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Abstract The effects of low temperature on germination characteristics of rice seeds were studied， using three rice varieties as materials. The results showed that the germination rate， germination energy， germination index， vigor index， shoot length and root length all reduced with the decrease of temperature. When the treatment temperature was 10 ℃， whether or not coldresistant varieties， the values of all parameters showed an extremely significant difference from the controls. When the stress temperature was 15 ℃， compared with the normal condition， the parameter values of the nontolerant variety （Changbai 9） had an extremely significant difference and the germination rate， germination potential， seedling length and root length of two coldtolerance varieties （Jisheng 202 and Tongjing 9） had little difference from the control. Therefore， the above four indicators at 15 ℃ can be considered as the evaluation indices of cold tolerance at early stage.
Key words Rice; Lowtemperature stress; Germination characteristic
Low temperature acts as an abiotic stress factor that has a strong impact on the survival， growth， reproduction and distribution of plants. Each plant is characterized by a certain genetically fixed level of resistance to low temperatures， which reduces its metabolic activity. This level of resistance can vary among individual plants and species[1].
Rice is a temperaturesensitive crop， and low temperature dramatically reduces its production. Early rice in South China and seedlings in North China always meet cold current that causes rot of plenty of seeds as well as seedling death， thereby resulting in a huge loss to agricultural production[2]. Good cold tolerance at the germination stage is an important character for stable rice production. Developing coldtolerant genotypes is one of the most effective ways to avoid the lowtemperature damage[3].
Different kinds of plants， even different varieties of the same species， show different coldtolerance ability. At the same time， the effect of low temperature on the crop depends on development stage， intensity， and exposure time. Identification on coldtolerance ability not only needs appropriate methods， but also quantitative indicators[4]. Researchers have made extensive efforts on improving cold tolerance in rice in the past 20 years and established many growthstage specific criteria to evaluate and select coldtolerant rice[5]. Evaluation of rice varieties for improving cold tolerance is mainly performed during seedling and reproductive stages[6]. There are fewer studies on cold tolerance at other stages. Directsowing rice is a developing trend. Cold tolerance at early stage is one key factor which affects development of directsowing rice. The parameters at seed germination stage are key indicators for identification of cold tolerance at early stage. Cold resistance can be predicted by the identification indices at seeds germination stage.
In this paper， with three rice varieties， which differ in cold tolerance， as test materials， the germination characteristics were surveyed under different temperature treatments. The objective of this study is to analyze the correlation between germination characteristics and coldtolerance. The study will provide a theoretical basis for early screening and identification of rice germplasm resources.
Materials and Methods
Materials
Oryza sativa， Coldsensitive （seedling stage） Changbai 9， and coldtolerant Jisheng 202 and Tongjing 9， were used in this experiment. The three rice varieties were collected by Rice Research Institute， Jilin Academy of Agricultural Science.
Methods
Evolution of cold tolerance in rice at the germination stage
Seeds were selected based on size uniformity and absence of spots. Total 100 full seeds from each material were put into a culture dish with two layers of germination filter paper. Then， the dishes were kept in an incubator for 48 h at 48 ℃ to make the seeds dry fully and break seed dormancy. After seed soaking， seeds were divided into three groups. One group was maintained in the incubator at 28 ℃. The other two groups were transferred to a chamber for cold treatment at 10 ℃ and 15 ℃， respectively.
A completely randomized experiment was completed with three replicates. The blocks consisted of different shelves in the germination chamber. The standard assessment of whether a rice grain has germinated was determined as the point at which the bud length equals half the length of the seed， and the root length equals the seed length[7]. As long as a seed germinated， it is recorded the beginning of seed germination. If the seeds dont germinate for four days， it means the end of germination[3]. After the end of the experiments， some indices were calculated： germinaton rate （Gr）， germination potential（Gp）， germinative index（Gi） and vigor index（Vi）. At the same time， the length of roots and shoots were measured.
Calculations for each seed sample
Germination percentage （%）=（Number of germinated grain/Number of total grain）*100 Germination potential （%）=（Number of germinated grain at special days/Number of total grain）*100
Germinative index （Gi）=∑GtDt
Vigor index （Vi）= =S∑GtDt
（Gt： the germinating number on t day; Dt： the days of relative germination）.
Statistical analysis
Experimental data were analyzed using the Statistical Product and Service Solutions （SPSS） software. Data obtained were subjected to analysis of variance （ANOVA）. Mean comparison was conducted using Duncans multiple range tests （P<0.05 and 0.01）.
