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a Dep. of Agronomy and Horticulture., Univ. of Nebraska, Lincoln, NE 68583
b U.S. Golf Assoc., P.O. Box 4717, Easton, PA 18043
c Dep. of Crop and Soil Sci., Michigan State Univ., East Lansing, MI 48824
* Corresponding author (rgoss{at}unlserve.unl.edu)
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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Abbreviations: ASTM, American Society of Testing Materials • ET, evapotranspiration • GA, gibberellic acid • PGR, plant growth regulators • RLC, reduced light conditions • RSR, root to shoot ration • TE, trinexapac-ethyl
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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Ultimately, turfgrass management practices must be altered in RLC to increase light interception and turf health. Although the best solution to improve turfgrass health and performance under RLC would be removal of trees or other obstructions, this may not be possible. Beyond tree removal, practices such as raising the mowing height and reducing traffic also would benefit turfgrass under RLC (Danneberger, 1993). Unfortunately, these practices are not conducive with today's game of increased ball roll speed and increased play.
Plant growth regulators (PGRs) recently have been studied to manage problems associated with turfgrass grown under RLC. Trinexapac-ethyl (TE) is a PGR that inhibits biosynthesis of GA1 from GA20. It has been utilized in turfgrass management for clipping reduction, annual bluegrass (Poa annua L.) seedhead suppression, germination enhancement of the desired species, increased ball roll speed and improvement of overall turf quality, including color and density (Landry and Murphy, 2000). Stier (1997) and Stier and Rogers (2001) compared the use of TE on Poa supina Schrad. and Kentucky bluegrass (Poa pratensis L.) under RLC. Trinexapac-ethyl improved P. supina color, density, tillering, and recovery from traffic. Shear resistance was increased indicating an increase in overall surface biomass. In addition, leaf area index, fresh weight, total chlorophyll, and chlorophyll a and b levels were increased following TE application. Qian and Engelke (1999) studied TE on zoysiagrass (Zoysia matrella L. Merr. cv. ‘Diamond’) under RLC. Bimonthly (0.096 kg a.i. ha-1) and monthly (0.048 kg a.i. ha-1) TE applications resulted in acceptable turf quality with a 73 to 76% reduction in shoot vertical growth, a 75 to 77% reduction in clipping yields, a 38 to 40% increase in total nonstructural carbohydrate content, a 50 to 60% increase in root mass, a 46 to 51% increase in root viability and a 42 to 48% increase in photosynthesis. Trimonthly applications (0.192 kg a.i. ha-1) decreased zoysiagrass quality. Effects of TE on creeping bentgrass grown on putting greens under RLC have not been studied.
Another management practice for turfgrasses grown under RLC is to supply nitrogen (N) at lower application rates than is required in full sunlight (Wilson, 1997). Excessive N can limit the number of carbon-containing molecules available for protein synthesis and reduce root growth and carbohydrate reserves (Blackman and Templeman, 1940; Schmidt and Blaser, 1967). Carbohydrates are decreased with increasing N because of reduced assimilation due to competition with respiration (Green and Beard, 1969; Powell et al., 1967; Mazur and Hughes, 1976; Westhafer et al., 1982). In addition, reduction and assimilation are high-energy demanding processes. Nitrogen is a critical element in any turf management program, regardless of light level. Therefore, it is important to document the effects of N in combination with TE under RLC.
The objectives of this study were to determine the effects of TE and N on shoots plus stem and root growth, and carbohydrate content of a creeping bentgrass putting green turf maintained under RLC. Other objectives were to compare two TE use rates and two application frequencies on creeping bentgrass grown under RLC.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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Shade Structures
In experiment 1 (1998–1999), two 7.6 by 7.6 m shade house frames were constructed on the putting green with 2.5 cm diam. aluminum tubing. The houses were positioned approximately 6 m apart in a north to south orientation and had a double pitch roof with a 15° slope with the base of the toe-slope 2.1 m above the turf surface. An 80%, knitted, poly-fiber black shade cloth (Omni Growing Systems; ON, Canada) was installed over the shade house frames on 14 July 1998 and 27 May 1999. Installation of the shade cloth was intended to coincide with tree leaf expansion, which normally occurs during May at this site. However, delays in receipt and construction of shade house materials postponed the installation of shade cloth in 1998. Cloth was removed following leaf drop of nearby oak trees (Quercus sp.) in October of both years. The cloth covered all four sides and the roof, and the north and south facing sides containing openings for mowing and other maintenance practices. The openings remained closed except during maintenance practices, and were timed not to alter light quality and quantity on the experimental area. Full sun treatments initially were monitored in 1998, however, they were abandoned in 1999 because of the inability to assess visible differences between full sun and RLC treatments due to light quality and quantity differences.
