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Bermudagrass Freeze ToleranceOklahoma State University researchers use laboratory and fieldevaluations to compare bermudagrass freeze tolerance.BY JEFF ANDERSON, CHARLES TALIAFERRO, DENNIS MARTIN,YANQI WU, AND MICHAEL ANDERSON

Regrowth of CIS-CD? seeded bermudagrass varied after exposure to a range ofsub-freezing temperatures.

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rfgraSS managersspend a considerableamount of time and

energy to establish andmaintain turf grasses foraesthetic, environmental,and recreational purposes.Both genetic and environ-mental components interactto determine how well achosen cultivar performs ina particular location. Anincreasing number of fine-textured bermudagrassesare being developed andevaluated for resistance toenvironmental stresses.Freeze damage is a primary concern inthe northern boundaries of thebermudagrass adaptation zone.

Some years are relatively mild andcause little or no damage, while otherwinters are sufficiently severe to causeextensive winterkill. The costs, interms of loss of use and dollars to re-establish turf following winterkill, canbe substantial. Therefore, our long-term goal is to develop seed- andvegetatively propagated bermudagrasseswith high turf quality and improvedfreeze tolerance.

A common way to compare relativefreeze tolerance of a group of cultivarsis to establish them in the field andwait for cold temperatures to sort themout. However, during a mild winter,temperatures may not be cold enoughto kill any cultivars of interest, and noprogress would be achieved. If evalu-ations were conducted at a northern orhigh-elevation location, low tempera-

tures may kill most or all of the ber-mudagrasses. Therefore, several yearsof observation may be required toexperience temperature conditions thatdistinguish different levels of freezetolerance within a group of bermuda-grass cultivars. Relying on test wintersmakes it difficult to repeat studies overtime and across climatic locations.

Another factor that comes into playduring natural freezes is the nature ofthe freeze itself Differences in freezingrate or duration, even with the sameminimum exposure temperature, canresult in different plant responses.4

Whether or not a snow cover is presentcan have marked influences on plantsurvival due to insulation effects.Developmental and morphologicalfeatures also can be factors in wintersurvival. The presence of rhizomes cancontribute to freeze avoidance by beingsufficiently deep in the soil profile toavoid temperature extremes. The well-

documented susceptibilityof newly seeded bermuda-grasses may involve physio-logical and/or morpho-logical factors such asstolon density.6

YEAR-ROUNDWINTER INDOORSLaboratory-based methodsto measure freeze tolerancehave been developed. Oneapproach has been toacclimate plants naturallyin the field, followed bylaboratory-based exposureto sub-freezing tempera-

tures. Studies also have been conductedentirely indoors, with plant materialsestablished and acclimated in growthchambers, followed by exposure to arange of temperatures in a freezechamber. Laboratory-based freeze-tolerance evaluations generally corre-spond well with field observationsand have provided useful informationon relative freeze tolerance ofturfgrasses.2

Our objective was to quantify freezetolerance of advanced lines, recentlyreleased cultivars, and standard varietiesentered in the 2002 National TurfgrassEvaluation Program (NTEP) bermuda-grass trial using laboratory-basedmethods. Standardized, quantitativeinformation on bermuda grass freezetolerance is vital to scientists to trackprogress in developing new cultivars.Freeze tolerance data also are beneficialto turf grass managers selecting turf-grasses for the transition zone.

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SWI-1046

Veracruz

Figure IDeviation temperature (OF) from AZ Common

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in freeze tolerance relative to the stan-dard. Transcontinental, SWI-1014,Riviera, and CIS-CD6 were signifi-cantly more cold hardy than ArizonaCommon. Although Yukon and Trans-continental differed from ArizonaCommon by the same amount, thedifference was not significant forYukon at the 5% level due to greatervariability in data from Yukon. Aprevious study that included these twocultivars found Yukon to be signifi-cantly more freeze tolerant thanArizona Common.!

