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TURFGRASS SCIENCE

Flurprimidol Effects on Kentucky Bluegrass under Reduced Irradiance

^a Dep. of Horticulture, University of Wisconsin, Madison, WI 53706-1590 USA
^b Dep. of Crop and Soil Sciences, Michigan State University, East Lansing, MI 48824-1325 USA

jstier{at}facstaff.wisc.edu

	ABSTRACT

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Turf management in shaded conditions is difficult in part because the reduced irradiance causes excessive shoot elongation, reduced tillering and root growth, and weak plants exceptionally prone to damage. Successful installation of natural grass athletic fields in covered or shaded stadia require improved management procedures. The objective of this study was to determine if flurprimidol [-(1-=-methylethyl)--[(trifluourmethoxy)phenyl]-5-pyrimidinemethanol], a gibberellic acid (GA)-biosynthesis inhibitor, could be used to increase turf quality for athletic fields in covered stadia and other reduced irradiance situations. Portable plots of Kentucky bluegrass (Poa pratensis L.) were established outdoors and moved inside a covered-stadium simulator facility (CSSF). Four rates of flurprimidol (0.56, 1.12, 1.68, and 2.80 kg ha^-1) were compared against a control. Studies were conducted under ambient (equivalent to 2–3% full summer sunlight) and supplemental (equivalent to 9–18% full summer sunlight) irradiance. The turf was evaluated with and without simulated soccer traffic. Under ambient irradiance conditions, turf quality generally declined to unacceptable levels within 30 d, although the low rate of flurprimidol extended this period for up to 60 d. Supplemental irradiance provided turf of acceptable quality for up to 70 d and the low rate of flurprimidol extended this period for >100 d. Applications repeated within 30 to 40 d virtually stopped measurable shoot growth under all irradiance and traffic regimes. Flurprimidol did not affect turf shear resistance or rooting but did increase tillering. Low rates of flurprimidol should be useful for maintaining turf in reduced irradiance conditions because turf quality is enhanced with minimal risk of phytotoxicity.

Abbreviations: ANOVA, analysis of variance • CSSF, covered-stadium simulator facility • GA, gibberellic acid • HPS, high pressure Na • PAR, photosynthetically active radiation • PGR, plant growth regulator • PPFD, photosynthetic photon flux density • RH, relative humidity

	INTRODUCTION

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
THE AMOUNT OF TURF managed under shaded conditions in the U.S. has been estimated to be as high as 25% (Beard, 1973). Lack of sunlight for vigorous growth and enhanced disease pressure make the maintenance of high quality turfgrass in the shade difficult. Consequently, suggested management practices for turf under shaded conditions are different than practices under full sun (Smith and Rieke, 1986). The recent use of turfgrass for indoor athletic facilities has accelerated the interest for development of management strategies for turf under reduced irradiance (Rogers et al., 1994). Several systems to install turf inside covered stadia have been proposed but do not include management plans (Anonymous, 1995; Keirle, 1995; Tracinski, 1993). Turf grown under reduced irradiance is especially subject to damage from disease (Beard, 1965) and traffic (Cockerham et al., 1994) because of the weakened nature of the turf plant.

Sufficient irradiance quality and quantity are necessary for normal photomorphogenic responses and photosynthesis. Although cool-season turfgrasses may be able to grow at as little as 5% sunlight (Beard, 1965), at least 25 to 35% sunlight is required for optimal turf growth (McBee and Holt, 1966). In densely shaded conditions, the quality of irradiance can be the most critical factor for acceptable turf growth (McBee, 1969). McBee (1969) reported that blue irradiance was reduced more than red irradiance under dense deciduous tree shade (post oak, Quercus stellata Wangenh.). McBee (1969) concluded that red irradiance caused the development of spindly, elongated bermudagrass (Cynodon spp.) plants and resulted in loss of turf quality compared with growth under blue irradiance. The elongation is apparently mediated by the production of excessive levels of GA due to the inactivation of phytochrome by far-red irradiance (Cooke et al., 1975; Rood et al., 1986).

In grasses, GA stimulates shoot elongation largely by increasing cell elongation (Rood et al., 1986; Watschke et al., 1992). Applications of GA reduce turf quality by causing a lighter green color (Leben et al., 1959; Juska, 1959), reduced tillering (Leben et al., 1959), root growth, and root/shoot ratios (Juska, 1959). Similar responses have been observed on turf grown under shaded conditions (Peacock and Dudeck, 1981), suggesting that GA synthesis is at least partially responsible. A sparse turf stand, easily damaged by traffic and disease, is characteristic under shaded conditions and proper management is difficult.

