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Published online 30 July 2007
Published in Crop Sci 47:1619-1627 (2007)
© 2007 Crop Science Society of America
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TURFGRASS SCIENCE

Zoysiagrass Species and Genotypes Differ in Their Winter Injury and Freeze Tolerance

Aaron J. Pattona,* and Zachary J. Reicherb

a Dep. of Horticulture, Univ. of Arkansas, 316 Plant Sciences Bldg., Fayetteville, AR 72701
b Dep. of Agronomy, Purdue Univ., 915 W. State St., West Lafayette, IN 47907-2054. Purdue Univ. Agric. Exp. Stn. Journal 2006-18055

* Corresponding author (ajpatton{at}uark.edu).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Lack of cold hardiness may limit widespread use of newly released zoysiagrass (Zoysia spp.) cultivars in the transition zone. Our objectives were to quantify differences in the winter injury of 35 zoysiagrass genotypes in field plots in West Lafayette, IN, and the freeze tolerance of 13 genotypes in a cold stress simulator as well as determine the relationship between leaf width, establishment rate, and autumn growth with winter injury. Winter injury varied between years and among genotypes in the field study. Zoysia japonica Steud. genotypes had less winter injury each year than Z. matrella (L.) Merr. genotypes. Genotypes of Z. japonica available as seed had less winter injury (2% in both years) than genotypes of Z. japonica (41%, 2005; 54%, 2006) and Z. matrella (51%, 2005; 73%, 2006) available only as vegetative propagules. ‘Meyer’, ‘Chinese Common’, and ‘Zenith’ were the commercially available cultivars exhibiting the least winter injury (<7%) in both years, whereas ‘Victoria’, ‘DeAnza’, ‘Diamond’, and ‘Empress’ had the most winter injury (>88%) both years. There was a relationship (r2 = 0.48, P = 0.0088) between freeze tolerance (LT50) in the cold stress simulator and winter injury in the field. Freeze tolerance ranged from –8.4°C (Diamond) to –11.5°C (Meyer and Zenith). Meyer has been the industry standard for zoysiagrass, but our research has identified other commercially available cultivars and genotypes with winter injury similar to Meyer.

Abbreviations: DAP, days after plugging • EL, electrolyte leakage • LT50 or freeze tolerance, lethal temperature killing 50% of the plants


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
ZOYSIAGRASS (Zoysia spp.) is a warm-season grass well adapted for lawns and golf turfs in the transitional and warm climatic regions of the USA and requires minimal maintenance inputs. Thus, expanded use of zoysiagrass could play a key role in making transition zone golf courses and lawns more environmentally friendly and sustainable. However, one barrier to widespread zoysiagrass use is a relative lack of cold hardiness in the transition zone, especially compared to cool-season grasses. Winter injury of zoysiagrass was first reported by Forbes and Ferguson (1947). They found that both Zoysia japonica Steud. and Z. matrella (L.) Merr. survived winters in Maryland, but that Z. japonica had better spring green-up and less winter injury than Z. matrella.

Zoysiagrass was first used on golf courses in the 1950s when the cultivar Meyer was released (Grau and Radko, 1951) and gained immediate popularity because of its heat, freezing, and drought tolerance in the transition zone (Grau, 1952). Rogers et al. (1977) evaluated differences in survival of zoysiagrass genotypes with an artificial freezing test using rhizomes and stolons and found that Meyer had superior cold tolerance compared to a selection of Z. matrella. Rogers et al. (1975) also evaluated the freezing tolerance of Meyer in Missouri and found that maximum survival occurred in early January with 50% survival occurring between –11.1 and –12.8°C. Warmund et al. (1998) evaluated recovery of Meyer rhizomes in response to extracellular freezing. Extracellular voids formed as a result of ice formation in Meyer rhizomes cooled to –7°C, but rhizomes survived temperatures as low as –11°C (Warmund et al., 1998).

