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© 2007 Plant Management Network.
Accepted for publication 4 January 2007. Published 8 May 2007.

Freeze Tolerance of Seed- and Vegetatively-Propagated Bermudagrasses Compared with Standard Cultivars

Jeffrey A. Anderson, Department of Horticulture and Landscape Architecture, and Charles M. Taliaferro and Yanqi Q. Wu, Department of Plant and Soil Sciences, Oklahoma State University, Stillwater 74078

Corresponding author: Jeffrey A. Anderson. jeff.anderson@okstate.edu

Anderson, J. A., Taliaferro, C. M., and Wu, Y. Q. 2007. Freeze tolerance of seed- and vegetatively-propagated bermudagrasses compared with standard cultivars. Online. Applied Turfgrass Science doi:10.1094/ATS-2007-0508-01-RS.

Abstract

Bermudagrasses, Cynodon sp., periodically sustain freeze damage in the transition zone for warm- and cool-season turfgrasses. Therefore, there is a need to develop and characterize bermudagrass cultivars with superior freeze tolerance. Our objective was to determine relative freeze tolerance levels of recently released cultivars, advanced lines, and standard cultivars from the 2002 National Turfgrass Evaluation Program bermudagrass trial using laboratory-based methodology. Twenty-seven seed-propagated cultivars were randomly divided into five groups with Arizona Common and Riviera serving as standards in each of the five groups. Tifway and Midlawn were used as standard cultivars for the three vegetatively-propagated groups. A range in freeze tolerance from -5.3°C (cv. SWI-1003) to -8.7°C (cv. CIS-CD6) was observed for seed-propagated cultivars. The most freeze tolerant seed-propagated cultivars were CIS-CD6, Riviera, Transcontinental, and SWI-1014. Freeze tolerance of vegetatively-propagated cultivars ranged from -6.2°C (cv. GN-1) to -11.5°C (cv. OKC 70-18). OKC 70-18, Ashmore, and Patriot were comparable or superior to the standard vegetatively-propagated cultivar Midlawn, reflecting potential to survive in the northern boundary of the transition zone with a low probability of winterkill.

Introduction

Bermudagrasses grown in the transition zone for warm- and cool-season turfgrasses are subject to freeze damage, resulting in loss of use and substantial costs in re-establishment (6,10). Therefore, development of cultivars with improved freeze tolerance and superior turf qualities has become a priority for both seed-propagated and vegetatively-propagated bermudagrasses. Breeding efforts require a rapid and reproducible means to evaluate freeze tolerance. Standardized, quantitative information on freeze tolerance is vital for scientists to track their progress in developing new cultivars. Freeze tolerance information is also important for turfgrass managers selecting bermudagrasses for the transition zone.

Winter survival in the field is frequently reported as percent area of live above-ground shoots in the spring (9). Often, several years of exposure are required to experience temperatures causing significant freeze damage. Different years may have contrasting temperature patterns during the acclimation period that can contribute to year-to-year differences in hardiness estimates. Similarly, differences in freezing rate or duration, even with the same minimum exposure temperature, could result in different plant responses (4). To overcome the unpredictable occurrence of test winters and to expand evaluations year-round, laboratory-based approaches to measure freeze tolerance have been developed. Plant material has been acclimated in growth chambers, followed by exposure to a range of temperatures spanning the presumptive killing temperature in a freeze chamber (2). The combined approach with plant material acclimating in the field, followed by laboratory-based exposure to sub-freezing temperatures also has been employed (8,13). Laboratory-based evaluations generally correspond well with field observations (5,12), and have provided useful information on relative freeze tolerance of turfgrasses. Our objective was to evaluate freeze tolerance of turf bermudagrasses, aligning with cultivars in the 2002 National Turfgrass Evaluation Program (NTEP) bermudagrass trial.

