Research Article  Journal of Agricultural Science and Botany (2017) Volume 1, Issue 1
Evaluation of sweet sorghum accessions for seedling cold tolerance using both lab and field cold germination test
Ming Li Wang^{1*}, Zhanguo Xin^{2}, Gloria Burow^{2}, Junping Chen^{2}, Phiffie Vankus^{1}, David Pinnow^{1}, Brandon Tonnis^{1}, Hugo Cuevas^{3} and Jianming Yu^{4}
^{1}USDAARS, Plant Genetic Resources Conservation Unit, Griffin, Georgia 30223, United States
^{2}USDAARS, Plant Stress and Germplasm Development Unit, Lubbock, Texas 79415, United States
^{3}USDAARS, Tropical Agriculture Research Station, Mayaguez, Puerto Rico 00680, United States
^{4}Department of Agronomy, Lowa State University, Ames, IA 50011, United States
 *Corresponding Author:
 Wang ML
USDAARS, Plant Genetic Resources Conservation Unit
Griffin, Georgia 30223, United States
Email: [email protected]
Accepted on November 6, 2017
Citation: Wang ML, Xin Z, Burow G, Chen J, Vankus P, et al. Evaluation of sweet sorghum accessions for seedling cold tolerance using both lab and field cold germination test. J Agric Sci Bot 2017;1(1):18.
DOI: 10.35841/25917897.1.1.411
Visit for more related articles at Journal of Agricultural Science and BotanyAbstract
Objective: Seedling cold tolerance is one of the important traits for sweet sorghum production. This study is to evaluate the method for identification of sweet sorghum accessions with seedling cold tolerance using both lab cold germination and field earlyspring cold planting.
Methodology: Sorghum seeds were germinated in growth chamber (under lab cold condition) at a constant 12°C for five weeks. After two weeks, germinated seeds and germination rates were counted and calculated at weekly intervals for four times. The same seed lots for lab cold test were also planted 45 days earlier than the normal planting time in the field (earlyspring cold planting). In addition to seedling dry weight, germinated seeds and seed germination rates were also counted and calculated at weekly intervals for three times.
Results: In this study, a high correlation coefficient between lab germination rate and field germination rate (R2=0.503, p<0.0001) was observed. In general, lab germination rate can predict the field germination performance; but some discrepancies between field and lab tests were also observed for some accessions. Among 212 sweet sorghum accessions tested, several sweet sorghum accessions with seedling cold tolerance were identified from both lab and field tests. These accessions will be useful materials for development of sweet sorghum cultivars with early spring cold tolerance.
Conclusion: Compared with the field test, the lab test is less labor intensive. For a largescale screening of seedling cold tolerance, the lab test may be conducted initially followed by selection of superior accessions from the lab test for further evaluation under field conditions.
Keywords
Sweet sorghum germplasm, Seed germination rate, Seedling cold tolerance.
Introduction
Early planting (or early sowing) may increase yield of sugar, grain or biomass; but it depends on many factors (e.g. rainfall, soil types and temperatures, cultivars, and crops). For example, early planting soybean in southwestern Japan can increase grain yield [1]. But planting peanut in early to midApril instead of early to midMay in the state of Georgia can increase the risk of tomato spotted wilt virus (TSWV) incidence [2] and lead to yield reduction. Sorghum seeds can be planted in early April for achieving a higher yield or earlier harvesting time for a doublecropping system in Texas, but selection of cultivars with early spring cold tolerance is important. Sorghum cultivars with early spring cold tolerance have several obvious advantages: faster and better seedling emergence and establishment, an extended growing season in the same region, and slightly expanded planting zones from the south to north regions [3]. Significant seedling cold tolerance exists in sorghum. Several studies have been conducted on seedling cold tolerance in grain sorghum [3,4]. Three SSR markers for different QTL for earlyseason cold tolerance have been identified, and they are mapped in different chromosome regions [57]. However, little research has been conducted on cold tolerance in sweet sorghum. The U.S. sweet sorghum collection is maintained by the Plant Genetic Resources Conservation Unit (PGRCU) at Griffin, GA. These sweet sorghum accessions are used as experimental materials for screening seedling earlyspring cold tolerance under both lab and field conditions. Therefore, the objectives of this study were to (i) determine the germination rates of sweet sorghum accessions under cold conditions in both lab and field; (ii) determine the correlation coefficients among investigated traits; (iii) determine the correlation coefficients of germination rates between lab and field conditions; and (iv) identify accessions with cold tolerance under both lab and field conditions and recommend them to sweet sorghum breeders as parental materials for developing new cultivars.
