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a Jacklin Seed Company, 5300 W. Riverbend Ave., Post Falls, ID 83854
b Dep. of Horticulture, D-136 Poole Ag. Center, Clemson University, Clemson, SC 29634-0319
c Dep. of Genetics and Biochemistry, Clemson University, Clemson, SC 29634-0318
d Dep. of Applied Economics and Statistics, Clemson University, Clemson, SC 29634-0318
* Corresponding author (haibol{at}clemson.edu).
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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40% lower (147 kg ha–1 yr–1) than the typical recommended rates for ultradwarf bermudagrass putting greens improved Champion TQ under reduced light compared to higher N rates. Applying TE resulted in a linear TQ increase for full sunlight and shade-grown Champion bermudagrass. Under reduced sunlight, a 15% chlorophyll concentration increase was noted for TE-treated plots compared to nonTE-treated plots. Shade reduced thatch accumulation 40% compared to sun-grown Champion, which suggests less aggressive cultivation practices are required for thatch control under reduced light. Champion bermudagrass did not provide an acceptable putting green quality when grown under 55% full-day shade, however, adjusting management practices enhanced Champion bermudagrass quality under reduced light.
Abbreviations: TE, trinexapac-ethyl • PGR, plant growth regulator • TNC, total nonstructural carbohydrates • TQ, turfgrass quality
Received for publication July 3, 2008.
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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Nitrogen is the most dynamic and important nutrient for turfgrasses because it improves color, density, recuperative ability, and plant health when applied at adequate rates (Carrow et al., 2001; Hull and Liu, 2005; Liu et al., 2008). Reducing N has previously been reported to enhance a turfgrass stand when light interception is limited. Burton et al. (1959) reported a high N rate (294 kg ha–1) in 64% shade decreased ‘Coastal’ bermudagrass carbohydrates 30% and decreased plant density and leaf area compared to a low N rate (36 kg ha–1). Bunnell et al. (2005a) also noted a 39% TNC reduction in heavily shaded ‘TifEagle’ bermudagrass with additional N (24.5 kg ha–1 as (NH4)2SO4) compared to TE-treated plots. Therefore, reduced N rates should enhance warm-season TQ under reduced light by reducing aboveground vertical growth; thereby, reducing mower scalping. Similar trends are noted for cool-season turfgrasses grown under reduced light (Schmidt and Blaser, 1967; Bell and Danneberger, 1999; Goss et al., 2002).
Trinexapac-ethyl has become a routine management practice for turfgrass managers with ultradwarf bermudagrass putting greens (McCullough et al., 2006, 2007). Since TE inhibits gibberellic acid (GA) production, vertical shoot growth is slowed (Adams et al., 1992). Excessive shoot growth in reduced light rapidly depletes plant carbohydrate reserves resulting in turfgrass thinning and TQ decline. Morphological limitations, such as reduced lateral stem growth, negatively impact warm-season turfgrass development when sunlight is blocked (Beard, 1997). Therefore, TE is an effective management tool to alleviate shade stress because vertical shoot growth is reduced. ‘Diamond’ zoysiagrass [Zoysia matrella (L.) Merr] grown under 86% shade with TE applied every month at 0.048 kg a.i. ha–1 or every other month at 0.096 kg a.i. ha–1 enhanced TQ, root production, root + rhizome tissue TNC, and photosynthetic efficiency (Qian and Engelke, 1999). Similarly, Qian et al. (1998) demonstrated TE prolonged Diamond zoysiagrass acceptable TQ ( >6) for 134 more days compared to nonTE-treated zoysiagrass under 88% shade. Also, TE-treated zoysiagrass had 113% greater TNC and 50% greater canopy photosynthetic rates compared to nonTE-treated zoysiagrass (Qian et al., 1998). Bunnell et al. (2005a) noted TE (0.0393 kg a.i. ha–1 3 wk–1), along with an increase in mowing height (4.7 mm), increased TifEagle bermudagrass TQ and chlorophyll concentration in 4 h of daily sunlight.
