DIVERSITY OF 42 BERMUDAGRASS CULTIVARS IN A REDUCED LIGHT ENVIRONMENT

Morphological limitations, such as reduced lateral stem growth, reduced stress resistances to unfavorable environments and pests, and overall reduction of carbohydrate synthesis contribute to bermudagrass (Cynodon spp.) decline under shade. Due to shade sensitivity and possible genetic variability of bermudagrasses, a two-year replicated greenhouse study in 2005 and 2006 determined the shade tolerance of 42 bermudagrass cultivars selected from the 2002 National Turfgrass Evaluation Program (NTEP). Cultivars were subjected to 64% continuous artificial shade for 60 days. Data collection included visual turfgrass quality (TQ), shoot chlorophyll concentration, root length, and total root biomass. Seeded cultivars included ‘Sunstar’, ‘B-14’, ‘SWI-1003’, ‘SWI-1012’, ‘SWI-1014’, ‘SWI-1041’, ‘SWI-1044’, ‘SWI-1045’, ‘SWI-1046’, ‘Arizona Common’, ‘Numex Sahara’, ‘Princess 77’, ‘Mohawk’, ‘FMC-6’, ‘SWI-1001’, ‘Sundevil’, ‘Southern Star’, ‘Riviera’, ‘Transcontinental’, ‘CIS-CD5’, ‘CIS-CD6’, ‘CIS-CD7’, ‘Panama’, ‘La Paloma’, ‘Yukon’, ‘TiftNo.1’, ‘TiftNo.2’, and ‘Sunbird’. Vegetative cultivars included ‘TiftNo.3’, ‘Tift No,4’, ‘Tifway’, ‘Midlawn’, ‘Tifsport’, ‘MS-Choice’, ‘Aussie Green’, ‘GN1’, ‘Premier’, ‘Ashmore’, ‘Patriot’, ‘OKC 70-18’, and ‘Celebration’. Cultivars with best shade tolerance were Celebration, TiftNo.4, TiftNo.1, and Transcontinental. Cultivars with intermediate shade tolerance included Aussie Green, MS-Choice, Princess 77, SWI-1045, SWI-1041, and SWI-1012. Most shade sensitive cultivars included SWI-1014, Arizona Common, Sundevil, SR 9554, GN-1, and Patriot. INTRODUCTION Bermudagrass (Cynodon spp.), a C4 plant, growth and development is interrupted when light interception is reduced. In shade, warm-season turfgrass decline is attributed to morphological limitations, such as reduced lateral stem growth (Beard, 1997). A shaded microenvironment initiates excessive shoot vertical growth, depleting turfgrass root carbohydrate status (Qian and Engelke, 1999). Previous warm-season and cool-season turfgrass evaluations have noted a linear relationship as shade increases, vertical shoot growth increases (Qian et al., 1998; Tegg and Lane, 2004). Bermudagrasses continue to be the preferred turfgrass species used as variable turf types in the southern United States (McCarty and Miller, 2002; McCarty, 2005), however, its use is often limited when optimal light interception is interrupted. For example, a ‘TifEagle’ bermudagrass putting green requires 32.6 mol m d for acceptable turfgrass quality (≥7) (Bunnell et al., 2005a), while ‘Floradwarf’ and ‘Tifdwarf’ bermudagrass requires 38.6 mol m d 1 for sustained growth (Miller et al., 2005). Cultural practices to enhance warm-season turfgrass TQ under shade includes raising mowing heights (Bunnell et al., 2005b; White, 2004), reducing nitrogen rates (Bunnell et al., 2005b; Goss et al., 2002; Bell and Danneberger, 1999; Burton et al., 1959), applying plant growth regulators (Bunnell et al., 2005b; Ervin et al., 2003; Qian et al., 1998; Qian and Engelke, 1999), and watering deeply and infrequently (Dudeck and Peacock, 1992). Variation among C4 turfgrasses shade sensitivity has been previously investigated. Bunnell et al. (2005c) reported ‘Meyer’ Japanese zoysiagrass (Zoysia japonica Steud.) had greatest shade tolerance maintaining acceptable TQ (>7) at 71%, ‘Celebration’ bermudagrass at 58%, and ‘TifSport’ and ‘Tifway’ bermudagrass at 41% continuous shade. Jiang et al. (2004) noted significant variation between seashore paspalum (Paspalum vaginatum Swartz) and bermudagrass entries under 70% and 90% shade. ‘Sea Isle 1’ and ‘Temple 1’ paspalum were shade tolerant, while TifSport and TifEagle bermudagrass showed least shade tolerance. In a similar study, Jiang et al. (2005) also noted greater shade tolerance of Sea Isle 1 compared to TifSport bermudagrass. Gaussoin et al. (1988) noted the diversity of 32 bermudagrass cultivars under 90% uninterrupted shade. According to results, ‘Boise’, ‘No Mow’, and ‘NM 2-13’ were relatively shade tolerant, while ‘Arizona Common’ and ‘Santa Ana’ were relatively shade sensitive. Due to the genetic variation and diversity among existing bermudagrass cultivars, it is critical to continue shade tolerance evaluation of new bermudagrass cultivars. Therefore, the