Many new cultivars of zoysiagrass released after Meyer have improved texture, shade tolerance, drought tolerance, establishment rate, divot recovery, and pest resistance compared to Meyer (White and Engelke, 1990; Reinert and Engelke, 2001; White et al., 2001; Patton et al., 2007; Karcher et al., 2005). Dunn et al. (1999) evaluated the low temperature tolerance of newer cultivars of zoysiagrass using a programmable freezer and rhizomes harvested in January. Although they did not report freezing tolerance (LT50) in their study, they found that ‘Meyer’ was among the most hardy entries, whereas ‘Emerald’, ‘El Toro’, ‘Cavalier’, and ‘Palisades’ were among the least hardy entries. Since other cultivars are less cold hardy than Meyer, extensive use of zoysiagrass in the transition zone is limited to Meyer (Dunn et al., 1999; Dunn and Diesburg, 2004).

Freeze tolerance of warm-season grasses is most often evaluated by measuring electrolyte leakage (EL) or regrowth of plant tissues after freezing. Lethal temperatures predicted by EL and regrowth are correlated (Gusta et al., 1980; Maier et al., 1994) and use of EL is more popular than regrowth measurements because freeze tolerance can be assessed rapidly. However, a drawback to EL is its questionable accuracy. Electrolyte leakage underestimated freezing tolerances of seashore paspalum (Paspalum vaginatum Swartz) (Cardona et al., 1997) and centipedegrass [Eremochloa ophiuroides (Munro) Hack.](Fry et al., 1993) and overestimated freezing tolerance of St. Augustinegrass [Stenotaphrum secundatum (Walter) Kuntze](Maier et al., 1994). Inconsistency in the EL procedure may make regrowth tests a better option to determine freeze tolerance of zoysiagrass genotypes.

Researchers at Oklahoma State University developed a standardized, rapid, reproducible regrowth method for quantifying freeze tolerance of bermudagrass (Cynodon spp.) producing results that agree with field observations (Anderson et al., 1993). This method cold acclimates plants using a controlled-environment chamber set at 8/2°C day/night cycles with a short photoperiod (10-h) and low light (~300 µmol m–2 s–1) for 4 wk. Controlled-environment chambers provide an acclimation setting for plants that is independent of yearly fluctuations in the field (Cardona et al., 1997) and have been used successfully to acclimate and test freeze tolerance of other warm-season grasses such as buffelgrass [Pennisetum ciliare (L.) Link] and seashore paspalum (Cardona et al., 1997; Stair et al., 1998).

Although many newer zoysiagrass genotypes have either previously been evaluated or are currently being evaluated in field trials (Morris, 1998; 2004), conditions necessary for separating cold hardiness among genotypes may occur only once per decade (Dunn et al., 1999). As a result, trials lasting 5 yr are usually insufficient to separate differences in cold hardiness among genotypes. In addition to testing for winter injury in field plots, it is necessary to use a standardized method to determine the relative freeze tolerance of zoysiagrass genotypes since winter injury can result in substantial reestablishment costs and loss of use. The objectives of our research were (i) to determine the differences in winter injury of commercially available cultivars and experimental zoysiagrass genotypes in field plots, (ii) to determine if winter injury is related to growth rate or leaf width, and (iii) to evaluate the freeze tolerance of zoysiagrass genotypes using controlled environment acclimation and freezing procedures.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
Winter Injury in Field Plots
Plant material of commercially available cultivars and experimental genotypes of zoysiagrass (Table 1) were collected in the fall of 2003 and propagated in the greenhouse (23 ± 5°C) in plug trays with 8 by 8 by 8 cm divisions filled with fritted clay (Turface, Profile Products LLC, Buffalo Grove, IL). Vegetatively established genotypes were planted into trays as plugs or stolons and seeded genotypes were seeded (49 kg ha–1) into trays. Plants in the greenhouse were fertilized monthly with 49 kg ha–1 N, 21 kg ha–1 P, and 40 kg ha–1 K using a soluble fertilizer (18N–7.9P–17.4K) and mowed weekly at 4.0 cm. Plants were transplanted into field plots at the W.H. Daniel Turfgrass Research and Diagnostic Center, West Lafayette, IN. Experimental plots were 1.0 by 1.0 m arranged in a randomized complete-block design with four replications. One vegetative plug (8 by 8 by 8 cm) of zoysiagrass was transplanted into the center of each plot for both seeded and vegetative genotypes on 7 June 2004 and again in 2005. Plots were irrigated four times daily for the first month to encourage establishment and then irrigated as needed to prevent wilting. Soil type was a Stark silt loam (fine-silty mixed mesic Aeric Ochraqualf) with a pH of 7.0, 224 kg ha–1 P, 808 kg ha–1 K, and 84 g kg–1 organic matter in 2004 and with a pH of 6.8, 224 kg ha–1 P, 639 kg ha–1 K, and 29 g kg–1 organic matter in 2005. Areas were fumigated with methyl bromide at 732 kg ha–1 before establishment each year to minimize weed competition. Plots received 49 kg ha–1 N from urea (46N–0P–0K) on 1 July and 1 August of each year, and weeds were manually removed during establishment. A weather station was located on the plots which recorded air temperatures and bare soil temperatures 2.5 cm deep.


