Waterlogging is a common environmental stress that limits plant growth and development. Under waterlogging conditions, soil pores are filled with water and hypoxia often occurs, thereby hindering the exchange of O2 and other gases between roots and the atmospheric environment (Armstrong, 1979). Oxygen deficiency is one of the primary root stresses in the waterlogged or flooded soils (Kozlowski, 1984). As a result, plant metabolism at various growth stages can be significantly affected due to lack of oxygen supply in the environment (Colmer and Voesenek, 2009).
In a water excess environment, one of the fundamental physiological alterations is waterlogged or flood-induced anaerobic respiratory pathways. In the absence of oxygen, roots rely on anaerobic respiration pathways to produce limited energy to maintain metabolic activity (Bailey-Serres and Voesenek, 2008). Thus, adaptation of roots to flooding stress is critical to whole-plant survival. Compared with flood sensitive species, flood tolerant species are better able to regulate their processes of glycolysis and fermentation to ethanol (Drew, 1997). However, depending on the species and duration of stress, alterations of anaerobic enzyme activity are not always consistent with waterlogging tolerance. The increased alcohol dehydrogenase (ADH) or lactate dehydrogenase (LDH) under waterlogging can be found in the tolerant and sensitive cultivars of a plant species (Kato-Noguchi and Morokuma, 2007; Wei et al., 2013; Yin et al., 2009, 2010). ADH and LDH activities were enhanced in the roots of a flood tolerant cultivar of sorghum (Sorghum bicolor) during 72 h of flooding but a transient increase in the activities was found in the roots of a flood sensitive cultivar up to 24 h flooding followed by a decline in activities of these enzymes (Jain et al., 2010). In addition, plant species vary in their selection of anaerobic respiration pathways for survival of flood stress. Flood tolerant Dendranthema zawadskii showed higher root ADH activities in favor of ethanol fermentation, whereas flood sensitive Dendranthema nankingense preferred lactic acid fermentation as a main way of anaerobic respiration (Yin et al., 2010). The ethanol produced by anaerobic respiration is composed of neutral molecules that can diffuse and may have a smaller negative effect on plants (Kato, 2000). Moreover, lactic acid fermentation produces high amounts of lactic acid, leading to cytoplasmic acidification, which may not be a long-term survival strategy of plants under stress. Collectively, responses of anaerobic enzymes to waterlogging stress can be influenced by the plant species, duration, and intensity of the stress. Their roles in waterlogging tolerance are not fully understood, especially for perennial grass species exposed to a relatively longer period of stress.
Waterlogging stress can block the chloroplast and mitochondrial electron transport chain inside the plant cell and cause decreases in cell energy charge and increases in the reducing power. As a result, production of active oxygen species (ROS) such as superoxide (O2−) and hydrogen peroxide (H2O2), hydroxyl radical (OH−), and singlet oxygen is enhanced, which can break cell homeostasis and is detrimental to the plant cell (Mittler, 2002). Plants have evolved defense systems to protect cells against oxidative injury by removing, decomposing or scavenging ROS. Antioxidant metabolisms play an important role in detoxification of ROS (Mittler, 2002). By de novo sequencing, assembly, and analysis of the roots and shoots transcriptome in response to short-term waterlogging, Qi et al. (2014) concluded that ROS detoxification and energy maintenance were the primary coping mechanisms of ‘Zhongshansa’, a hybrid of baldcypress (Taxodium distichum) and montezuma cypress (Taxodium mucronatum) in surviving waterlogging. In enzymatic defense systems, SOD plays a central role in catalyzing the dismutation of O2·- to H2O2 and O2 (Bowler et al., 1992). In creeping bentgrass (Agrostis stolonifera), SOD activities in the roots increased 83% and 44% in the waterlogging tolerant ‘PennG-6’ and increased 32% and 26% for intolerant ‘Penncross’ when waterlogging occurred at 15 and 1 cm below soil level, respectively (Wang and Jiang, 2007). The decreased and unchanged SOD activities as well as differential responses of other antioxidant enzymes such as catalase (CAT), POD, and ascorbate peroxidase (APX) to waterlogging or submergence have also been found in different plant species (Ahmed et al., 2002; Arbona et al., 2008; Lin et al., 2004; Tan et al., 2010; Wang and Jiang, 2007). In addition, significant correlations between SOD and APX and between SOD and CAT were observed in the waterlogging tolerant citrus genotype ‘Carrizo’ citrange (Poncirus trifoliata × Citrus sinensis), supporting the idea of synergistic action in the positive antioxidant response (Arbona et al., 2008). Similar to anaerobic metabolism, differential responses of antioxidant enzymes to waterlogging vary with species and stress duration and intensity. The enhanced activity of antioxidant enzymes during waterlogging stress could contribute to waterlogging tolerance in the tolerant cultivar; however, responses of antioxidant enzymes to waterlogging tolerance are not fully understood, especially for warm-season turfgrasses.
Turfgrasses are often subjected to water excess environments due to frequent, heavy rainfall or over-irrigation followed by slow drainage. Although growth and physiological responses to waterlogging or submergence have been investigated in cool-season turfgrass species (Jiang and Wang, 2006; Wang and Jiang, 2007; Yu et al., 2012), alterations in growth and anaerobic and antioxidant enzymatic activities are not well understood in perennial grasses under waterlogging stress, especially for warm-season turf species under an extended period of waterlogging. Therefore, the specific objectives of this study were to determine growth response of four warm-season turfgrass species exposed to waterlogging stress and to examine anaerobic and antioxidant metabolism in relation to waterlogging tolerance. The outcome of this study would provide a basis for selecting appropriate species for turfgrass sites adjacent to flood plains or for vegetation restoration.
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