Waterlogging is defined as the saturation of the soil with water around the roots (Anton et al., 2002). Excess water in the root environment blocks the transfer of oxygen and other gases between the soil and the atmosphere; as a result, root injury occurs as a result of oxygen deficiency. Oxygen deficiency is one of the primary root stresses in the waterlogged or flooded soils (Kozlowski, 1984). Short-term or even transient waterlogging could have adverse effects on the growth of dry land plants (Condona and Giunta, 2003). Also, the intensity of stress increases during long-term waterlogging of up to weeks or months (Naidoo et al., 1992). Plant tolerance to waterlogging or flooding can vary from only several hours to days or weeks depending on the plant species.
Waterlogging stress and soil saturation happen in turfgrass as a result of high precipitation, poor soil quality, or overirrigation followed by slow drainage. The reduced soil oxygen availability under waterlogging decreased the turf quality, photosynthetic rate, and chlorophyll and carbohydrate concentration of turfgrass (Huang et al., 1998; Jiang and Wang, 2006). Although perennial grasses are considered to be tolerant to a shortage of oxygen induced by waterlogging or flooding (Jones and Etherington, 1970), large variations in waterlogging tolerance occur in different turfgrass species and cultivars. Creeping bentgrass (Agrostis stolonifera L.) is relatively tolerant to waterlogging-induced hypoxia. However, creeping red fescue (Festuca rubra L.) is sensitive, and orchard grass (Dactylis glomerata L.) is unusually sensitive to waterlogging (Cooper, 1982; Gray and Scott, 1977). Fry (1991) reported that bahiagrass (Paspalum notatum Flügge) and bermudagrass (Cynodon dactylon L.) had higher shoot survival after submersion than did St. Augustinegrass [Stenotaphrum secundatum (Walt.) Kuntze] and zoysaigrass (Zoysia japonica Steud.), whereas centipedegrass [Eremochla ophiuroides (Munro) Hack.] showed no survival under the same stress condition. Within creeping bentgrass, tolerant cultivars of G-6 and L-93 showed less reduction in chlorophyll concentration and root dry weight than did the intolerant cultivars of Penncross and Pennlinks when the water level was at 15 or 1 cm below the soil surface (Jiang and Wang, 2006). A wide range in flood damage among perennial ryegrass cultivars was observed with ‘All Star’ and ‘Premier’ being relatively tolerant to flooding (Razmjoo et al., 1993). These results suggest that variations in the waterlogging tolerance of turfgrass species and cultivars are important for turfgrass management for selection of a grass for a particular use.
The physiological responses of plant species to waterlogged or flooded soil are diverse, including changes in nonstructural carbohydrate content (Barta, 1988; Castonguay et al., 1993; Su et al., 1998), antioxidant enzyme activity and isoenzyme profile (Biemelt et al., 2000; Lin et al., 2004), anaerobic metabolisms (Chen and Qualls, 2003; Kennedy et al., 1992), expression of oxygen-related stress protein (Dordas et al., 2003; Igamberdiev et al., 2004), and levels of plant hormones that include ethylene, abscisic acid, and cytokinin (Olivella et al., 2000; Schravendijk and Van Andel, 1986; Zhang and Davis, 1987). Some of these mechanisms may allow plants to adapt or be tolerant to oxygen deficiency to a certain degree; however, they are influenced by species and cultivars and the duration and depth of waterlogging stress as well as other environmental factors such as temperature. The responses of shoots and roots to waterlogging are not well documented in different turfgrass species or cultivars. Thus, more detailed research is needed to assess the waterlogging tolerance of turfgrass and to explore the physiological traits associated with stress tolerance.
Kentucky bluegrass (Poa pratensis L.) is a widely used turfgrass species that adapts to well-drained soil conditions over cool climate regions. Large variations in growth habits, leaf texture, color, and some stress resistance have been found in Kentucky bluegrass; however, little information is known about the waterlogging tolerance of this grass, except that reduced root growth is found under poor aeration conditions (Waddington and Baker, 1965). A better understanding of the diversity of waterlogging tolerance among cultivars would provide a basis for managing this grass and would further reveal the mechanisms of grass adaptation to saturated or flooded soils. Thus, the objectives of this study were to compare the relative waterlogging tolerance of Kentucky bluegrass cultivars and to investigate the physiological responses of shoots and roots to waterlogging.
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