Materials and Methods
Sweet sorghum accessions: Seed regeneration for sweet sorghum accessions was conducted every year from 2013 to 2015 at St. Croix, Puerto Rico. Fresh seeds, after breaking dormancy, were used for germination testing. Four checks (Rio, a good sweet sorghum cultivar; BTx623, a good grain sorghum cultivar; GT26056, a good early spring cold tolerance cultivar; and PI 610727, a very good early spring cold tolerant cultivar from Shanxi, China) were used for comparison with the sweet sorghum accessions. A total of 212 sweet sorghum accessions plus four checks used for both field and lab germination tests are listed in (Table 1).
PI  Identifier  PI  Identifier 

PI 17548  RED AMBER  PI 173971  JAWAR 
PI 22913  CHINESE AMBER  PI 174381  KARADARI 
PI 48191  SACCALINE  PI 175919  IS 12833 
PI 52606  MN 2680  PI 176766  MN 2873 
PI 88000  Mokutakususu  PI 177156  MN 2742 
PI 88007  Bangu manguisusu  PI 177553  AKDARI 
PI 92270  MN 2740  PI 177554  MN 2894 
PI 144134  Inyangentombi  PI 179504  AKDARI 
PI 144331  ISIDOMBA  PI 179747  JAWAR 
PI 145619  ISIDOMBA  PI 179749  Juar 
PI 145622  Jiba  PI 180004  JAWAR 
PI 145632  TEGEVINI  PI 180005  JAWAR 
PI 145633  Tugela Ferry  PI 180348  Juar 
PI 146890  SUGAR DRIP  PI 180489  Juar 
PI 147026  Nagro  PI 181077  DEPAR 
PI 147200  W. 21  PI 181080  HONEY SORGHUM 
PI 147224  B. 35  PI 181083  KAMANDRI 
PI 147573  MN 600  PI 181899  Aleppo No. 41 
PI 149830  IS 2462  PI 181971  MN 2939 
PI 149832  IS 2464  PI 182303  AKDARI 
PI 152596  ANKOLIB TEQUIL  PI 183149  JUAR 
PI 152629  Feterita Fayoumi D.S. 8  PI 189114  MN 2972 
PI 152630  FETERITA FAYOUMI D.S. 10  PI 195754  KAOLIANG 
PI 152633  FETERITA FAYOUMI D.S. 13  PI 196049  IS 2131 
PI 152646  FETERITA GEZIRA  PI 196592  MN 3089 
PI 152650  FETERITA FULLI  PI 196598  MN 3095 
PI 152651  Feterita Geshaish  PI 197542  SUCRE DROME 
PI 152671  GISHISH  PI 198885  SWEET SACCALINE 
PI 152675  Heger Taie  PI 201723  FETERITA LA ESTENZUELA 
PI 152676  HEGIRI 1  PI 217691  NAGAD EL MUR 
PI 152683  HEMAISI RED SHENDI SHERSHER  PI 217770  BARGOWI 
PI 152692  KAFIR PINK  PI 218112  IS 2352 
PI 152714  LUEL  PI 221560  BALAKA 
PI 152725  MALWAL AWEIL  PI 247136  MN 4052 
PI 152733  MERISSA (BARI)  PI 247744  U. g. 6. 7. 
