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Dep. of Horticulture, Univ. of Wisconsin, Madison, WI 53706
* Corresponding author (jstier{at}facstaff.wisc.edu)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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-hydroxy-methylene)-3,5-dioxo-cyclohexane-carboxylic acid ethyl ester] (TE) and two N types, liquid and granular. Supina bluegrass provided the best overall turf quality among the three species tested. Liquid urea (foliar uptake) improved turf quality of creeping bentgrass while Kentucky bluegrass responded better to granular urea (root uptake). Supina bluegrass response to fertilizer type was seasonally dependent. Both monthly and bimonthly applications of TE (0.05 kg ha-1) significantly improved turf quality, density, and chlorophyll levels of all species, though visual effects of bimonthly applications dissipated before subsequent applications. Creeping bentgrass divot recovery was faster than for either Kentucky bluegrass or supina bluegrass. Divot recovery was not influenced by TE or N source. Supina bluegrass had significantly greater growth rates during late autumn through spring than either Kentucky or creeping bentgrass. The best turf quality was achieved using supina bluegrass, treated monthly with TE, and fertilized with granular N in the spring and liquid N in the summer and autumn.
Abbreviations: ET, evapotranspiration • FIA, flow injection analysis • PAR, photosynthetically active radiation • PGR, plant growth retardant • RLC, reduced light conditions • TE, trinexapac-ethyl
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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One management technique to improve turf in shaded conditions is the application of plant growth retardants (PGRs) that inhibit gibberellic acid biosynthesis. In addition to reducing clipping yields, multiple applications of PGRs increased turf density, color, and quality of trafficked Kentucky bluegrass and supina bluegrass under RLC of <20% full sunlight (Stier et al., 1999; Stier and Rogers, 2001). Trinexapac-ethyl also increased tillering and density of creeping bentgrass turf mowed at greens height under 80% shade, but turf quality was still unsatisfactory due to low irradiance (Goss et al., 2002). A 60% shade treatment was insufficient to cause turf discoloration or loss of density. In Goss et al. (2002), neither TE application rate nor interval (2 vs. 4 wk) affected turf density at either 80 or 60% shade. The effect of application interval may be species-specific, as monthly and bimonthly treatments of TE enhanced the shade tolerance of zoysiagrass [Zoysia matrella (L.) Merr.] compared with trimonthly applications and untreated turf (Qian and Engelke, 1999).
Reduced fertility, particularly N, has long been a recommended practice for maintaining turf in RLC. Goss et al. (2002) used liquid applications of N to confirm previous reports that lower N rates (150–185 kg ha-1 annually) resulted in better quality turf than higher N rates (212–235 kg ha-1). It is not known if the form of N can influence turf growth under shaded conditions. Granular forms of N are absorbed through the roots and transported to the shoots of the plant, a process that could be energy-inefficient by forcing roots to use their carbohydrates for energy to assimilate and transport N to the shoots (Jiang and Hull, 1999). In a shaded environment, turfgrass root development and energy budgets are stressed due to low photosynthetically active radiation (PAR). Since the majority of N is utilized in the shoots, increased foliar absorption from liquid applications of N may increase N use efficiency and allow more photosynthate to be allocated to the roots of the plant, enabling the turfgrass to attain more nutrients and water (Bushoven and Hull, 2001). Spangenberg et al. (1986) showed the effects of liquid-applied and granular urea on color of Kentucky bluegrass in full sunlight were dependent on the season. Comparisons between granular and liquid N applications on shaded turf have not been reported. Similarly, documentation is lacking on the potential interactive effects between N type and PGRs on turf quality and recovery in shaded conditions.
The objectives of this study were to compare the following parameters to develop a management system for shaded tee boxes: (i) species selection (supina bluegrass, Kentucky bluegrass, and creeping bentgrass), (ii) TE application intervals (none, 28 d, and 56 d), and (iii) liquid (foliar-applied) vs. granular (soil-applied) N.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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A shade structure was constructed over the site in April 2000 using 80% shade cloth (Catalog Number 27-8025, Hummert International, Earth City, MO). The shade cloth was placed 2.4 m above the plots and enclosed the top 2.1 m on all sides of the shade structure. Shading coincided with leaf development and loss on local maple trees (Acer spp.). Shade cloth was installed when local trees achieved an estimated 80% leaf development (8 May 2000 and 9 May 2001) and removed when trees lost 80% of their leaves (25 Oct. 2000 and 29 Oct. 2001).
