How researchers are developing freeze-resistant Bermuda grass

How researchers are developing freeze-resistant Bermuda grass

Winterkill is used to determine grass loss during the winter. Winterkill or winter survival depends on various factors, including crown hydration, drought, immediate low temperatures, ice sheets, and snow rot disease.

Replacing damaged grass lost to winter is labor-intensive and expensive. Intensively managed areas such as golf courses are vulnerable to winter kill due to aggressive fertilization programs, low mowing heights and vehicular and foot traffic.

Bermuda grass (Cynodon spp.) is the most widely used warm-season turf grass on transition zone golf courses in the United States. Bermuda grass has excellent heat and drought tolerance but has a low tolerance to freezing temperatures. Developing bermudagrass with better freeze tolerance is a priority in bermudagrass breeding programs.

Previous research at Oklahoma State University (Oklahoma State) and North Carolina State University (NC State) reported significant variation in freeze tolerance of bermudagrass cultivars. Many of these experiments determining the freezing tolerance of bermudagrass use controlled environmental chambers to estimate the temperature to kill 50 percent of the population (LT50). Research results show that genetic improvement is possible in bermudagrass breeding programs.

LT50 values ​​obtained in controlled environment experiments showed a significant negative correlation with spring greening and a positive correlation with winter kill estimated in the field. Although field evaluations are typical for breeders to evaluate genotypes' ability to survive the winter in large nurseries, environmental conditions in field trials are unpredictable and difficult to replicate.

Controlled environmental experiments can quickly identify genetic differences in freezing tolerance based on exposure to direct freezing temperatures. Therefore, the objective of this experiment was to determine LT50 values ​​for two experimental hybrid bermudagrass genotypes and two commercially available cultivars by exposing them to 11 freezing target temperatures (25 to 7°F) under controlled environmental conditions.

Plant materials and growing conditions

Entries in this trial consisted of two experimental genotypes, OKC1873 and OKC1406, developed by the OK State bermudagrass breeding program, and two industry standards, Tifway (freeze-sensitive) and Tahoma 31 (freeze-tolerant).

The experiment was repeated in time, with overlapping cultivations to allow uniform fixation periods. We propagated all entries clonally in potting mix inside containers (8.25 inches deep and 1.5 inches in diameter). The propagation material used in each container was a single shoot consisting of root material, crown and shoots.

Bermuda grass accessions were established in a growth chamber at 32/28°F day and night for 13 weeks. (Photo: Lakshmi Gopinath, Ph.D.)

We established bermudagrass accessions in a PGC Flex growth chamber at the OK State Controlled Environmental Research Laboratory, Stillwater, Oklahoma (Image 1). The growth chamber was maintained at 32/28°F day and night for 13 weeks with a 14-h photoperiod and photosynthetically active radiation (PAR) of 900 μmol m-2 s-1.

Containers were adequately fertilized weekly with a 20-10-20 NPK general purpose fertilizer (J.R. Peters) and pruned to maintain a 1-inch height. During the establishment phase, containers were treated every 14 days with Talstar (bifenthrin) as a precaution.

At the end of 13 weeks, temperatures were reduced to 75/68°F day/night for 1 week to pre-acclimate the containers prior to cold acclimation. We then subjected the containers to cold acclimation by reducing the temperature to 46/36°F day/night for four weeks with a 10-hour photoperiod and an exposure rate of 400 μmol m-2 s-1.

Containers were placed in a freezer chamber with 10 randomly placed thermal sensors inserted 1 inch into the potting medium.  Ice flakes are located in all containers to prevent supercooling and induce freezing.  (Photo: Lakshmi Gopinath, Ph.D.)

Containers were placed in a freezer chamber with 10 randomly placed thermal sensors inserted 1 inch into the potting medium. Ice flakes are located in all containers to prevent supercooling and induce freezing. (Photo: Lakshmi Gopinath, Ph.D.)

Freezing therapy

After cold acclimation, we placed the containers in a freezer chamber (Conviron E8). Ten thermocouple sensors were inserted 1.0 inches into the potting medium at the center of randomly selected containers to monitor soil temperature. We place small ice chips in all containers to prevent supercooling and induce freezing (Image 2).

The freezer chamber is programmed to remain at 27°F for 18 hours to dissipate latent heat and then cool linearly at a rate of 1.8°F per hour. The 11 target temperatures (1.8°F, 25°F to 7°F intervals) covered a range expected to extend from the limits of complete survival to complete death.

We removed four bins from each entry (16 bins total) immediately at each target temperature. These containers were then placed in a plant growth chamber at 39°F overnight to induce thawing.