In experiment 2 (1999), two 11 by 11 m shade houses were constructed as described in experiment 1 in spring 1999. The houses were positioned approximately 6 m apart in a north to south orientation. A 60%, knitted, poly-fiber green shade cloth (Omni Growing Systems) was installed on 28 May 1999. A different shade cloth was used in experiment 2 because of the severity of stress observed in experiment 1.
Environmental Data Collection
Dataloggers (LiCor 1400; Lincoln, NE) monitored soil temperature, air temperature, light intensity, and relative humidity inside and outside of the shade houses. Conditions for experiment 1 were monitored 42 d for three different periods beginning in August 1998 and September 1999 with the same instruments. Environmental data were collected for experiment 2 inside and outside the shade houses from 10 Sept. 1999 to 21 Oct. 1999 with the same equipment described for experiment 1. Data for each parameter were collected on 1 min intervals and averaged every 15 min during each experimental period. Daily maximum light intensity was measured with LI-190SA Quantum Sensors (LiCor; Lincoln, NE) under RLC and full sun. Relative humidity, average maximum and minimum daily air temperatures were measured with 1400-104 sensors (LiCor; Lincoln, NE). Light quality was measured with a ZSpec spectrometer (range: 330 to 1150 nm; Zeiss, Germany) under RLC and a nearby maple (Acer campestre L.) tree.
Treatments
For experiment 1 in 1998, TE was applied at 0.042 or 0.070 kg a.i. ha-1 for a total of 0.210 or 0.350 kg a.i. ha-1; respectively, on 14- to 21-d intervals (11 and 27 Aug., 11 and 28 Sept., 12 Oct.). Applications of TE began one month after shade cloth was installed and ended on 10 Oct. when the shade cloth was removed. In 1999, TE was applied on 14- to 21-d intervals (4 and 18 June, 3 and 24 July, 16 Aug., 18 Sept., and 7 Oct.). Applications began one week after shade cloth was installed for a total of 0.294 and 0.490 kg a.i. TE ha-1 for each respective date.
Prior to installation of shade cloth in 1998, the experimental area received a total of 135 kg N ha-1, 14 kg P ha-1, 27 kg K ha-1 over six applications. Following installation of shade cloth in 1998, N was applied at a rate of 10 (low N) and 20 (high N) kg N ha-1 using a liquid N formulation (30% N - 0% P - 0% K; 15% urea, 15% water soluble N) on 11 and 27 Aug., 11 and 28 Sept., and 12 Oct. Total N applied in 1998 was 185 (low N) and 235 (high N) kg N ha-1. In 1999, the experimental area received applications of 25 kg N ha-1 (12% N - 25% P2O5 - 13% K2O) on 14 Apr. and 6 May before shade was imposed. Following installation of shade houses, N was applied at a rate of 25, 38, or 50 kg N ha-1 on 15 June, 4 July, 2 Aug., and 4 Sept. Total N applied in 1999 was 150 (low N) and 213 (high N) kg N ha-1. The total N applied throughout the experiment was 335 (low N) and 448 (high N) kg N ha-1. The experimental area received 23 kg P2O5 ha-1 and 22 kg K2O ha-1 in 1999. All TE and N treatments were applied with a backpack CO2 pressurized sprayer (275 kPa) in 209 L ha-1 of water. Applications of N and TE were made in separate applications.
In experiment 2, TE was applied at rates of 0.025 and 0.050 kg a.i. ha-1 on 2- or 4-wk intervals beginning 4 June 1999 (7 d after shade cloth installation), and ending on 8 Oct. (early season treatment). A total of 0.175 and 0.350 kg a.i. TE ha-1 were applied in 1999, respectively. In addition, a third TE treatment was applied on 31 Aug. to previously untreated turf at a rate of 0.050 kg a.i. ha-1 on 2-wk intervals until 8 Oct. (mid-season treatment). All TE applications were applied as previously described.
