Published online 1 September 2007
Published in Crop Sci 47:2145-2151 (2007)
© 2007 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
TURFGRASS SCIENCE
Influence of Sequential Trinexapac-Ethyl Applications on Cytokinin Content in Creeping Bentgrass, Kentucky Bluegrass, and Hybrid Bermudagrass
E. H. Ervin* and
Xunzhong Zhang
Dep. of Crop and Soil Environmental Sciences, Virginia Polytechnic Institute and State Univ., Blacksburg, VA 24061-0404
* Corresponding author (ervin{at}vt.edu).
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ABSTRACT
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Trinexapac-ethyl (TE) is a popular plant growth regulator in the turfgrass industry not only for its effectiveness in reducing mowing requirements but for its positive effects on turf density and appearance. We hypothesized that reported side effects of TE such as increases in color, photochemical efficiency (PE) of photosystem II, tillering, and chlorophyll, may be related to changes in leaf cytokinin content. Our objective was to determine if TE influences leaf trans-zeatin riboside (t-ZR) content of three common turfgrass species. Sods of Kentucky bluegrass (Poa pratensis L.), creeping bentgrass (Agrostis stolonifera L.), and hybrid bermudagrass (Cynodon dactylon x C. transvaalensis) were transplanted and grown in flats under a greenhouse mist system. Label-rate TE treatments were applied every 2 wk for 2 mo. Canopy PE, leaf color, canopy height, and leaf t-ZR contents were measured every 2 wk in 2003, with leaf total nonstructural carbohydrates (TNC) measurements added in 2004. A significant increase in t-ZR content was measured following sequential TE treatment in all three species in both years. Leaf TNC was consistently increased following the second TE application in creeping bentgrass and hybrid bermudagrass. While our data are nonspecific as to how TE increases leaf t-ZR, it appears that a shift in assimilate partitioning to basal organs could be a contributing factor. Whatever the mechanism, increased leaf t-ZR is likely to confer aesthetic, as well as functional, advantages to treated turfgrasses.
Abbreviations: EU, enzyme units • GA, gibberellic acid • PAR, photosynthetically active radiation • PE, photochemical efficiency • PSII, photosystem II • TE, trinexapac-ethyl • TNC, total nonstructural carbohydrate • t-ZR, trans-zeatin riboside • WAIT, weeks after initial TE treatment
Influence of Sequential Trinexapac-Ethyl Applications on Cytokinin Content in Creeping Bentgrass, Kentucky Bluegrass, and Hybrid Bermudagrass
E. H. Ervin* and
Xunzhong Zhang
Dep. of Crop and Soil Environmental Sciences, Virginia Polytechnic Institute and State Univ., Blacksburg, VA 24061-0404
* Corresponding author (ervin{at}vt.edu).
Trinexapac-ethyl (TE) is a popular plant growth regulator in the turfgrass industry not only for its effectiveness in reducing mowing requirements but for its positive effects on turf density and appearance. We hypothesized that reported side effects of TE such as increases in color, photochemical efficiency (PE) of photosystem II, tillering, and chlorophyll, may be related to changes in leaf cytokinin content. Our objective was to determine if TE influences leaf trans-zeatin riboside (t-ZR) content of three common turfgrass species. Sods of Kentucky bluegrass (Poa pratensis L.), creeping bentgrass (Agrostis stolonifera L.), and hybrid bermudagrass (Cynodon dactylon x C. transvaalensis) were transplanted and grown in flats under a greenhouse mist system. Label-rate TE treatments were applied every 2 wk for 2 mo. Canopy PE, leaf color, canopy height, and leaf t-ZR contents were measured every 2 wk in 2003, with leaf total nonstructural carbohydrates (TNC) measurements added in 2004. A significant increase in t-ZR content was measured following sequential TE treatment in all three species in both years. Leaf TNC was consistently increased following the second TE application in creeping bentgrass and hybrid bermudagrass. While our data are nonspecific as to how TE increases leaf t-ZR, it appears that a shift in assimilate partitioning to basal organs could be a contributing factor. Whatever the mechanism, increased leaf t-ZR is likely to confer aesthetic, as well as functional, advantages to treated turfgrasses.