PI 152751  NYTWAL  PI 247745  Tjolotjo 
PI 152755  POTCH 4  PI 250232  MN 4118 
PI 152764  QUERY 3  PI 250234  MN 4120 
PI 152771  RAHMETALLA GALLABAT  PI 250402  MN 4126 
PI 152813  Wad Aker Red  PI 250521  MN 4122 
PI 152816  WAD FUR WHITE  PI 250582  MN 4124 
PI 152828  U.T. 23  PI 250897  MN 4133 
PI 152860  MERASI  PI 250898  MN 4134 
PI 152872  FETERITA ABDEL MAGID  PI 251672  MN 4135 
PI 152880  LWEL FADIANG  PI 253795  MN 4136 
PI 152898  BILICHIGAN  PI 253796  MN 4137 
PI 152909  Mahananga  PI 253986  MN 4138 
PI 152914  WAXY CLUB  PI 255239  CAXA 
PI 152923  Duro El Jack  PI 257599  NO. 5 GAMBELA 
PI 152953  CHIKKORI  PI 257600  NO. 6 GAMBELA 
PI 152961  MALNAL  PI 257602  NO. 8 GAMBELA 
PI 152963  Thok (B)  PI 260210  Darso 28 
PI 152966  Ayuak  PI 248298  CHINESE AMBER 
PI 152971  AWANLEK  PI 267476  Tseta 27/51 
PI 152998  GUMBILU  PI 273955  MN 4566 
PI 153871  MUBEYA  PI 273969  MN 4578 
PI 154750  Serere  PI 287625  MN 48 
PI 154787  MN 1344  PI 287627  MN 12 
PI 154796  NKUMBA  PI 302120  MN 4155 
PI 154800  Wenabu  PI 302122  IS 13718 
PI 154844  GRASSL  PI 302131  MN 4179 
PI 154846  KABIRI  PI 302198  MN 4243 
PI 154929  J56 Akouangok  PI 302199  MN 4369 
PI 154943  L28 Lawere  PI 302252  IS 13726 
PI 154944  L31 Emiroit  PI 302264  MN 4330 
PI 154962  V3 Nakyeru  PI 303658  Nerum Boer 
PI 154980  Wheatland  PI 511355  SMITH 
PI 154987  S. A. 1  PI 533998  Brawley 
PI 154988  S. A. 2  PI 535783  N98 
PI 154990  P 127 (S.A. 5)  PI 535785  N100 
PI 155336  MUYO  PI 535792  N107 
PI 155485  Maila  PI 535796  N111 
PI 155516  MASAKA  PI 562716  HONEY NO. 2 
PI 155543  Hasesa  PI 563295  RIO 
PI 155556  MAILA  PI 566819  DELLA 
PI 155571  LONGWE  PI 583832  TOP 766 
PI 155609  MAPIERA  PI 584989  POPSORGHUM 
PI 155721  WAQUEMA  PI 586443  MN 818 
PI 155760  Namuse  PI 586541  TRACY 
PI 155805  MAPIRA  PI 641806  AMES AMBER 
PI 155845  MN 2077  PI 641807  ATLAS 
PI 155902  MN 2103  PI 641815  EARLY FOLGER 
PI 155924  CHIFUNGO  PI 641817  EARLY SUMAC 
PI 156136  MAILA  PI 641821  HONEY DRIP 
PI 156203  MN 2089  PI 641834  PLANTER 
PI 156217  MN 2109  PI 641835  REX 
PI 156252  Nefee  PI 641848  TEXAS SEEDED RIBBON 
PI 156352  MN 2238  PI 641862  COLLIER 
PI 156356  Sonkwe  PI 641893  DWARF ASHBURN 
PI 156393  MN 2277  PI 641904  H.C. 4113 
PI 156890  Dura Huria  PI 641909  Red Losinga 
PI 157030  Andiwo III 57  PI 642999  LeotiPeltier 
PI 157033  Ifube No. 18  PI 643008  MN 2751 
PI 157035  Nyagwang No. 56  PI 643013  MN 2756 
PI 157804  Feterita Abu Derega  PI 643016  MN 2761 
PI 167047  AKDARI  PI 643017  MN 2762 
PI 167352  AKDARI  PI 643464  IS 3986 
PI 170783  AKDARI  PI 651493  RAMADA 
PI 170787  MN 2826  PI 651495  DALE 
PI 170788  MN 2827  PI 651497  Theis 
PI 170799  MN 2838  PI 653616  WRAY 
PI 170802  IS 12807  PI 653617  KELLER 
PI 170805  IS 12810  PI 655983  SUGAR DRIP 
PI 173112  7392  PI 655983  M81 E 
PI 173118  8371  GT26056  Cold tolerance check 
PI 173120  8493  PI 610727  Cold tolerance check, China 
PI 173121  MN 2857  Rio  Sweet sorghum, TX 
PI 173808  GILGIL  BTx 623  Grain sorghum, TX 
Table 1. Information on PI number and identifier for the selected sweet sorghum accessions.
