Crop Science 42:1621-1626 (2002)
© 2002 Crop Science Society of America
TURFGRASS SCIENCE
Turf Quality and Freezing Tolerance of ‘Tifway’ Bermudagrass as Affected by Late-Season Nitrogen and Trinexapac-Ethyl
M. D. Richardson*
Dept. of Horticulture, Univ. of Arkansas, 316 Plant Sci. Building, Fayetteville, AR 72701
* Corresponding author (mricha{at}uark.edu)
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ABSTRACT
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Bermudagrass [Cynodon dactylon (L.) Pers.] is the most widely used species for intensively managed turf sites in the southern United States and in the transition zone. However, the lack of cold tolerance in many cultivars can result in significant winter injury. There is a limited body of information in the literature regarding management of bermudagrass to enhance cold tolerance, especially as it relates to N nutrition and the use of plant growth regulators (PGRs). As such, a 2-yr field study (1998–1999 and 1999–2000) was conducted to examine the effects of late season N fertilization and trinexapac-ethyl (TE) applications on morphology, quality, and freezing tolerance of ‘Tifway’ bermudagrass. During both years, monthly N applications were terminated on either 15 July, 15 August, or 15 September, while applications of TE were made on 15 August; 15 August and 15 September; or 15 August, 15 September, and 15 October. Late season applications of N and TE enhanced the fall green color retention of bermudagrass and promoted early spring green-up (SGU). Neither N nor TE had a consistent effect on growth and development of bermudagrass rhizomes or stolons, and neither treatment had a consistent effect on the freeze tolerance of rhizomes. However, a positive attribute of these treatments is a significant increase in the overall green period of bermudagrass (20–25 d), which can prolong the playability of high maintenance sports facilities. From these studies we have concluded that, contrary to what is commonly believed, late season applications of N did not affect the freeze tolerance of bermudagrass rhizomes.
Abbreviations: PGR, plant growth regulator • SGU, spring green-up • TE, trinexapac-ethyl
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INTRODUCTION
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THE TRANSITION ZONE presents golf and sports turf managers with an array of problems relative to turfgrass stress tolerance and survival. Although the use of cool season turfgrasses such as creeping bentgrass {Agrostis palustris Huds. [= A. stolonifera var. palustris (Huds.) Farw.]}, tall fescue (Festuca arundinacea Schreb.), and Kentucky bluegrass (Poa pratensis L.) continues to expand in the region, warm season grasses such as bermudagrass and zoysiagrass (Zoysia japonica Steud.) predominate on most intensively managed turf sites such as golf course fairways and tees and athletic fields. Zoysiagrass is generally more cold tolerant than bermudagrass and is more frequently grown in the northern extremes of the transition zone (Dunn et al., 1999a). However, the primary weakness of zoysiagrass is a lack of vigor during establishment and poor recovery from injury, which favors the more aggressive bermudagrasses in areas where winter injury is not a concern (Carrow, 1994).
The most extensively used bermudagrass cultivars are hybrids between C. dactylon and C. transvaalensis. These hybrids generally lack adequate cold tolerance, and winter injury results in a severe loss of stand in 
1 out of every 5 yr in the upper regions of the transition zone (John King, 1998, personal communication). Winter injury is especially prevalent in intensively managed areas such as golf course fairways and tees, where aggressive fertilization programs, low mowing heights, and extensive traffic predispose the turfgrass to winter injury (Gilbert and Davis, 1971; Chalmers and Schmidt, 1979; Schmidt et al., 1989).
Management practices that enhance cold hardiness of bermudagrass have focused primarily on avoiding late season N applications and increasing K fertilization (Reeves et al., 1970). Late season N applications are believed to promote excessive shoot growth, which hinders the accumulation of storage carbohydrates and other protective osmolytes. However, several studies have suggested that high N fertilization does not predispose bermudagrass to winter injury as much as was previously thought (Gilbert and Davis, 1971; Goatley et al., 1994, 1998). In addition, the positive effects of late season K fertilization on cold hardiness have not been demonstrated in warm season turfgrasses (Miller and Dickens, 1996a, b).
