| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Dep. of Natural Resources and Environmental Sciences, Univ. of Illinois, 1102 S. Goodwin Ave., Urbana, IL 61801
* Corresponding author (bbranham{at}uiuc.edu)
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
-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] are routinely used to suppress clipping production. Single-leaf turfgrass C exchange rates (CERs) in response to plant growth regulator (PGR) treatment have not been characterized. Individual Kentucky bluegrass (KBG, Poa pratensis L.) plants received label rates of TE or PAC and were placed in growth chambers at 23/18 or 30/25°C. Photosynthetic efficiency and CER measurements were recorded every 4 d for 44 d. Total root length (TRL), root surface area (SA), and average root diameter were measured at the end of the study. Reductions in CERs of TE- or PAC-treated plants were short lived with CERs suppressed 17 to 29% of control (POC) at 4 and 12 days after treatment (DAT), respectively. Plants treated at 23/18°C with PGRs typically had short-lived increases in CERs following CER suppression. A similar pattern of CER response to PGR treatment was observed at the 30/25°C temperature regime. Quantum efficiency was unaffected, but plants treated with PGRs had reduced root growth. PAC caused the greatest reduction in TRL and SA while increasing root diameter. A decline in TRL and SA in conjunction with increased tillering indicates that PGR reduced TRL and SA on a tiller basis. Changes in single-leaf CERs do not fully explain PGR-induced changes in plant growth.
Abbreviations: CER, carbon exchange rate • DAT, days after treatment • gs, stomatal conductance • KBG, Kentucky bluegrass • PAC, paclobutrazol • POC, percentage of control • PGR, plant growth regulator • PPFD, photosynthetic photon flux density • SA, root surface area • TA, trinexapac acid • TE, trinexapac-ethyl • TRL, total root length • WAT, weeks after treatment
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
Gibberellin inhibitors have become essential components of many sports turf maintenance programs. Though routinely applied, the effects of PGRs on CERs are poorly understood. The results of past studies on turfgrasses and other agronomic commodities have shown conflicting results. Han et al. (1998) reported sequential applications of TE at various application rates and timings had no effect on creeping bentgrass (Agrostis stolonifera L.) canopy CERs on a ground area basis from 4 to 22 wk after treatment (WAT). However, they observed creeping bentgrass total nonstructural carbohydrate levels increased shortly after TE application followed by carbohydrate depletion. Spokas and Cooper (1991) measured canopy CERs on a ground area basis weekly for 23 wk on KBG treated with amidochlor (N-[(acetylamino)methyl]-2-chloro-N-(2,6-diethylphenyl)-acetamide) or mefluidide (N-[2,4-dimethyl-5-[[(trifluoromethyl)sulfonyl]amino]phenyl]acetamide). Amidochlor increased KBG CERs 32 to 68 POC 7 of 23 wk while mefluidide had no effect on canopy CERs. Gaussoin et al. (1997) reported that flurprimidol (-(1-methylethyl)--(4-(trifluoromethoxy)phenyl)-5-pyrimidinemethanol) reduced CERs on a leaf area basis for creeping bentgrass and annual bluegrass (Poa annua L.). Increasing flurprimidol rates linearly decreased canopy CERs while mefluidide had no effect on creeping bentgrass or annual bluegrass CERs. In a field study conducted by Beasley and Branham (2006) over a period of 8 wk, single TE or PAC applications altered leaf areas and CERs on a ground area basis. PAC altered KBG CERs in spring and to a lesser extent in summer. Changes in leaf area corresponded to alterations in CERs on a ground area basis. On a leaf area basis, average CERs were not affected by TE treatment, whereas PAC caused both increased and decreased CERs. Increases in KBG CERs for PAC may be due to increased photoassimilate sink demand to support increased tiller initiation (Stier and Rogers, 2001; Hanson and Branham, 1987). Under shade, researchers have reported TE increased canopy CERs on a ground area basis for zoysiagrass (Zoysia japonica Steud). Differences in canopy cover between TE-treated and control shaded swards were attributed to a combination of increased tillering, leaf area, and reduced plant stature.