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  		Table 1. Zoysiagrass cultivar or genotype, species, typical type of establishment method, and source of plant material.

 

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  		Table 2. Zoysiagrass mean winter injury by cultivar or genotype from a field trial established in June 2004 and 2005 in West Lafayette, IN.

 
Digital images were taken of each plot once weekly with a Nikon Coolpix 3200 (Nikon, Melville, NY) digital camera mounted on a monopod to ensure a consistent height from the lens to the soil surface (1.05 m). Coverage of zoysiagrass was determined using digital image analysis (SigmaScan Pro, Systat Software Inc., Richmond, CA) (Richardson et al., 2001). To selectively identify green leaves in the images, hue range was set from 47 to 107 and saturation was set from 10 to 100. Images were taken of a calibration disk and data were converted from selected green pixels to zoysiagrass coverage (cm2).

Coverage was determined from weekly digital image analysis of plots and establishment rate was determined and reported previously (Patton et al., 2007). To determine an establishment rate for each genotype, coverage was transformed using the natural logarithm so the data could be fit to the linear model, coverage = [K(DAP) + I], where K is the rate of increase (establishment rate, loge coverage d–1), DAP is days after plugging, and I was equal to the natural logarithm of 64 cm2 which was the starting coverage for all plots. An autumn growth rating was determined as the increase in coverage between 6 September (91 DAP) and 28 September (113 DAP) each year. Leaf blade width of three randomly selected mature leaves per plot was measured with a vernier caliper at 64 DAP each year.

The start of winter dormancy (leaf discoloration) occurred 112 and 113 DAP in 2004 and 2005, respectively. Images with the maximum green coverage before winter dormancy and images taken after winter dormancy (spring green-up) were used to calculate winter injury. Winter injury was determined using the following formula:

Formula
Maximum green coverage before winter was collected on 28 September in 2005 and 2006, and spring green-up data were collected on 23 May 2005 and 20 May 2006. Some genotypes with good cold tolerance and earlier spring green-up had winter injury <0% (minimum –24%) based on our method of calculation due to new growth before spring green-up data recording. Therefore, plots with <0% winter injury were set to 0% before analysis.

Winter injury data were arcsine transformed before analysis to improve homogeneity of the error variance and are presented as backtransformed means for easier interpretation (Steel et al., 1997). Data were analyzed using PROC ANOVA, PROC CORR, and PROC TTEST (SAS Institute, Cary, NC). Error variances were tested for homogeneity between years and for year x genotype interactions before analysis of variance. When differences were examined between species, ‘J-14’ (Z. sinica Hance) was grouped with Z. japonica and ‘Emerald’ [Z. japonica x Z. pacifica (Goudsw.) Hotta and Kuroki] was grouped with Z. matrella because of the similarities in leaf color, leaf width, and plant density with respective species. Means were separated using Fisher's protected least significant difference when F tests were significant at {alpha} ≤ 0.05.

Freeze Tolerance
A cold stress simulator was constructed by modifying a 0.55-m3 chest freezer similar to the method of Beard et al. (1980, 1991). The freezer was modified by adding an elevated rack inside the chamber with attached 120-mm-diameter computer cooling fans (Philmore Manufacturing Company, Inc., Rockford, IL) to circulate air within the chamber. A programmable controller (Watlow 981, Watlow Electric Manufacturing Co., St. Louis, MO) and a type T Teflon-tipped thermocouple (ThermoWorks, Alpine, UT) connected to the controller were used to control temperature inside the chamber. Soil temperatures are more important than air temperatures when assessing freezing stress (Beard et al., 1991), so the thermocouple was inserted 2.5 cm into the potting mix to control temperature.