The objective of this study was to evaluate the effects of flurprimidol on turf in reduced irradiance conditions. Flurprimidol is a plant growth regulator (PGR) which inhibits GA biosynthesis (Sterret and Tworkoski, 1987). When flurprimidol is used under normal sunlight conditions to reduce mowing requirements, side effects include a transient period of phytotoxicity, followed by darker green color of new growth and increased tillering and turf density (Dernoeden, 1984; Diesburg and Christians, 1989). The effects of GA-inhibiting PGRs on turf under conditions of reduced irradiance have not been documented, except for one preliminary report (Rogers and Stier, 1993).

	Materials and methods

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Portable plots were established by filling wooden boxes (1.2 by 1.2 by 0.15-m depth) with a sand/peat mix (80:20, v/v) of pH 7.9 and initial P and K levels of 46 and 49 kg ha^-1, respectively. The sand/peat mix was compacted and a starter fertilizer (13-25-12) supplying 7.6 g N m^-2, 6.4 g P m^-2, and 5.8 g K m^-2 was raked into the top 1.25 cm. On 30 Sept. 1992, the plots were sodded using a washed Kentucky bluegrass blend (20% each of `Trenton', `Midnight', `Aspen', `Rugby', and `Kelly'). Plots were sodded 10 Sept. 1993 with a washed Kentucky bluegrass blend (20% each of Trenton, Midnight, Aspen, `Glade', and `Parade'). Plots were mown once to twice weekly at a 3.8-cm height during establishment and irrigated as necessary to prevent moisture stress. Urea (2.4 g N m^-2) was applied to aid establishment at 3 wk after sodding in 1992 and at 2 and 5 wk after sodding in 1993.

Plots were moved inside the CSSF on 21 Nov. 1992 and on 22 Nov. 1993. The CSSF, constructed of Sheerfill II fiberglass fabric (Chemical Fabrics, Buffalo, NY), was an air-supported structure (18 by 31 m) designed to simulate the conditions inside the Pontiac Silverdome (Stier et al., 1993). Automated furnaces were used to heat the facility. Temperature and relative humidity were recorded daily with a sling psychrometer. Studies were conducted inside the CSSF in both ambient and supplemental irradiance conditions.

Supplemental irradiance was supplied by 15 Son-Agro high pressure Na (HPS) lamps (430 W) suspended 2.7 m above the turf surface (3.7 by 6.1 m). In 1992, white polyethylene sheets were suspended on two sides of the plot area to separate the lighted plots from the unlighted plots. In 1993, mylar was used instead of white polyethylene because it reflected irradiance more uniformly. Radiation data were collected weekly from each plot within 1 h of the solar zenith. From November 1992 through February 1993, illumination was measured weekly with a photometer (Greenlee Textron, Rockford, IL) and occasionally with a Li-Cor LI-1800 spectroradiometer (Li-Cor, Lincoln, NE). Photometric units (lux) were converted to quantum units by multiplying against a constant derived from data collected concurrently with the Greenlee photometer and the spectroradiometer. The constants used were 0.1613 for ambient irradiance and 0.2215 for supplemental irradiance from the HPS lamps. From December 1993 through March 1994, data were collected weekly within 1 h of the solar zenith using only the spectroradiometer.

The hourly proportions of solar radiation throughout the day of each measurement were collected with an onsite outdoor pyranometer (PY 14226, Li-Cor) linked to a Maxi weather station (Rain Bird Sales, Glendora, CA). The photosynthetic photon flux densities (PPFDs) determined inside the CSSF were converted to a total daily PPFD based on the hourly proportion of solar radiation.

A randomized complete block design with three replicates was used to evaluate the effects of flurprimidol rates on turf quality and growth in both ambient and supplemental irradiance conditions. Spatial constraints forced the irradiance treatments to be conducted independently of one another, preventing direct comparisons between irradiance levels. Flurprimidol was applied at sublabel (0.56 kg ha^-1), label (1.12 and 1.68 kg ha^-1), and above-label (2.8 kg ha^-1) rates after moving the plots inside and again 6 wk later (25 Nov. 1992, 5 Jan. 1993, 30 Nov. 1993, and 7 Jan. 1994). In 1993, a third treatment was applied (11 Feb.) toward the end of the experiment. The plots were irrigated with 1.25 cm of water following the flurprimidol application according to label instructions. Experiments were conducted at each irradiance with and without simulated soccer traffic. Traffic applications began 3 wk after the first flurprimidol application. Traffic was applied to one-half of each plot by having a person wearing molded soccer cleats walk a total of 50 passes on the following dates: 15 Dec. 1992, 7 Jan. 1993, 22 Jan. 1993, 29 Jan. 1993, 21 Dec. 1993, 29 Dec. 1993, 12 Jan. 1994, 18 Jan. 1994, 1 Feb. 1994, 8 Feb. 1994, 22 Feb. 1994, 2 Mar. 1994, 15 Mar. 1994.