Plant growth regulators are playing an increasing role in turf management programs (Watschke et al., 1992). Although originally introduced to reduce mowing and suppress seed head development, PGRs have also been shown to affect turfgrass population dynamics and improve the tolerance of turfgrasses to abiotic stresses (Nabati et al., 1994). Of the PGRs used on turf, TE, a gibberellic acid inhibitor, has been the most widely accepted chemical for preparing turfgrasses for various types of stress. Some of the beneficial effects of TE include enhanced tolerance to drought (Hongfei and Fry, 1998) and shade (Qian and Engelke, 1999).
Recent studies demonstrated that TE applied to ‘Diamond’ zoysiagrass reduces leaf elongation, enhances root development, enhances photosynthesis, and increases carbohydrate levels, especially under low light conditions (Qian and Engelke, 1999). Han et al. (1998) also reported increased levels of total soluble carbohydrates in creeping bentgrass following applications of TE, although their increases were transient and were only observed for 4 wk. The authors speculated that reduced consumption of fixed carbohydrates, as a result of reduced leaf growth, allows photosynthate to be stored for later use.
The increased soluble carbohydrates observed in TE-treated grasses suggest that turfgrasses treated with TE would be better able to withstand dessication-related stresses such as drought, salinity, and freezing. In addition, turfgrasses treated with TE may also have a partitioning of growth from leaves to structures such as rhizomes or stolons. To test this hypothesis, an experiment was conducted to assess the effects of late season applications of TE and N on morphology and freezing tolerance of ‘Tifway’ bermudagrass.
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METHODS
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This study was conducted at two sites in northwest Arkansas during 1998–1999 (Year 1) and 1999–2000 (Year 2). The Year 1 study was conducted at the University of Arkansas Agricultural Research and Extension Center, Fayetteville, AR, and the Year 2 study was located at the Springdale Country Club, Springdale, AR. The soil at the Research and Extension Center was a Captina silt loam soil (fine-silty, siliceous, active, mesic Typic Fragiudults) with a pH of 6.2, while the soil type at the Springdale Country Club was a Pickwick silt loam (fine-silty, mixed, semiactive, thermic Typic Paleudults) with a pH of 6.4. Both sites were established to a mature (>5 yr) ‘Tifway’ bermudagrass turf. Prodiamine (N3,N3-Di-n-propyl-2,4-dinitro-6-(trifluoromethyl)-m-phenylenediamine) was applied at 1.12 kg ha-1 in early March and mid-September at both sites to control annual grassy weeds. The fertilization program at both sites included monthly applications from April to July of soluble N (urea or ammonium nitrate) at a rate equivalent to 7.5 g N m-2, while P, K, and other nutrients were maintained at sufficiency levels according to soil tests. Irrigation was applied as needed to prevent drought and plots were maintained at a cutting height of 1.9 cm (Year 1) and 1.25 cm (Year 2) throughout the experiments.
Treatments were arranged in a split plot block design with fertilization treatments as whole plots and PGR treatments as subplots. Nitrogen treatments included: (i) no N fertilizer after 15 July (control), (ii) 5 g N m-2 on 15 August, and (iii) 5 g N m-2 on 15 August and 15 September. The treatments were chosen based on the bermudagrass fertilization practices of the region, where no N fertilization is generally applied after Aug. 1. Trinexapac-ethyl treatments included: (i) nontreated control, (ii) TE (15.3 mg a.i. m-2) on 15 August, and (iii) TE (15.3 mg a.i. m-2) on 15 August and 15 September. In Year 2 of the study, a fourth TE treatment was added and included applications at 15.3 mg a.i. TE m-2 on 15 August, 15 September, and 15 October. All treatment combinations were replicated four times.
Plots were visually rated for turf color on a biweekly schedule after treatments were imposed and continued until turf was completely dormant. In addition, plots were rated for green-up on 15 April using a scale of 1 to 9, with 9 being complete green-up. On 5 Jan. 1999 and 3 Jan. 2000, plots were sampled with a core sampler (7.35-cm diam. by 6.35-cm height) and tissues were separated as shoots, roots, rhizomes, or stolons. Rhizomes and stolons were identified according to their growth habit and were only included in the measurements if they had produced a single, distinct node. Tissue measurements included rhizome number and mass, shoot mass, root mass, crown and stolon density, and rhizome and stolon internode length. Stolon and rhizome internode length were an average of a single internode measurement from 10 random stolons and rhizomes. Two additional plugs were harvested from each plot near 10 January of each year and the rhizomes were separated from the remainder of the plugs and used for freeze tolerance experiments.