Zoysiagrass plant material was collected in the fall of 2003 and propagated in the greenhouse in trays filled with fritted clay. Vegetatively established genotypes were planted into trays as plugs or stolons and seeded genotypes were seeded (49 kg ha–1) into trays. Starting in the fall of 2005, plants were established in 2.5-cm-diameter, 12-cm-deep Ray Leach cone-tainers (Stuewe & Sons, Inc., Corvallis, OR) filled with potting mix (BM-2, Berger Peat Moss Ltd., Saint-Modeste, PQ, Canada) using a 2- to 4-cm stolon or rhizome containing root, crown, and shoot material. One or more commonly used zoysiagrass genotypes with low, medium, and high winter injury were selected from each group based on winter injury results from the spring of 2005 (Z. japonica genotypes established vegetatively, Z. japonica genotypes established from seed, and Z. matrella genotypes established vegetatively). ‘Midlawn’ bermudagrass [Cynodon dactylon L. (Pers.) x C. transvaalensis Burtt-Davy] and ‘Penncross’ creeping bentgrass (Agrostis stolonifera L.) were used as controls in this experiment because their freeze tolerances were known. After 10 wk of establishment in the greenhouse (25 ± 5°C) with supplemental lighting (14-h photoperiod), plants were cold acclimated for 4 wk using a controlled-environment chamber (PGR15, Controlled Environments Inc., Pembina, ND) set at 8/2°C day/night cycles and a 10-h photoperiod of 300 µmol m–2 s–1 photosynthetically active radiation based on the procedure of Anderson et al. (1988).

Plants were placed in the cold stress simulator to test for relative freeze tolerance of each genotype after 4 wk of acclimation. The cold stress simulator was programmed to cool 1°C h–1 after 15 h at –3°C similar to Anderson et al. (1993). Plants were randomized in the growth chamber and cold stress simulator within temperature treatments. Target soil temperatures (1°C intervals, –6 to –15°C) covered a range anticipated to span the limits from complete survival to complete mortality based on preliminary testing. To increase sample size near the anticipated freeze tolerance temperature, six cone-tainers were removed at each test temperature from –8 to –12°C for each genotype, and only three cone-tainers were removed for each genotype for test temperatures –6 and –7°C where complete survival was expected and at –13, –14, and –15°C where complete mortality was expected. Plants were thawed in a walk-in refrigerator (5°C) overnight after freezing. Plants were then transferred to a greenhouse (26 ± 6°C) and evaluated for regrowth for a period of 4 wk after freezing. The temperature resulting in no regrowth from 50% of the plants (LT50, freeze tolerance) was determined by nonlinear regression using an equation from Ingram and Buchanan (1984) which was also used by Anderson et al. (2002), Cardona et al. (1997), Stair et al. (1998), and Väinölä et al. (1999) to predict survivability of bermudagrass, seashore paspalum, buffelgrass, and Rhododendron spp., respectively. Freeze tolerance was determined by the following equation:

Formula
where a is the base line of survival, b is the maximum survival, c is a function of the slope of the line at the inflection point, Tm is the temperature at the inflection point which is also the LT50, and T is the treatment temperature. This experiment was replicated for a total of six experimental replications. Data were fit to sigmoidal curves using PROC NLIN (Gauss–Newton method) (Fig. 1 ) and LT50 values were analyzed using PROC ANOVA (SAS Institute, Cary, NC). Means were separated using Fisher's protected least significant difference when F tests were significant at {alpha} ≤ 0.05.


Figure 1
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  		Figure 1. Sample sigmoidal curve fit to regrowth data from a pilot study using ‘Meyer’ zoysiagrass. Freeze tolerance (LT50, temperature resulting in no regrowth from 50% of the plants) was determined by nonlinear regression (PROC NLIN [Gauss-Newton method]) using the following equation: Survival = a + (b – a)/{1 + exp [c(Tm – T)]} where a is the base line of survival, b is the maximum survival, c is a function of the slope of the line at the inflection point, Tm is the temperature at the inflection point (LT50), and T is the treatment temperature.