Data Collection
For experiment 1, visual ratings were made at 3 to 18 d intervals when noticeable differences appeared. In 1998, color and percent cover ratings were made. Percent annual bluegrass, moss (species unknown), and algae (species unknown) cover ratings were obtained in 1999. Color was visually assessed on a 1 to 9 scale with 1 being yellow turf, 9 being dark green turf, and 5 being the minimum value for acceptable turf quality. Percent plot area covered with creeping bentgrass, annual bluegrass, algae, and moss were visually assessed on a linear 0 to 100% scale. Clipping yields were collected on 14- to 21-d intervals with a Greensmaster 1000 mower (Toro Co., Bloomington, MN). A mower pass was made in a north-south direction on both the east and west edge of the plots. One mower pass in the east-west direction removed clippings from approximately the center 70% of each plot. Clippings then were placed in a paper bag, oven-dried for 48 h at 60°C and weighed. Collections included both annual bluegrass and creeping bentgrass clippings.
Root mass and tiller count measurements were obtained on 14 Oct. 1998 and 12 Oct. 1999. In 1998, 6 cores (2.5 cm diam x 15 cm deep) were extracted randomly from creeping bentgrass areas of each plot. Cores were kept at -20°C until analyzed. Tiller number was determined for each plug by count during removal and thatch (approximately 1 cm thick) was discarded. The remaining soil was placed into water and agitated to separate soil from roots. The solution was poured over a 25 mesh screen constructed to American Society for Testing and Materials (ASTM; West Conshohocken, PA) specifications, and roots were removed and separated from remaining organic matter. Collected roots were dried for 12 h at 80°C and weighed. In 1999, two cores were extracted randomly from each plot. Cores were kept at -20°C until analyzed. Tiller number was determined and thatch was discarded. Roots and soil were shaken for 12 h in 100 mL water and 5.0 g sodium hexametaphosphate to remove soil from root surfaces. The solution then was rinsed over a series of screen meshes (25, 35, and 40 ASTM specifications), and roots were removed and separated from the remaining organic matter. Collected roots were dried for 12 h at 80°C and weighed.
For experiment 2, color, quality, and clipping yield data were collected on 14- to 21-d intervals as described for experiment 1. Root mass and tiller counts were not collected in experiment 2.
Starch and Carbohydrate Analysis
In experiment 1, two cores (7.5 cm diam x 15 cm deep) were randomly removed from creeping bentgrass areas from each plot between 14 and 17 Oct. 1999 and analyzed for starch and carbohydrate levels. Samples were rinsed to remove soil, thatch was discarded, and cores were separated into roots (below thatch) and shoots plus stems (leaves, sheaths, stolons, and crowns). Samples were frozen at -20°C until analyzed. Each sample was lyophilized, then ground with a coffee grinder along with a mortar and pestal. Randomly selected 100 mg subsamples were extracted 3 times with 3.5 mL 80% ethanol and the tissue was dried with a rotary evaporator (Savant Speedvac SC200; Farmingdale, NY). The aqueous extract then was mixed with 5.0 mL chloroform, the aqueous phase was removed with a transfer pipette, and dried with a rotary evaporator. The dried tissue and supernatant also were used further for starch and carbohydrate analyses, respectively.
For starch analysis, acetate buffer (0.1 M, pH = 5.0, 2.0 mL) was added to each aforementioned dried tissue sample and incubated at 100°C for 1 h. After incubation, 1.66 mg amyloglucosidase and 0.10 mL acetate buffer were added to each sample solution and incubated for 16 h at 55°C. Aliquots of 200 µL for shoot plus stem tissue and 400 µL for root tissue were added to 1.0 mL of ddH2O. Aliquots of 250 µL of the solution in triplicate were added to 2.5 mL of color reagent solution [1 PGO capsule, 1.6 ml o-dianisidine dihydrochloride solution filled to 100 ml volume with ddH2O; Glucose Kit 510-A (Sigma; St. Louis, MO)] and allowed to incubate at room temperature for 40 min. Absorbance for samples, a starch standard and a water blank were measured at 440 nm with a U-3110 spectrophotometer (Hitachi; Tokyo, Japan) with a standard starch curve.