Bermudagrasses have been noted for their poor performance in a shaded microenvironment. For example, TifEagle, ‘Floradwarf’, and Tifdwarf bermudagrass putting greens require 36 mol m–2 d–1 of sunlight for an acceptable appearance (Bunnell et al., 2005c; Miller et al., 2005). Gaussoin et al. (1988) and Baldwin et al. (2008) noted color and above/belowground biomass reductions of bermudagrass cultivars when subjected to shade stress. Also, Bunnell et al. (2005b) suggested zoysiagrass was more shade-tolerant than bermudagrass, while Jiang et al. (2004, 2005) noted seashore paspalum (Paspalum vaginatum Swartz.) cultivars were more shade tolerant than selected bermudagrass cultivars. Although bermudagrass putting greens perform poorly under reduced light, superintendents in the eastern part of the transition zone are considering switching from predominately creeping bentgrass (Agrostis stolonifera L.) greens to ultradwarf bermudagrass greens in an attempt to alleviate summer stress management challenges (Hartwiger and O'Brien, 2006). Creeping bentgrass greens showed minimal deleterious effects under moderate shade (Bell and Danneberger, 1999; Goss et al., 2002), however, conversion to bermudagrass greens will be problematic where moderate shade is present. Unless tree removal is a viable option, reduced light will be a constraint for maintaining an ultradwarf bermudagrass putting green. Therefore, sound management recommendations will become more important enabling turfgrass managers to appropriately manage bermudagrass greens under reduced light.
Field research of Champion bermudagrass response to reduced light, N, and PGRs is limited and needed as Champion bermudagrass is gaining popularity as a golf course putting green in the southeastern United States (McCullough et al., 2005; Long, 2006; personal communication with superintendents and U.S. Golf Association [USGA] agronomists from 2002–2008). Therefore, objectives of this research were to determine (i) Champion bermudagrass performance under reduced light in the field, (ii) optimum N rates when growing an ultradwarf bermudagrass cultivar when shading is prevalent, (iii) interactive effects of TE, various N rates, and Fe in a reduced light environment, and (iv) N rates, TE, Fe, and various light intensities effect on Champion bermudagrass thatch accumulation over a 2-yr study period.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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Nitrogen was applied every 2 wk as liquid urea N at 147, 293, and 440 kg ha–1 yr–1 using a CO2–pressurized backpack sprayer calibrated at 767 L ha–1. Iron was tank mixed with N at 2.7 kg ha–1 2 wk–1 provided as 10% chelated Fe. Additional P and K requirements were provided as potassium phosphate (K2HPO4) at an equal rate K of 98 kg ha–1 applied in July, August, and September. Trinexapac-ethyl was applied at 0.02 kg ha–1 using the emulsifiable concentrate (11.3% a.i.) every 2 wk from 15 June to 31 August 2006 and 2007 using a CO2–pressurized backpack sprayer calibrated at 767 L ha–1.
Plots were mowed daily at 3.2 mm with clippings collected throughout the study period. Hollow tine aerification (1.3 cm diam. tines 10 cm in length with 5.0 cm spacing), with cores pulled and collected, occurred late June and early August for both years. Irrigation was provided as needed equally over all plots to prevent wilt, while no fungicides, insecticides, or other pesticides were applied.
Data Collection
Data collected included microenvironment conditions, visual TQ, clipping yield, clipping chlorophyll concentration, thatch accumulation, thatch depth, and root TNC. Microenvironment parameters included canopy and soil temperature, wind movement, and light quality and quantity. Canopy and soil temperature, light quality, and PPFD (mmol m–2 s–1) were recorded on a clear, cloudless day at solar noon using a thermometer (model no.1455 and model no. 9840, Taylor, Oakbrook, IL), spectroradiameter (Model LI-1800; LiCor, Inc., Lincoln, NE), and quantum radiometer (Model LI-250, LiCor, Lincoln, NE), respectively. Wind movement was recorded twice on days with a consistent breeze using an anemometer (model no. CS-800, Clark Solutions, Hudson, MA).
Visual TQ ratings were recorded at Weeks 4, 8, and 12 based on color, density, texture, and uniformity of the bermudagrass surface (www.ntep.org). Quality was visually evaluated from 1 to 9, 1 = brown, dead turfgrass, 6 = minimal acceptable turfgrass, 9 = ideal green, healthy turfgrass.
Clipping yield (g m–2) was collected at Week 6 and 12 for both years. Shoot tissue was collected using a Toro walk behind greens mower (Greenmaster 800, The Toro Company, Bloomington, MN) following 1 d of growth. Harvested clippings were then oven dried at 80°C for 48 h and weighed to quantify clipping yield.
Clipping chlorophyll (mg g–1) concentration was measured on the same dates as clipping yield using a portion of clippings (0.1 g). Fresh clippings were collected (as described above) from each plot and immediately placed in a plastic bag inside a covered bucket to prevent sunlight degradation. Clippings were weighed (0.1 g) 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) to eliminate shoot tissue decomposition (Hiscox and Israelstam, 1979). Samples were incubated in 65°C water on a hot plate (PC-600, Corning, Corning, NY) for 1.5 h and continuously shaken. Upon completion, samples were passed through filter paper (Whatman 41, Whatman, England) and remaining extract (2 mL) transferred into cuvettes. Absorbance values were recorded at 663 and 645 nm wavelengths using a spectrophotometer (Genesys 20, ThermoSpectronic, Rochester, NY). Blanks were initially run and also after every sixth sample as an internal control. The following formula was used to calculate total shoot chlorophyll: (mg g–1) = (8.02 x D663 + 20.2 x D645) x 0.1 (Arnon, 1949).
