Hybrid bermudagrass (Cynodon dactylon × C. transvaalensis) such as ‘TifSport’ and ‘TifGrand’ are widely used on golf courses, athletic fields, as well as commercial and residential landscapes because of their tolerance to lower and more frequent mowing, high temperature and drought, and fast recuperative growth (Hanna et al., 2010). However, hybrid bermudagrass tends to have less shade tolerance when compared with other species such as zoysiagrass (Zoysia sp.) and St. Augustinegrass (Stenotaphrum secundatum) (Gardner and Goss, 2013), largely explained by its requirement of greater light intensities to reach a balance of photosynthesis and cellular respiration (Alexander and McCloud, 1962). Shaded conditions, caused either by artificial constructions or trees, have always been a great challenge for turfgrass managers. These challenges are becoming more common as roofed stadiums are being built. Shade results in reduced irradiance and reduced levels of total nonstructural carbohydrates (TNCs) (Schnyder and Nelson, 1989), which makes turfgrass more vulnerable to disease and pests (Giesler et al., 2000). Shaded microenvironments are also typically comprised of greater humidity and limited airflow, which only exacerbates the condition (Giesler et al., 2000). ‘TifGrand’, as an improved cultivar, has demonstrated better shade tolerance than ‘Tifway’ and ‘TifSport’ when grown under 70% continuous shade (Hanna et al., 2010).
Shoot elongation under shade is a typical response of turfgrass as a result of increased levels of gibberellic acid (Tan and Qian, 2003). This shade avoidance mechanism increases leaf area and light capture, but the drawback is that the photosynthates have been removed when the turfgrass is mowed regularly, leading to weakening physiological status. TE is a plant growth regulator that is formulated to reduce plant vigor by decreasing the biosynthesis of gibberellic acid (King et al., 1997). It is often applied to turfgrass for several beneficial purposes, but in recent years it has been used to reduce shoot elongation on turfgrasses growing under shaded conditions (Fagerness et al., 2002; Wherley and Sinclair, 2009). Several studies have shown increased zoysiagrass performance under shade by inhibiting shoot elongation through TE application (Ervin et al., 2002; Qian and Engelke, 1999; Qian et al., 1998; Riffell et al., 1995). Qian and Engelke (1999) reported an increase of canopy photosynthesis, TNC, tiller density, root weight, and TQ as a result of TE application in ‘Diamond’ zoysiagrass grown in a shaded environment. In bermudagrass, Bunnell et al. (2005b) reported that ‘TifEagle’ bermudagrass maintained acceptable TQ under 4 h sunlight and TE application when mowed at 4.7 mm. Applying TE resulted in a linear TQ increase for ‘Champion’ bermudagrass grown under 55% shade (Baldwin et al., 2009). The TE application was also reported to improve ball lie in ‘TifSport’ when maintained at a 1.5-inch height (Kowalewski et al., 2012), although the effect was not significant throughout the entire year.
Increasing mowing heights within the recommended range of each turfgrass species can increase turf performance under shaded conditions (Dudeck and Peacock, 1992). Cutting the grass at a lower mowing height not only removes the most photosynthetic actively tissues, but also alters the leaf orientation, leading to a modification of light compensation point (Beard, 1973; Gardner and Goss, 2013). ‘TifEagle’ hybrid bermudagrass had better TQ when maintained at 4.7 mm than at 3.2 mm under 4 h daily sunlight (Bunnell et al., 2005b). Turfgrass quality of bermudagrass was greater at 25 mm than at 16 mm under 71% shade, but there was no difference in TQ under 0% or 41% shade (Bunnell et al., 2005a). However, maintaining turfgrass at a higher height of cut has adverse effects on turf performance resulting from greater respiration rate, increased shading within the turf system, and less traffic tolerance (Gardner and Goss, 2013). Zhang et al. (2017) determined the minimum daily light integral for bermudagrass was ≈20 mol·m–2·d–1 in summer months. Specifically, they suggested that ‘TifGrand’ required slightly less light than ‘Tifway’ bermudagrass to maintain acceptable performance.
To ensure the successful long-term performance of individual hybrid bermudagrass under shaded conditions, field studies are needed to determine best management practices. Therefore, the objective of this research was to evaluate whether mowing height and TE growth regulator application could improve the turfgrass performance of ‘TifGrand’ and ‘TifSport’ under shaded conditions that have been reported too heavy for healthy bermudagrass growth in the southeastern United States.
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