Detailed information about sweet sorghum accessions can be obtained from the USDAARS Germplasm Research Information Network (GRIN) website.
Lab cold germination test: The growth chamber (Percival Scientific, Inc., model GR36L) was set at a constant 12°C and 60% humidity with eight hours of light and 16 hours of dark.
Seeds were treated with thiram before testing. Fifty seeds were evenly spread out onto a double layer of wetted germination paper towels, wrapped up, and bound in place with a rubber band. The bound paper towels were then transferred standing upright into a lattice plastic tray (to allow air circulation around towels) and placed in the growth chamber. Whenever the paper towels needed to be rewetted, 12°C water was used from a carboy stored within the same growth chamber. High humidity (~60%) was maintained by keeping an open plastic basin containing clean tap water at the bottom of the growth chamber at all times. After two weeks, the wrapped paper towels were opened on a clean table to count germinated seeds, which were removed from the towels. Seeds not yet germinated were then wrapped up again and put back into the growth chamber for further germination. Following the same procedure, seeds were recounted at weekly intervals three more times. Germination rates for each accession were calculated cumulatively for each counting time.
Field earlyspring planting evaluation: Normal planting time for sorghum in Lubbock, TX is around May 15^{th}, but the planting time for earlyspring cold tolerance test in this study was April 1^{st} (45 days earlier than the normal planting time). The same seed lots for the growth chamber test from 212 accessions were also used for the field planting evaluation. Twentyfive seeds were planted in a 6 x 1 meter row, and two replicates were planted for each accession. The seedling emergence was first counted fourteen days after planting. Seedling emergence was recounted at weekly intervals two more times. Similar to lab germination rates, the seed germination rates in the field were also calculated cumulatively.
Seedling dry weight and emergence index: At 28 days after counting, the aboveground seedling tissues from five seedlings for each accession were cut, harvested, and then were dried in an oven at 80°C for 72 hours. After drying, the seedling tissues were weighed and the dry weight recorded (g/5 seedlings). Emergence index (EI), a measurement of rate of emergence, was calculated using the following formula: EI=Σ (Ej x Dj)/E where Ej=emergence on day j, Dj=days after planting, and E=final stand. The final stand counts were taken at 28 days and 35 days for field test and lab test, respectively [8].
Statistical analysis: An analysis of variance was performed on the data and means were using minimum significant difference (MSD) comparison procedure (SAS, 2008, Online Doc^{®} 9.2. Cary, NC: SAS Institute Inc.). Significant correlations between investigated traits were determined using Pearson correlation coefficients.
Results and Discussion
Variation in germination rates from lab and field, emergence index, and dry weight of field seedlings: The germination rates from lab and field conditions counted at different times plus dry weight of five field seedlings are listed in (Table 2) and shown in (Figure 1). Overall, at the beginning the seeds germinated more slowly in the field (2.01% at 14 days) than the lab (18.71% at 14 days). After 21 days, the germination rate was higher in the field (35.69% at 21 days, 43.20% at 28 days) than in the lab (28.86% at 21 days, 36.92% at 28 days) (Table 2 and Figure 1). Some accessions (e.g. PI 610727, a very cold tolerant check in replicate 2) reached 100% germination in the field at 21 days; while in the lab 100% germination was reached by some accessions (e.g. PI 154800 in replicate 1) at 28 days. Sorghum seeds appear to germinate more quickly in the field than in the lab. This observation is supported by the emergence index (EI). The average emergence index was lower in the field test (19.45) than in the lab test (20.02). Temperature fluctuation (between daytime high and nighttime low) may be required for a better germination. In addition to lab/field environmental conditions, differences in accessions (genotypes) also significantly affected the germination rate. Significant differences in germination rates for both lab (0–100%) and field conditions (0–100%) were identified among sweet sorghum accessions. The results from this study are consistent with the results from a previous study in which different cultivars differed significantly between lab germination and field emergence9. For example in the lab, the germination rate of the sweet sorghum cultivar Rio was only 42% at 14 days (Figure 2a), while the germination rate of GT26056 reached 94% at 14 days (Figure 2b). In the field, the germination rate of sweet sorghum accession PI 653617 at 28 days was only 68% (Figure 3a), while the rate of PI 152751 at 28 days reached 89% (Figure 3b).