Abbreviations: EU, enzyme units • GA, gibberellic acid • PAR, photosynthetically active radiation • PE, photochemical efficiency • PSII, photosystem II • TE, trinexapac-ethyl • TNC, total nonstructural carbohydrate • t-ZR, trans-zeatin riboside • WAIT, weeks after initial TE treatment
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INTRODUCTION
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PLANT GROWTH REGULATORS like trinexapac-ethyl (TE) are used extensively in the turfgrass industry to reduce mowing frequency and improve aesthetics. Trinexapac-ethyl [4-(cyclopropyl-ß-hydroxymethylene)-3,5-dioxocyclohexanecarboxylic acid ethyl ester, as a structural mimic of 2-oxoglutarate] competitively inhibits the 3ß-hydroxylase conversion of gibberellic acid-20 (GA20) to gibberellic acid-1 (GA1), resulting in reduced cell elongation (Rademacher, 2000). Inhibition of cell elongation has been shown to increase mesophyll cell density and chlorophyll concentration, resulting in dwarfed shoots that are darker green (Ervin and Koski, 2001; Heckman et al., 2005; McCullough et al., 2006). Net photosynthesis may be increased by TE as photosynthetic rates are reportedly unchanged due to TE on cool-season (Stier et al., 1997; Beasley and Branham, 2007) and warm-season (Qian et al., 1998) turfgrasses, while maintenance respiration may be decreased (Heckman et al., 2001). Photosynthate not used for leaf elongation must be stored or transported to other organs for growth and maintenance. Hybrid bermudagrass Cynodon dactylon x C. transvaalensis) treated with 15N-labeled ammonium nitrate allocated approximately 50% more N to roots and rhizomes when treated with TE (Fagerness et al., 2004). These data and reports of postinhibition shoot growth enhancement (Fagerness and Yelverton, 2000; Lickfeldt et al., 2001) support the hypothesis that total nonstructural carbohydrates (TNCs) may accumulate during the period of TE inhibition or be used to fuel other developmental events such as increased tillering, stem growth, or rooting.
Han et al. (1998, 2004) reported increased TNC content in the verdure of creeping bentgrass (Agrostis stolonifera L.) at 2 wk after the initial TE application (WAIT), with decreased TNC levels from weeks 4 to 16. Waltz and Whitwell (2005) reported increased TNC in ‘Tifway’ hybrid bermudagrass root and shoot tissues during sequential TE applications. A shift of photoassimilates to crowns, stems, and roots during regulation may be responsible for greater rooting reported in hybrid bermudagrass (McCullough et al., 2004, 2005a,b) and greater tillering reported for almost all species tested (Ervin and Koski, 1998, 2001; Fagerness and Yelverton, 2001; Ervin et al., 2002).
Formation of new roots and tillers requires cell division. Cell division requires a hormonal signal (cytokinins) and available carbohydrates. Previous research with GA-inhibiting compounds such as triazoles and prohexadione-calcium showed increased shoot cytokinin levels in cucumber (Fletcher and Arnold, 1986) and wheat (Grossmann, 1990). Which change comes first in GA-inhibited plants, increased crown and root carbohydrates or increased cytokinins? In an extensive review of the physiological effects of GA-inhibitors, Rademacher (2000) concluded that increased assimilate availability in the roots most likely results in stimulation of root biosynthesis of cytokinins, which are then transported in the xylem to shoots.
On the basis of this evidence, we hypothesized that sequential TE application would increase leaf cytokinin content of cool- and warm-season turfgrasses. Our objectives were to determine if TE influences endogenous leaf trans-zeatin riboside (t-ZR) content of creeping bentgrass, Kentucky bluegrass (Poa pratensis L.), and hybrid bermudagrass and to associate any changes with TE effects on leaf color, photochemical efficiency, and carbohydrate status.