For carbohydrate analysis, aqueous extracts from the aforementioned supernatant were derivatized with 2.0 mL pyridine, 60.0 mg hydroxylamine hydrochloride and 10.0 mg b-phenyl-D-glucopyranoside (internal standard) in a combined solution and agitated overnight. After carbohydrates were dissolved, samples were heated at 72°C for 1 h while being vortexed twice during incubation. Aliquots of 1.0 mL were added to 1.0 mL of hexamethyldisilazane followed by 0.10 mL trifluroacetic acid and allowed to sit at room temperature for 1 h. Sample solutions then were transferred to gas chromatography (GC) vials, removing precipitate by carefully extracting the solution with transfer pipettes. A Hewlett-Packard (HP; Wilmington, DE) 5890 GC with DB17 (J&W Scientific; Folsom, CA) diphenyldimethylpolsiloxane column (30 m length) was used with a computed standard curve of 0 to 1000 µM for each carbohydrate (Bach Knudsen and Li, 1991). Samples of 2.5 mL were injected into the GC column at 285°C with an initial oven temperature of 150°C. Oven temperature was ramped 5°C min-1 to 210°C with a 5 min plateau, then ramped 10°C min-1 to 285°C. Data were acquired with an HP 59970 MS ChemStation. Retention times were determined for mannitol (8.4 min), sorbitol (8.8), fructose (9.5), glucose (11.0), inositol (12.4), sucrose (24.8), and raffinose (32.7). Concentration was determined by area under the peak and converted from mM to mg-carbohydrate g-1 tissue.
Experimental Design and Data Analyses
In experiment 1, TE and N treatments were placed in a factorial arrangement within a randomized complete block design with three replicates of each treatment in each of the two shade house locations. Plots were 0.9 by 1.8 m and blocked from east to west because of perceived differences in light quantity or quality with respect to shade house architecture and sun angle.
In experiment 2, TE rates and timing of application were placed in a factorial arrangement within a randomized complete block design with three replicates of each treatment in each of the two shade house locations. Plots were 1.2 by 2.4 m and were blocked from east to west.
Data for both experiments were analyzed with analysis of variance and SAS mixed procedures (SAS Institute, Inc., Cary, NC). Fishers protected means separation tests were computed for significant effects (P = 0.05, except where otherwise indicated). Dates were analyzed separately for percent cover, quality and color ratings.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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In general, turf was more adversely affected by RLC in the southern shade house, when compared with the northern house. One possible explanation for this difference was that the southern shade house received additional shade during late afternoon in the spring and autumn from a Zelcova serrata L. tree positioned approximately 35 m to the southwest of the house. Therefore, data were analyzed by repeated measures in space (Keselman and Keselman, 1993). In addition, results confirmed initial observations that turf was more adversely affected by RLC in the eastern block, when compared with the central or western blocks of each shade house due to perceived differences in light quantity throughout the day (data not shown). Analysis of variance revealed no significant interactions between TE and N for all data collected.
In 1998, turfgrass cover decreased rapidly within 1–2 wk following installation of the shade cloth in July 1998 (data not shown). Initially, turfgrass shoots were observed to become chlorotic and etiolated followed by a reduction in stand density. In 1998, initial TE applications were applied one month after shade cloth was installed. At that time, turf cover under RLC was reduced to approximately 40% of complete cover (data not shown). By 23 Sept. 1998, turf cover increased to as high as 73% of complete cover (Table 1). On 12 Sept. 1998, cover data under RLC indicated an increase in turf cover by both TE treatments compared with non-TE treated plots (Table 1). In addition, plots treated with 0.042 kg a.i. ha-1 TE continued to exhibit increased turf cover when compared with non-TE treated plots on 23 Sept. 1998. Turf grown under RLC with low N had 3.3 and 7.2% greater cover ratings, when compared with high N plots on 12 and 23 Sept. 1998, respectively.