Researchers have reported alteration in CERs from PGR application in other agronomic commodities. In a study evaluating growth regulators on ‘Mahti’ wheat (Triticum aestivum L.), TE was reported to suppress single-leaf CERs for 3 DAT while increasing plant tillering (Rajala and Peltonen-Sainio, 2001). Hawkins et al. (1985) investigated CERs of PAC-treated soybean [Glycine max (L.) Merr.] leaves. Mature, fully expanded leaves showed no changes in CERs, while developing leaves had increased CERs. Differences in leaf age and associated stomatal conductance (gs) were cited as the primary reasons for differing CERs. In corn (Zea mays L.), Kasele et al. (1995) showed increases in CERs using growth retardants BAS 110.W (1-(2,4-dichlorophenyl)-2-methoxy-1-methyl-2-(1H-1,2,4-triazol-1-yl) ethanol) and ethephon (2-chloroethylphosphonic acid). Overall, plant biomass and leaf size were reduced, while chlorophyll was concentrated and CERs enhanced. PGRs have been reported to reduce leaf area and plant stature of stone fruit crops (Huang et al., 1995; Vu and Yelenosky, 1992; Curry and Reed, 1989). Reduction in total leaf area is thought to reduce overall plant CERs even though leaves within the inner canopy may have higher CERs with increased light penetration compared to inner canopy leaves of untreated plants.
The effect of PGRs on turfgrass CERs are generally measured on turfgrass canopies rather than single leaves due to the tedious nature of turfgrass leaf area measurements. However, gibberellin-inhibiting PGRs have been reported to increase turfgrass canopy density through increasing tillering, widening leaf blades, and delaying leaf senescence (Heckman et al., 2001; Ervin and Koski, 2001; Ervin and Koski, 1998; Wiles and Williams, 1985). Therefore, calculation of turfgrass CERs on a ground area basis without taking into account changes in leaf area may not accurately reflect the effects of PGRs on single plant and/or sward CERs.
This study examined the effects of TE and PAC on KBG single-leaf CERs while characterizing single-plant shoot and root growth. Evaluations at two different temperatures were made to more fully characterize turfgrass responses to PGRs.
|
MATERIALS AND METHODS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
Fifty-six days after germination, nine KBG plants of similar size were placed in each growth chamber. Growth chambers were maintained at day/night temperatures of 23/18 or 30/25°C (Environmental Growth Chambers, Chagrin Falls, OH) with a 16-h photoperiod. Plants were allowed to acclimate for 14 d before treatments were applied. Plants were irrigated with 50 mL water three times per week and mowed and fertilized as in the greenhouse.
After the acclimation period, plants were treated with TE at 0.27 kg ai ha–1 or PAC at 0.42 kg ai ha–1 or were untreated. PGRs were applied using a chamber sprayer (Allen Machine Works, Midland, MI) with air as the propellant and 375 L water ha–1 as the carrier. Immediately after treatment, plants were returned to their respective growth chambers and PAC-treated plants watered to ensure PAC uptake. Each plant represented one replicate per treatment per growth chamber. The experiment was repeated 60 d after completion of the first experiment with temperature regimes switched between growth chambers.
Data Collected
Individual plant height from the soil surface and color ratings based on a scale of 1 to 9 (1 = brown turf, 6 = minimally acceptable color, and 9 = dark green) were recorded every 7 d for 56 DAT. At the end of experiment, tillers were counted and leaves for each treatment were excised for image analysis. Leaves were scanned (Hewlett-Packard 7400c, Palo Alto, CA) at 500 dots per inch and leaf area calculated using image analysis software (National Scientific Inc, Claremont, CA). The software filtered out particles less than 0.5 mm2 before converting the image to black and white. Area in black was calculated and subtracted from the total scanned area (white) to determine total leaf area.
Gas Exchange Measurements
Carbon exchange rate measurements were taken using a LI-COR 6400 (LI-COR Inc., Lincoln, NE) fitted with an Arabidopsis leaf chamber (0.785 cm2). General procedures for the LI-COR 6400 included zeroing and calibration checks using two CO2 standards, 340 and 500 µL L–1 CO2, in each growth chamber before actual measurements. Carbon exchange and gs measurements were made between 1100 and 1400 h every 4 DAT for 44 d. When the experiment was repeated, CERs and gs were measured using the same schedule with an additional measurement collected at 2 DAT.
Leaves chosen for CER measurement were the youngest fully expanded leaves, and measurements were made at least 2.0 cm above the base of the leaf. Conditions for CER measurements included a constant air flow of 3.0 mmol s–1 with a concentration of 370 µmol CO2. Irradiance levels at canopy height were between 650 and 700 µmol m–2 s–1 photosynthetic photon flux density (PPFD) from high-intensity fluorescent bulbs in the growth chambers. The leaf chamber or block temperature was maintained at the corresponding growth chamber temperature. To ensure irradiance levels were saturating for CER measurements a light curve from 0 to 1000 µmol m–2 s–1 PPFD was produced in each growth chamber using the LI-COR 6400 and standard leaf chamber with attached light source. Measurements were collected every 200 µmol m–2 s–1 PPFD on untreated KBG leaves.
