Plants under water deficit have a limited capacity for carbon fixation. Continued light absorption under drought stress may result in excessive energy that cannot be used for carbon fixation but may cause reduction of molecular oxygen, generating active oxygen species (AOS) such as singlet oxygen (1O2), superoxide (O2 .−), hydrogen peroxide (H2O2), and hydroxyl radicals (OH·) (Asada, 1999). These species can cause oxidative damage to lipids, nucleic acids, and proteins (Smirnoff, 1993). The production of AOS is not limited to drought stress and also occurs as a result of extremes in temperature (Mishra and Singhal, 1992; Schoner and Krause, 1990), high irradiance levels (Halliwell and Gutteridge, 1989a), disease (Apostol et al., 1989), and nutrient deficiency (Cakmak and Marschner, 1988).
Plants have developed an antioxidant defense system in response to the generation of AOS within plant tissues. The antioxidant defense system is comprised of enzymatic and nonenzymatic components that can be divided into two different types of repair mechanisms: 1) production of antioxidants or antioxidant enzymes that directly react with and scavenge AOS, including superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), peroxidase (POD), and α tocopherol; and 2) production of enzymes that regenerate oxidized antioxidants such as glutathione, glutathione reductase, ascorbate, and ascorbate reductase (Smirnoff, 1993). Accumulation of AOS results in increased antioxidant enzyme activities, providing indirect evidence for the extent of generation of AOS and the importance of these enzymes in scavenging free radicals (Smirnoff, 1993).
Drought stress resistance has been associated with either maintenance or increase in antioxidant enzyme activity levels in various plant species (Fu and Huang, 2001; Price and Hendry, 1989; Tanaka et al., 1990; Zhang and Schmidt, 1999). Antioxidant enzyme activity levels have been positively related to drought resistance in some species (Bowler et al., 1992; Jagtap and Bhargava, 1995; Lascano et al., 2001; Price and Hendry, 1989). Levels of SOD and CAT were maintained for a greater duration of combined drought and heat stress for a drought-tolerant cultivar of kentucky bluegrass (Poa pratensis L.) compared with a drought-susceptible cultivar (Wang and Huang, 2004). Zhang and Schmidt (1999) also found that kentucky bluegrass plants exhibiting higher levels of antioxidant enzyme activities maintained better turf quality under drought stress.
Under conditions of severe plant stress, the production of AOS may exceed the scavenging capacity of the antioxidant defense system (van Breusegem et al., 1998). As a consequence, AOS can accumulate and cause cellular damage such as lipid peroxidation or the oxidation of phospholipids and other unsaturated lipids. Peroxidation results in the breakdown of lipids and membrane function by causing loss of fluidity, lipid crosslinking, and inactivation of membrane enzymes (Girotti, 1990). The extent of lipid peroxidation can be calculated by measurement of malondialdehyde (MDA) content, which is a secondary breakdown product of lipid peroxidation (Halliwell and Gutteridge, 1989b). MDA content is a commonly used measurement for assessing lipid peroxidation and oxidative damage in both leaves and roots (Queiroz et al., 1998; Zhou and Zhao, 2004), and its maintenance of low levels has been associated with increased drought stress resistance in many plant species (Lima et al., 2002; Moran et al., 1994; Sairam et al., 1998; Zhang and Kirkham, 1994).
Identifying and understanding the function of antioxidant defense mechanisms are important for developing drought-tolerant plants. Our previous studies have found that velvet bentgrass is more tolerant to drought stress than creeping bentgrass or colonial bentgrass as exhibited by higher turfgrass quality, leaf water content, and osmotic adjustment under drought stress (DaCosta and Huang, 2006a, 2006b). Whether bentgrass species variation in drought resistance is also associated with differences in antioxidant mechanisms is not well understood. The objectives of this study were to determine whether bentgrass species variation in drought resistance could be associated with differences in antioxidant enzyme levels in response to drought stress.
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