objectives of this research were to evaluate bermudagrass growth and determine the diversity of 42 bermudagrass cultivars maintained at 64% reduced irradiance. MATERIALS AND METHODS This two-year greenhouse study was conducted from 15 June 2005 to 15 August 2005 and repeated in 2006 at Clemson University (Clemson, SC). Greenhouse conditions averaged 30.8C /26.2C high/low air temperature and 73% relative humidity for both years. Environmental conditions (temperature and humidity) were maintained by an automated computer recording system (Argus Controls, Whiterock, British Columbia V4B 3Y9). This study consisted of two treatments including a control (full-sun) and 64% continuous shade applied daily using a neutral density, polyfiber black shade cloth (Glenn Harp and Sons, Inc., Tucker, GA). Shade clothes were placed on a polyvinyl chloride (PVC) structure 15.2 cm in length and 12.7 cm in diameter with 2.5 cm diameter PVC pipes. Shade tents were 30.48 cm above the turf surface to reduce early morning and late afternoon sunlight encroachment. Photon flux density (μmol m s) and light quality were measured on a clear, cloudless day using a quantum radiometer (Model LI-250, LiCor, Lincoln, NE) and spectroradiameter (Model LI1800; LiCor, Inc., Lincoln, NE). Surface temperatures for both treatments were recorded twice on a clear/cloudless day using an indoor/outdoor thermometer (model# 1455, Taylor, Oakbrook, IL). Plugs were collected from the 2002 Bermudagrass National Turfgrass Evaluation Program (NTEP) field research plots located at Clemson University and transplanted into conetainers with 85% sand and 15% peat as growth media. Cone-tainer dimensions were 25.4 cm in height and 5.1 cm in diameter. Following potting, plugs were established one month prior to treatment initiation. Fertilizers were provided at 49 kg N ha on 1 d and 30 d using a complete fertilizer (16N-1.7P-6.6K) with a slow release urea and micronutrients (kg•ha): Calcium, 6; Magnesium, 3; Sulfur, 9; Boron, 0.06; Copper, 0.03; Iron, 0.3; Manganese, 0.15; Molybdenum, 0.0015; and Zinc, 0.15. Cone-tainers were mowed at a height of 12.8 mm four times a week with clipping removal. Data collection was identical in year I and II. Turfgrass quality ratings were recorded after 4 and 8 weeks of shade exposure based on a subjective combination of color, density, texture, and uniformity of the bermudagrass surface. Quality was visually evaluated from 1 to 9, 1 = brown, dead turf, 6 = minimal acceptable turf, 9 = ideal green, healthy turf. Root biomass (g) and length (cm) were determined at the end of the study. Roots were extracted from the soil and thoroughly washed until all soil was removed. Following soil removal, root length was measured from the base of the thatch to the longest fully extended root present. Roots were then clipped from the base of thatch and dried in an oven at 80.0 C for 48 hours. Once dried, samples were weighed to determine total root biomass. Chlorophyll (mg g) was recorded at week 4 and 8. Clippings were collected from each cone-tainer and immediately placed in a plastic bag inside a covered bucket to prevent sunlight degradation. Clippings were weighed (0.1g) and placed in a glass test tube (1.0 cm in width and 14.8 cm in length) with 10 ml of dimethyl sulfoxide (DMSO) (Hiscox and Israelstam, 1979). Samples were incubated in 65 C water on a hot plate (PC-600, Corning, Corning, NY) for 1.5 hours and continuously shaken. Upon completion, samples were passed through filter paper (Whatman 41, Whatman, England) and remaining extract (2 ml) was transferred into cuvettes. The absorbance values were recorded at 663 nm and 645 nm wavelengths using a spectrophotometer (Genesys 20, ThermoSpectronic, Rochester, NY). The following formula was used to calculate total chlorophyll = (8.02 * D663 + 20.2 * D645) * 0.1 (Arnon, 1949). Data Analysis Each treatment was replicated three times in a randomized complete block design. Data from both years were combined as year I x year II interactions were not significant. All statistical computations were conducted using analysis of variance (ANOVA) within the Statistical Analysis System (SAS Institute, 2005) with means separated by Fisher’s Least Significance Difference (LSD) test. An alpha of 0.05 was used to determine statistical significance.

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