Results and Analysis
Effects of low temperature on germination rate and germination potential
As shown in Table 1， under the condition of normal temperature， the germination rate （Gr） and germination potential （Gp） values of three varieties were higher. The Gr values of Changbai 9， Jisheng 202 and Tongjing 9 were 99%， 98% and 99%， respectively. The Gp values of the three cultivars were 86%， 90% and 91%， respectively. There were no significant differences between them. With the temperature decreasing， the Gr and Gp of the three materials were on the decline. The lower the temperature， the greater the effects on germination rate and germination potential. The Gr was 96% and 98% in Jisheng 202 and 80% and 76% in Tongjing 9 at 15 and 10 ℃， respectively. The Gr was 82% and 28% in Changbai 9 at 15 and 10 ℃， respectively. Jisheng 202 and Tongjing 9 showed no significant differences in germination rate between the control and the 15 ℃ treatment. However， the germination rate of Changbai 9 showed a significant difference from the control at 15 ℃. The germination potential also showed the same trend. The three varieties had Gr and Gp values decreased significantly at 10 ℃， especially for the nonresistant varieties. The values of Changbai 9 dropped extremely. The Gr and Gp of all cultivars， whether it is coldresistant or noresistant， showed extremely significant difference from the control at 10 ℃. The Gr of Changbai 9， which is a nonresistant variety， decrease to 28%， and its Gp decreased even more， to 7%. So germination rate and germination potential at 15 ℃ can serve as coldtolerance indicators of rice germplasms at early stage.
Effects of low temperature on germination index and vigor index
Seen from table 2， under low temperature treatment， the values of germination index （Gi） and vigor index （Vi） were lower than normal values. The Gi value of Changbai 9 was 25.23 at 28 ℃， which decreased by 17.48% and 61.40% at 15 and 10 ℃， respectively. The Gi of the two treatments showed extremely significant differences from the normal condition. For the coldtolerance varieties， Jisheng 202 and Tongjing 9， compared with the control， the Gi values decreased by 11.82% and 7.29% at 15 ℃， respectively. There were significant differences on Gi between the 10 ℃ treatments and the controls. The values of Jisheng 202 and Tongjing 9 at 10 ℃ were significantly different from the value of corresponding control. Effects of low temperature on root length and shoot length
The data in Table 3 showed that low temperature had certain effects on root length and shoot length. Under normal condition， the shoot lengths of the three varieties were 4.01， 4.91 and 4.33 cm， respectively. The lengths dropped at 15 ℃， respectively， to 2.95， 4.57 and 4.01 cm， which were reduced by 26.43%， 6.92% and 7.39%， respectively. The values at 10 ℃ were respectively， 1.23， 3.80 and 3.63 cm， which decreased by 68.9%， 22.6% and 68.9%， respectively.
The root length of Changbai 9 reduced at 15 ℃， from 3.95 cm to 2.76 cm， with a significant difference from the control. Meanwhile， the root lengths of Jisheng 202 and Tongjing 9 decreased by 2.16% and 5.5% at 15 ℃， respectively. There were no significant differences between the control and the two 15 ℃ treatments. The lower the temperature， the greater the effects on root length of nonresistant varieties. The root length of Changbai 9 dropped to zero at 10 ℃. As for the coldtolerance varieties， the root length values decreased by 49.37% and 51.23% at 10 ℃， respectively. Therefore， shoot and root lengths at 15 ℃ can also be the evaluation indices of cold tolerance at early stage.
Yadong ZHAO et al. Effect of Low Temperature on Germination Characteristics of Rice Cultivars
Discussion
Cold tolerance in plant is controlled by multigenes[8-9] and is a very complex trait[4]. The researches on coldtolerance mainly focus on the screening and identification of coldresistant resources[10-16]， and compared with the seedling and booting stage， fewer researches have been conducted on rice tolerance at germination stage. The selection of appropriate indices and suitable temperature plays an important role in the accuracy of identification.