Plant Culture

Experiments were divided into vegetatively-propagated and seed-propagated cultivars. For studies with seed-propagated cultivars, seed from the lots used in the 2002 NTEP bermudagrass trial was obtained from the sponsors. Twenty-seven of the twenty-nine seed-propagated entries were included in this study. Plants were established using approximately 50 mg of seed per Cone-tainer (Ray Leach Cone-tainer Nursery, Canby, OR) with the intention of having seedlings that were representative of the population. Vegetatively-propagated cultivars were initially established in pots from stolons collected from the Stillwater site for the 2002 NTEP bermudagrass trial. Cone-tainers were then planted with individual phytomers from the pots. The potting medium (BM-1, Saint-Modeste, Quebec, Canada) was supplemented with dolomite (2.97 g/liter), superphosphate (0.74 g/liter), micromax (The Scotts Co., Marysville, OH) (0.45 g/liter), KNO₃ (0.59 g/liter), and FeSO₄ · 7H₂O (0.24 g/liter). Plants were watered daily and fertilized weekly with full-strength soluble fertilizer (Peters 20N-8.6P-16.6K, The Scotts Co., Marysville, OH). Plants were trimmed with scissors as needed. After establishment for 7 weeks in a growth chamber (model PGW36, Conviron, Ashville, NC) at 32°/28°C (day/night) temperatures with a 14-h photoperiod, plants were transferred to a chamber at 24°/20°C for 1 week (14-h photoperiod) before acclimation at 8°/2°C (day/night) temperatures for 4 weeks with a 10-h photoperiod. Photosynthetic photon flux densities were about 400 μmol/m²sec during establishment and about 350 μmol/m²sec during acclimation.

Temperature Treatments and Data Analysis

Due to space limitations in the freeze chamber, experiments with seed-propagated bermudagrasses were divided into five groups. Each group contained five randomly selected entries and two standards (Arizona Common and Riviera), allowing the potential for limited comparisons across groups. Vegetatively-propagated bermudagrasses were randomly assigned to three groups with Tifway and Midlawn as standard cultivars. Experiments were conducted on three dates for each group, constituting replications in time, with staggered plantings allowing uniform establishment periods.

Plant shoots were trimmed to Cone-tainer height (even with the top), then thermocouple sensors attached to wooden dowels were inserted to a measuring depth of 2 cm. Crushed ice was placed on top of the shoots and medium before Cone-tainers were placed into a freeze chamber (model CEC23, Rheem Scientific, Asheville, NC) for low temperature exposure. The freeze chamber was rapidly cooled to -2.5°C and sample freezing was confirmed after about 4 h by observation of exothermic release of the heat of fusion in the temperature records from the datalogger (model 21X, Campbell Scientific, Logan, UT). Chamber temperature was held overnight at -2.5°C after loading samples, then cooled linearly at 1°C per hour. Four Cone-tainers from each genotype were removed when they reached the target temperature. Target temperatures (1°C intervals) covered a range anticipated to span from complete survival to complete mortality. Cone-tainers were held overnight at about 4°C after removal from the freeze chamber. Non-frozen controls are treated the same, except without freeze chamber exposure. Following thawing, plants were returned to a growth chamber at 24°/20° for 1 week before 4 weeks at 32°/28°C (day/night) temperatures with a 14-h photoperiod. Plant response was based on regrowth potential (survival indicated by shoot emergence, growth, and development) and fresh weight of regenerated shoots excised at Cone-tainer height. Data were reported as the midpoint of the sigmoidal temperature response curves (T_MID) determined by nonlinear regression (1,7) using Proc NLIN (SAS Institute Inc., Cary, NC) for survival and log₁₀ (fresh weight + 0.0001). Log transformations of mass data resulted in sigmoidal response curves. Calculations involving values of zero (Cone-tainers without regrowth) required adding an offset (0.0001) to all values. Mean comparisons within each experiment were conducted as appropriate using LSD at P ≤ 0.05 following ANOVA.