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MATERIALS AND METHODS
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Turfgrass Culture and Trinexapac-Ethyl Treatment
‘L-93’ creeping bentgrass, ‘Midnight’ Kentucky bluegrass, and ‘Tifway’ hybrid bermudagrass sod (0.3 m x 0.6 m) were harvested from mature field plots and transplanted into flats (6 cm deep) on 28 March 2003. The sod was grown under a greenhouse mist system at 22 ± 2°C; Kentucky bluegrass and creeping bentgrass were grown under natural sunlight (average 
350 µmol m–2 s–1 photosynthetically active radiation [PAR], 11-h photoperiod), while supplementary light (410 µmol m–2 s–1 PAR, 11-h photoperiod) was provided over the hybrid bermudagrass. Before the experiment initiation, clipping occurred twice weekly, with Kentucky bluegrass being clipped to 3.8 cm, creeping bentgrass to 0.65 cm, and hybrid bermudagrass to 1.3 cm. At the initiation of each treatment cycle, each species was clipped in the morning and then treated with TE in the afternoon at label rates appropriate for the cutting height of each and the 14 d application frequency (a.i.: 108 g ha–1 for Kentucky bluegrass; 23 g ha–1 for creeping bentgrass; and 45 g ha–1 for bermudagrass); retreatment occurred every 14 d for a total of four applications during the duration of the each experiment. No mowing occurred between the TE treatment dates. Foliar TE applications were delivered with a CO2 pressurized sprayer at a volume of 784 L ha–1 and a pressure of 290 kPa. Nitrogen from a 20–20–20 fertilizer with micronutrients (J.R. Peters, Inc., Allentown, PA) was applied at 6.1 kg ha–1 every 14 d in a mixture with TE. In 2003 each turfgrass species was tested during a different eight-week period. The Kentucky bluegrass TE application cycle was from 3 April to 30 May, the creeping bentgrass cycle was from 2 May to 27 June, and the bermudagrass cycle was from 30 May to 26 July. The grasses were maintained on the greenhouse mist bench through 2003, with the 2004 experimental cycle occurring over the same eight-week period, 18 February to 14 April, for each species.
Measurements
Leaf Color, Canopy Height, and Canopy Photochemical Efficiency
Leaf color was evaluated, and canopy height and photochemical efficiency were measured every 14 d. Turfgrass visual color was rated based on a scale of 1 to 9, with 9 indicating the darkest green color and 1 indicating no green color. Canopy height was measured with a floating disk ruler method in which a round paper disk was placed on the canopy and distance between the paper disk and the base of the canopy was measured and reported as the average of three readings per experimental unit (Sharrow, 1984). Photochemical efficiency of photosystem II (PSII) was measured with a chlorophyll fluorometer according to the methods of Zhang and Schmidt (2000). Following these measurements, scissors were used to remove the top one-third of the turfgrass canopy, with the resulting leaf tissues being frozen with liquid N and kept at –80°C for subsequent cytokinin and carbohydrate assays. Once these tissue samples had been collected, creeping bentgrasses were clipped to a height of 2.5 cm, Kentucky bluegrasses to 3.8 cm, and hybrid bermudagrasses to 1.3 cm. We did not clip the bentgrass back down to its original 0.65-cm height because we perceived that this would be too severe of a defoliation event for healthy growth to be maintained.
Cytokinin
Extraction and purification procedures for one plant-active cytokinin, t-ZR, were conducted on leaf samples following the methods of Turnbull et al. (1997), with minor modifications. The t–ZR in leaf tissue (500 mg) was extracted in 5 mL of 80% methanol containing 0.01% butylated hydroxytoluene and analyzed using ELISA as described by Trione et al. (1985). A series of t-ZR concentrations (0, 2.5, 5, 10, 25, 50 ng mL–1) were made from the stock solution. Each standard or sample was repeated three times, and the averages were used for data analysis. Antibody t-ZR3 (for t-ZR; 1:200 dilution) and an alkaline phosphatase-labeled goat anti-mouse IgG (1:1000 dilution; Sigma Chemical Co., St. Louis, MO) were used as primary and secondary antibodies. After a series of reactions, 100 µL of substrate solution (3 mg mL–1 of p-nitrophenyl phosphate in 10% diethanolamine buffer, pH 9.8, 0.5 mM MgCl2) were added to each well, and the plates were incubated at 37°C for 50 min. The color reactions in each well were determined by measuring absorbance at 405 nm by an enzyme immunoassay microplate reader (Opsys MR, Thermo Labsystems, Chantilly, VA). Zeatin riboside concentration was calculated based on the standard curve after logic conversion of the data. The antibodies for t-ZR were kind gifts from Dr. Gary Banowetz (USDA, Corvallis, OR).