Environmental data under the shade canopy were monitored with a LI-1400 datalogger (Li-Cor, Lincoln, NE). Data collected included PAR, air temperature, relative humidity, and soil temperature at a 5-cm depth. Similar data were also collected in full sun using the on-site weather station at the research facility (Model CR-21X datalogger, Campbell Scientific, Logan, UT).
Plots were mowed three times weekly with a reel mower at 1.3 cm, with clippings removed. Beginning in spring 2000, turf was irrigated once weekly to replenish 50% of the moisture lost to ET. Plots were topdressed once monthly with a 0.16-cm depth of an 80:20 sand:peat mixture (Waupaca Sand & Solutions, Waupaca, WI) which met conventional specifications for putting green construction (Green Section Staff, 1993). Fungicide applications were applied as needed to control disease outbreaks. Azoxystrobin [methyl (E)-2-{2-[6-(2-cyanophenoxy)pyrimidin-4-yloxy]phenyl}-3-methoxyacrylate] (0.64 kg ha-1), iprodione [3-(3,5-dichlorophenyl)-N-(1-methylethyl)-2,4-dioxo-1-imidazolidinecarboximide] (5.93 kg ha-1), and chlorothalonil [tetrachloroisophthalonitrile] (5.49 kg ha-1) were applied at 14- to 21-d intervals during the growing season to control disease, particularly microdochium patch [Microdochium nivale (Fr.) Samuels & I.C. Hallett]. Mefenoxam {(R)-2-[(2,6-dimethylphenyl)-methoxyacetylamino]-propionic acid methyl ester} (0.68 kg ha-1) was used to control Pythium blight [Pythium aphanidermatum (Edson) Fitzp.] once in July and once in August of both years. Halofenozide [benzoic acid, 4-chloro-, 2 benzoyl-2-(1, 1-dimethylethyl) hydrazide] (7.64 kg ha-1) was applied once in July of both years to control black cutworm [Agrotis ipsilon (Hufnagel)].
The experimental design was a split-plot randomized incomplete block with four replications. Each replication contained 18 experimental units measuring 1.5 by 1.8 m. Main plots consisted of grass species with TE and N sources serving as subplots. Main plots (4.6 by 3.5 m) were split vertically, and each half received 12 kg N ha-1 every 14 d from either a granular or liquid application of urea (46-0-0). Each main plot was split horizontally to evaluate three TE application intervals.
Trinexapac-ethyl treatments were applied at three intervals during the growing season: no TE, 0.05 kg a.i. ha-1 at 28-d intervals, and 0.05 kg a.i. ha-1 at 56-d intervals. Treatments were applied in 407 L H2O ha-1 using a CO2 backpack sprayer equipped with 8004XR flat fan nozzles (Teejet, Minneapolis, MN). Trinexapac-ethyl application dates were as follows: 23 May, 23 June (28-d treatment only), 24 July, 26 August (28-d treatment only), and 21 September 2000; 23 May, 19 June (28-d treatment only), 19 July, 22 August (28-d treatment only), and 20 September 2001.
The N source was a 100% water soluble feed-grade urea (46-0-0) applied at 12 kg N ha-1 at 
14-d intervals during the growing season as either granular or liquid treatments. Nitrogen application dates were as follows: 24 May, 8 June, 22 June, 5 July, 20 July, 4 August, 16 August, 1 September, 14 September, and 3 October in 2000; and 22 May, 6 June, 20 June, 2 July, 17 July, 31 July, 15 August, 1 September, 13 September, and 28 September in 2001. Granular treatments were applied first and irrigated for 15 min after each application. After turf dried, liquid N applications (Wesely et al., 1985; Carrow et al., 2001) were applied in 407 L H2O ha-1 using a CO2 backpack sprayer equipped with 8004XR flat fan nozzles (Teejet).
Turf evaluations were conducted at 7- to 17-d intervals as plots were rated visually for turf quality (1 = necrotic turf or bare soil, 9 = dense, uniform turf; 6 = acceptable) and turf density (0–100% ground cover). Quantitative turf density ratings were collected each spring, summer, and autumn using a visual point quadrat with 100 points per plot. At each grid intersection (5-cm spacing), the presence or absence of a plant was recorded to quantify density (Woolhouse, 1976).
A divot tool was used to make divots (10- by 8-cm by 1-cm depth) each summer to allow evaluation of divot recovery. Divots were made on 31 July 2000 and 25 July 2001 and were filled with an 80:20 sand:peat topdressing mixture. Divot recovery was evaluated at 3, 7, and 10 wk after initial divoting and again on 1 May of the following year using a visual point quadrat with 40 1-cm2 squares per plot. The number of squares with at least 50% turf coverage were recorded and used to calculate the total percentage recovery.