 

   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
Winter Injury in Field Plots
Winter injury was greater (P = 0.09) in 2006 (36%) than 2005 (26%) (Table 2) when averaged across genotypes even though the minimum soil temperature was lower in 2004–2005 (–7.4°C, 28 Jan. 2005) than in 2005–2006 (–3.4°C, 19 Feb. 2006) (Fig. 2 ). Differences in winter injury between years could be attributed to differences in plant cold acclimation status, snow cover, or soil moisture. Zoysiagrass is not as susceptible to air temperature fluctuations in winter as stoloniferous warm-season grasses such as centipedegrass and St. Augustinegrass since it also has rhizomes that lie protected below the soil surface (DiPaola and Beard, 1992).


Figure 2
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  		Figure 2. (A) Air and (B) soil temperatures at a 2.5-cm depth during the winter and spring of 2004–2005 and 2005–2006. Mean of two soil probe measurements.

 
There were significant year x genotype interactions for winter injury and thus results from 2004–2005 and 2005–2006 are presented separately (Table 2). ‘Chinese Common’, BMZ 230, DALZ 0102, J-36, PST-R7LT, Meyer, VJ, J-37, PZB 33, Emerald, J-14, and PZA 32 had the least amount (<4%) of winter injury in the spring of 2005. With the exception of PZB 33, VJ, DALZ 0102, and Emerald, these same genotypes along with PST-R7MA, PST-R7ZM, PST-R7TH, and ‘Zenith’ had the least amount (<7%) of winter injury in the spring of 2006. ‘Diamond’, DALZ 0105, 6186, DALZ 0104, ‘Empress’, ‘Victoria’, and ‘DeAnza’ had the highest (>91%) winter injury in the spring of 2005. With the exception of Victoria, these same genotypes along with ‘Empire’ had the highest (>92%) winter injury in the spring of 2006. ‘Companion’, DALZ 0101, Himeno, ‘Zorro’, Cavalier, ‘Zeon’, El Toro, 6136, Palisades, ‘GNZ’, and ‘Royal’ were among the genotypes whose winter injury was intermediate (14–79%) both years. Winter injury of genotypes in our study are comparable to previous reports of winter survival across the nation (Morris, 1996, 2001; Dunn et al., 1999; Fry, 2001).

Overall, genotypes with low winter injury in 2005 also had low winter injury in 2006. Additionally, genotypes with high winter injury in 2005 also had high winter injury in 2006. However, genotypes with intermediate winter injury in 2005 usually had more winter injury in 2006. For instance, Emerald and Empire exhibited dramatically more winter injury in 2006 than 2005. Emerald also had significant winterkill in Kansas in 2001 and Kentucky in 1997 (Morris, 2001) and is not considered well adapted to the transition zone compared to other cultivars (Dunn and Diesburg, 2004). However, Himeno exhibited the opposite effect with more winter injury in 2005 (36%) than 2006 (14%). This demonstrates the inconsistency typical when assessing winter injury in the field (Anderson et al., 1988; Dunn et al., 1999).

Chinese Common, Meyer, and Zenith were the only commercially available cultivars with less than 7% winter injury in both years (Table 2). Among these cultivars, Meyer is the only one commercially available as vegetative propagules. Several improved cultivars commercially available as vegetative propagules were tested in our study, but winter injury of these cultivars was higher than Meyer. Meyer is the most widely used cultivar in the transition zone (Dunn and Diesburg, 2004), and our studies indicate that apart from Zenith and Chinese Common, there are few additional commercially available options for use in the northern transition zone based on the criteria of winter injury. However, many genotypes exhibited levels of winter injury similar to Meyer in both years including BMZ 230, PST-R7LT, PST-R7MA, PST-R7ZM, PST-R7TH, J-14, J-36, J-37, and PZA 32.