Total Nonstructural Carbohydrates
Nonstructural carbohydrates, including glucose, fructose, sucrose, and starch in leaves, were extracted and analyzed according to the procedures of Hendrix (1993), with modifications. The leaf samples were dried at 100°C for 1 h, and then at 65°C for another 24 h. Sugars were extracted from ground dry samples (20 mg) in 2 mL 80% ethanol in an 80°C water bath for 15 min and then centrifuged at 3000 gn for 10 min. The supernatant was collected. The residue was re-extracted, and the supernatant was collected. The supernatants from two extractions were pooled and brought up to 5 mL with 80% ethanol. To 1.5 mL supernatant, 20 mg active charcoal was added. The extract was shaken and centrifuged at 2200 gn for 15 min, and the supernatant was stored at –80°C for later analysis.
To the residue after sugar extraction, 1.0 mL 0.1 M KOH was added. The sample in the tube was heated in boiling water for 60 min and cooled. To each tube, 0.2 mL of 1 M acetic acid was added followed by 100 µL 1.0 M Tris buffer (pH 7.2). After adding 200 µL alpha-amylase, the tube was kept in an 85°C water bath for 30 min. After reducing pH to less than 5 with 1.0 M acetic acid, 1.0 mL amyloglucosidase was added, and the samples were then kept in a 55°C water bath for 60 min. Enzyme action was stopped by heating at 100°C for 4 min, with the extract brought up to 6 mL with distilled H2O, mixed, and centrifuged at 3000 gn for 10 min. The extract was stored at –80°C until analysis for the sugars.
For the sugar analysis, the extract (20 µL) was transferred to a microtitration plate and dried at 50°C for 1.5 h. After drying, 20 µL deionized–distilled H2O was added to each well, and the plate was covered for 1 h. A series of standard solutions (a mixture of glucose, fructose, and sucrose; 0, 0.005, 0.025, 0.05, 0.125, 0.25 g L–1) were added to separate wells for standard curve development. To each well, 100 µL glucose reagent solution (Sigma GAHK-20, Sigma Chemical Co., St. Louis, MO) was added, and the plate was kept at room temperature (18–25°C) for 30 min. The glucose was measured on a microplate reader at 340 nm (under reduced light). Next, 10 µL of 0.25 enzyme units (EU) phosphoglucose isomerase was added to each well and incubated at room temperature for 30 min. After measurement of reducing sugars (glucose + fructose) at 340 nm, 10 µL of 83 EU invertase solution was added and incubated for 30 min before measurement at 340 nm for total sugars (glucose + fructose + sucrose).
For bermudagrass, the starch in the residue following sugar extraction was degraded into glucose under action of alpha-amylase and amyloglucosidase enzymes according to the procedure of Hendrix (1993). Glucose was measured as described previously and its content calculated on the basis of the standard curve. Starch content was determined on the basis of the glucose content. Fructan content in creeping bentgrass and Kentucky bluegrass was determined by analyzing fructose content after hydrolysis of fructan into fructose (Hendrix, 1993).
Experimental Design and Data Analysis
A randomized complete block design was used with four replications for each turfgrass species. The response of each turfgrass species to TE treatment was subjected to ANOVA, and separation of means was performed with a Fisher's protected LSD test at a 5% probability level, and Spearman correlation coefficients between the parameters were calculated (SAS Institute, 2001). The experiments for each species were done in 2003 and repeated in 2004.
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RESULTS
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Because significant year-by-environment interactions occurred, yearly data are presented separately. Turfgrass species was not considered a factor in our experimental design, so these data are also presented separately.
Leaf Color
Creeping bentgrass leaf color was darker at 2 and 4 WAIT in 2003 and at 4 and 6 WAIT in 2004 (Table 1
). Leaf color was significantly darker in Kentucky bluegrass due to TE only at 6 WAIT in both years (Table 2
). Biweekly TE application on hybrid bermudagrass had a more consistent effect, showing darker green color at 4 and 8 WAIT in 2003 and at 2, 4, 6, and 8 WAIT in 2004 (Table 3
).
