Bermudagrasses (Cynodon spp.) are commonly used on athletic fields in the U.S. transition zone, because they offer increased recuperative potential and heat tolerance compared with other turfgrass species (Christians, 2004). Traffic is the most frequent and damaging stress imposed to bermudagrass athletic fields, because it induces two forms of damage: wear and soil compaction (Carrow and Petrovic, 1992; Minner et al., 1993). Wear is characterized by injury (tearing) of leaf tissues, whereas soil compaction negatively alters soil physical properties (Carrow and Petrovic, 1992).
Plant growth regulators (PGRs) have been defined as organic compounds that alter turfgrass growth or development by targeting the actions of plant hormones (DiPaola, 1988). Gibberellins, auxins, cytokinins, and ethylene are examples of naturally occurring plant hormones that affect growth (Arteca, 1996). Applications of PGRs can affect turf growth and development by inhibiting or stimulating hormonal action (Serensits, 2008).
Trinexapac-ethyl (TE), a Class A PGR, acts late in the gibberellin biosynthesis pathway by inhibiting the conversion of GA20 to GA1, reducing cell elongation and in turn vertical growth (Adams et al., 1992). Although applications of TE at rates of 0.1 kg·ha−1 or less have been shown to suppress growth and increase the turf quality of several bermudagrasses (Baldwin et al., 2009; Fagerness and Yelverton, 2000; McCullough et al., 2005a, 2005b, 2006), certain physiological responses have also been observed after treatment with TE that may improve turfgrass traffic tolerance. Waltz and Whitwell (2005) reported that TE increased the non-structural carbohydrate content (TNC) of hybrid bermudagrass (C. dactylon × C. transvaalensis Burtt-Davey) root and shoot tissues. Increased TNC may improve traffic tolerance in that TNC can be used as an energy reservoir to promote stress tolerance and regrowth after leaf tissue removal (Huang and Jiang, 2002). TE has also been reported to increase the content of structural carbohydrates in cell walls (Heckman et al., 2005), which has been associated with improved wear tolerance in several cool- and warm-season turfgrass species (Brosnan et al., 2005; Hoffman et al., 2010; Trenholm et al., 2000).
Applications of TE have been shown to increase tiller density (Beasley et al., 2005; Ervin and Koski, 1998), which has been associated with improved traffic tolerance (Trenholm et al., 2000). Ervin and Zhang (2007) suggested that elevated cytokinin content in hybrid bermudagrass crowns after treatment with TE may lead to increased cell division and consequently increased tiller density. However, the influence of TE on tiller density in the field has been erratic (Ervin and Koski, 2001; Lickfeldt et al., 2001; Stier and Rogers, 2001). Serensits (2008) reported that sequential applications of TE at 0.17 kg·ha−1 before traffic stress increased the tiller density of several kentucky bluegrass (Poa pratensis L.) cultivars established on a sand-based root zone but did not affect traffic tolerance.
Paclobutrazol and flurprimidol are Class B PGRs that act early in the gibberellin biosynthesis pathway by preventing the conversion of ent-kaurene to ent-kaurenoic acid (Buchanan et al., 2000; Sponsel, 1995). Both of these PGRs are absorbed primarily by turfgrass roots (Anonymous, 2009; Tukey, 1986), whereas TE absorption occurs primarily through the foliage (Fagerness and Penner, 1998). Applications of flurprimidol and paclobutrazol have been shown to suppress both common and hybrid bermudagrass growth, but significant injury has also been observed after application (Johnson, 1989, 1990a, 1990b, 1992, 1994; McCullough et al., 2005c, 2005d). Applications of flurprimidol and paclobutrazol may negatively affect bermudagrass traffic tolerance, as McCullough et al. (2005c, 2005d) observed reductions in shoot density, root mass, and root length after application. These morphological changes would likely render bermudagrass less tolerant of traffic stress; however, data describing the effects of flurprimidol and paclobutrazol on bermudagrass traffic tolerance have not been reported.
Ethephon, a Class E PGR that affects plant growth by releasing the hormone ethylene, is mainly used to suppress seed head formation (Serensits, 2008). McCullough et al. (2005b) reported a 22% reduction in growth (measured as clipping reduction) with sequential ethephon applications to ‘TifEagle’ hybrid bermudagrass; however, the researchers also reported linear reductions in bermudagrass quality with increasing rates of ethephon and as high as a 33% reduction in root mass. Although shoot density was not measured, both McCullough et al. (2005b) and Shatters et al. (1998) observed discolorations in bermudagrass leaf tissue and severe shoot thinning after treatment with ethephon. These responses suggest that ethephon may reduce bermudagrass traffic tolerance, but data describing the effects of ethephon on bermudagrass athletic field turf have not been reported.
Mixtures of TE with ethephon and flurprimidol have been shown to mitigate the negative effects that have been reported after treatment with flurprimidol and ethephon alone. McCullough et al. (2005b) reported that mixtures of ethephon + TE yielded increased bermudagrass quality, root length, and root mass values compared with ethephon alone. Kane and Miller (2003) reported that mixtures of ethephon + TE reduced the level of creeping bentgrass (Agrostis stolonifera L.) discoloration that had been observed with applications of ethephon alone. Totten et al. (2006) did not report improvements in clipping reduction or lateral regrowth for mixtures of flurprimidol + TE compared with TE or flurprimidol applied alone, but note that their findings do not rule out the potential benefits of applying flurprimidol + TE. Data describing the traffic tolerance of bermudagrass treated with mixtures of ethephon + TE and flurprimidol + TE have not been reported.
Although applications of PGRs have been shown to improve bermudagrass quality and alter plant morphology, data on bermudagrass traffic tolerance after treatment with various PGRs are limited. The objective of this study was to evaluate the traffic tolerance of ‘Riviera’ bermudagrass (Cynodon dactylon L.) after treatment with various PGRs commercially marketed for use in turfgrass management.
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