The temperature was then increased to 75/68°F for a week and then to 90/82°F to encourage recovery (Image 3). Regrowth was assessed based on shoot appearance visually after five weeks using binary values ​​(1 = alive, 0 = dead).

The temperature was then increased to 75/68°F for a week and then to 90/82°F to encourage recovery.  Regrowth was assessed based on the visual appearance of shoots after five weeks using binary values ​​(1 = alive, 0 = dead).  A = Tahoma 31, B = OKC 1406, C = Tifway and D = OKC 1873. (Photo: Lakshmi Gopinath, Ph.D.)

The temperature was then increased to 75/68°F for a week and then to 90/82°F to encourage recovery. Regrowth was assessed based on the visual appearance of shoots after five weeks using binary values ​​(1 = alive, 0 = dead). A = Tahoma 31, B = OKC 1406, C = Tifway and D = OKC 1873. (Photo: Lakshmi Gopinath, Ph.D.)

Experimental design and statistical analysis

We determined LT50 values ​​for each entry using a logistic regression procedure. This statistical procedure generated a table of expected survival at each temperature, and the temperatures corresponding to 50% survival are LT50 estimates for each entry.

We repeated the freezing test three times to produce three estimated LT50 values ​​for each entry. LT50 for each replicate was treated as a response variable and tested for statistical significance. Entry means were separated using Fisher's protected LSD when F tests were significant at P ≥ 0.05.

Results and discussion

There were significant differences between entries in LT50 values. Tifway had the highest LT50 or low freeze tolerance (Table 1), similar to the 18 to 17.6°F range previously reported for this cultivar in OK State. However, the LT50 value was 22.3°F obtained by NC State, and the discrepancy may be due to differences in acclimation temperatures and recovery periods between the two experiments. The samples in their experiment were shorter

(Graphic: Golfdome Crew)

(Graphic: Golfdome Crew)

establishment period, and they acclimatized the samples at a higher temperature.

The lower acclimation temperatures in our experiment could have caused a greater level of acclimation. Also, in the NC State trial, multi-year field test results (2011-2015) indicated that the Tifway achieved the highest winter survival rate among four commercial standards (Patriot, TifwaySport, Quickstand, and Tifway) in 2013, 2014, and 2015 . This contradicts other national standards reports that have shown that Tifway has high rates of winter kill in Indiana and Kentucky. The inconsistency in winter survival of Tifway may be due to differences in environmental conditions and environmental interactions of genotype X during the acclimation and wintering period.

The lower LT50 value for Tahoma 31 is consistent with field observations, showing lower winter kill of 4 percent and 25 percent in Indiana and Kentucky, respectively, with superior regrowth after dormancy. Tahoma 31 recovered quickly and reached 75 percent green coverage within 22 days after cold stress was removed, indicating high recovery potential after freezing temperatures. The LT50 values ​​for Tahoma 31 in this trial were similar to results obtained in previous trials at OK State.

Tahoma 31 received a turf quality rating above six at five out of seven sites in preliminary data from the National Turfgrass Assessment Program warm-season trial, indicating its ability to withstand a cutting height greater than 0.125 inch. With the lower LT50 reported in this trial and the ability to handle a cutting height of 0.125 inch, the Tahoma 31 could serve as an ideal fairway and low-maintenance putting green in the US transition zone.

The LT50 of OKC1873 was not significantly different from Tifway, suggesting that its scope of use should be similar to Tifway (Table 1). OKC1406 was in the same statistical group as Tahoma 31. The result is consistent with a previous report in which OKC1406 ranked sixth out of 53 experimental winter survival genotypes tested in Kansas (17). The OKC1406 had a higher winter survival rate (88.3 percent) than industry standards Tifway (0 percent), Latitude 36 (20 percent), NorthBridge (25 percent), Patriot (30 percent), and TifTuf (23 percent).

A winter survival percentage of OKC1406 higher than some current industry standards and an LT50 value similar to Tahoma 31 in this trial indicate its high freeze tolerance. However, multi-site, multi-year testing of experimental genotypes is required to evaluate turf quality, mowing tolerance, and pest and disease resistance.

Conclusion

Investigation in the controlled environment revealed that OKC1406 had a freeze tolerance similar to Tahoma 31, while OKC1873 had a freeze tolerance similar to Tifway. These evaluations provide valuable information for plant breeders to decide whether experimental bermuda genotypes tested should be subject to further evaluation. Using freeze-tolerant genotypes of bermudagrass will help golf courses reduce the costs associated with reestablishing turfgrass lost due to winter injury.

Thanks and appreciation

The authors would like to thank the following supporters of this research: Oklahoma Center for the Advancement of Science and Technology, United States Golf Association, Oklahoma Agricultural Experiment Station, USDA NIFA, and Hatch Project OKL03146.

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