Overall, Z. japonica genotypes had less winter injury than Z. matrella genotypes (Table 3) which is consistent with earlier reports (Forbes and Ferguson, 1947; Daniel, 1955). Zorro and Cavalier had the least amount of winter injury among commercially available Z. matrella cultivars over both years, but their winter injury was greater than Meyer. Zoysia japonica is sold commercially both as seed or as vegetatively propagules, but Z. matrella is only available as vegetative propagules. All genotypes were established by vegetative plugs in the field for this study, but it was found that Z. japonica genotypes with commercial seed availability had winter injury lower than genotypes of Z. japonica and Z. matrella available as vegetative propagules (Table 3). This is consistent with winter injury observations in Kansas where seeded genotypes had the lowest winter injury (Fry, 2001) and indicates that genotypes with seed availability are likely genetically similar.


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  		Table 3. Differences in zoysiagrass winter injury by species and by typical type of plant material available for establishment.

 
We explored the relationship between winter injury (Table 2) and leaf width (data not shown). Zoysia japonica genotypes have wider leaves (2–4 mm) than Z. matrella (<2 mm) (Anderson, 2000). Winter injury decreased with increasing leaf width (r = –0.57, P < 0.001, 2005; r = –0.60, P < 0.001, 2006) when analyzed across species (Table 4). However, when each species was analyzed separately, this correlation only occurred in Z. japonica (r = –0.70, P < 0.001, 2005; r = –0.53, P = 0.005, 2006) (Table 4). This correlation was strongly influenced by a group of Z. japonica genotypes available as seed that exhibit low winter injury and have wide leaf blades. Therefore, species not leaf width likely influences winter injury.


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  		Table 4. Correlation coefficients between winter injury and growth and leaf width for 2005 and 2006.

 
Some crops such as alfalfa (Medicago sativa L.) are prone to winter injury if late fall growth rate is high (Schwab et al., 1996). Using data on the establishment rate of zoysiagrass genotypes from our earlier study (Patton et al., 2007) and data on autumn growth (data not shown), we found no relationship between winter injury and autumn growth in either year of the study and a weak correlation between winter injury and establishment rate (r = –0.37, P = 0.03) in 2006 (Table 4). This correlation between winter injury and establishment rate in 2006 is likely an artifact of the use of two species of zoysiagrass in our study. Zoysia japonica genotypes have a faster establishment rate (Patton et al., 2007) and lower winter injury than Z. matrella genotypes. As a result, two groups are formed where Z. japonica is fast establishing with low winter injury and Z. matrella is slow establishing with high winter injury. Within species there was no relationship between winter injury and establishment rate (Table 4). Therefore, the correlation between winter injury and establishment rate in 2006 was likely strongly influenced by species.

Zoysiagrass cold hardiness is likely genetically controlled (Engelke and Anderson, 2003). Although Z. japonica genotypes as a whole had less winter injury than Z. matrella genotypes, some cultivars of Z. japonica such as Victoria and DeAnza had severe winter injury, while others such as Palisades and El Toro exhibited intermediate winter injury. Engelke and Anderson (2003) proposed that many commercially available Z. japonica cultivars are not a single species as classified by their morphological characteristics, but instead are interspecific hybrids. This is supported by Anderson (2000) who found probable interspecific hybrids based on restriction fragment length polymorphism fingerprint characterization and by Yaneshita et al. (1997) who found interspecific hybridization of Zoysia spp. among natural populations. Cultivars such as Victoria, DeAnza, El Toro, and Palisades are likely crosses of Z. japonica with Z. matrella and/or Z. pacifica (Anderson, 2000; Engelke et al., 2002; Gibeault, 2003), but are classified as Z. japonica because of their predominant morphological characteristics. Zoysia pacifica and Z. matrella have poor cold hardiness compared to Z. japonica (Daniel, 1955). Therefore, the genetic makeup of Victoria, DeAnza, El Toro, and Palisades may help explain why they had more winter injury than most other Z. japonica cultivars.