Variable  N  Mean  Std Dev  Minimum  Maximum 

Field 1^{st} count germination rate (%) 
425  2.012  7.670  0  80.00 
Field 2^{nd} count germination rate (%) 
425  35.69  25.565  0  100.00 
Field 3^{rd} count germination rate (%) 
425  43.20  26.177  0  100.00 
Dry weight (g/5 seedlings) 
324  0.72  0.320  0.23  1.74 
Lab 1^{st} count germination rate (%) 
430  18.71  21.057  0  94.00 
Lab 2^{nd} count germination rate (%) 
430  28.86  24.239  0  94.00 
Lab 3^{rd} count germination rate (%) 
430  36.92  26.808  0  100.00 
Lab 4^{th} count Germination rate (%) 
430  38.26  27.088  0  100.00 
Table 2. Simple statistics for the lab germination rate, field germination rate, and seedling dry weight. (N=Number of samples; Std Dev=Standard Deviation).
Figure 1. Comparison of germination rates between field and lab at different timecounting intervals. The xaxis indicates days after planting or growing in the field or growth chamber, the yaxis is the germination rate (%). Blue curve represents the lab experiment and red curve represents the field experiment.
Figure 2. Comparison of lab germination rates between sweet sorghum cultivar Rio and cold tolerance line GT26056. 2A) Seeds from sweet sorghum Rio in the growth chamber at 12°C for 14 days with a low germination rate of 42% and 2B) seeds from cold tolerance line GT26056 in the growth chamber at 12ºC for 14 days with a high germination rate of 94%.
Figure 3. Comparison of field germination rates between sweet sorghum PI 653617 and PI 152751. Seeds from sweet sorghum PI 653617 28 days after field planting with a moderately high emergence rate (68%). 3A) Number of seeds emerged is shown in the lower panel and a closer image of some seedlings is shown in the upper panel. 3B) Seeds from sweet sorghum PI 152751 28 days after field planting with a high emergence rate (89%). Number of seeds emerged is shown in the lower panel and a closer image of some seedlings is shown in the upper panel (B).
In sweet sorghum growing regions where early spring cold may be an issue, accessions with good tolerance to early spring cold should be selected and used as parental materials to make crosses for developing new cultivars. The average dry weight of five seedlings in the field was 0.72 g, ranging from 0.23 to 1.74 g. PI 201723 (1.74 g/5 seedlings) had a significant higher (P<0.05) dry weight than PI 155516 (0.23 g/5 seedlings). Seedling dry weight may relate to early spring cold tolerance. This issue will be discussed in the following section of unique germplasm accessions identified.
Correlation coefficients among investigated traits: The results of Pearson correlation coefficients, probability, and number of observations among lab germination rates, field germination rates, and seedling dry weight are listed in (Table 3). In the field test, all the correlation coefficients among different countings were significant (p<0.0001). But the correlation coefficient values between the first counting and the second and third countings were low (R2=0.287 and R2=0.266, respectively), while the correlation coefficient value between the second counting and the third counting was very high (R2=0.944). In the lab test, all correlation coefficients among the different countings were significant (p<0.0001), and all correlation coefficient values were also high (all R2>0.84). The germination conditions in the lab are very controlled compared to the field. This may partly explain why there was some inconsistency in the correlation coefficient values among the different countings in the field. A high correlation coefficient between lab germination rate and field germination rate (R2=0.503, p<0.0001) was also observed. The correlation coefficient of seedling dry weight with the field germination rate was much higher (R2=0.257, p<0.0001) than with the lab germination rate (R2=0.109, p<0.05).
