara et al., 2003). Several researchers have reported that dehydrin accumulation is correlated with freezing tolerance in some plant species (Marian et al., 2003; Zhu et al., 2000). Danyluk et al. (1998) noted that the WCOR410 dehydrin protein accumulated near the plasma membrane during CA of wheat (Triticum aestivum L.) and suggested that this accumulation protected integrity of the plasma membrane when plants were subjected to stress. Patton et al. (2007) found that dehydrin polypeptides of 23 and 25 kDa increased during CA and that the 23 kDa dehydrin polypeptide was associated with freezing tolerance of zoysiagrass (Zoysia Willd.).
The electrolyte leakage technique is commonly used to assess the level of cell injury caused by low temperatures and to test the relative freezing tolerance of turfgrasses (Anderson et al., 1988, 2002; Cardona et al., 1997; Fry et al., 1991; Miller and Dickens, 1996; Shashikumar and Nus, 1993; Zhang et al., 2006). The concept of lethal temperature for 50% loss of electrolytes (LT50) has been used as a measure of cold hardiness and is defined as the predicted test temperature resulting in 50% or greater loss of total electrolytes (Shashikumar and Nus, 1993). When interpreting these results, it has been assumed that an electrolyte leakage (EL) of 50% or more is lethal (Fry et al., 1993). Predicted LT50 based on EL and glasshouse regrowth evaluations have been reported to be in close agreement when bermudagrass was tested (Anderson et al., 1988; Miller and Dickens, 1996; Zhang et al., 2006).
Bermudagrass cultivars vary largely in freezing tolerance (Anderson et al., 2003; Taliaferro et al., 2004). ‘Patriot’ (2n = 4x = 36 chromosomes) and ‘Tifway’ (2n = 3x = 27 chromosomes) are vegetatively propagated hybrid cultivars, whereas ‘Riviera’ and ‘Princess’ are common seeded cultivars (2n = 4x = 36 chromosomes). ‘Patriot’ and ‘Riviera’ were top performers among all cultivars in the 2003 to 2006 National Turfgrass Evaluation Program (NTEP) bermudagrass test and have excellent cold tolerance and high turf quality ratings (NTEP, 2008; Taliaferro et al., 2004). ‘Tifway’ and ‘Princess’ are widely used cultivars with highly rated visual quality but a medium level of cold tolerance. Enhancement of cultivar freezing tolerance as a means of reducing risk of winterkill has been a major goal of many bermudagrass improvement programs (Anderson et al., 2003). However, there are few reports on the physiological mechanisms of cultivar variation in freezing tolerance. Investigations concerning the physiological bases of cultivar differences in freezing tolerance would provide valuable selection information for turfgrass breeders and practitioners, especially in the transition zone. The objectives of this study were: 1) to examine changes in the levels of endogenous ABA and dehydrin proteins during CA and to investigate relationships of ABA and dehydrins with freezing tolerance; and 2) to examine if exogenous ABA affects endogenous ABA level and dehydrin expression associated with freezing tolerance in bermudagrass.
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