Creeping bentgrass (Agrostis stolonifera L.) is a turfgrass species highly suitable for use on golf course tees, greens, and fairways. As a result of its ability to provide exceptional quality playing surfaces when mowed short, it is used worldwide. Because of golf course construction features, bentgrass is often maintained under reduced light conditions. Although it tolerates partial shading, it grows best in full sunlight (Beard, 1973; Bell and Danneberger, 1999).
Turfgrasses can be subjected to both natural and artificial (neutral) shade from vegetation and building structures, respectively. While under neutral shade, turfgrasses respond to reduced light intensity; shade under vegetation canopy can reduce light intensity and alter spectral composition, which act in concert to determine turf performance. Reduction of PPF was shown to induce excessive vertical shoot growth in turfgrass plants at the expense of tiller formation and lateral spread, thereby resulting in a poor density of the turfgrass stand (Bell and Danneberger, 1999; Dudeck and Peacock, 1992; Koh et al., 2003; Wherley et al., 2005). Moreover, turfgrasses grown in low PPF environments were characterized by longer, thinner, and more succulent leaves (Allard et al., 1991; Wherley et al., 2005; Wilkinson and Beard, 1974). Alteration in spectral composition and specifically reduced R:FR further contribute to aforementioned morphological changes (Casal et al., 1990; Dudeck and Peacock, 1992; Frank and Hofman, 1994; Wherley et al., 2005). However, Wherley et al. (2005) reported that leaves of plants grown under low PPF but high R:FR were wider compared with those grown under low PPF and low R:FR environments.
Under golf course conditions, shaded creeping bentgrass is maintained by frequent mowing at reduced heights. As a result, the bentgrass suffers decreased photosynthetic capacity, which ultimately leads to poor stand quality. Wilson (1997) suggested that when selecting species for shaded environments, the focus should include: compact growth morphology, relatively insensitive to changes in PPF and R:FR, and lax, horizontally oriented leaves.
Plant responses to light stimuli, including light quality, quantity, and duration, are in part mediated by gibberellins (GAs) (Hedden and Kamiya, 1997; Sponsel and Hedden, 2004). GAs are phytohormones that are involved in many developmental processes including stem elongation (Davies, 2007). GAs act by inducing genes involved in cell elongation and division (Sun, 2004). GA levels can be reduced in plants through application of growth regulators or biotechnological manipulation of genes involved in the biosynthetic pathway (Busov et al., 2003; Coles et al., 1999; Tan and Qian, 2003).
Among GA-inhibiting growth regulators, trinexapac-ethyl (TE) suppresses vertical growth and improves overall turf quality under low light conditions (Goss et al., 2002; Steinke and Stier, 2003). TE competitively inhibits the conversion of GA20 to GA1 × 3-β-hydroxylase, reducing leaf cell elongation (Adams et al., 1992) but not cell division (Ervin and Koski, 2001). However, to ensure consistent and lasting effects, frequent applications of TE are required.
In plants, inactivation of bioactive gibberellins GA1 and GA4 is ensured by GA2-oxidases (GA2ox) that catalyze their 2β-hydroxylation yielding GA8 and GA34 (Hedden and Proebsting, 1999). Overexpression of OsGA2ox1 in rice caused a dwarf phenotype with leaves that were darker green, shorter, and wider than those of the wild-type plants and adversely affected development of reproductive organs (Sakamoto et al., 2001). A similar phenotype was obtained by expressing GA2ox in transgenic tobacco plants (Nicotiana tabacum) (Biemelt et al., 2004; Schomburg et al., 2003), poplar trees (Populus tremula × Populus alba) (Busov et al., 2003), and Arabidopsis (Arabidopsis thaliana) (Radi et al., 2006). Overexpression of AtGA2ox1 in bahiagrass (Paspalum notatum L.) produced a semidwarf phenotype with increased tillering, delayed flowering, and shorter inflorescence, thus enhancing its overall quality (Agharkar et al., 2007).
Creeping bentgrass plants containing the runner bean (Phaseolus coccineus) GA2-oxidase gene (PcGA2ox) have been developed, and through preliminary greenhouse and field studies (Yan, 2005), superior lines were chosen. These superior lines were characterized by more horizontal growth habit, inhibited vertical growth, internode extension, and leaf growth when grown under restricted light conditions. The objective of this study was to determine the effect of genetically induced dwarfism on creeping bentgrass performance under different shade treatments while being maintained at a low mowing height.
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