Biosolids are valued as sources of plant nutrients, soil organic matter, and, in the case of alkaline-stabilized materials, liming agents (U.S. EPA, 2007; Zhang et al., 2009). The addition of organic materials enhances the biological, physical, and chemical properties of the soil with yield improvements usually being associated with increased nutrient availability (Akrivos et al., 2000; Boquet et al., 1999; Bugbee, 2002; Gilmour et al., 2003; Tester, 1989) and, sometimes, to improved soil physical properties (Klock-Moore, 2000). Recent evidence indicates that biosolids may contain BAS that enable crops to withstand environmental stresses (e.g., drought, salinity, pathogens) and/or positively affect crop growth and quality (Subler et al., 1998; Zhang et al., 2005, 2007, 2009).
Several groups of BAS (e.g., humic substances, amino acids, vitamins, hormones) have been identified and isolated from biosolids (Lemmer and Nitschke, 1994; Sanchez-Monedero et al., 1999; Zhang et al., 2005). The presence of BAS in biosolids may enhance crop production by providing plant growth regulators directly or by stimulating the activity of microbes that supply substrates and hormones. Auxin, an important phytohormone, has been shown to improve root growth and stress tolerance in turfgrasses (Zhang et al., 2009). It was found that biosolids contained auxin [indole-3-acetic acid (IAA)] at physiologically active levels (Zhang et al., 2009). Microbial production of IAA from tryptophan has been shown in a number of cases (Arshad and Frankenberger, 1993; Barea et al., 1976; Lebuhn et al., 2007). In addition, Zhang et al. (2005) measured IAA contents of the humic acid fraction of variously processed biosolids to range from 0.5 to 2.4 μg·g−1. Plant hormones, particularly IAA in the biosolids, may account for some of the beneficial effects previously reported on plant growth and physiological fitness.
Environmental stresses such as drought may cause damage to plant cells through production of excess reactive oxygen species (ROS) such as singlet oxygen (1O2), superoxide radical (O2–.), hydrogen peroxide (H2O2), and hydroxyl radical (OH·) (Zhang and Ervin, 2004). During drought stress, an abscisic acid signal causes stomatal closure and light-exposed, over-reduced photosynthetic systems may experience oxidative stress and overproduction of ROS. Excess ROS may cause severe damage to important cellular components such as proteins, lipids, and nucleic acids, leading to cell death. Plants have evolved a complex antioxidant defense system (i.e., enzymatic and non-enzymatic detoxification systems) for protecting cells from ROS injury.
SODs are metalloenzymes that convert O2–. to H2O2 and are considered the “primary defense” against ROS (Perl-Treves and Perl, 2002; Zhang and Schmidt, 1999, 2000). In this system, H2O2 is further reduced to water by CAT and APX. Catalase, localized in peroxisomes, scavenges H2O2 produced by glycolate oxidase in the C2 photorespiratory cycle (Perl-Treves and Perl, 2002). POD is also an important antioxidant enzyme for scavenging ROS. There is a complex signaling network governing antioxidant defense with plant hormones playing an important role in signaling plant defense responses (Strivastava, 2002).
Tall fescue, a cool-season turfgrass, is widely used for home lawns, recreational surfaces, and roadsides in the temperate to semitropical United States and experiences frequent summer drought stress. Little previous research has been reported on the impact of biosolids on antioxidant metabolism associated with drought tolerance in tall fescue. The objectives of this study were to investigate effects of biosolid applications on drought tolerance associated with antioxidant metabolism and root growth under differential soil moisture availability.
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