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a 226 J.C. Miller Hall, Louisiana State Univ., Baton Rouge, LA 70803
b Dep. of Natural Resources and Environmental Sciences, Univ. of Illinois, 1102 S. Goodwin Ave., Urbana, IL 61801
* Corresponding author (bbranham{at}uiuc.edu).
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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-hydroxy-methylene)-3,5-dioxocyclohexanecarboxylic acid ethyl ester] and paclobutrazol (PAC) [(2RS, 3RS)-1-(4-chlorophenyl)-4,4-dimethyl-2-(1H-1,2,4-triazol-1-yl) pentan-3-ol] has been reported during warmer summer months. Plant growth regulator dissipation and Kentucky bluegrass (KBG; Poa pratensis L.) clipping production, canopy leaf area, and carbon exchange rates (CERs) were measured following single TE or PAC applications applied at three rates and three application timings. Warmer summer conditions reduced the intensity and duration of TE and PAC growth suppression compared to spring applications. Plant growth regulator applications above the manufacturers label rate provided no additional suppression. Residues of PGRs were analyzed by HPLC-UV. Warmer temperatures increased TE and PAC uptake, but accelerated the rate of PGR dissipation. Carbon exchange rates were measured weekly on ground (CERG) and leaf (CERL) area bases. Treatment with PGRs resulted in decreased leaf area early during inhibition followed by increased rate of leaf area production. Changes in canopy leaf area were greatest in spring and PAC caused the greatest change in leaf area. Changes in CERG reflected changes in leaf area. Paclobutrazol generally increased CERL during periods of clipping suppression, while TE had no effect on CERL.
Abbreviations: CER, carbon exchange rate • CERG, carbon exchange rate per ground area • CERL, carbon exchange rate per leaf area • DAT, days after treatment • KBG, Kentucky bluegrass • LAI, leaf area index • PAC, paclobutrazol • PIGE, post-inhibition growth enhancement • PGR, plant growth regulator • POC, percent of control • TE, trinexapac-ethyl • TA, trinexapac acid • WAT, weeks after treatment.
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Wiecko (1997) reported differences in the period of suppression for TE-treated bermudagrass [Cynodon dactylon (L.) Pers.] in Guam compared to studies conducted in Georgia. He concluded that the variation in TE efficacy resulted from differences in soil temperatures, photoperiod, and rainfall levels at the two locations. In a study conducted by Fagerness et al. (2002), bermudagrass was treated with TE and maintained in growth chambers at one of two temperature regimes, 35°/25°C or 20°/10°C. The duration of clipping suppression for TE-treated bermudagrass was increased at 20°/10°C while lateral stolon growth was increased by TE at 35°/25°C. Differences in TE efficacy between bermudagrass growth at the two temperature regimes were attributed to decreased plant growth in conjunction with decreased TE metabolism at 20°/10°C.
Similar changes in PGR efficacy have also been reported for cool-season turfgrasses. Lickfeldt et al. (2001) reported shorter periods of clipping suppression from TE applications in summer months compared to cooler spring months for Kentucky bluegrass (KBG, Poa pratensis L.). They posited PGR metabolism was accelerated with warmer summer temperatures resulting in decreased PGR activity and shortened periods of suppression.
Several studies have examined PGR effects on plant growth and shown increased canopy density as measured through tiller counts or visual ratings (Fagerness and Yelverton, 2001; Ervin and Koski, 1998). Direct evidence of PGR impact on individual leaves includes an increase in specific leaf weight, greater cell density, and wider leaf blades (Heckman et al., 2001; Ervin and Koski, 2001; Gaussoin et al., 1997). However, the effects of PGR application on canopy leaf area have not been reported.