The optimum temperature for seed germination and early seedling growth ranges from 25 to 35 ℃[17]. 10 and 12 ℃ are the lowest temperatures for seed germination of japonica and indica rice， respectively. The varieties used in this experiment belong to the japonica rice. 10 ℃ is the lowest germination temperature. Moreover， the directsowing time recommended in Jilin Province is early May， a period with an average temperature around 15 ℃. Therefore， this experiment selected 10 and 15 ℃ as stress temperatures. The results showed that the critical temperature has a very significant impact on its germination characteristics， no matter whether its a coldresistant variety. The germination characteristics of different varieties were extremely different at 15 ℃， and 15 ℃ can be identification temperature whether the varieties are coldresistant. Germination rate is the main criteria used for the evaluation of cold tolerance in rice at the germination stage[7]. Previous studies have shown that germination rate was strongly affected by low temperature. Stress of low temperature at the germination stage caused a reduction in final germination rate and the lengths of coleoptile and radical[17]. Besides， average sprouting days， germinating coefficient， germinating index are also considered as indicators of coldresistant identification. Germination potential measures the speed and uniformity of seed germination and is a key evaluation indicator in other plants. Meanwhile， the research has not been reported about the effects of low temperature on rices shoot length at germination stage. This study analyzed the effects of different low temperature treatments on above indices and showed that the germination potential， shoot length， root length and germination rate could be used as coldtolerance indicators.
References
[1] AGHAEE A， MORADI F， ZAREMAIVAN H， et al. Physilogical responses of two rice （Oryza sativa L.） genotypes to chilling stress at seedling stage[J]. African Journal of Biotechnology， 2011， 10（39）： 7617-7621.
[2] ZHANG HQ， ZOU YB， XIAO GC， et al. Effect and mechanism of cold tolerant seedcoating agents on the cold tolerance of early indica rice seedlings[J]. Agricultural Sciences in China， 2007， 6（7）： 792-801.
[3] LOU QJ， CHEN L， SUN ZX， et al. A major QTL associated with cold tolerance at seedling stage in rice （Oryza sativa L.）[J]. Euphtica， 2007， 158（1-2）： 87-94.
[4] GENG GD， CHENG ZH， ZHANG SQ. Appraisal of eggplant chillingresistant by low temperature germination[J]. Seed， （2006）， 25（6）： 43-46.
[5] MARUYAMA K， URANO K， YOSHIWARA K， et al. Integrated analysis of the effects of cold and dehydration on rice metabolites， phytohormones， and gene transcripts[J]. Plant Physiol， 2014， 164（4）： 1759-1771.
[6] ZHANG Q， CHEN QH， WANG SL， et al. Rice and cold stress： Methods for its evaluation and summary of cold tolerancerelated quantitative trait loci[J]. Rice， 2014， 7： 24.
[7] QI DL， ZHANG SY， CAO GL， et al. Studies on screening methods for alkaline tolerance at germination period and early seedling stage in rice[J]. Journal of Plant Genetic Resources， 2006， 7（1）： 74-80.
[8] LU MH， CHEN JF. Genetic engineering of plant for cold tolerance[J]. Acta Botanica Borealioccidentalia Sinica， 2004， 24（10）： 1953-1958
[9] JENA KK， KIM SM， SUH JP， et al. Identification of coldtolerant breeding lines by quantitative trait loci associated with cold tolerance in rice[J]. Crop Science， 2012， 52： 517-523. [10] HAN LZ， CAO GL， AN YP， et al. Characterization and evaluation for cold tolerance at the budburst period of rice germplasm[J]. Journal of Plant Genetic Resource， 2004， 5（4）： 346-350.
[11] SUN JIANCHANG， et al. A study of cold tolerance of seeds of japonica rice varieties of different grain shape in germination period[J]. Ningxia Journal of Agri and Fores Sci&Tech， 2015， 56（11）： 1-3， 22.
[12] RANAWAKE AL， NAKAMURA C. Cold tolerance of an inbred line population of rice （Oryza sativa L） at different growth stage[J]. Tropical Agricultural Research & Extension， 2011， 14（2）：25-33.
[13] SANG IK， THOMAS HT. Evaluation of seedling cold tolerance in rice cultivars： a comparison of visual ratings and quantitative indicators of physiological changes[J]. Eupthytica， 2011， 178： 437-447.
[14] XIAO YL， ZHONG JY， HU QF， et al. Identification of major cultivated early rice varieties in Jiangxi Province for cold tolerance at bud stage under controllable conditions[J]. Acta Agriculture Universitatis Jiangxiensis， 2014， 36（2）： 243-248.
[15] LIU HL， SUN SC， WANG JG， et al. Identification of cold tolerance of rice germplasm resource at germinating stage in cold region[J]. Journal of Northeast Agricultural University， 2008， 15（2）： 1-4.