LapalomaSWI-IOOI

Tift #2SR 9554

CIS-CDSNuMex Sahara

SunbirdSouthern Star

SWI-1044SWI-1012Sundevill

Transcontinental *Yukon

SWI-1014Riviera

CIS-CD6-1.5 -0.5 0.5 1.5

.. Less Freeze Tolerance

Contessa

SWI-1003Tift #1AZ CommonFMC 6MohawkPrincess 77

Freeze tolerance of seed-propagated bermudagrasses relativeto Arizona Common. Deviationtemperatures represent the Tmidvalue (midpoint ofthe survival-temperatureresponse curve) ofthecultivarminus the Tmidvalue for Arizona Comm~n. Cultivarssignificantlydifferentfrom ArizonaCommon are indicatedby an asterisk.Adapted from Anderson et al.5

CONSIDERABLE VARIATIONIN FREEZE TOLERANCESeed-propagated bermudagrassesranged in freeze tolerance from22.5°F (-5.3°C) (SWI-1003) to 16.3°F(-8.7°C) (CIS-CD6). Even thoughthree cultivars were numerically lessfreeze tolerant than Arizona Common,none of the three was significantlydifferent. FMC 6, Mohawk, Princess77, and SWI -1046 were identical infreeze tolerance to Arizona Common.Fifteen cultivars had numericallygreater, yet non-significant differences,

Bermudagrass plants were establishedand maintained in growth chambers.For studies with seed-propagated culti-vars, seed from the lots used in the2002 NTEP bermudagrass trial wasobtained from the sponsors. Twenty-seven of the 29 seed-propagated entrieswere included in this study. Experi-ments with seed-propagated bermuda-grasses were divided into five groups.Entries were randomly selected andassigned to groups with ArizonaCommon included as a standard ineach, allowing the potential for com-parisons across groups. Vegetativecultivars were propagated from indi-vidual phytomers using Tifway as thestandard cultivar in each of the threegroups. Experiments were conductedon three dates for each group, consti-tuting replications in time, withstaggered plantings allowing uniformestablishment periods andplant age.

After plants had acclimated to fall-like temperatures, they were trimmedof top-growth and placed in a freezechamber with a temperature sensor ineach pot. The chamber was pro-grammed to slowly cool the plants,allowing them to be removed over arange of temperatures. Ideally, nodamage would occur at the warmesttemperatures, and all plants would bekilled by exposure to the coldesttemperatures.

After being removed from thefreeze chamber, plants were thawedand returned to the growth chamberto observe regrowth. Non-frozencontrols were treated the same, exceptwithout the freeze chamber exposure.Evaluating the temperature-survivalcurve allowed estimation of a T mid

value, similar to the LDso (lethal dosefor 50% of the subjects) in a toxicityscreen. Data were combined intoseeded and vegetative types. Perfor-mance relative to the standard cultivar(Arizona Common or Tifway) wasdetermined by subtracting the T mid foreach cultivar from the T mid value forthe standard in that group.

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Figure 2Deviation temperature (OF) from Tifway

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Freeze Tolerance

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Laboratory-based methods have been developedto measure turfgrass freeze tolerance. Thermo-couple temperature sensors are used tomeasure soil temperatures.

Bermudagrass plants are acclimated to fall-liketemperatures, trimmed of top growth, andplaced in a programmable freeze chamber to beexposed to sub-freezing temperatures.

Freeze tolerance of vegetatively propagated bermudagrasses relative to Tifway. Deviation temperaturesrepresent the Tmidvalue (midpoint ofthe survival-temperature response curve) of the cultivar minusthe Tmidvalue for Tifway. Cultivars significantly different from Tifway are indicated by an asterisk.Adapted from Anderson et al.5

Vegetatively propagated bermuda-grasses ranged in freeze tolerance from20.8°F (-6.2°C) (GN-l) to 11.3°F(-11.S0C) (OKC 70-18). Three culti-vars, GN-l, Celebration, and MS-Choice, were significantly less freezetolerant than Tifway. Tift #4, Tifsport,Premier, Tift #3, and Aussie Greenhad cold hardiness levels similar toTifway. Midlawn, Ashmore, Patriot,and OKC 70-18 were significantlymore freeze tolerant than Tifway.