Thatch accumulation (g m–2) and depth (cm) were measured at Week 12 for both years. A bulk density sampler extracted one 206 cm3 core from each plot. Roots were clipped at the base of the thatch layer and the remaining thatch sample was placed in an 80°C oven for 96 h and weighed. Thatch samples were then placed in a muffle furnace (Benchtop Muffle Furnace LMF-A550, Omega Engineering, Inc., Stamford, CT) at 525°C for 3 h to provide ash free weight (Snyder and Cisar, 2000). Samples were weighed again and then subtracted from the original dry weight, which determined thatch accumulation (g m–2). Thatch depth (cm) was measured from five points on the soil core and averaged using a ruler. Following oven drying, measurements were taken from the top of the turfgrass surface to the thatch layer base.
Root TNC (mg g–1) was collected at Week 12 for both years. Root tissue was harvested using a bulk density sampler which extracted 206 cm3 cores before sunrise to minimize any diurnal fluctuations. Following soil removal, root tissue samples were stored at –75°C until freeze dried at –40°C for 2 wk to cease all metabolic activity. Samples were then ground using an A-10 plant grinder (IKA Works, Inc., Wilmington, NC). Grounded samples were rehydrated with 100 mL of 80% ethanol (EtOH) and 2 mL of 0.1 M sodium acetate buffer (pH 4.5) in glass test tubes 13 x 100 mm. Rehydrated samples were placed in boiling water for 1 h, cooled for 1 h and repeated. Two milliliters of invertase (Sigma I-4753, 433 units mg–1) and amyloglucosidase (Sigma A-7255, 23,000 units g–1) were then added to solution. Samples were placed in water bath (40– 45°C) for 3 d and vortexed three times daily. The TNC was analyzed using Nelson's Assay (Nelson, 1944), which determines glucose and fructose in plant tissue (Nelson, 1944; Somogyi, 1945). A 25 mL of aliquot was removed and two reagents (copper and arsenomolybdate) were added to the solution. Absorbance values were measured at 520 nm using a spectrophotometer.
Data Analysis
Treatment factors were arranged in a split-block design with three replications. Management practices were arranged in a randomized complete block design (RCBD), while shade was the split-plot factor. Treatment effects were evaluated using analysis of variance within the Statistical Analysis System (version 9.1, SAS Institute, Cary, NC). At Week 4, no meaningful year by treatment interaction was noted for all responses measured; therefore, data for the 2 yr were pooled. Orthogonal contrasts examined linear and quadratic responses for all parameters measured. Means separation was performed using Fisher's protected least significant difference (LSD) test with alpha = 0.05.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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Turfgrass Quality
Overall, Fe minimally impacted TQ of full sunlight or shaded plots (data not shown). Munshaw et al. (2006) also noted Fe was largely ineffective in consistently enhancing bermudagrass color and quality. However, Xu and Mancino (2001) reported Fe increased two cool-season turfgrasses leaf color. The inability of Fe to increase TQ may be related to daily clipping removal. Symptoms of Fe deficiency are often linked to sites where daily mowing occurs (Turgeon, 2005). Also, Fe is quickly converted to insoluble forms in the soil, which typically results in short-term visual responses following Fe applications (Munshaw et al., 2006). Monitoring leaf tissue Fe concentration following multiple seasonal applications on ultradwarf bermudagrass greens may provide insight into why Fe was largely ineffective in enhancing TQ.
At Week 4, increasing N under 55% full-day shade linearly decreased TQ (Table 1 ). However, in the absence of shade, increasing N linearly increased TQ. Applying N at 147 kg ha–1 yr–1 resulted in unacceptable TQ (5.4) in full sunlight. After two TE applications, minimal impacts on TQ scores were noted under reduced light. However, TE linearly increased TQ of sun-grown Champion bermudagrass.
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After 3 mo of 55% full-day shade, applying N at 147 kg ha–1 yr–1 (5.6) linearly increased TQ compared to N at 293 (4.8) and 440 kg ha–1 yr–1 (3.8) (Table 1). Also, in full sunlight, N at 147 kg ha–1 yr–1 (6.5) had lower TQ scores than applying N at 293 (7.4) and 440 kg ha–1 yr–1 (7.6), however, all TQ scores remained above acceptable threshold. Trinexapac-ethyl linearly increased TQ, regardless of light environment. Bunnell et al. (2005a) noted TE-treated TifEagle bermudagrass mowed at 3.2 mm had greater TQ scores compared to nonTE-treated TifEagle when grown under 4 h of sunlight.