Freeze Tolerance of Seed-Propagated Bermudagrasses

The first group of seed-propagated bermudagrasses ranged in freeze tolerance from -6.0°C (Arizona Common) to -7.7°C (Riviera) based on survival (Table 1). Similarly, results calculated from mass of regrowth indicated that Riviera was more freeze tolerant than Arizona Common. Numex Sahara, Southern Star, SWI-1044, SWI-1012, and Yukon formed a group with intermediate freeze tolerance and were not significantly different from either standard cultivar.

Table 1. Freeze tolerance of seed-propagated bermudagrasses.

 ^x T_MID values (midpoints of sigmoidal temperature response curves)
based on survival and Log₁₀ (mass) are presented for five groups
of bermudagrass cultivars.

 ^y Mean separation by columns within each group by LSD at P ≤ 0.05.

T_MID values from the second group of seed-propagated bermudagrasses exhibited a range in freeze tolerance from -5.3°C (SWI-1003) to -6.7°C (Riviera) based on survival (Table 1). Riviera was more freeze tolerant than all other cultivars except SR 9554 (-6.0°C) based on survival, and was significantly more hardy than the other six cultivars in the second group based on mass of regrowth. SWI-1003, Mohawk, FMC 6, Lapaloma (SRX 9500), and SR 9554 were not significantly more freeze tolerant than Arizona Common.

The third group of seed-propagated bermudagrasses ranged in freeze tolerance from -5.7°C (Tift No.1) to -7.2°C (Riviera) based on survival, while T_MID values from regrowth spanned from -5.5° to -6.8°C (Table 1). Survival of Tift No.1 was not significantly different from Arizona Common, Princess 77, Panama, or CIS-CD7. Riviera survived to a lower temperature than all other cultivars except Sundevil II (-6.5°C), and was significantly hardier than the other cultivars in this group based on mass of regrowth.

T_MID values from the fourth group of seed-propagated bermudagrasses exhibited a range in freeze tolerance from -6.1°C (SWI-1041) to -8.7°C (CIS-CD6) based on survival (Table 1). Survival and regrowth of SWI-1041, SWI-1046, and Contessa (SWI-1045) were not significantly different from Arizona Common. CIS-CD6 had exceptional freeze tolerance for a seed-propagated bermudagrass, surviving to a lower temperature than all other cultivars except Riviera. Transcontinental had an intermediate level of freeze tolerance based on survival, but had a T_MID value based on mass of regrow similar to Contessa.

The fifth group of seed-propagated bermudagrasses ranged in freeze tolerance from -6.2°C (Arizona Common) to -8.5°C (Riviera) based on survival and from -5.7° to -7.5°C based on regrowth (Table 1). In addition to Riviera, SWI-1014 was also significantly more freeze tolerant than Arizona Common. SWI-1001, Tift No.2, CIS-CD5, and Sunbird (PST-R68A) were not significantly more hardy than Arizona Common according to survival and regrowth data.

The seed-propagated cultivars CIS-CD6, Riviera, Transcontinental, and SWI-1014 exhibited significantly greater freeze tolerance than Arizona Common and would be less likely to experience winterkill in northern locations of the transition zone. Arizona Common was also reported to have more winterkill than these cultivars at Olathe, KS (11). Although comprehensive information on freeze tolerance of recently developed bermudagrass cultivars is limited, greater freeze tolerance of Riviera than Princess 77 is consistent with previous findings (4,10). Previous studies of seeded bermudagrasses found Yukon to be significantly hardier than Arizona Common in laboratory (3) and field experiments (11), but the difference was not significant at P = 0.05 in the current report. The standards for the seed-propagated cultivars, Arizona Common and Riviera, had freeze tolerance levels based on survival of -6.0° and -7.6°C, respectively, averaged over the five groups. Values were slightly warmer (Arizona Common = -5.7°C and Riviera = -7.4°C) when T_MID was based on fresh weight of survivors. Twenty-two out of twenty-five seed-propagated cultivars had freeze tolerance levels similar to Arizona Common, but none were significantly less hardy. Although Riviera was significantly more freeze tolerant than Arizona Common in each group, T_MID values for the standards varied from group to group. Therefore, comparisons made across groups should be limited to performance relative to the standards.