[16] WANG JM， LIN XY， SUN Q， et al. Evaluation of cold tolerance for japonica rice varieties from different country[J]. Advance Journal of Food Science & Technology， 2013， 5（1）： 54-56.
[17] PEYMAN SHARIFI. Evaluation on sixtyeight rice germplasms in cold tolerance at germination stage[J]. Rice Science， 2010， 17（1）：77-81.
Key words Rice; Lowtemperature stress; Germination characteristic
Low temperature acts as an abiotic stress factor that has a strong impact on the survival， growth， reproduction and distribution of plants. Each plant is characterized by a certain genetically fixed level of resistance to low temperatures， which reduces its metabolic activity. This level of resistance can vary among individual plants and species[1].
Rice is a temperaturesensitive crop， and low temperature dramatically reduces its production. Early rice in South China and seedlings in North China always meet cold current that causes rot of plenty of seeds as well as seedling death， thereby resulting in a huge loss to agricultural production[2]. Good cold tolerance at the germination stage is an important character for stable rice production. Developing coldtolerant genotypes is one of the most effective ways to avoid the lowtemperature damage[3].
Different kinds of plants， even different varieties of the same species， show different coldtolerance ability. At the same time， the effect of low temperature on the crop depends on development stage， intensity， and exposure time. Identification on coldtolerance ability not only needs appropriate methods， but also quantitative indicators[4]. Researchers have made extensive efforts on improving cold tolerance in rice in the past 20 years and established many growthstage specific criteria to evaluate and select coldtolerant rice[5]. Evaluation of rice varieties for improving cold tolerance is mainly performed during seedling and reproductive stages[6]. There are fewer studies on cold tolerance at other stages. Directsowing rice is a developing trend. Cold tolerance at early stage is one key factor which affects development of directsowing rice. The parameters at seed germination stage are key indicators for identification of cold tolerance at early stage. Cold resistance can be predicted by the identification indices at seeds germination stage.
In this paper， with three rice varieties， which differ in cold tolerance， as test materials， the germination characteristics were surveyed under different temperature treatments. The objective of this study is to analyze the correlation between germination characteristics and coldtolerance. The study will provide a theoretical basis for early screening and identification of rice germplasm resources.
Materials and Methods
Materials
Oryza sativa， Coldsensitive （seedling stage） Changbai 9， and coldtolerant Jisheng 202 and Tongjing 9， were used in this experiment. The three rice varieties were collected by Rice Research Institute， Jilin Academy of Agricultural Science.
Methods
Evolution of cold tolerance in rice at the germination stage
Seeds were selected based on size uniformity and absence of spots. Total 100 full seeds from each material were put into a culture dish with two layers of germination filter paper. Then， the dishes were kept in an incubator for 48 h at 48 ℃ to make the seeds dry fully and break seed dormancy. After seed soaking， seeds were divided into three groups. One group was maintained in the incubator at 28 ℃. The other two groups were transferred to a chamber for cold treatment at 10 ℃ and 15 ℃， respectively.
A completely randomized experiment was completed with three replicates. The blocks consisted of different shelves in the germination chamber. The standard assessment of whether a rice grain has germinated was determined as the point at which the bud length equals half the length of the seed， and the root length equals the seed length[7]. As long as a seed germinated， it is recorded the beginning of seed germination. If the seeds dont germinate for four days， it means the end of germination[3]. After the end of the experiments， some indices were calculated： germinaton rate （Gr）， germination potential（Gp）， germinative index（Gi） and vigor index（Vi）. At the same time， the length of roots and shoots were measured.
Calculations for each seed sample
Germination percentage （%）=（Number of germinated grain/Number of total grain）*100 Germination potential （%）=（Number of germinated grain at special days/Number of total grain）*100
Germinative index （Gi）=∑GtDt
Vigor index （Vi）= =S∑GtDt
（Gt： the germinating number on t day; Dt： the days of relative germination）.
Statistical analysis
Experimental data were analyzed using the Statistical Product and Service Solutions （SPSS） software. Data obtained were subjected to analysis of variance （ANOVA）. Mean comparison was conducted using Duncans multiple range tests （P<0.05 and 0.01）.