Plant growth regulator induced changes in canopy leaf area would directly affect canopy CERs. In a study conducted by Morgan and Brown (1983), the importance of leaf area to CERs was examined as a function of mowing frequency on bermudagrass swards. They reported bermudagrass mowed weekly had less leaf area resulting in decreased light interception and reduced canopy carbon exchange rate per leaf area (CERL) compared to a monthly mowing regime. However, most turfgrass studies that measure CERs, calculate carbon exchange rate per ground area (CERG), which would not account for leaf area. Disregarding canopy leaf area as well as the effects of treatments on leaf canopy dynamics could lead to erroneous conclusions of the treatment effects on canopy CERs.
Research on PGR effects on turfgrass CERs has yielded conflicting results. Spokas and Cooper (1991) reported no difference in CERG for KBG treated with amidochlor{N-[(acetylamino)methyl]-2-chloro-N-(2,6-diethylphenyl)-acetamide} but reported mefluidide {N-[2,4-dimethyl-5-[[(trifluoromethyl)sulfonyl]amino]phenyl]acetamide} increased CERG at several times throughout the 39 d measuring period. Han et al. (1998) reported no difference in CERG for various rates and timings of TE on creeping bentgrass (Agrostis stolonifera L.) from 4 wk after treatments (WAT) through 22 WAT. In contrast, Gaussoin et al. (1997) found mefluidide and flurprimidol {-(1-methylethyl)--[4-(trifluoromethoxy)phenyl]-5-pyrimidinemethanol} reduced CERs on a leaf area basis of PGR-treated bentgrass and annual bluegrass (Poa annua L.). Increasing PGR application rates resulted in decreasing CERL. Researchers in other agronomic commodities have also reported conflicting effects of PGRs on plant CERL (Kasele et al., 1995; Archbold and Houtz, 1988; Hawkins et al., 1985).
The primary objective of this research was to examine the effect of different application timings of a PGR application on the duration and intensity of KBG clipping suppression. A second objective was to determine whether PGR applications affect canopy leaf area and CERs. Carbon exchange rate measurements were calculated on both a leaf and ground area basis to more fully determine PGR effects on canopy CERs.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Each experiment was a randomized complete block design with three replications and a plot size of 1.2 by 2.4 m. Fixed factors for the experiment included PGR treatment and application rate. Single PGR applications at 0.5x, 1x, and 1.5x the manufacturer's recommended rate were made at each application timing. Plant growth regulator treatments were TE at 0.13, 0.27, and 0.40 kg ai ha–1 or PAC at 0.28, 0.56, and 0.84 kg ai ha–1. In 2003 and 2004, treatments were applied 15 May and 21 April for spring; 7 July and 29 June for summer; and 9 and 12 September for fall, respectively. Treatments were applied using a CO2–pressurized backpack sprayer that delivered 468 L ha–1 at 220 kPa. All plots were fertilized with urea at 25 kg N ha–1 mo–1. Plots were irrigated as needed to ensure high quality turf.
Clippings were collected weekly for 10 WAT using a reel mower (McLane Inc., Paramount, CA) at a height of 3.2 cm. Clippings were collected by making a single pass over 0.7 m2 of each plot. Collected clippings were oven-dried at 60°C for 48 h, and the dry mass recorded. Color ratings were recorded every 2 wk on a scale of 1 to 9 (1 = brown turf; 6 = minimally acceptable turf color; 9 = dark green).
Canopy Carbon Exchange Rates
Canopy CERs were measured weekly for 8 WAT on TE and PAC label rate treatments (TE = 0.27 kg ai ha–1; PAC = 0.56 kg ai ha–1) for the spring and summer experiments. Measurements were taken between 1100 and 1400 h and just before clipping collection. Measurements were taken ±1 d from the date scheduled if weather forecasts indicated cloudy conditions.