Each replicate in the freeze test consisted of four to five rhizomes with 20 total nodes. The number of nodes in each replicate was recorded and used to determine percentage survival in the freeze tests. Each replicate was grouped and wrapped in moist cheese cloth to prevent desiccation and maintained at 4°C for 24 h prior to being moved into an experimental freeze chamber (Tenney Jr., Tenney Engineering Inc., New Brunswick, NJ). The freeze chamber was initially set at 0°C and the temperature was dropped from 0 to -2°C during a 2-h period. Samples were held at -2°C for 2 h, at which time four replicate samples of each treatment were removed and slowly thawed in a refrigerated cooler at 4°C for 24 h. The remaining tissues were subsequently dropped to -4, -6, and -8°C in 2-h increments, with units removed at each target temperature and moved to the cooler. After the 24-h thaw period, rhizomes were potted in a commercial greenhouse mix (High Porosity Mix, Strong-lite Co., Seneca, IL) and maintained under greenhouse conditions (maximum temperature, 28°C; natural photoperiod) until regrowth occurred. Percentage rhizome survival for each treatment was assessed 20 d after planting by comparing the number of nodes that initiated growth with the total number of nodes at the beginning of the freeze test.
Data were analyzed by general analysis of variance procedures using the PROC ANOVA model in SAS Systems Software V7.0, (SAS Inst., Cary, NC). In the split plot model, replicate x N was used as the error term to test the main plot effects of N. Where appropriate, Fisher's Protected LSD test was used to separate main effect means. There was a year x N and year x TE interaction for several parameters (data not shown), so all data were subsequently analyzed by year.
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RESULTS
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Both growing seasons were abnormally warm into the fall, with the first killing frost occurring in late November of both years (Fig. 1)
. In addition, both winters of the study were mild for the region, with weekly average low temperatures above freezing for all but 1 or 2 wk each year. Because of these mild conditions, natural winter injury was not observed in any of the plots.
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Fig. 1. Weekly average minimum and maximum air temperatures in Fayetteville, AR, during the experimental periods.
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As dormancy progressed, turf color was enhanced by N applications in both years of the study (Tables 1 and 2)
, but especially during Year 2, when turf color was enhanced by N across most fall evaluation dates (Table 2). Color differences were only seen in November of the first year of the study (Table 1). TE also had a significant effect on turf color of ‘Tifway’ bermudagrass during both years of the study (Tables 1 and 2). In Year 1, either one or two applications of TE resulted in increased turf color compared with the control. In October and averaged across all dates, there was an added advantage of the double application compared with a single application (Table 1). During Year 2, a single application of TE was not as effective as three sequential applications and, in most cases, not as effective as two applications (Table 2).
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Table 1. Turf color and spring green-up (SGU) of ‘Tifway’ bermudagrass, as affected by late-season applications of N and trinexapac-ethyl (TE) (1998–1999 data).
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Table 2. Turf color and spring green-up (SGU) of ‘Tifway’ bermudagrass, as affected by late-season applications of N and trinexapac-ethyl (TE) (1999–2000 data).
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Spring green-up was enhanced by both late season applications of N and TE across both years of the study (Tables 1 and 2). The application of N in August did not enhance SGU more than the controls. However, a second application of 5 g N m-2 in September (Tables 1 and 2) significantly enhanced SGU more than the controls and a single August application of N. All TE treatments enhanced SGU compared with the controls, but two applications of TE did not enhance SGU more than a single application in either year of the study (Tables 1 and 2). In Year 2 of the study, three late season applications of TE enhanced SGU compared with a single August application. There were no significant N x TE interactions for SGU during either year of the study. It was observed that plots fertilized with either a single or sequential N application and two or three TE applications exhibited green color for 15 d longer in the fall and began to break dormancy 10 d prior to control plots in the spring.