Results and Analysis
Effects of low temperature on germination rate and germination potential
As shown in Table 1， under the condition of normal temperature， the germination rate （Gr） and germination potential （Gp） values of three varieties were higher. The Gr values of Changbai 9， Jisheng 202 and Tongjing 9 were 99%， 98% and 99%， respectively. The Gp values of the three cultivars were 86%， 90% and 91%， respectively. There were no significant differences between them. With the temperature decreasing， the Gr and Gp of the three materials were on the decline. The lower the temperature， the greater the effects on germination rate and germination potential. The Gr was 96% and 98% in Jisheng 202 and 80% and 76% in Tongjing 9 at 15 and 10 ℃， respectively. The Gr was 82% and 28% in Changbai 9 at 15 and 10 ℃， respectively. Jisheng 202 and Tongjing 9 showed no significant differences in germination rate between the control and the 15 ℃ treatment. However， the germination rate of Changbai 9 showed a significant difference from the control at 15 ℃. The germination potential also showed the same trend. The three varieties had Gr and Gp values decreased significantly at 10 ℃， especially for the nonresistant varieties. The values of Changbai 9 dropped extremely. The Gr and Gp of all cultivars， whether it is coldresistant or noresistant， showed extremely significant difference from the control at 10 ℃. The Gr of Changbai 9， which is a nonresistant variety， decrease to 28%， and its Gp decreased even more， to 7%. So germination rate and germination potential at 15 ℃ can serve as coldtolerance indicators of rice germplasms at early stage.
Effects of low temperature on germination index and vigor index
Seen from table 2， under low temperature treatment， the values of germination index （Gi） and vigor index （Vi） were lower than normal values. The Gi value of Changbai 9 was 25.23 at 28 ℃， which decreased by 17.48% and 61.40% at 15 and 10 ℃， respectively. The Gi of the two treatments showed extremely significant differences from the normal condition. For the coldtolerance varieties， Jisheng 202 and Tongjing 9， compared with the control， the Gi values decreased by 11.82% and 7.29% at 15 ℃， respectively. There were significant differences on Gi between the 10 ℃ treatments and the controls. The values of Jisheng 202 and Tongjing 9 at 10 ℃ were significantly different from the value of corresponding control. Effects of low temperature on root length and shoot length
The data in Table 3 showed that low temperature had certain effects on root length and shoot length. Under normal condition， the shoot lengths of the three varieties were 4.01， 4.91 and 4.33 cm， respectively. The lengths dropped at 15 ℃， respectively， to 2.95， 4.57 and 4.01 cm， which were reduced by 26.43%， 6.92% and 7.39%， respectively. The values at 10 ℃ were respectively， 1.23， 3.80 and 3.63 cm， which decreased by 68.9%， 22.6% and 68.9%， respectively.
The root length of Changbai 9 reduced at 15 ℃， from 3.95 cm to 2.76 cm， with a significant difference from the control. Meanwhile， the root lengths of Jisheng 202 and Tongjing 9 decreased by 2.16% and 5.5% at 15 ℃， respectively. There were no significant differences between the control and the two 15 ℃ treatments. The lower the temperature， the greater the effects on root length of nonresistant varieties. The root length of Changbai 9 dropped to zero at 10 ℃. As for the coldtolerance varieties， the root length values decreased by 49.37% and 51.23% at 10 ℃， respectively. Therefore， shoot and root lengths at 15 ℃ can also be the evaluation indices of cold tolerance at early stage.
Yadong ZHAO et al. Effect of Low Temperature on Germination Characteristics of Rice Cultivars
Discussion
Cold tolerance in plant is controlled by multigenes[8-9] and is a very complex trait[4]. The researches on coldtolerance mainly focus on the screening and identification of coldresistant resources[10-16]， and compared with the seedling and booting stage， fewer researches have been conducted on rice tolerance at germination stage. The selection of appropriate indices and suitable temperature plays an important role in the accuracy of identification.
The optimum temperature for seed germination and early seedling growth ranges from 25 to 35 ℃[17]. 10 and 12 ℃ are the lowest temperatures for seed germination of japonica and indica rice， respectively. The varieties used in this experiment belong to the japonica rice. 10 ℃ is the lowest germination temperature. Moreover， the directsowing time recommended in Jilin Province is early May， a period with an average temperature around 15 ℃. Therefore， this experiment selected 10 and 15 ℃ as stress temperatures. The results showed that the critical temperature has a very significant impact on its germination characteristics， no matter whether its a coldresistant variety. The germination characteristics of different varieties were extremely different at 15 ℃， and 15 ℃ can be identification temperature whether the varieties are coldresistant. Germination rate is the main criteria used for the evaluation of cold tolerance in rice at the germination stage[7]. Previous studies have shown that germination rate was strongly affected by low temperature. Stress of low temperature at the germination stage caused a reduction in final germination rate and the lengths of coleoptile and radical[17]. Besides， average sprouting days， germinating coefficient， germinating index are also considered as indicators of coldresistant identification. Germination potential measures the speed and uniformity of seed germination and is a key evaluation indicator in other plants. Meanwhile， the research has not been reported about the effects of low temperature on rices shoot length at germination stage. This study analyzed the effects of different low temperature treatments on above indices and showed that the germination potential， shoot length， root length and germination rate could be used as coldtolerance indicators.