Trait  Field 2^{nd}  Field 3^{rd}  Dry wt.  Lab 1^{st}  Lab 2^{nd}  Lab 3^{rd}  Lab 4^{th} 

Field 1^{st} count  0.287 <.0001 422 
0.266 <.0001 422 
0.005 0.9354 321 
0.306 <.0001 422 
0.217 <.0001 422 
0.189 <.0001 422 
0.184 0.0001 422 
Field 2^{nd} count  0.944 <.0001 425 
0.337 <.0001 324 
0.393 <.0001 425 
0.417 <.0001 425 
0.471 <.0001 425 
0.462 <.0001 425 

Field 3^{rd} count  0.257 <.0001 324 
0.408 <.0001 425 
0.443 <.0001 425 
0.504 <.0001 425 
0.503 <.0001 425 

Dry weight 
0.119 0.0320 324 
0.104 0.0617 324 
0.115 0.0390 324 
0.109 0.0499 324 

Lab 1^{st} count  0.913 <.0001 430 
0.842 <.0001 430 
0.829 <.0001 430 

Lab 2^{nd} count  0.934 <.0001 430 
0.929 <.0001 430 

Lab 3^{rd} count  0.992 <.0001 430 
Table 3. Pearson correlation coefficients, probability, and number of observations for lab germination rate, field germination rate, and seedling dry weight.
Consistency and discrepancy in germination rates between lab and field conditions: In general, the lab germination rate can reflect or predict the field germination rate. The results from this study were consistent with the results from an earlier report on sorghum hybrids. But some discrepancies between the field and lab tests were observed. For example, the lab germination rates (84%, 90%, 96%, and 94%) of four checks (Rio, BTx623, GT26056, and PI 610727) were consistent with their field germination rates (80%, 80%, 91%, and 94%), respectively (Table 4).
Classified type  PI or cultivar name  Field germination (%)  Lab germination (%) 

Field rate high Lab rate low 
PI 302199  78  28 
PI 653617  82  35  
PI 146890  92  25  
Lab rate high Field rate low 
PI 247745  47.5  91 
PI 154800  58  93  
PI 154750  60  91  
Field and lab rate similar  PI 173112  78  65 
PI 195754  80  70  
PI 152751  74  92  
Field and lab rates for Check (selected controls) very consistent 
Rio (sweet sorghum)  80  84 
BTx623 (grain sorghum)  80  90  
GT26056 (check for cold tolerance)  91  96  
PI 610727 (check for cold tolerance)  94  94 
Table 4. Difference in seed germination rates between field and lab from some selected accessions.
Thus, the lab germination rates predict well the field germination rates for these four checks. We also observed that the germination rate for some accessions was high in the lab but low in the field. For example, the lab germination rate for PI 247745 was 91%, but its field germination rate was only 48%. The field conditions are more variable (e.g. lower nighttime temperature in the field) than the controlled lab conditions. Emergence of the seedlings from soil in the field can be adversely affected by soil type and level of moisture, whereas moistened germination papers in the lab offer no resistance to emergence. Some accessions may be cold tolerant to the lab conditions (12°C) but less tolerant to the field conditions where the temperature can dip much lower than 12°C. This may explain the difference in cold tolerance between the lab and field tests. Conversely, the lab germination rates for some accessions (PI 146890 and PI 653617) were low (25% and 35%) while their field germination rates were high (92% and 82%). These big differences between lab and field germination rates are difficult to interpret. One possible explanation could be that these accessions may require a lower temperature for breaking seed dormancy. Additionally, since fresh seeds were used, these accessions may have not completely overcome their seed dormancy yet before the start of the germination experiment. The lab experimental temperature of 12°C may not be low enough to break dormancy for some accessions, but the field low temperature can easily break its dormancy resulting in better germination rates. This explanation is supported by the second year lab germination test. After receiving the fresh sweet sorghum seeds from Puerto Rico, the seeds were stored at room temperature for less than two and half months before the beginning of the lab cold germination test. Overall the germination for most accessions from the second year was postponed by about two weeks. The postponed lab germination can only be explained by seed dormancy. To draw a final conclusion, the lab germination test for these accessions needs to be repeated.