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Table 1. Influence of TE treatment on leaf color, canopy height, and canopy photochemical efficiency in ‘L-93’ creeping bentgrass.
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Table 2. Influence of TE treatment on leaf color, canopy height, and canopy photochemical efficiency in ‘Midnight’ Kentucky bluegrass.
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Table 3. Influence of TE treatment on leaf color, canopy height, and canopy photochemical efficiency in ‘Tifway’ bermudagrass.
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Canopy Height
Trinexapac-ethyl reduced canopy height of creeping bentgrass at 2 and 8 WAIT in 2003 and 4, 6, and 8 WAIT in 2004 (Table 1). Maximum canopy height reductions of about 23% were measured at 8 WAIT in both years. Repeated TE application consistently reduced Kentucky bluegrass canopy height at 2, 4, 6, and 8 WAIT in 2003 and at 4, 6, and 8 WAIT in 2004 (Table 2); canopy height reductions ranged from about 20 to 40% in both years. For bermudagrass, sequential TE application significantly reduced canopy height at 2, 4, 6, and 8 WAIT in 2003 and at 4, 6, and 8 WAIT in 2004 (Table 3).
Canopy Photochemical Efficiency
Estimates of the efficiency of electron capture in PSII of the canopy of TE-treated creeping bentgrass indicated significantly higher photochemical efficiency (PE) at 4, 6, and 8 WAIT in 2003 and at 2, 4, and 6 WAIT in 2004 (Table 1). Similar results were found with Kentucky bluegrass, where TE treatment resulted in greater PE at 4, 6, and 8 WAIT in 2003 and 2004 (Table 2). Trinexapac-ethyl treated hybrid bermudagrass also had greater PE at 4 and 8 WAIT in 2003 and at 4, 6, and 8 WAIT in 2004 (Table 3).
Leaf Trans-Zeatin Riboside Content
Trans-zeatin riboside content of creeping bentgrass leaf tissue was increased (10–50%) due to sequential TE treatment at 4, 6, and 8 WAIT in 2003 and 2004 (Fig. 1A and 1B
). For Kentucky bluegrass, t-ZR was increased (15–30%) due to TE at 2, 4, 6, and 8 WAIT in 2003 (Fig. 2A
) and at 4, 6, and 8 WAIT in 2004 (Fig. 2B). Hybrid bermudagrass responded similarly to the other two species as leaf t-ZR was increased (25–50%) due to TE at 4, 6, and 8 WAIT in both years (Fig. 3A and 3B
).
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Figure 1. trans-Zeatin riboside response to trinexapac-ethyl (TE) application in creeping bentgrass in (A) 2003 and (B) 2004. Bars marked with an asterisk indicate a significant difference between the TE treatment and the control at P £ 0.05.
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Figure 2. trans-Zeatin riboside response to trinexapac-ethyl (TE) application in Kentucky bluegrass in (A) 2003 and (B) 2004. Bars marked with an asterisk indicate a significant difference between the TE treatment and the control at P  0.05.
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Figure 3. trans-Zeatin riboside response to trinexapac-ethyl (TE) application in bermudagrass in (A) 2003 and (B) 2004. Bars marked with an asterisk indicate a significant difference between the TE treatment and the control at P 0.05.
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Leaf Total Nonstructural Carbohydrate Content
Given the positive response of leaf t-ZR in all three species to TE in 2003, we expanded our 2004 investigations to include monitoring leaf TNC content. Sequential TE treatment resulted in 15 to 30% greater leaf TNC content in creeping bentgrass at 4, 6, and 8 WAIT (Fig. 4A
). A significant increase in leaf TNC content due to TE was only measured at 4 WAIT for Kentucky bluegrass (Fig. 4B). However, bermudagrass had a response similar to creeping bentgrass with TNC increases of about 15% at 4, 6, and 8 WAIT due to TE (Fig. 4C).
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Figure 4. Total nonstructural carbohydrate (TNC) response to trinexapac-ethyl (TE) application in (A) creeping bentgrass, (B) Kentucky bluegrass, and (C) bermudagrass in 2004. Bars marked with an asterisk indicate a significant difference between the TE treatment and the control at P 0.05.