Fluorescence Measurements
Fluorescence measurements were taken using a Hansatech FMS-100 (Hansatech Instruments Ltd., Norfolk, UK) to determine photosynthetic efficiency. Before actual measurements, a series of tests were performed to gauge the proper saturating irradiance level (Anonymous, 2000).
Leaves used for CER measurements were also used for fluorescence measurements after completion of CER measurements within each growth chamber. Leaves were dark adapted for a period of 20 min before fluorescence measurement. Data collected included fluorescence baseline and fluorescence maximum to calculate photosynthetic efficiency.
Stomatal Measurements
Leaf impressions for stomatal measurements were taken 14 DAT on leaves used for CER and fluorescence measurements from the 23/18°C growth chamber treatment. Impressions were made on dry leaves using a nitrocellulose adhesive (Duco cement, ITW Devcon, Danvers, MA). The adhesive was applied using a small brush and allowed to dry for 20 min after which impressions were carefully peeled away from the leaf surface. Impressions were stored on glass slides at room temperature until image analysis.
Stomatal density and stomatal width and height were measured at 175x and 5600x, respectively, using an environmental scanning electron microscope (FEI Company, Hillsboro, OR). Impressions were sputter coated with silver using the DESK II TSC (Denton Vacuum, Moorestown, NJ) for ESEM analysis. Stomatal density measurements were scaled to a leaf area of 1 mm2. Stomatal apertures were not measured because impressions were not clean enough to accurately determine pore edges.
Root Architecture
At 56 DAT, plant roots were thoroughly washed to remove any debris and roots excised as close to the crown as possible. Roots were placed in a 5:95 (v/v) ethanol/deionized water solution and stored at 4°C until image analysis.
The WinRhizo system (Regent Instruments Inc., Version 5.0A, Quebec City) was used to analyze root architectural parameters. Guidelines for image analysis followed a combination of procedures outlined by Bouma et al. (2000) and Costa et al. (2001), and was modified to account for turfgrass roots by setting the exclusion filter to 0.25 mm and increasing the resolution to 500 dots per inch. Roots were washed using deionized water and arranged in a water-filled tray to minimize root overlap. The water level in the scanning tray was maintained at 2 cm for each image. Larger root samples were cut into 5-cm segments to reduce root volume and overlap. Roots were analyzed for total root length (cm), diameter (mm), and estimated surface area (cm2).
Statistical Design
Measurements made over time including plant height, CER, gs, color, and photosynthetic efficiency were analyzed as completely randomized designs with three replications using repeated measures. Fixed factors of the study were experimental run, temperature, PGR type, and date. Results of the experiments were combined when the experiment interaction term was nonsignificant ( > 0.1). Parameters analyzed over time were separated using Fisher's least significant difference (FLSD) ( < 0.05). One-time measurements such as tiller number, leaf area, stomatal density and width, and root architecture data were combined when the experiment interaction term was nonsignificant ( > 0.1) Means were separated using FLSD ( < 0.05). Repeated measure analyses were performed using SAS mixed procedure and single measurements using the general linear model procedure (SAS Institute, 2003).
|
RESULTS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
0.0001).
|
Tiller numbers and leaf area data between the two experiments were combined (experimental run* temperature*PGR, 2 df, P = 0.8954 and 2 df, P = 0.7670, respectively). Plant growth regulator treatment resulted in plants with increased tiller numbers and greater leaf area compared to controls at each temperature (Table 1). Tiller numbers were greater for both treated and untreated plants at 23/18°C compared to plants at 30/25°C. Plant leaf area was increased 13 or 25 POC for TE- or PAC-treated plants, respectively. On average, plants grown at 23/18°C increased leaf area 15% compared to plants at 30/25°C. In addition to changes in leaf area, stomatal density was increased 44 POC for PAC-treated leaves at 14 DAT, however, PAC treatment did not change stomatal morphology (Table 2). Stomatal width was increased 15 POC for TE plants.
|
|
|
|
|
Under the 23/18°C temperature regime TE initially reduced CERs 26 POC at 4 DAT followed by a 13 to 17 POC increase in CERs at 16 and 20 DAT before returning to levels found in the control plants (Fig. 2 ). However, when the experiment was repeated, initial decreases in CERs from TE application were not observed (Fig. 2). In the second experimental run, TE initially increased CERs 10 POC at 4 and 8 DAT. TE-treated plants at 30/25°C also had decreased CERs in the first experimental run but the same pattern was not observed when the experiment was repeated. Single-leaf gs measurements fluctuated in accordance with CERs (Fig. 2 and 3) . During periods of suppressed or enhanced CER, gs increased or decreased compared to control levels.
|
|
Over the course of the experiments, control plants maintained at 30/25°C tended to have a slight reduction in CERs. However, photosynthetic efficiency did not differ among treatments within either temperature regime nor over time (Table 3).
|
DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
Patterns of PGR-treated plant growth at each temperature did not appear to be directly related to fluctuations in single-leaf CERs. Changes in KBG CERs from TE or PAC applications were generally short lived compared to PGR effects on plant growth. CERs of plants maintained at 30/25°C gradually declined compared to CERs at the 23/18°C regime. Increased photorespiration at 30/25°C likely caused an overall reduction in single-leaf CERs of the C3 KBG plants since no changes in photosynthetic efficiency were observed.