Freeze Tolerance of Vegetatively-Propagated Bermudagrasses

Freeze tolerance evaluations of the first group of vegetative entries ranged from -6.6°C (Celebration) to -11.5°C (OKC 70-18) (Table 2). Midlawn (-9.1°C) was less tolerant than OKC 70-18, but significantly hardier than all other cultivars. Tift No. 3 (-7.6°C), Tifway (-7.7°C), and Aussie Green (-8.2°C) formed a group of intermediate hardiness.

Table 2. Freeze tolerance of vegetatively-propagated bermudagrasses.

 ^x T_MID values (midpoints of sigmoidal temperature response curves)
based on survival and Log₁₀ (mass) are presented for three groups
of bermudagrass cultivars.

 ^y Mean separation by columns within each group by LSD at P ≤ 0.05.

GN-1 was the least hardy, and Midlawn and Patriot were the most freeze tolerant bermudagrasses in the second group of vegetatively-propagated cultivars (Table 2). Tift No. 4, Tifsport, and Tifway, were intermediate in hardiness based on mass of regrowth. Survival data were similar, except GN-1 was not significantly different from Tift No. 4.

T_MID values from the third group of vegetatively-propagated bermudagrasses indicated a range in freeze tolerance from -7.1°C (MS Choice) to -9.7°C (Ashmore) based on survival (Table 2). Midlawn and Ashmore were significantly more freeze tolerant than the other three vegetatively-propagated cultivars in the third group. Tifway was significantly more freeze tolerant than MS Choice based on survival, but MS Choice, Premier (OR 2002), and Tifway were not significantly different based on mass of regrowth.

Freeze tolerance of vegetatively-propagated cultivars ranged from -6.2°C (GN-1) to -11.5°C (OKC 70-18). Ashmore and Patriot were similar to Midlawn, but OKC 70-18 was significantly more freeze tolerant than the standard cultivar Midlawn. Winterkill results from Olathe, KS (11) were generally similar, except Premier was more tolerant and Patriot was less tolerant at that location, compared with our results. Midlawn was significantly more freeze tolerant than Tifway in all three experiments, consistent with previous findings (3,4). Similarly, Patriot had freeze tolerance similar to Midlawn (4) and GN-1 was reported to be significantly less freeze tolerant than the standard cultivar Midlawn (3). OKC 70-18, Ashmore, and Patriot were comparable or superior to the standard vegetatively-propagated cultivar Midlawn, reflecting potential to survive in the northern boundary of the transition zone with a low probability of winterkill.

Whole plant responses to freezing stress are frequently measured as survival, treating strong and weak regrowth equally. Dunn et al. (5) reported freeze tolerance of zoysiagrass (Zoysia japonica Steud.) based on shoot number and mass. In the present study, T_MID values based on mass were 0.4°C warmer than T_MID values derived from survival, reflecting instances of weak regrowth. Additional studies will be required to determine whether one parameter is superior in predicting freeze tolerance or whether a composite provides the most useful information. Estimates based on regrowth mass may provide a means to distinguish between cultivars that respond to low winter temperatures with strong regrowth, cultivars that respond weakly but can fill in with time, and those that do not survive.

Conclusions

Substantial progress is being made by turfgrass breeders to develop seed-propagated and vegetatively-propagated bermudagrasses with improved freeze tolerance. Although many factors in addition to freeze tolerance will be assessed in making cultivar selections, choices are now available with freeze tolerance suitable for areas of the transition zone requiring superior winter hardiness. However, cultivars with limited freeze tolerance may still be excellent choices for locations that do not experience severe winters.

Acknowledgments

We gratefully acknowledge use of the Controlled Environment Research Laboratory and assistance from Shakuntala Fathepure, Dennis Martin, and Xi Xiong. Approved for publication by the Director, Oklahoma Agricultural Experiment Station. Research supported by the Oklahoma Agricultural Experiment Station and the United States Golf Association.

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