There were no consistent effects of either N or TE on any morphological parameters measured in this study (Tables 3 and 4)
. During Year 1, rhizome and crown density were enhanced by N fertilization (Table 3), but these effects were not observed in Year 2 (Table 4). In addition, late season applications of TE had no effect on any morphological parameter in Year 1 of the study, but did enhance rhizome weight and stolon density in Year 2 (Table 4). There were no N x TE interactions for any morphological parameter in either year of the test.
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Table 3. Morphological characteristics of ‘Tifway’ bermudagrass, as affected by late-season applications of N and trinexapac-ethyl (TE). All data were collected in January from replicated plugs 5.08 cm in diameter and 7.6 cm deep. Plots were established at the University of Arkansas Research Station in Fayetteville, AR, in the autumn of 1998, and plugs were sampled on 5 Jan. 1999.
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Table 4. Morphological characteristics of ‘Tifway’ bermudagrass, as affected by late-season application of N and trinexapac-ethyl (TE). All data were collected from replicated plugs 5.08 cm in diameter and 7.6 cm deep. Plots were established at Springdale Country Club in Springdale, AR, in the autumn of 1999, and plugs were sampled on 3 Jan. 2000.
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In the rhizome freeze tests, the number of rhizome nodes that initiated growth following freezing treatments was low for both years of the study, even in those from control plots (Table 5) . Late season N applications had minimal effects on freezing tolerance of rhizomes, although survival was reduced by added N at one temperature in the second year. Late summer applications of TE slightly enhanced rhizome survival at -2 and -4°C in Year 1, but had no effect on rhizome survival in the second year of the test. There were no significant N x TE interactions for any freeze test data.
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Table 5. Survival of ‘Tifway’ bermudagrass rhizomes under freezing temperatures, as affected by late-season application of N and trinexapac-ethyl (TE). Rhizomes were sampled on 5 Jan. 1999 and 8 Jan. 2000.
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DISCUSSION
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Late season N nutrition is often avoided by warm season turfgrass managers, especially in the transition zone, where significant winter injury can routinely occur. In the present study, late season applications of soluble N had no effect on either winter injury in field plots or rhizome freezing tolerance (Table 5). However, late season N applications did extend the fall color of ‘Tifway’ bermudagrass and promote earlier SGU following the dormancy period, similar to earlier studies on bermudagrass (Reeves et al., 1970; Goatley et al., 1994, 1998) and zoysiagrass (Dunn et al., 1993). Although the positive effects of an extended green period are obvious, a potential negative of fall N fertilization is increased winter annual weeds (Dunn et al., 1993). However, winter annual weeds are easily managed in highly maintained bermudagrass through the use of traditional preemergence herbicides, triazine herbicides, or nonselective herbicide applications to dormant turf.
The most common fertility schedule used on warm season turfgrass is to shift from a high N fertilization program in summer to a high K program in the fall to enhance cold hardiness. However, this practice was largely adapted from forage production systems (Adams and Twersky, 1960) and there is no published data on bermudagrass turf that firmly supports this practice. Studies by Miller and Dickens (1996a)(b) indicated that high rates of K fertilization had no effect on bermudagrass root or rhizome freezing tolerance or total nonstructural carbohydrates. Gilbert and Davis (1971) demonstrated that adequate, balanced ratios of N and K were more effective at promoting cold hardiness than either high N or high K fertilization programs. Goatley et al. (1994) also demonstrated that fall N fertilization did not reduce rhizome total N content nor enhance winter injury. The present study, in which K was maintained at sufficiency levels throughout the experiments, supports these earlier studies and suggests that continued N fertilization into the later months of the growing season does not significantly affect freeze tolerance of bermudagrass rhizomes (Table 5) as long as other macronutrients such as K are not limiting.