Differences in CERs of treated plants between experiments were most likely due to PGR uptake. Even though conditions were similar between growth chambers, PGR uptake has been reported to vary in previous growth chamber experiments (Beasley and Branham 2005). However, overall plant growth for each treatment was similar across experimental runs. As for the lack of change in photosynthetic efficiency between PGR-treated plants and controls, these findings indicate plants were not stressed. Results were consistent with those reported by Stienke and Stier (2003).
TE effects on CERs and plant growth were similar to the results of Rajala and Peltonen-Sainio (2001) on TE-treated Mahti wheat. CER of Mahti wheat was suppressed less than 3 DAT while plant tiller number increased. However, we also observed that CERs increased for a short period following suppression of CERs during the first experimental run but not the second.
The effect of PAC on CERs followed a pattern similar to TE but with CERs suppressed for a slightly longer duration. However, PAC did not decrease CERs sufficiently to account for the differences that occurred between PAC and controls in reducing plant stature. PAC application resulted in increased stomatal density which may have contributed to recovery of CERs at later dates. Other published accounts of PAC effects on single-leaf CERs have reported conflicting results varying by species and leaf age (Huang et al., 1995; Archbold and Houtz, 1988; Hawkins et al., 1985). However, most studies have not established strong correlations in single-leaf CERs and overall plant growth.
The results of this study in conjunction with previously conducted experiments examining PGR effects on turf CERs fail to provide a clear cause-and-effect relationship between PGR altered CERs and reduced clipping production (Beasley and Branham, 2007; Gaussoin et al., 1997; Spokas and Cooper, 1991). Patterns of CER alteration from PGR applications appear to fluctuate greatly for gibberellin inhibitors. The effects of PGRs on CERs are the result of, or a combination of, leaf and tiller production, changes in leaf morphology, increased chlorophyll density, and changes in sink demand for photoassimilates (Beasley and Branham, 2007; Gaussoin et al., 1997; Spokas and Cooper, 1991). Since TE and PAC are metabolized quickly within plant tissues, fluctuations in single-leaf CERs from TE and PAC applications may be in response to changes in plant photoassimilate demand and growth patterns and not the cause of altered growth.
In contrast to the CER data, the combination of shoot and root data clearly shows PAC-treated shoot growth in terms of tiller number and leaf area was favored over root growth. Increased tiller number compared to root mass has been observed in application of PGRs to shaded turfs (Goss et al., 2002). Because no link in CERs and changes in plant growth are evident, altered photoassimilate partitioning seems a more plausible explanation. Changes in photoassimilate partitioning from PGR application have been previously reported on other species (Huang et al., 1995; El-Hodairi et al., 1988). Hanson and Branham (1987) reported PAC to shift photoassimilates into adjacent tillers weeks after treatment. Changes in plant tillering and root growth from this study fully support a PGR-induced shift of photoassimilates in treated plants.
Plant growth regulators have proven useful tools in maintaining highly managed turfgrasses. Understanding PGR effects on turfgrass growth from the leaf level to overall plant growth can serve in developing novel uses of PGRs on turfs. Because PGRs do not appear to severely limit CERs, future research should focus on the ability of PGRs to shift photoassimilates to manipulate plant growth. Shifting photoassimilates into lateral growth using PGRs may be advantageous during turf establishment, to increase verdure biomass, or to aid in the recovery of trafficked turf.
Received for publication December 5, 2005.
|
REFERENCES |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
This article has been cited by other articles:
|
|
 |
|
  E. H. Ervin and X. Zhang Influence of Sequential Trinexapac-Ethyl Applications on Cytokinin Content in Creeping Bentgrass, Kentucky Bluegrass, and Hybrid Bermudagrass Crop Sci., September 1, 2007; 47(5): 2145 - 2151. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| The SCI Journals | Agronomy Journal | Vadose Zone Journal | |||
| Journal of Natural Resources and Life Sciences Education |
Soil Science Society of America Journal | ||||
| Journal of Plant Registrations | Journal of Environmental Quality |
The Plant Genome | |||