Carbon exchange rates were measured with a LI-COR 6200 CO2 gas analyzer (LI-COR Biosciences Inc., Lincoln, NE) using a positive pressure open-system design. The chamber was constructed from polycarbonate with a height of 7.66 cm and diameter of 10.16 cm. The chamber was fitted with a propafilm (UCB Inc., DuPont, DE) top to reduce increases in air temperature during canopy enclosure. To secure the chamber to the soil, thin-walled conduit (7.5-cm height x 10-cm diameter x 0.6-cm wall thickness) was pushed into the ground to a depth of 4 cm for each treatment 3 d before CER measurements to reduce any canopy–soil disturbance. At the time of measurement, a rubber gasket was placed around each metal rim and the chamber pushed down to form an air-tight seal.
Ambient air from a baffle box was pumped into the chamber and reference side simultaneously through Bev-A-Line IV tubing (Cole-Parmer, Vernon Hills, IL). The flow rate of the air entering the chamber was maintained at 0.5 L min–1 using a volumetric flowmeter (Dywer Instruments, Inc., Michigan City, IN). The volumetric flowmeter was used as a flow controller and backup to the thermal mass flowmeter (Omega Inc., Stamford, CT). Temperature of the air entering and exiting the chamber was measured using copper-constantine thermocouples (Omega Inc.). Both the reference and exiting chamber gas lines entered a condenser to reduce discrepancies in water vapor. Exiting gas line pressures were measured and equalized using a needle valve on the reference gas line before entering the gas analyzer.
Carbon exchange rates were recorded 45 to 60 s after chamber attachment and CERs were stable. Temperatures during measurement never increased more than 1.7°C. The gas analyzer was zeroed and the span checked daily under measurement conditions.
At the conclusion of CER measurements, all green leaf tissue within the sampling chamber was excised. Leaves were scanned at 500 dpi on a flatbed scanner and leaf area calculated using National Scientific software (National Scientific Inc., Claremont, CA). Six hours after leaf CER measurements, devegetated soils were measured for soil CERs to determine apparent canopy CER. It was determined in preliminary studies the lapse in time between canopy CERs and soil CERs had no major effect on soil CERs.
Plant Growth Regulator Residue Analysis
Plant growth regulator residue analysis was performed on 2-g KBG samples from TE and PAC label rate applications (TE = 0.27 kg ai ha–1; PAC = 0.56 kg ai ha–1) for the spring and summer studies in 2004. Trinexapac-ethyl samples were collected 2, 5, 8, 11, and 14 d after treatment (DAT) while PAC samples were collected 2, 10, 18, 26, and 34 DAT. All dates for each PGR tissue collection were based on previously conducted TE and PAC residue analysis studies (Beasley and Branham, 2005). Samples were stored at –20°C until analysis.
Extraction and cleanup procedures for PAC were based on procedures outlined by Stahly and Buchanan (1986) and Mauk et al. (1989, 1990) with slight modifications (Beasley and Branham, 2005). Trinexapac-ethyl samples were analyzed for the active primary metabolite, trinexapac acid (TA) (Advisory Committee on Pesticides, 1995). Trinexpac acid extraction and cleanup followed procedures developed by Beasley and Branham (2005). Paclobutrazol and TA samples were analyzed using reverse-phase HPLC-UV.
Statistical Analysis
Each season was independently analyzed as an RCBD over time for clipping production, color ratings, CER measurements, and leaf area indices (LAIs). Contrasts were used to detect differences between treatments and controls for seasonal clipping production, color ratings, LAI, and CERs. Regression was performed on PGR residue analysis data. All statistical analysis was performed using SAS statistical software (SAS Institute, 2003).
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RESULTS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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). For 2003 and 2004, label rates of TE provided 5 wk inhibition with clipping production suppressed between 44 and 73 percent of control (POC) in spring and fall compared to 4 wk inhibition and 58 to 66 POC clippings suppressed in summer (Fig. 1, 2, and 5
). Application of PAC at the label rate followed a similar pattern to that of TE (Fig. 3–5). Paclobutrazol provided 6 wk of clipping inhibition in spring and fall compared to 4 to 5 wk in the summer. Additionally, greater levels of clipping suppression were attained in spring and fall at 22 to 42 POC compared to 49 to 63 POC with PAC applied in summer. Application temperatures for the spring period of clipping suppression were cooler than summer temperatures (Fig. 6
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Canopy color was similar between controls and both PGR label rates for each season with two exceptions. Spring PAC applications at the label rate elicited a slightly phytotoxic response early in the inhibition period in 2003 while the summer PAC application enhanced canopy color during inhibition in 2003 and 2004 (data not shown).