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DISCUSSION
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Darker green leaves and reduced canopy heights due to TE were expected because these effects have been detailed in a number of previous reports. Inhibition of GA1 due to TE action restricts leaf cell elongation and increases cell density, resulting in more chlorophyll per unit area and darker green leaves (Ervin and Koski, 2001; Heckman et al., 2005; McCullough et al., 2006). One would assume that increased chlorophyll concentration would be associated with increased efficiency of light capture in PSII, but Beasley and Branham (2007) reported no change in Kentucky bluegrass single-leaf PE due to a single application of TE. However, results of repeated TE applications to field-maintained creeping bentgrass (Zhang and Schmidt, 2000) and the species in the study herein indicate that PE (when measured on a canopy, rather than a single-leaf basis) was consistently higher relative to the untreated species. Turfgrass canopies consist of photosynthetic tissues in various levels of physiological function from newly emerged leaf blades to those undergoing multiple stages of senescence. Cytokinins are commonly known as antisenescence agents because of their action of preventing oxidation of unsaturated fatty acids in membranes (Salisbury and Ross, 1992). Thus, increased leaf tissue cytokinin (t-ZR) levels due to sequential TE treatments, across the sampled canopy, may have been associated with less overall leaf senescence contributing to our measurements of greater sustained canopy PE. More detailed research wherein control and TE-treated leaves in various stages of phenological development are tagged and monitored for changes in single-leaf PE over time is required to clarify this issue further.
Greater efficiency of the turfgrass canopy for radiant energy capture in PSII due to TE could imply greater conversion to chemical energy resulting in more net carbohydrate production. Further, less leaf demand for TNC to fuel cell elongation in TE-regulated tissues could, given constant regulation, correspond to enhanced phloem transport of carbohydrates to basal sink tissues. Greater available carbohydrates in the crown could then function to increase root growth. Increased root metabolism may, in turn, result in increased cytokinins (such as t-ZR) biosynthesis and their transpirational transport into crown and shoot tissues. Higher crown content of t-ZR would supply the signal necessary to coordinate the increased formation of tillers often noted to occur in response to sequential TE applications. The observation that the leaves of these new tillers remain nonelongated merely shows the continued active presence of TE. Our picture of this dynamic process comes full circle when we note that higher t-ZR levels in these TE-regulated leaf tissues most likely make these tissues sinks for phloem transport of sugars, amino acids, and other mobile nutrients (Salisbury and Ross, 1992). Thus, in our study greater leaf TNC was consistently found in two of the three species tested following two or more sequential TE applications. Solid evidence for this supposed causative series of events will need to be tracked in a subsequent experiment by exposing the plants to 14CO2 and analyzing 14C sucrose concentration in TE-regulated and non-regulated leaf, crown, and root tissues over time.
Previous research has indicated that supplementation of endogenous cytokinin levels via exogenous cytokinins application improves the antioxidant capacity of creeping bentgrass to resist drought (Zhang and Ervin, 2004) and heat stress (Liu and Huang, 2002). Monthly TE applications on a creeping bentgrass fairway have been reported to increase antioxidant (superoxide dismutase) activity and photochemical efficiency, resulting in higher summer quality (Zhang and Schmidt, 2000). Jiang and Fry (1998) reported that TE improved drought resistance of perennial ryegrass (Lolium perenne L.), while Baldwin et al. (2006), noted 25% greater ultradwarf bermudagrass rooting and improved salinity tolerance resulting from TE. It is plausible that the membrane and pigment stabilizing (antisenescence) effects commonly attributed to enhanced tissue cytokinins could be playing a significant role in these reports of TE-improved abiotic stress performance.
Our data indicate that two or more sequential applications of TE result in more leaf t-ZR in creeping bentgrass, Kentucky bluegrass, and hybrid bermudagrass. Although our data do not provide a direct mechanistic explanation for this result, it appears that altered carbohydrate partitioning during regulation may play a role. The discovery that TE applications can increase leaf cytokinin levels may be an initial step toward our understanding of how TE increases turfgrass tiller density and, in some instances, abiotic stress tolerance.
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NOTES
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All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.
Received for publication January 31, 2007.
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INTRODUCTION