One core (10.8-cm diam. by 10-cm depth) was collected from each subplot in November of 2000 and 2001 to determine treatment effects on root mass. The top 1.27 cm of each core was removed to eliminate verdure and thatch. Samples were split into two sections (0-to-5- and 5-to-10-cm depths) and air-dried for 24 h. Samples were incubated in 100 mL of a 3.75-M NaOH stock solution and 250 mL of distilled water at room temperature for 12 h then placed on a no. 18 sieve and washed with tap water. The remaining soil/root mixture was incubated an additional 15 min in 50 mL of the stock solution and 250 mL of distilled water then washed on a no. 35 sieve. After removing all soil, the samples were allowed to air-dry for 24 h then oven-dried at 60°C for 48 h. Oven-dried root samples were weighed then ashed in a muffle furnace to 600°C to eliminate all organic matter. Ash samples were weighed and this weight was subtracted from the pre-ashing weight to determine the amount of organic matter.
The percentage total N retained in the leaves was evaluated for each subplot. Clippings were collected one day before fertilization from a 56- by 175-cm strip through the center of each plot on 3 Aug. 2000 and 14 Aug. 2001. Samples were dried in a forced-air oven at 60°C for 48 h and then ground with a Wiley mill to pass a 40-mesh screen. Total plant N was determined by the University of Wisconsin Soil and Plant Testing Laboratory using flow injection analysis (FIA). Tissue samples (0.15 g) were digested in H2SO4, diluted with deionized water, adjusted for pH with a buffer containing NaOH, and degassed with helium. Samples were preheated to 160°C, then heated at 389°C for 120 min. After cooling, deionized water was added and the ammonium concentration determined by FIA (Ruzicka, 1983; Lachat Instruments, 1995). Nitrogen content was reported using the following formula, where WS stands for the weight of the sample (g) and CD stands for the concentration in the digest (mg N L-1): %N = (50/WSCD/1000).
Clippings were collected from a 56- by 175-cm strip through the center of each subplot for chlorophyll analysis on 10 Aug. and 10 Sept. 2001. Chlorophyll was extracted with N,N-dimethylformamide [3 mL g-1 of plant tissue (fresh weight)] (Moran and Porath, 1980). A double-beam spectrophotometer was used to determine absorbance values at 647 and 664.5 nm. Total chlorophyll amounts were calculated as described by Moran (1982).
In both years of the study, chlorophyll fluorescence measurements were collected from each subplot during spring, summer, and autumn to evaluate stress by measuring photochemical efficiency (Maxwell and Johnson, 2000). Measurements were collected in situ with a modulated chlorophyll fluorometer (Model OS5, Opti-Sciences, Tyngsboro, MA) from five subsamples in each subplot. The youngest mature leaf of five plants was taped, with clear tape, side-by-side on one of five fluorometer leaf clips per plot and dark-acclimated for five minutes. An actinic light pulse was used to saturate the photosystems and the resulting Fv:Fm ratio for each subplot was used to compare the mean photosynthetic efficiencies of the turf across treatments (Maxwell and Johnson, 2000; Steinke and Stier, 2002). The average Fv:Fm of the five subsamples in each plot was used to compare photosynthetic efficiencies of treatments.
Data were subjected to ANOVA to determine if significant treatment effects existed (MSTAT-C, 1988). Fisher's protected LSD values (P = 0.05) were calculated to separate treatment means when appropriate.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Turf Quality
Turf species and TE each significantly affected turf quality on most rating dates, while N effects were significant during June, August, and September of both years (data not shown). Most important was an interaction between species and N, which occurred on nine of 11 dates in 2000 and all 10 dates in 2001. Main effects of turf species are not discussed due to the interaction with N.
Trinexapac-Ethyl
Both monthly and bimonthly applications of TE significantly improved turf quality on most rating dates compared to untreated turf (Table 1). Monthly applications consistently resulted in superior turf quality compared to untreated turf. The benefits of bimonthly applications diminished within 6 wk after every bimonthly application. No TE treatment caused any phytotoxicity from May through August. The final autumn application of TE in 2000 caused slight phytotoxicity to both supina and Kentucky bluegrass beginning 1 to 2 wk following application and lasting for 3 wk but did not delay spring green-up the following year (data not shown).
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Divot Recovery
Species main effects proved significant on all rating dates for divot recovery. Creeping bentgrass recovered from divots significantly faster than the bluegrasses at 3, 7, and 10 wk after the divots were made except for Kentucky bluegrass in Week 10 of 2000 (Fig. 1). Kentucky bluegrass had a greater recovery rate than supina bluegrass during the summer and autumn time period after divoting. Recovery rates slowed for all species in August, while Kentucky bluegrass recovery rates increased again in September. Most recovery in supina bluegrass occurred from October to May. By late spring, supina bluegrass had recovered from most of the previous summer divot injury and ranked second in overall recovery behind creeping bentgrass. Supina bluegrass had earlier spring green-up than either creeping bentgrass or Kentucky bluegrass (Table 3). Trinexapac-ethyl and N type did not affect divot recovery of any species.