Fertilizer was applied to all plots immediately preceding the flurprimidol applications. Fertilizer was applied as urea (2.4 g N m^-2) on the first application date in 1992 and 1993. A complete fertilizer (18-3-18) was applied on the succeeding dates in both years to supply 3.7 g N m^-2, 0.6 g P m^-2, and 3.0 g K m^-2. Plots received additional irrigation as necessary to prevent moisture stress. Plots in the ambient irradiance study received 1.27 cm water every 14 to 21 d, while plots in the supplemental irradiance study received 1.27 cm water every 7 to 10 d. Chlorothalonil (tetrachloroisophthalonitrile) was applied once each year to control leafspot disease caused by Drechslera/Bipolaris spp. (13.8 kg a.i. ha^-1 on 8 Jan. 1993 and 8.9 kg a.i. ha^-1 on 14 Jan. 1994).

Plots were mowed at a 3.2-cm height once to twice weekly as needed to prevent removal of more than one-third of the leaf tissue. Clippings were collected weekly beginning 3 wk after the first flurprimidol application, oven-dried at 60°C for 48 h, and weighed. Visual turf quality ratings (scale of 1 to 9, where 1 is completely brown turf or bare soil and 9 is dark green, dense turf) were recorded at 14-d intervals. An Eijkelkamp Shearvane Type 1B apparatus was used to assess turf shearing strength three times during each study (Rogers et al., 1998). Two torque readings were collected from each plot and averaged for analysis. At the end of the studies, three cores (10-cm diameter, 15-cm depth) were collected from each plot and used to estimate plant biomass. Plant density was determined by counting the number of plants in each core. Rooting depth was determined by measuring the length of an estimated top 75% of the root mass. Root mass was quantified using roots harvested below the turf–soil interface. The roots were washed using the hydropneumatic elutriation system (Smucker et al., 1982), oven-dried at 60°C, and weighed. The results were averaged across the three subsamples and used for analysis of variance (ANOVA). In the second year, the average number of tillers per plant were obtained from a random sample of five plants from each of the three cores. Verdure biomass was estimated by separating green tissue from dead or necrotic tissue in each core. Verdure biomass was oven-dried at 60°C for 48 h and weighed.

Data were analyzed using the one factor randomized complete block design program in MSTAT (MSTAT, 1988) for ANOVA. Despite the split-plot nature of the experiments (whole plots = flurprimidol rates, subplots = traffic), each of the four irradiance–traffic combinations were analyzed separately. Treatment means were separated using Fisher's protected LSD values when appropriate .

	Results and discussion

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Environmental Data
The fiberglass fabric of the CSSF transmitted 11 ± 2% solar radiance. The temperature inside the CSSF was normally maintained at 16.8 ± 0.9°C; temperature extremes ranged from 3 to 23°C. Relative humidity (RH) averaged 44.8% ± 6.2 with a range of 24 to 70% RH. The fiberglass fabric of the CSSF provided relatively neutral shading (Fig. 1) . The HPS lamps provided a high proportion of their radiation in the yellow to red wavelengths. Ambient PPFD was greater the second year of the study compared with the first year due in part to less cloud cover (Table 1) . During the second year of the study, the higher PPFD and longer establishment time resulted in improved turf quality, particularly in ambient irradiance conditions. A preliminary study inside the Pontiac Silverdome stadium during summer in 1992 suggested turf with a poorly developed root system would not survive as long in reduced irradiance conditions as turf with a well-established root system (unpublished data, 1992).

View larger version (26K): [in this window] [in a new window]   Fig. 1 Photon flux density of solar radiation compared with ambient and supplemental irradiances inside the Covered Stadium Simulator Facility (CSSF) at 1330 h, 30 Dec. 1993 in East Lansing, MI

View this table: [in this window] [in a new window]   Table 1 Photosynthetic photon flux density (400–700 nm) of irradiance inside the Covered Stadium Simulator Facility (CSSF) in East Lansing, MI.

Experiment 1 — No traffic
Turf Quality
In conditions of ambient irradiance, turf quality was not affected by flurprimidol and declined to unacceptable levels within 4 wk after being moved into the CSSF the first year (Table 2) . In the second year, the turf was dormant when it was moved into the CSSF, but the turf resumed foliar growth within 1 wk, which coincided with the first flurprimidol treatments. Turf quality was better in the second year, probably due to the greater irradiance of 1993-1994, and acceptable turf quality was maintained throughout the study at the sublabel flurprimidol rate. The highest label rate and the above-label rates caused phytotoxicity 4 wk after the second flurprimidol application. These results agree with Dernoeden (1984), in which two applications of flurprimidol at label and above-label rates caused phytotoxicity to Kentucky bluegrass–red fescue (Festuca rubra L. subsp. rubra) turf in normal sunlight conditions. Diesburg and Christians (1989) reported phytotoxicity at sublabel rates, but this was dependent on the season and temperature, with the least phytotoxicity during cool conditions in the spring. Phytotoxicity did not occur at the sublabel rate in our studies under reduced light and a steady, cool temperature of 17°C.

