Bermudagrass is widely used in the turf systems and is equally relevant in animal husbandry because of its high protein content and relatively low cost. Bermudagrass is a typical warm-season grass, which grows best under air temperature ranging from 29.4 to 37.8 °C and soil temperature ranging from 23.9 to 35 °C. The minimum air temperature required for growth is 12.8 °C (Zhou, 1996). Therefore, below optimum temperature is a significant factor that may limit resource use in bermudagrass. Low temperature can influence growth, development, and yield of botanical species (Zhu et al., 2007). Hughes and Dunn (1996) reported that when exposed to low but not freezing temperature, plants can obtain chilling and freezing tolerance (cold acclimation) (Hughes and Dunn, 1996). Some studies have reported that cold stress led to biochemical and physical changes in plants, particularly causing freezing injury, which was accounted for by the damage on the plasma membrane. Other studies found that low temperature damaged cell membrane systems and lipid peroxidation by decreasing the fluidity of cell membranes of most plants (Levitt, 1980).
Malondialdehyde (MDA) and electrolyte leakage (EL) served as indicators of lipid peroxidation. Södergren (2000) reported that MDA was higher under low-temperature conditions (Södergren, 2000). In Forsythia species treated at 4 °C, MDA contents increased by ≈66.7% (Yan et al., 2010). Similarly, MDA levels were higher in wheat (Triticum aestivum) seedlings and strawberry (Fragaria ananassa) leaves subjected to 4 and 0 °C, respectively (Hou et al., 2010; Luo et al., 2011). Electrical conductivity is widely applied to detect membrane injury caused by various biotic and abiotic stresses in plants (Whitlow et al., 1992). EL declined in bermudagrass cultivars after 8 °C day temperature and 4 °C night temperature cold acclimation (Zhang et al., 2006). EL of leaves significantly increased by 55% and 26.3% in naked oats (Avena nuda) treated by –10 and 1 °C, respectively (Liu et al., 2013), indicating that low temperature could affect cell membrane penetrability.
Sufficient evidence suggests that extremely low temperature can induce oxidative stress through the generation of reactive oxygen species (ROS) in plants (Yan et al., 2010). Reactive oxygen species damage many vital cellular components such as lipids, proteins, and DNA (Södergren, 2000). Reactive oxygen species could be scavenged by enzymatic detoxification mechanisms consisting of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) (Asada, 2006; Gupta et al., 1993; Kono and Fridovich, 1982). SOD including copper/zinc (Cu/ZnSOD), manganese (MnSOD), and iron (FeSOD) catalyzes the superoxide free radical ion to H2O2 according to the equation: 2O2– + 2H+ → H2O2 + O2.
Superoxide dismutase content increased during the first 7 d and then declined, whereas CAT and ascorbate peroxidase (APX) activity decreased when bermudagrass was subjected to 8/4 °C (day/night) temperature (Zhang et al., 2006). SOD activity increased dramatically in Euonymus radicans (Guo et al., 2004) and Citrus species (C. unshiu, C. sinensis, C. limon) by prolonged exposure to cold treatment (Mohammadian et al., 2012). POD is also an essential enzyme in plant response to abiotic stress by reducing hydrogen peroxides through energy transfer from reactive peroxides to glutathione. Rice (Oryza sativa) treated with 6 °C temperature had a 56% to 72% higher level of POD activity than the control (Wang and Cai, 2011). APX exists as an isoenzyme and is vital for catalyzing of H2O2 reduction in higher plants. APX activity in strawberry leaves was also elevated after low temperature (0 °C) for 72 h (Luo et al., 2011). Another crucial enzyme necessary for H2O2 decomposition in plant tissues is CAT (Brian et al., 1984). Luo et al. (2011) reported that CAT activity increased in strawberry (Luo et al., 2011). However, APX subjected to 3 °C equaled that of the control where leaves were exposed to 15 °C in the dark for 24 h in cucumber (Cucumis sativus) cultivars (Shen et al., 1999). These observations suggested that CAT response might depend on species, cold duration, and other undocumented factors. Previous research suggests that oxidative stress as indicated by lipid peroxidation may occur when ROS production overwhelms the active oxygen species scavenging (Mittler, 2002).
Alterations in enzymatic detoxification mechanisms correlated positively with the expression of corresponding genes (Levitt, 1980). Elevated cold tolerance can be accompanied by the increase in expression of specific genes encoding antioxidant enzymes (Baek and Skinner, 2003). Kayihan et al. (2012) investigated the effects of cold acclimation and freezing on Cu/Zn superoxide dismutase activity and respective gene expression in barley (Hordeum vulgare) cultivars and found that leaf Cu/ZnSOD expression levels were unchanged during cold acclimation but increased evidently at –3 °C freezing stress. Seedling leaves of manioc (Jatropha curcas) subjected to chilling shock exhibited a higher level of SOD, APX, CAT, and glutathione reductase than the control (Ao et al., 2013).
Most studies on the effects of cold on bermudagrass have focused on growth and antioxidant enzyme activities. Therefore, there was insufficient information on the combined mechanism of antioxidant enzyme activity and gene expression levels. A chitinase gene of bermudagrass CynCHT1 was reported in revolving freezing tolerance (Anderson et al., 2005). This implies that studies on mechanisms of response to cold stress in bermudagrass are limited; therefore, more investigation is mandatory. Consequently, the mechanism of antioxidant metabolism contribution to bermudagrass adaptation to low temperature is undocumented. Therefore, the comprehension of antioxidative metabolic responses to cold will improve our understanding of the contribution of biochemical mechanisms under low temperature to resistance in bermudagrass. Thus, the objective of this study was to investigate the physiological and molecular alteration in wild bermudagrass under cold stress, particularly the changes of transpiration rate, soluble sugar content, enzyme activities, and expression of antioxidant genes.
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