Shade tolerance is an important factor for selecting turfgrasses for home lawns and recreational facilities. Beard (1973) estimated that 20% to 25% of established turfgrass stands are impacted by some type of light restriction. Cool-season (C3) turfgrass species generally have lower light compensation points than warm-season (C4) turfgrass species. Greater photochemical efficiency at low light intensities contributes to the generally superior shade adaptability of C3 grasses (Kephart and Buxton, 1996). However, only a few of these cool-season species have shown potential for adaptability to the southern or transition zones of the United States. In recent years, there has been renewed interest in developing heat, drought, and shade-tolerant cool-season turfgrass species that may offer year-round growth in these climates.
Tall fescue (Festuca arundinacea Schreb.) is one such species that displays a wide range of genetic variability and greater adaptation to heat, drought, and shade relative to other cool-season turfgrasses (Hurley, n.d.). It is the predominant cool-season species for shaded areas in the transition zone with good to moderate tolerance (Wu et al., 1985). Semidwarf- and dwarf-type tall fescues with fine leaf textures and high turfgrass quality are most popular but require frequent mowing and are highly susceptible to brown patch (Rhizoctonia solani Kühn) and other diseases (Watkins and Meyer, 2004; Wu et al., 1985). For these reasons, breeders have recently been investigating the potential for alternative cool-season species in southern environments.
Kentucky bluegrasses (Poa pratensis L.) are genetically and morphologically diverse. Compact america, compact midnight, and mid-atlantic types display greater tolerances to heat, drought, and shade, but are generally not well adapted to southern environments (Brilman, 2009; Hall, 1996; Morris, 2010). Genes conferring heat and drought tolerances have been introgressed from texas bluegrass (Poa arachnifera Torr.), a native species adapted to the southern plains of the United States (Hitchcock, 1950), into kentucky bluegrass through interspecific hybridization. Some of these hybrid bluegrasses (Poa arachnifera × Poa pratensis) have demonstrated markedly better performance when compared with kentucky bluegrass and tall fescue (Abraham et al., 2008; Meeks et al., 2015; Su et al., 2007) and have allowed for a greater range of adaptability into more southern environments, but their potential under shaded environments have not been evaluated.
Methods of evaluating the shade tolerance of cool-season grasses include shade structures covered with shadecloth either in the glasshouse or in the field (Burner and West, 2010; Feldhake et al., 1985; Lin et al., 1999; Tan and Qian, 2003; Tegg and Lane, 2004; Watson et al., 1984), under tree canopies (Gardner and Taylor, 2002; Wherley et al., 2005), and in growth chambers (Wood, 1968). In most cases, full sun, moderate (≈50%), or heavy (≈80%) shade have been used for comparisons; however, establishment methods and experiment duration vary depending on the objectives and location.
Parameters typically used to characterize turfgrass growth responses under shaded environments include clipping yield, turfgrass quality, and color (Burner and West, 2010; Cockerham et al., 2002; Feldhake et al., 1985; Lin et al., 1999; Watson et al., 1984; Wood, 1968). In cool-season grasses, clipping biomass production has been shown to decrease with decreasing light intensity (Cockerham et al., 2002; Lin et al., 1999; Watson et al., 1984). This may result from increased leaf succulence and narrower, thinner leaves in shade. Leaf elongation on the other hand is one of the most documented shade avoidance mechanisms in genotypes that are intolerant (Beard, 1965; Tan and Qian, 2003; Tegg and Lane, 2004). High leaf elongation rates result in accelerated energy depletion in plant tissue (Qian and Engelke, 1999), increased mowing frequency by the turfgrass manager or homeowner, reduced stand density, and green cover (Gardner and Taylor 2002; Wherley et al., 2013). All of these effects negatively impact turfgrass quality. Plant growth regulators such as flurprimidol or TE have been used to reduce leaf elongation and mowing frequency under full sunlight and shaded conditions (Lickfeldt et al., 2001; Stier et al., 1999; Tan and Qian, 2003), but reports on enhanced turfgrass quality in shade have been inconsistent (Ervin and Koski, 2001; Gardner and Wherley, 2005; Qian et al., 1998; Steinke and Stier, 2003; Stier and Rogers, 2001; Wang et al., 2009).
The objectives of this study were to 1) evaluate the growth responses of interspecific bluegrass hybrids in comparison with kentucky bluegrasses and a shade-tolerant cultivar of tall fescue under full sunlight and shaded environments, 2) identify optimum times to evaluate shade tolerance using the selected growth measurements, 3) calculate the minimum daily light requirements to retain acceptable turfgrass quality, and 4) determine if TE applications enhance hybrid bluegrass quality under shade.
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