Clipping Yields and Biomass
All rates of flurprimidol significantly reduced clipping yields beginning 3 to 4 wk after the initial treatment (Table 3) . Under ambient irradiance all rates of flurprimidol reduced clipping yields similarly. Vertical growth stopped following the second flurprimidol application regardless of rate.

Flurprimidol treatments increased tillering but did not affect root growth (Table 4) . In supplemental irradiance conditions, flurprimidol had a tendency to reduce plant density, although verdure was relatively unaffected due to the enhanced tillering. In ambient irradiance conditions, sublabel and label rates of flurprimidol effectively increased verdure . Tiller and verdure data were not collected the first year.

View this table: [in this window] [in a new window]   Table 4 Effect of flurprimidol on root and shoot growth of Kentucky bluegrass maintained under ambient (1.0 and 1.7 mol m-2 d-1 average photosynthetically active radiation (PAR) in 1992–1993 and 1993–1994, respectively) and supplemental (5.6 and 9.0 mol m-2 d-1 average PAR in 1992–1993 and 1993–1994, respectively) irradiance conditions inside the Covered Stadium Simulator Facility in East Lansing, MI#

Experiment 2 — Simulated Soccer Traffic
Turf Quality
Flurprimidol ameliorated the effect of traffic under both ambient and supplemental irradiance (Table 5) . Quality of untreated turf became unacceptable within 4 to 6 wk under ambient irradiance and within 8 wk under supplemental irradiance. The sublabel rate of flurprimidol was particularly effective, providing acceptable turf quality for up to 10 wk under ambient irradiance and for >14 wk under supplemental irradiance. Multiple applications of the higher rates of flurprimidol hastened turf death under ambient irradiance and were phytotoxic with supplemental irradiance.

View this table: [in this window] [in a new window]   Table 5 Quality ratings of Kentucky bluegrass treated with flurprimidol, and subjected to simulated soccer traffic under ambient and supplemental irradiance conditions inside the Covered Stadium Simulator Facility in East Lansing, MI.

View this table: [in this window] [in a new window]   Table 6 Clipping yields of Kentucky bluegrass treated with flurprimidol and subjected to simulated soccer traffic under ambient and supplemental irradiance conditions inside the Covered Stadium Simulator Facility in East Lansing, MI.

View this table: [in this window] [in a new window]   Table 7 Effect of flurprimidol on root and shoot growth of Kentucky bluegrass subjected to traffic under ambient (1.0 and 1.7 mol m-2 d-1 average photosynthetically active radiation (PAR) in 1992–1993 and 1993–1994, respectively) and supplemental (5.6 and 9.0 mol m-2 d-1 average PAR in 1992–1993 and 1993–1994, respectively) irradiance conditions inside the Covered Stadium Simulator Facility in East Lansing, MI.#

	Conclusions

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Untreated turf was flaccid, providing a shaggy appearance, and had a light green color. Above-label rates of flurprimidol caused phytotoxicity after the second application. Multiple applications of sublabel rates of flurprimidol were effective at maintaining acceptable turf quality for extended periods in the severely reduced irradiance conditions of the CSSF. Sublabel rates of flurprimidol were effective at maintaining turf quality for at least 100 d, even under ambient irradiance of 1.7 mol m^-2 photosynthetically active radiation(PAR) d^-1 and no traffic, provided the turf was sufficiently established before being placed into the CSSF. Turf of acceptable quality was maintained for a maximum of 60 d under ambient irradiance when subjected to traffic. Supplemental irradiance extended the time period for maintaining acceptable turf quality, under traffic conditions, to a minimum of 100 d (the maximum amount of time tested). Supplemental irradiance enhanced turf response to flurprimidol. Insufficient irradiance was the most limiting factor for turf inside the CSSF, particularly during December and January when solar radiation was lowest for the year.

All rates of flurprimidol significantly reduced clipping yields for a minimum of 6 wk. Flurprimidol improved turf density through increased tillering, although two or more applications of flurprimidol effectively voided clipping yields. Further research is warranted to examine the interaction between fertility and PGRs and the length of time necessary between PGR applications in reduced irradiance situations.

Received for publication October 8, 1998.

	REFERENCES

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 

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