Clipping Yield
By Week 6, under full sunlight, increasing N linearly increased shoot biomass (Table 2
). Under shade, clipping yield was reduced 93% when applying N at 147 kg ha–1 yr–1 compared to N at 293 and 440 kg ha–1 yr–1. Also, under 55% shade and full sunlight, TE linearly reduced shoot growth 110 and 63%, respectively. In full sunlight, McCullough et al. (2006, 2007) also noted clipping yield reductions in field grown TifEagle bermudagrass following TE applications, while Ervin and Zhang (2007) noted a reduction in ‘Tifway’ bermudagrass plant height when treated with TE. In shade, TE effectively suppresses warm-season turfgrass clipping yield and reduces plant height (Qian et al., 1998; Ervin et al., 2002; Bunnell et al., 2005a).
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Chlorophyll
Iron did not impact chlorophyll concentration of Champion bermudagrass (data not shown). Similar results were noted by Stier and Rogers (2001) when Kentucky bluegrass (Poa pratensis L.) and supine bluegrass (Poa supina Schrad.) were subjected to shade stress. Previous investigations have noted increased Fe availability in a nutrient solution medium enhanced chloroplast development of Kentucky bluegrass, which increased chlorophyll b production (Lee et al., 1996). In this study, Fe uptake was inhibited, Fe rates and application frequency were insufficient, or a reduced light environment possibly restricted Fe uptake. Iron uptake might have occurred; however, foliar absorbed Fe may have been removed through daily mowing resulting in lower Fe usage. The fate and movement of Fe in the turf-soil system under reduced light is unknown and warrants further research.
At Week 6, under reduced light, 147 kg ha–1 yr–1 of N increased chlorophyll concentration 5% compared to 440 kg ha–1 yr–1 of N; however, a linear or quadratic response did not occur (Table 3 ). In full sunlight, applying N at 440 kg ha–1 yr–1 increased chlorophyll concentration 17 and 11% compared to 147 and 293 kg ha–1 yr–1 of N, respectively. Applying TE linearly increased chlorophyll concentration 15% compared to nonTE-treated Champion bermudagrass under 55% full-day shade.
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Thatch
Under reduced light, Champion had 40% less thatch accumulation than sun-grown Champion bermudagrass (Table 4
), which suggests a less aggressive cultivation approach is needed to control thatch buildup in a shaded ultradwarf bermudagrass putting green. Similar to thatch accumulation, full-sunlight plots increased thatch depth 29% compared to shade grown plots. Excessive thatch accumulation typically occurs when organic matter production, such as clippings or stolons, is greater than organic matter decomposition (Beard, 1973), which is often linked to accelerated shoot growth. Therefore, shoot biomass reductions (Table 2) of shade-grown Champion bermudagrass likely reduced thatch accumulation and depth. Trinexapac-ethyl and higher N both resulted in slightly greater thatch depth compared to nonTE-treated and lower N plots. However, Fagerness et al. (2001) indicated repeated TE applications (0.11 kg a.i. ha–1) did not affect thatch development, rather increased shoot density and percent green canopy tissue accelerated thatch accumulation. In full sunlight, higher N linearly and quadratically increased thatch depth. Nitrogen applied at 293 kg ha–1 yr–1 increased thatch depth 26 and 5% compared to N at 147 and 440 kg ha–1 yr–1, respectively.
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CONCLUSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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40% lower than the recommended N requirements (McCarty and Miller, 2002) for sun-grown ultradwarf bermudagrass putting greens, significantly improved Champion TQ under reduced light compared to higher N rates. Low N reduced vertical shoot growth, thereby, minimizing shoot tissue removed from daily mowing. Also, 440 kg N ha–1 yr–1 reduced Champion bermudagrass chlorophyll concentration 20% compared to 147 kg N ha–1 yr–1 under shade. Similar to reducing N, applying TE every 2 wk increased TQ and chlorophyll concentrations of shade-grown Champion bermudagrass.
Champion bermudagrass quality was enhanced by reducing N rates and routinely applying TE; however, TQ will inevitably decline if shade intensity is too great or shade duration is too long. It has been suggested that ultradwarf bermudagrass greens require 36 mol m–2 d–1 of sunlight (Bunnell et al., 2005c; Miller et al., 2005). Also, time of shading is a relevant consideration when planting a bermudagrass green (Bunnell et al., 2005c). Therefore, selective tree thinning or removal, along with traffic reductions, should also be considered in attempting to prolong Champion bermudagrass TQ under reduced light (Ervin, 2002).
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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