Freeze tolerance estimates generallycorresponded well with previousexperience.3 Both Midlawn and Patriotexhibited greater freeze tolerance thanTifway as previously reported.4 Greaterfreeze tolerance of Riviera thanPrincess 77 is consistent with earlierfindings.5 In a previous report, we also

found GN-l to be significantly lessfreeze tolerant than Midlawn.3

It is important to distinguishbetween T mid temperatures determinedin the laboratory and air temperaturesexperienced during a natural freeze. Inthe laboratory, conditions are set toensure that plants reach the targettemperatures. Critical tissues, such ascrowns, of plants in the field willusually be considerably warmer thanair temperature due to the thermalbuffering capacity of the soil.

Substantial progress is being madeby turf grass breeders to develop seed-propagated and vegetatively propagatedbermudagrasses with improved freezetolerance. Although many factors inaddition to freeze tolerance will beassessed in making cultivar selections,

choices are now available with freezetolerance suitable for areas of thetransition zone requiring superiorwinter hardiness.

LITERATURE CITED1. Anderson, J., C. Taliaferro, M. Anderson,D. Martin, and A. Guenzi. 2005. Freezetolerance and low temperature-induced genesin bermudagrass plants. USGA Tuljgrass andEnvironmental Research Online. 4(1):1-7.2. Anderson, J. A., C. M. Taliaferro, and D. L.Martin. 1993. Evaluating freeze tolerance ofbermudagrass in a controlled environment.HortScience. 28:955.3. Anderson, J. A., C. M. Taliaferro, and D. L.Martin. 2002. Freeze tolerance of bermuda-grasses: vegetatively propagated cultivarsintended for fairway and putting green use,and seed-propagated cultivars. Crop Sci.42:975-977.

4. Anderson, J. A., C. M. Taliaferro, and D. L.Martin. 2003. Longer exposure durations

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A Q&A with DR. JEFF ANDERSON regarding the use of arti{tcialfreezetesting to evaluate turf (or cold hardiness.

Q: Arti{tcialfreeze testing seems like a good method to screen turf-grass selections for freeze tolerance, but how often do laboratoryfreeze-testing methods disagree with {teld testing? What other factorsbesides temperature affect {teld-grownplants that may lead to thesedifferences?A: Research conducted at several universities has shown that fieldand laboratory results on turfgrass freeze tolerance are usually inagreement. While there are instances when rankings from fieldstudies do not completely match laboratory studies, there alsohave been cases when results from one field study do not matchanother. Different locations could have different environmentalconditions before a freezing episode, leading to different patternsof acclimation. It also is possible for a cultivar exposed to oneenvironmental stress to be more susceptible to other stresses.

Q: How long have laboratory freeze-testing methods been used onturfgrasses? Are the methods the same as they were in earlier tests?A: Laboratory-based methods of freeze tolerance evaluation havebeen available for many decades. Ongoing research has led torefinements in testing methods, resulting in greater precision andreproducibility. Important refinements include ice nucleation tonegate supercooling and monitoring the temperatures of eachexperimental unit. Use of microprocessor-controlled chambersand precision monitoring equipment has further improved theprecision and reproducibility of the testing procedures.

Q: Growing plants in the greenhouse to evaluate survival aftersubjecting those plants to low temperature seems time-consuming.What additional tests are available that scientists can use to measuretissue viabilityafter freezing that don't take as much time?A: Viability testing has been a major focus of plant stress studiesfor many years. Approaches range from whole plant responses tobiochemical assays, with each procedure having its strengths andweaknesses. Assays such as electrolyte leakage can be performedmuch more rapidly than regrowth analysis and have been appliedto turfgrasses. When compared, the two procedures are ingeneral agreement. However, there have been instances whenelectrolyte leakage has either overestimated or underestimatedfreeze tolerance when compared to regrowth results. One reasonmay be that freezing stress yields a more gradual electrolyteleakage versus temperature response compared with heat stress,which is very well suited to electrolyte leakage assays. One of thechallenges of using electrolyte leakage for freeze tolerance ofbelow-ground structures like crowns and rhizomes is therequirement that tissues be separated from soil/media withoutintroducing artifacts.