The physiological basis of cold tolerance in bermudagrass remains poorly defined, which makes it difficult to design management schemes to enhance cold hardiness. Several factors have been associated with cold hardiness, including morphology (Dunn et al., 1980), carbohydrate reserves (Dunn and Nelson, 1974), protein accumulation (Gatschet et al., 1994), and lipid desaturation (Samala et al., 1998). The ability of plant cells to survive low temperature stress is often related to the osmotic characteristics of the plant tissue and the ability of osmolytes to prevent the freezing of inter- and extracellular water (Hsiao, 1973). The principle osmolytes found in plants are soluble carbohydrates, amino acids, soluble proteins, and free ions such as K+ or Cl-. However, the actual effects of these osmolytes on bermudagrass freeze tolerance have not been clearly defined. Dunn and Nelson (1974) sampled three bermudagrass cultivars during the dormancy period and found very little change in the total carbohydrate pool, and only a slight shift in the soluble carbohydrate fraction (sucrose + reducing sugars) relative to the insoluble fraction (starch). Gatschet and co-workers (1994) found that a more cold hardy cultivar, ‘Midiron’, had significantly higher soluble protein levels compared with a cold sensitive cultivar, "Tifgreen". Collectively, these data suggest that protein may be a more important osmolyte than carbohydrate in bermudagrass, especially considering that 50 to 80% of the total carbohydrate pool in bermudagrass is insoluble starch (Dunn and Nelson, 1974). Assuming this is the case, it is not surprising that most studies, including the present investigation, have shown no deleterious effects of fall-applied N. However, further work into the area of osmotic freeze protection with an emphasis on soluble proteins and other N-containing osmolytes such as glycine-betaine and proline (Hellebust, 1976) would be valuable.
Late season N fertilization and repeat applications of TE significantly reduced the dormancy period of bermudagrass by both extending fall color and promoting early SGU (Tables 1 and 2). A reduction in the dormancy period can have several positive effects on bermudagrass turf. The most obvious benefit would be the increase in days of functional green turf available to the user. This is a major issue for many turf managers who are pressured to maintain the highest possible quality for the longest period of time. Secondly, grasses that break dormancy early can more effectively recover from damage caused by winter traffic or low temperature injury, as demonstrated by Schmidt et al. (1989). This is especially important when this species is used for fall activities such as football, where injury from traffic can be severe in certain areas of the turf. A final aspect of bermudagrass performance that could be affected by a shortened dormancy period is the overall potential to reduce winter injury. Dunn et al., (1980) suggested and Chalmers and Schmidt (1979) found that the freeze tolerance of bermudagrass rhizomes decreased as the post freeze dormancy period increased. In the present study, it was predicted that the dormancy period was reduced by 20 to 25 d in plots fertilized with both N and TE during the predormancy period. Comparing this to the work of Chalmers and Schmidt (1979), this reduction in the dormancy period may enhance rhizome survival by as much as 10 to 15%. On a negative note, earlier spring greenup can predispose the turf to damage from a late season frost.
The effects of TE applications on freezing tolerance have been investigated previously on both bermudagrass (Yelverton et al., 2000) and zoysiagrass (Dunn et al., 1999b), but the results have been inconsistent. In two studies from North Carolina (Fagerness and Yelverton, 2000; Yelverton et al., 2000), one study reported a slight increase in freezing tolerance of bermudagrass stolons when TE was applied late season, but the other reported no effects of late season TE application on whole-plant cold tolerance. In addition, Dunn et al., (1999b) reported no effect of late season TE applications of cold tolerance of two zoysiagrass cultivars. The present study also found either no effect or a positive effect of late season TE applications on freezing tolerance of bermudagrass rhizomes (Table 5). Collectively, these data suggest very little positive effect of TE on cold tolerance of bermudagrass. However, none of the studies reported here or described previously in the literature (Dunn et al., 1999b; Fagerness and Yelverton, 2000; Yelverton et al., 2000) reported any negative effect of TE on freezing tolerance. Since many turfgrass managers apply TE to bermudagrass turf during the late summer and early fall to prepare for winter overseeding (Menn et al., 1998), the fact that TE has not been shown to reduce winter hardiness is in itself positive.
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ACKNOWLEDGMENTS
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The author graciously acknowledges the financial support of Syngenta, Inc.
Received for publication May 20, 2001.
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REFERENCES
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TOP
ABSTRACT
INTRODUCTION