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Algae cover ratings were inversely proportional to turf cover. Turf treated with low N and TE exhibited a decrease in algae cover, which appeared to be related to improved turfgrass cover provided by TE (data not shown). No visible change in algae color or algae phytotoxicity was observed with any N or TE treatment. Moss cover was not affected by TE or N treatments (data not shown).
Clipping yield was not affected by TE or N on most dates (data not shown). Although TE visibly suppressed creeping bentgrass vertical leaf growth, clipping yields were likely confounded by higher cover in TE-treated plots maintained under low N. Similarly, more vertical leaf growth and less turfgrass cover was observed in high N-treated bentgrass, when compared with low N-treated turf.
There were no significant root mass, tiller count or root:shoot ratio (RSR) differences among TE and N treatments in 1998 (data not shown). This may have been due to the short period of time that the treatments were applied in relation to the tissue sampling date.
Significant differences, however, were found in 1999 between N rates (Table 2). In 1999, the low N rate increased the number of tillers by 0.21 tillers cm-2 and reduced RSR by 5.26 mg tiller-1. The increase in the number of tillers by low N rate significantly lower the RSR because the additional tillers had the same total root mass per unit area. The reduction in tillers might be attributed to the competition for energy used for N assimilation and reduction at the expense of carbohydrate and tiller production (Beard, 1973; Green and Beard, 1969; Mazur and Hughes, 1976; Schmidt and Blaser, 1967; Westhafer et al., 1982).
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No significant differences were found in starch content between shoots plus stem (leaves, sheaths, stems and stolons) and root tissues, regardless of TE and N treatment (data not shown). Shoot plus stem tissue, however, contained a ten-fold greater starch concentration than root tissue, but the trend was not significant. Starch is a major storage form of carbohydrates in plants and is stored in vegetative tissues and stems (Smith, 1968; Smith, 1972). Therefore, larger quantities of starch would be expected in shoots plus stems rather than roots. Regardless, TE and N had no significant effect on starch concentration in this study.
A comparison of carbohydrates found in shoot plus stem tissue under RLC showed high levels of fructose, glucose, sucrose, and raffinose in all treatments (Table 3). Mannitol, sorbitol, and inositol were extracted in small quantities. The concentrations of these carbohydrates in creeping bentgrass were consistent with previous findings in turfgrasses (Smith, 1972; Westhafer et al., 1982; Zamski and Schaffer, 1996). Shoot plus stem tissue contained both the leaf and stems and would be expected to contain large quantities of glucose, sucrose and fructans (Smith, 1968). Very high levels of fructose and sucrose were found in shoot plus stem tissue. There were much higher total carbohydrate levels in shoots plus stem versus root tissue. Furthermore, TE-treated plants had higher total sugar levels than non-TE treated plants. There was no effect of N on carbohydrate levels. There was, however, a non-significant trend for increased fructose, sucrose, and raffinose levels in turf treated with low N. The trend may suggest that larger quantities of carbohydrates are available for plant function when N levels are low under RLC.
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No turf discoloration or stand loss was observed due to the 60% shade imposed regardless of TE treatment. Results under the more intense shade in experiment 1 (80% shade) would be expected to differ from the results of experiment 2. In general, all treatments had acceptable turfgrass quality (Table 5). Bentgrass treated with 0.050 kg a.i. TE ha-1 every 2 wk generally exhibited highest quality on all dates. The higher turfgrass quality was attributed to a darker green color rather than an increase in density or uniformity (Table 6). Trinexapac-ethyl has been shown to increase total chlorophyll content per unit leaf tissue (Stier, 1997; Stier and Rogers, 2001). Bentgrass that was not treated with TE and grown under RLC had the lowest quality and color ratings. No consistent quality or color differences were found in bentgrass treated with 0.025 kg a.i. TE ha-1 on 2- or 4-wk intervals or with 0.050 kg a.i. TE ha-1 on 4-wk intervals, when compared with the other treatments. Spray initiation timing did not affect bentgrass quality following 15 Sept. in TE-treated plots (Table 5). The mid-season TE spray initiation date provided similar results when compared with early season spray initiation with regards to turfgrass quality after 2 TE applications. Bentgrass color, however, was highest following the early season spray initiation on 30 Aug. and 10 Oct. (Table 6).
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CONCLUSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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Received for publication January 29, 2001.
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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