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Foliar Effects
Chlorophyll Content
No significant interactions occurred, but species, TE, and N each affected chlorophyll content (P ≤ 0.05). Creeping bentgrass had chlorophyll concentrations of 28 to 29 µg g-1 of leaf tissue while supina bluegrass and Kentucky bluegrass had similarly lower concentrations of 23 to 24 and 22 µg g-1, respectively. Chlorophyll concentrations of turf treated with TE were 26 µg g-1 compared with <23 µg g-1 for untreated turf. The color of turf treated with TE was significantly darker than untreated turf (data not shown). Liquid N applications increased chlorophyll levels compared with the granular N applications (26 vs. 23.5 µg g-1 fresh leaf tissue, respectively), but color was not significantly affected.
Photochemical Efficiency
No significant treatment effects or interactions occurred during 2000; however, a species main effect was significant (P < 0.05) in spring, summer, and autumn 2001. Kentucky bluegrass averaged the highest photochemical efficiency across the year (0.839), while supina bluegrass (0.829) and creeping bentgrass (0.828) were significantly lower (LSD0.05 = 0.04). In a practical sense the differences are trivial, as nonstressed plants typically have values near 0.8, while values well below 0.8 indicate stress (Maxwell and Johnson, 2000). Nitrogen and TE treatments did not affect photochemical efficiency.
Leaf Nitrogen Content
There were significant species and N source main effects on leaf N content in addition to an interaction between species and N source (Table 4). Supina bluegrass had a lower total leaf N content than creeping bentgrass and Kentucky bluegrass. Both supina bluegrass and Kentucky bluegrass foliage had greater total N with granular N treatments, while N type did not affect N concentration in creeping bentgrass.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Supina bluegrass did have earlier spring green-up than either creeping bentgrass or Kentucky bluegrass, which could have increased its perceived shade tolerance. Early spring green-up should result in greater annual photosynthate production, as such turf would have an extended time period of maximum PAR before shading by tree leaves. Minimal traffic and cool air temperatures during early spring are important for carbohydrate storage. The subsequent growth and potentially increased carbohydrate production/retention due to minimal traffic and low respiration and photorespiration rates would confer an advantage compared with turf with delayed spring green-up. The dramatic growth exhibited by supina bluegrass between autumn and spring would also be advantageous for recovering from damage caused by late or early season traffic and winter disease injury. There are likely additional mechanisms for shade tolerance in supina bluegrass such as an enhanced photosynthesis:respiration ratio (Stier, 1997).
The beneficial impacts of TE were expected based on previous data which showed TE enhanced turf quality in RLC (Qian and Engelke, 1999; Stier and Rogers, 2001; Goss et al., 2002). The increased chlorophyll concentration resulting from TE did not affect photochemical efficiency, suggesting there was no physiological benefit despite the potential for increased photon capture (Prezelin and Nelson, 1995). An increase in chlorophyll content may potentially increase irradiance harvesting and increase the photosynthetic potential of a shaded turf (Heckman et al., 2001). Stier and Rogers (2001) found Kentucky bluegrass had a higher chlorophyll content than supina bluegrass under RLC, but chlorophyll levels were similar in our study. Differences between the two studies may have been due to cultivar differences or the effect of field conditions (changes in temperature, humidity, and increased PAR) vs. the semicontrolled environment and lower PAR used by Stier and Rogers (2001).
The reduction in turf quality between applications of the bimonthly TE application in our study was not unexpected, as label instructions note the effects dissipate between 4 and 8 wk. Higher rates may increase the length of growth regulation, but the phytotoxicity potential may also increase. Although Qian and Engelke (1999) found no difference in turf quality between monthly and bimonthly applications of TE at the same rate we used, zoysiagrass has a relatively slow growth rate and may have more slowly metabolized the TE.
Previous data showed TE substantially reduced clipping yields under RLC (Stier and Rogers, 2001; Goss et al., 2002), which presumably could delay divot recovery since growth was slowed. Calhoun and Branham (1995), however, reported PGR application did not reduce divot recovery of creeping bentgrass under full sun. Gibberellic acid inhibitors like TE decrease leaf elongation rates (Ervin and Koski, 1998) but not other forms of growth. Part of the reason divot recovery was not decreased by TE in our study may have been due to additional tillering stimulated by the TE (Stier and Rogers, 2001; Goss et al., 2002). Divot recovery in the Kentucky bluegrass plots occurred primarily from leaf development of surrounding plants which masked the divot, rather than from rhizome growth and new plant production. Both creeping bentgrass and supina bluegrass appeared to fill in divots by a combination of leaf development and stoloniferous growth.