Canopy Leaf Area and Apparent Carbon Exchange Rates
Both TE and PAC altered canopy leaf area and CERs to a greater extent in spring than summer (Fig. 7–10
). Trinexapac-ethyl reduced canopy leaf area 13 POC 1 WAT followed by increases of 21 and 9 POC 5 and 7 WAT in spring 2003 (Fig. 7). A similar decrease in canopy leaf area the week following TE application was seen in spring 2004 but was not followed by significant increases in canopy leaf area during PIGE (Fig. 9). Trinexapac-ethyl applied in summer did not affect canopy leaf area in either 2003 or 2004 (Fig. 8 and 10). Paclobutrazol reduced canopy leaf area 10 to 17 POC during the inhibition period for spring 2003 and 2004 followed by an increased rate of leaf production beginning in latter stages of inhibition and continuing through PIGE. Increases in canopy leaf area from spring PAC applications were between 13 and 16 POC. Summer PAC applications had little effect on canopy leaf areas in 2003, although, leaf area was reduced 15% at 1 WAT. Paclobutrazol applied in summer 2004 reduced leaf area 21% 2 WAT followed by a 14 POC increase in leaf area 5 WAT.
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). In subsequent weeks, apparent CERG for TE and PAC-treated swards increased in conjunction with increasing canopy leaf areas. Apparent CERG in spring 2003 and 2004 increased 7 to 22 POC for TE and 9 to 15 POC for PAC-treated turf. In summer 2004, PAC treatment increased apparent CERG 13 POC at 5 WAT. Changes in apparent CERL occurred only for PAC-treated swards. In spring 2003, PAC reduced CERL by 12 POC 3 WAT. However, generally PAC increased CERL 13 to 20 POC 2 to 6 WAT as seen in spring 2003 and 2004 and summer 2004.
Trinexapac Acid and Paclobutrazol Residue Analysis
Plant growth regulator residue analysis was conducted in 2003, but the initiation and interval between sample collections were too long to yield meaningful data. In 2004, TA dissipated more rapidly than PAC for treated KBG swards in spring and summer (Table 1). The spring application of TE had a half-life of 5.8 d compared to 4.2 d for TE applied during summer. Initial plant concentrations of TA were greater in summer, when temperatures were warmer, than in spring. Paclobutrazol plant uptake was greater in summer compared to spring (Table 1). The half-life of PAC was 15.4 d in spring compared with 11.5 d in summer.
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DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Increased application rates of TE and PAC did not result in greater suppression levels or longer durations of growth inhibition compared to label rates. Though TE and PAC residues were not analyzed at other PGR application rates, clipping data suggest higher TE or PAC rates do not further reduce clippings nor extend the suppression period. Conversely, reduced PGR application rates are not as effective as label rates in reducing clippings nor providing the same duration of inhibition.
The analysis of TA and PAC is complicated in turfgrass plants due to the nature of the products, uptake mechanisms, and clipping removal. Higher initial TA levels for summer versus spring could be caused by greater leaf absorption and increased TE to TA conversion due to higher leaf surface temperatures.
Paclobutrazol uptake was increased in the summer compared to spring due to warmer temperatures. Paclobutrazol is primarily root absorbed, and uptake should be correlated with transpiration rates. Because PAC is persistent in the soil, plant uptake can continue for days following application, making the determination of a true rate of PAC metabolism difficult. Therefore, PAC half-life calculations in turf are confounded by prolonged PAC uptake.