References
[1] AGHAEE A， MORADI F， ZAREMAIVAN H， et al. Physilogical responses of two rice （Oryza sativa L.） genotypes to chilling stress at seedling stage[J]. African Journal of Biotechnology， 2011， 10（39）： 7617-7621.
[2] ZHANG HQ， ZOU YB， XIAO GC， et al. Effect and mechanism of cold tolerant seedcoating agents on the cold tolerance of early indica rice seedlings[J]. Agricultural Sciences in China， 2007， 6（7）： 792-801.
[3] LOU QJ， CHEN L， SUN ZX， et al. A major QTL associated with cold tolerance at seedling stage in rice （Oryza sativa L.）[J]. Euphtica， 2007， 158（1-2）： 87-94.
[4] GENG GD， CHENG ZH， ZHANG SQ. Appraisal of eggplant chillingresistant by low temperature germination[J]. Seed， （2006）， 25（6）： 43-46.
[5] MARUYAMA K， URANO K， YOSHIWARA K， et al. Integrated analysis of the effects of cold and dehydration on rice metabolites， phytohormones， and gene transcripts[J]. Plant Physiol， 2014， 164（4）： 1759-1771.
[6] ZHANG Q， CHEN QH， WANG SL， et al. Rice and cold stress： Methods for its evaluation and summary of cold tolerancerelated quantitative trait loci[J]. Rice， 2014， 7： 24.
[7] QI DL， ZHANG SY， CAO GL， et al. Studies on screening methods for alkaline tolerance at germination period and early seedling stage in rice[J]. Journal of Plant Genetic Resources， 2006， 7（1）： 74-80.
[8] LU MH， CHEN JF. Genetic engineering of plant for cold tolerance[J]. Acta Botanica Borealioccidentalia Sinica， 2004， 24（10）： 1953-1958
[9] JENA KK， KIM SM， SUH JP， et al. Identification of coldtolerant breeding lines by quantitative trait loci associated with cold tolerance in rice[J]. Crop Science， 2012， 52： 517-523. [10] HAN LZ， CAO GL， AN YP， et al. Characterization and evaluation for cold tolerance at the budburst period of rice germplasm[J]. Journal of Plant Genetic Resource， 2004， 5（4）： 346-350.
[11] SUN JIANCHANG， et al. A study of cold tolerance of seeds of japonica rice varieties of different grain shape in germination period[J]. Ningxia Journal of Agri and Fores Sci&Tech， 2015， 56（11）： 1-3， 22.
[12] RANAWAKE AL， NAKAMURA C. Cold tolerance of an inbred line population of rice （Oryza sativa L） at different growth stage[J]. Tropical Agricultural Research & Extension， 2011， 14（2）：25-33.
[13] SANG IK， THOMAS HT. Evaluation of seedling cold tolerance in rice cultivars： a comparison of visual ratings and quantitative indicators of physiological changes[J]. Eupthytica， 2011， 178： 437-447.
[14] XIAO YL， ZHONG JY， HU QF， et al. Identification of major cultivated early rice varieties in Jiangxi Province for cold tolerance at bud stage under controllable conditions[J]. Acta Agriculture Universitatis Jiangxiensis， 2014， 36（2）： 243-248.
[15] LIU HL， SUN SC， WANG JG， et al. Identification of cold tolerance of rice germplasm resource at germinating stage in cold region[J]. Journal of Northeast Agricultural University， 2008， 15（2）： 1-4.
[16] WANG JM， LIN XY， SUN Q， et al. Evaluation of cold tolerance for japonica rice varieties from different country[J]. Advance Journal of Food Science & Technology， 2013， 5（1）： 54-56.
[17] PEYMAN SHARIFI. Evaluation on sixtyeight rice germplasms in cold tolerance at germination stage[J]. Rice Science， 2010， 17（1）：77-81.