The increased performance of Kentucky bluegrass with granular N instead of liquid N was surprising, as cool-season grasses are considered inefficient users of soil N (Hull and Jiang, 1998). Their inefficiency is likely due to root loss caused by heat stress during summer, which restricts NO3 uptake (Hull and Jiang, 1998); however, this may not occur at low N application rates. In shade, soil temperatures are buffered compared with full sun, but the combination of shade and moderately warm soil temperatures may be enough stress to decrease root growth, making foliar N applications useful. In our study, the favorable response of Kentucky bluegrass to granular N and the response of creeping bentgrass to liquid N could have been partly due to differences in root mass, as Kentucky bluegrass had significantly greater root mass in the second year of the study than either creeping bentgrass or supina bluegrass. The shift in preference between granular N in the spring and foliar N by mid-summer in supina bluegrass may also be explained by the same phenomena. In addition, Horgan et al. (2002) found ideal conditions for denitrification were created with adequate summer rainfall coupled to increased soil temperatures. In a shaded environment, soils tend to remain moist for extended periods and soil temperatures may not reach maximum levels until mid to late summer. The increased performance of supina bluegrass fertilized with liquid (foliar) N compared with granular N treatments during mid-summer may have correlated to denitrification levels. Though root mass of supina bluegrass was significantly less than Kentucky bluegrass by the autumn, supina bluegrass may have had greater root mass in the spring due to its early spring green-up and growth which diminished during the season due to combined shade and heat stress.
Factors such as root morphology, nitrate transporters, and source–sink relationships for carbohydrate energy may impact N uptake and assimilation (Sullivan et al., 2000b). The perennial root system of Kentucky bluegrass could result in thicker roots that are better adapted for N absorption (Sullivan et al., 2000b). Nitrate can either be assimilated by roots for transport to shoots or transported through xylem to shoots for assimilation. At high rhizosphere concentrations NO3 is primarily assimilated in shoots, which reduces C:N and subsequently root:shoot ratios (Jiang and Hull, 1998). Since roots use energy from the pentose phosphate pathway to reduce NO3, species that reduce a greater proportion of NO3 in roots rather than shoots may have a stronger sink for carbohydrate translocation to roots, thus enhancing root growth (Jiang et al., 2001). Low rhizosphere NO3 levels result in root assimilation and enhances root:shoot ratios (Jiang et al., 2001), but it is unknown if high NO3 concentrations in leaves could have a similar effect.
Not all species absorb foliar N equally. Wesely et al. (1985) found foliar N absorption ranged between 31 to 61% for six species of cool-season turfgrass, with differences between Kentucky bluegrass and creeping bentgrass being cultivar-dependent. Differences in cuticle thickness could affect absorption, but no data are available on cuticle differences among turf species. In our study, granular applications resulted in significantly greater N levels in both supina bluegrass and Kentucky bluegrass compared with liquid N treatments even though high N levels in shoots may not be desirable. Jiang and Hull (1999) suggested an N-efficient turfgrass should assimilate N in roots and transport it to shoots on demand for maximum N use efficiency. Supina bluegrass may utilize such a mechanism as part of its shade tolerance, as clippings had significantly less N than either Kentucky bluegrass or creeping bentgrass regardless of N source. Additional information on N absorption and localization of N assimilation is needed to determine why N types have different effects between species.
The choice of N type for fertilization may depend on the time of year and species as indicated by the response of supina bluegrass. In addition, Spangenberg et al. (1986) reported Kentucky bluegrass in full sun obtained a better color rating with granular urea early in the year and ended the year performing better with the foliar-applied urea. In our study under shade, supina bluegrass paralleled this trend, but Kentucky bluegrass did not and retained better color, quality, and density with the granular urea throughout the entire year. Supina bluegrass was able to maintain acceptable turf density and color with both granular and liquid N applications; however, turf quality varied with the N source. The effect of N carrier needs to be determined on foliar absorption under shade, as Sullivan et al. (2000a) showed N from foliar-applied NO3 was more readily absorbed by tall fescue (Festuca arundinacea Schreb.) than N from urea.
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ACKNOWLEDGMENTS |
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Received for publication August 22, 2002.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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affecting turf quality under reduced irradiation (80% shade).