Studies that have controlled PAC uptake have reported high variability of PAC dissipation based on dose and plant age for many stone fruit crops (Early and Martin, 1988; Sterrett, 1990; Shearing and Batch, 1982). Early and Martin (1988) concluded leaves were the primary site for PAC metabolism and that differences in PAC dissipation were due to younger tissues metabolizing PAC at a faster rate compared to more mature plant tissues. In this study and previously conducted PGR degradation studies, both TE and PAC showed increased uptake and dissipation with warmer temperatures that coincided with changes in PGR efficacy.
Trinexapac-ethyl and PAC applied in spring altered canopy leaf area and CER to a greater degree than when applied in summer. Both TE and PAC reduce canopy leaf areas early during the suppression period through decreased vertical growth. In the later stages of suppression and into PIGE, PGR-treated canopies had leaf areas similar or greater than controls. Based on several studies that have examined the effect of PGR application on plant morphology and growth, changes in canopy leaf area are most likely due to a combination of decreased canopy leaf scenescence, widened leaf blades, increased tillering, and/or resumed vertical growth (Heckman et al., 2001; Ervin and Koski, 1998, 2001; Wiles and Williams, 1985, Breuninger and Watschke, 1989). Leaf areas measurements in this study were within the ranges reported by Simon and Lemaire (1987) and Kopec et al. (1987).
Changes in canopy architecture of PGR-treated swards affected the amount of plant material available for light interception and photosynthetic processes. In spring when both TE and PAC were the most effective at reducing clippings, changes in CER followed changes in canopy leaf area. This demonstrates the importance leaf area has on canopy CER. As a result, the combination of clipping mass, LAIs, and apparent CERG data show PGRs may actually increase CERG during periods of inhibition if CERG is measured shortly after mowing instead of before clipping collection. During the latter stages of inhibition, PGR-treated turf swards had a greater proportion of leaf area below the mowing height as demonstrated through decreased clipping masses yet LAI similar to controls. Increased verdure leaf area during PGR inhibition should increase light interception efficiency and enhance CERG.
Trinexapac-ethyl did not affect CERL. A similar finding regarding TE effects on CERL has been reported in wheat (Triticum aestivum L.) (Rajal and Peltonen-Sainio, 2001). This suggests TE has no direct physiological effect on turf single-leaf CER or the effects are short-lived. Rather, gibberellic inhibitors such as TE alter photoassimilate partitioning and/or total nonstructural carbohydrate accumulation to support alterations in canopy leaf area and CERG (Han et al., 1998; Hanson and Branham 1987). In a study examining PGR effects on turfgrass photoassimilate partitioning, several PGR increased photoassimilate partitioning to adjacent tillers (Hanson and Branham, 1987). In other agronomic crops similar changes in photoassimlate distribution in PGR-treated plants have been observed (Huang et al., 1995; El-Hodairi et al., 1988).
Unlike TE, the effect of PAC on canopy CER is more complex than changes in canopy leaf area. Decreases in CERL from PAC were relatively short-lived followed by increased CERL. Paclobutrazol-induced changes in CERL have been reported by many researchers examining PGR effects on single-leaf CER of agronomic commodities (Kasele et al., 1995; Archbold and Houtz, 1988; Hawkins et al., 1985). Possible causes for changes in average leaf CER include increased leaf chlorophyll concentrations, increased stress tolerance, altered stomatal conductances, and changes in photoassimilate demand.
Plant growth regulators have proven to be a useful tool to reduce clipping production of highly managed turfgrasses. This research shows PGR application during warmer summer temperatures may not provide acceptable levels of clipping suppression on cool-season turfgrasses. However, during periods of increased PGR efficacy, single TE or PAC applications may serve to modify canopy growth not only to reduce clipping production but also increase canopy density and CER. Enhancement of leaf production could be beneficial during turf establishment or periods of recovery for trafficked turf.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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); and TE high rate of 1.5x label rate and TE label rate (
). The line at the 100% represents the control. WAT, weeks after treatment.