Q: In other articles, the bermudagrass germplasm that Dr. Yanqi Wucollected in China has been mentioned. Have you freeze-tested thisChinese collection and/or evaluated its cold tolerance? From yourexperience, is it likely that the Chinese bermudagrass germplasm willhelp improve freeze tolerance of yet-to-be-released bermudagrasscultivars?A: The addition of Chinese bermudagrass germplasm to theOklahoma State University collection provides additionalvariability that can be used to develop new stress-tolerant, high-quality bermudagrass cultivars. Characterization of this collectionand subsequent progeny for freeze tolerance is a priority and willproceed as funding permits. Based on geographic locations ofwhere these plant materials were collected, there is a highprobability that a portion of the collected material contains genesthat will make plants suitable for locations that experience coldwinters.

Q: How does the "rate of freeze" affect freeze-tolerancemeasurements?A: Plant survival during freezing stress is favored by slow rates ofcooling. Most studies use cooling rates of about rF per hour,similar to natural conditions. The rate of tissue cooling is notalways the same as the rate of air temperature decline, especiallyfor below-ground plant tissues. In addition to the buffering effectof the soil, plant temperatures will be moderated by the heatreleased when soil moisture freezes. Therefore, the rate oftemperature change, the temperature minimum, and the durationof the low temperature exposure will all contribute to theintensity of freezing stress.

Q: From your experience, does the maturity of the turfgrass standimpact its cold tolerance? Can superintendents expect seededbermudagrasses to be less cold tolerant the {trst winter (ollowingseeding, and more cold tolerant in subsequent winters?A: Although the mechanisms are not fully understood, thelong-held belief that seeded bermudagrasses are more freezesusceptible shortly after planting has been reinforced bycompelling evidence from research at the University of Arkansasand other locations.

Q: What should superintendents learn as a "take home" message fromyour work, Dr.Anderson?A: Plant breeding programs around the country are doing anexcellent job in developing new bermudagrass cultivars. It is nolonger necessary to sacrifice turf quality to achieve stressresistance. Increased freeze tolerance in fine-textured bermuda-grass lowers the probability of winter injury in traditional plantinglocations. While use can be extended to colder locations, eventhe most freeze-tolerant varieties currently available will besusceptible to winterkill under extreme conditions.

increase freeze damage to turfbermudagrasses.Crop Sci. 43:973-977.

5. Munshaw, G. c., E. H. Ervin, D. Parish,C. Shang, S. D. Askew, X. Zhang, and R. W.Lemus. 2006. Influence oflate-season iron,nitrogen, and seaweed extract on fall colorretention and cold tolerance of four bermuda-grass cultivars. Crop Sci. 46:273-283.

6. Richardson, M. D., D. E. Karcher, and]. W.Boyd. 2004. Seeding date and cultivar affectwinter survival of seeded bermudagrasses.USGA Tuifgrass and Environmental ResearchOnline. 3(13):1-8.

EDITOR'S NOTE: An expanded versionof this paper can be found at USGATutjgrass and Environmental ResearchOnline (http://usgatero.msu.edu/v06/n18.pdf).

JEFF ANDERSON, PH.D., Prifessor, Dept.Horticulture & LA, Oklahoma StateUniversity, Stillwater, Okla.; CHARLESTALIAFERRO, PH.D., Emeritus Regents

Prifessor, Dept. Plant & Soil Sciences,Oklahoma State University, Stillwater,Okla.; DENNIS MARTIN, PH.D., Prifessor,Dept. Horticulture & LA, OklahomaState University, Stillwater, Okla.; YANQIWu, PH.D., Assistant Prifessor, andMICHAEL ANDERSON, PH.D., AssociatePrifessor, Dept. Plant & Soil Sciences,Oklahoma State University, Stillwater,Okla.

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