Unique germplasm accessions identified: Besides the four checks (Rio, BTx623, GT26056, and PI 610727) which had high germination rates under the field cold conditions, three other accessions (PI 146890, PI 653617, and PI 195754) were identified with high field germination rates of 92%, 82%, and 80%, respectively. These three accessions also had lower emergence indexes (EI) (18.29, 17.63, and 18.52 respectively) than the average EI (19.45). The lower the EI value is, the earlier the seeds germinate. This means that these three accessions not only had high germination rates but also had earlier emergence. Compared with other accessions, the seedlings from these three accessions could establish more robustly than other lines within the same time window. For seedling dry weight, PI 146890 and PI 653617 averaged 0.838 g and 0.826 g, respectively, which is similar to the average (0.72 g). PI 195754, however, was 1.363 g, significantly higher than the average. Seedling dry weight (obtained from the early spring planting) may be an important indicator for early spring cold tolerance. PI 195754 was originally curated in China (GRIN database). The Chinese sorghum germplasm collection is known to contain accessions with good tolerance to early spring cold temperature [911].
Conclusion
Early spring cold tolerance is an important and complex trait. When freshly harvested seeds are to be used for screening early spring cold tolerance, seed dormancy should be considered. If the tested seeds have not completely overcome dormancy, the seed germination time and rate can be significantly postponed and reduced. Significant variability in the early spring cold tolerance exists among sweet sorghum accessions. There are over 2,100 sweet sorghum accessions in the USDA germplasm collection. Although some accessions with good tolerance to early spring cold were identified, we only tested 212 accessions (10%). In order to fully characterize the sweet sorghum collection for early spring cold tolerance, all accessions need to be first tested under lab conditions. Then the superior accessions selected from the lab test will be further evaluated in the field. The best sweet sorghum accessions will be selected and recommended for use by sweet sorghum breeders. At the same time, based on the published information of cold tolerance genetics (such as genetic heredity, QTLs, and existing genetic markers) and multiple harvests, crosses will be made between accessions with contrasting cold tolerance to establish biparental mapping populations to map the cold tolerance traits to specific chromosome regions for eventually cloning and identifying genes for early spring cold tolerance.
Conflict of Interest
The authors have declared that no competing interest exists.
Significance Statement
This study discovered the relationship between lab cold germination test and field seedling cold tolerance performance that can be beneficial for sweet sorghum breeders. This study will help sweet sorghum breeders to select parents from the unique accessions to make crosses for developing new cultivars in their breeding programs.
Acknowledgement
The authors would like to thank Mr. Jerry Davis from the Department of Statistics, University of Georgia for his excellent assistance in statistical analysis.
References
 Matsuo N, Fukami K, Tsuchiya S. Effects of early planting and cultivars on the yield and agronomic traits of soybeans grown in southwestern, Japan. Plant Prod Sci. 2016;19:37080.
 Hagan A, Weeks R. Tomato spotted wilt virus on peanuts. Alabama Cooperative Extension System, ANR. 1998;574:14.
 Yu M, Tuinstra MR. Genetic analysis of seedling growth under cold temperature stress in grain sorghum. Crop Sci. 2001;41:143843.
 Yu J, Tuinstra MR, Claassen MM, et al. Analysis of cold tolerance in sorghum under controlled environment conditions. Field Crops Res. 2004;85:2130.
 Knoll J, Ejeta G. Markerassisted selection for earlyseason cold tolerance in sorghum: QTL validation across populations and environments. Theor Appl Genet. 2008;116:54153.
 Knoll J, Gunaratna N, Ejeta G. QTL analysis of earlyseason cold tolerance in sorghum. Theor Appl Genet. 2008;116:57787
 Burow G, Burke JJ, Xin Z, et al. Genetic dissection of earlyseason cold tolerance in sorghum (Sorghum bicolor (L.) Moench). Mol Breed. 2011;28: 391402
 Smith PG, Millet AH. Germinating and sprouting response of tomato at low temperature. J Am Soc Hortic Sci. 1964;84:48084.
 Brar GS, Stewart BA. Germination under controlled temperature and field emergence of 13 sorghum cultivars. Crop Sci. 1994;34:133640.
 Cisse N, Ejeta G. Genetic variation and relationships among seedling vigor traits in sorghum. Crop Sci. 2003;43:82428
 Franks CD, Burow GB, Burke JJ. A comparison of U.S. and Chinese sorghum germplasm for early season cold tolerance. Crop Sci. 2006;46:137176