Freeze Tolerance
Freeze tolerance (LT50) of zoysiagrass genotypes ranged from –8.4°C (Diamond) to –11.5°C (Meyer and Zenith) with a mean of –10.2°C (Table 5). Zoysia matrella cultivars Diamond, Royal, and Zorro and the Z. japonica cultivar Victoria had poor tolerance to freezing. Zoysia japonica genotypes Meyer, Zenith, Palisades, El Toro, Companion, and J-36 were all similar and had better freeze tolerance. The LT50 of the experimental control Midlawn bermudagrass was –8.6°C which is similar to previous reports ranging from –8.4 to –10.3°C (Anderson et al., 1993, 2002, 2003). Freeze tolerance of Meyer (LT50 = –11.5°C) in our study was similar to that in a previous report of –11.1 to –12.8°C (Rogers et al., 1975) and consistent with studies showing rhizome survival at –11°C (Warmund et al., 1998; Dunn et al., 1999; Zhang and Fry, 2006).


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  		Table 5. Freeze tolerance (LT50, temperature resulting in no regrowth from 50% of the plants) of zoysiagrass plants as determined by nonlinear regression. Plants were grown in a greenhouse for at least 10 wk, cold acclimated in a growth chamber at 8/2°C day/night cycles and a 10-h photoperiod of 300 µmol m–2 s–1 for 4 wk and then placed in a cold stress simulator and subjected to temperatures ranging from –6 to –15°C.

 
There was a relationship (r2 = 0.48, P = 0.0088) between LT50 and winter injury in the field (Fig. 3 ). However, El Toro and Palisades were more freeze tolerant in the cold stress simulator than anticipated based on our winter injury data and a previous report (Dunn et al., 1999), whereas Zorro was less freeze tolerant than anticipated. Freeze tolerance does not always reflect absolute cold hardiness (Anderson et al., 2002) due to additional environmental factors in the field such as snow cover, soil moisture, and temperature fluctuations. Additionally, environmental conditions in the field may induce a greater level of acclimation in some genotypes compared to the induced growth chamber acclimation (Anderson et al., 2002).


Figure 3
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  		Figure 3. Relationship (r2 = 0.48, P = 0.0088) between zoysiagrass freeze tolerance (LT50, temperature resulting in no regrowth from 50% of the plants; growth chamber cold-acclimated and frozen in a cold stress simulator) and winter injury in the field for 13 genotypes.

 
Mechanisms involved in the cold hardiness of zoysiagrass are not well understood. Membrane stability (Samala et al., 1998; Cyril et al., 2002; Munshaw et al., 2006), carbohydrate composition (Ball et al., 2002; Shahba et al., 2003; Cai et al., 2004), proline concentration (Cai et al., 2004; Lu et al., 2005; Munshaw et al., 2006), and cold-regulated proteins (Gatschet et al., 1996) are known to be important in the freezing tolerance of other warm-season turfgrasses. We are currently investigating the physiological basis for differences in zoysiagrass cold hardiness.

In summary, our results show large differences in response to low temperatures and winter injury between species and genotypes of zoysiagrass. Zoysia japonica genotypes generally exhibited less winter injury and better freeze tolerance than Z. matrella genotypes. Additionally, Z. japonica genotypes that can be propagated by seed exhibit less winter injury and have wider leaves than other genotypes, which may indicate that this group is more genetically similar than vegetatively established Z. japonica genotypes. Our results indicate that there are a limited number of currently available cultivars (Meyer, Chinese Common, and Zenith) well adapted to northern areas of the transition zone, but that experimental genotypes with low winter injury are being developed. Relative rankings of the winter injury of zoysiagrass in West Lafayette, IN, which is north of the transition zone will be useful for practitioners selecting zoysiagrass cultivars for use in the transition zone. Selecting and establishing cultivars well adapted to the transition zone will help reduce winter injury and ensuing reestablishment costs, thus increasing golf course revenues and sustainability of turfs.


   ACKNOWLEDGMENTS
 
This research was supported by the Midwest Regional Turfgrass Foundation. The authors thank Jon Trappe for helping with planting, Judy Santini for help with statistical analysis, and Jack Banker, Thermtech Systems Inc., for technical assistance in constructing and programming the cold stress simulator.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
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Received for publication November 22, 2006.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 




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A. J. Patton, S. M. Cunningham, J. J. Volenec, and Z. J. Reicher
Differences in Freeze Tolerance of Zoysiagrasses: II. Carbohydrate and Proline Accumulation
Crop Sci., September 1, 2007; 47(5): 2170 - 2181.
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