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Abstract The selection of cold tolerance identification indices for bitter gourd germplasms is an important basis for the breeding of cold??tolerant bitter gourd varieties. In this study， with six bitter gourd germplasms as experimental materials， the cold injury index of bitter gourd was analyzed under low temperature treatment， the changes in physiological indices were revealed， and the main agronomic traits were measured. The results showed that the cold injury index of bitter gourd was in very significant negative correlation with SOD activity， POD activity and PRO content， in significant negative correlation with CAT activity， in very significant negative correlation with flowering node and commodity rate， but in significant positive correlation with female flower ratio. It was found that that cold injury index， SOD activity， POD activity， CAT activity and PRO content could serve as the cold tolerance identification indices for bitter gourd at seedling stage， and flowering node and commodity rate could be used as the auxiliary indices for field cold tolerance identification.
Key words Bitter gourd； Germplasm； Low temperature； Physiological index； Agronomic trait； Cold tolerance identification
Bitter gourd （Momordica charantia L.， 2n=2x=22） is an annual vine climbing plant in Momordica of Cucurbitaceae， also known as bitter melon[1]. It is probably to suffer from seasonal low??temperature damage during plantation in winter and spring， leading to problems including quality reduction， yield decrease and lagged marketing[2]. The breeding of cold??tolerant bitter gourd varieties through the identification of cold tolerance is considered to be the most effective way of reducing low??temperature damage[3]. The identification of cold tolerance in bitter gourd mostly adopts morphological index or physiological index[4-7]. However， comprehensively identifying the cold tolerance in germplasms through the comparative analysis of morphological indices， physiological indices and main agronomic traits is more accurate and reliable. Therefore， the cold injury index of bitter gourd at seedling stage was statistically analyzed， the changes in physiological indices were detected， and the main agronomic traits of germplasms were measured， followed by correlation analysis. The screening of more indices suitable for cold tolerance identification of bitter gourd is of great significance to the breeding of bitter gourd for cold tolerance.
Materials and Methods Experimental materials
There were six experimental materials， which were all homozygous inbred lines obtained by Cucurbits and Vegetables Laboratory， Tropical Crop Germplasm Research Institute through self??cross selective breeding： T1： 07??20 （Guangdong）， T2： 08??41（Guangdong）， T3： 11??11 （Fujian）， T4： 08??45 （Guangxi）， T5： 08??23 （Guangxi）， T6： T06 （Thailand）.
Experimental method
Determination of cold damage index and physiological indices at seedling stage The experiment was carried out in the solar greenhouse of the eight team of Cucurbits and Vegetables Laboratory， Tropical Crop Germplasm Research Institute. Full seeds of bitter gourd with uniform size were selected and subjected to pregermination. After the emergence of white buds， the seeds were sown into 50??hole trays filled with Peilei medium （Zhenjiang Peilei Organic Fertilizer Co.， Ltd.） for raising seedlings. At the stage of two leaves and one heart， uniform seedlings were selected and cultured in an illumination incubator for 2 d under the day/night temperature at 28 ??/18 ??， light period of 14 h and dark period of 10 h， and illumination intensity of 6 000 lx. The seedlings were then placed in an manual climatic box （Shanghai Yiheng Scientific Instrument Co.， Ltd.， MGC??450HP??2） for low temperature treatment at 6 ?? for 1 d， and during the period of treatment， the cold injury characteristics were investigated， and the cold injury index was investigated. The experiment had three replicates， each including 25 plants.
Determination of physiological indices： During the determination， 0.1 g of the second main leaf of bitter gourd was obtained and determined with the kit purchased by Suzhou Comin Biotechnology Co.， Ltd. for superoxide dismutase （SOD） activity， peroxidase （POD） activity， catalase （CAT） activity， malondialdehyde （MAD） content， hydrogen peroxide （H2O2） content and proline （PRO） content.
Cold injury index was calculated according to Cold injury index=??（Grade of cold injury??Number of plants of the grade）/4??Total number of investigated plants??100%， and the grading standards of cold tolerance at seedling stage are shown in Table 1.
Investigation of main agronomic traits of bitter gourd All the materials were subjected to pregermination on November 18， 2016. Field planting was performed on November 30. And harvesting was completed totally on March 14， 2017. Each plot had an area of 6.0 m??4.0 m=24.0 m2. The experiment had three replications in random arrangement. Each plot included 16 plants planted according to a plant spacing of 0.8 m. Flat frames were constructed to support vines. Field management was unified. Data processing and analysis Data analysis was performed in Excel， and analysis of significance and correlation coefficient was performed with SPSS software.
Results and Analysis
Comparative analysis of cold injury index and physiological indices at seedling stage
After treating at 6 ?? for 1 d， the different bitter gourd germplasms differed significantly in cold injury index （Table 2）. Among them， germplasms T1 and T2 had smaller cold injury index values， exhibiting stronger cold tolerance. T6 had the largest cold injury index of 84.38%， exhibiting weaker cold tolerance. Different bitter gourd germplasms differed in SOD activity， though the differences were not significant. T1 had the strongest SOD activity of 225.64 U/g FW， while T5 had the weakest activity of 219.18 U/g FW. The germplasms exhibiting cold tolerance overall had stronger SOD activity. Different bitter gourd germplasms had different POD activity. Among them， T1 and T2 had significant differences from T4， T5 and T6. The germplasms with stronger cold tolerance also had stronger POD activity. Germplasms with stronger cold tolerance had relatively higher CAT activity， though the differences were not significant. T2 had the strongest activity of 28.99 U/g FW， while T3 had the weakest activity of 26.36 U/g FW. There were no significant differences in H2O2 content between germplasms. Among them， T6 had the highest content of 2.48 mol/g， while T4 had the lowest content of 2.37 mol/g， and there were no obvious laws between germplasms. There were significant differences in PRO content between germplasms. Among them， T1 had the highest content of 51.16 mg/g， while T6 had the lowest content of 39.79 mg/g. The germplasms exhibiting stronger cold tolerance had higher PRO contents. There were significant differences in MDA content between germplasms. Among them， T4 had the highest content of 4.25 mol/g， while T3 had the lowest content of 3.53 mol/g.
Correlation analysis between cold injury index and physiological indices
Correlation analysis was performed between the cold injury index and physiological indices of bitter gourd under low temperature stress. The results showed that the cold injury index of bitter gourd germplasms were in very significant negative correlation with SOD activity， POD activity and PRO content of leaves （P
Comparative analysis on main agronomic traits of bitter gourd germplasms
Different bitter gourd germplasms differed in the first female flower node. Among them， T1 had the highest first female flower node， the 24 node averagely， while T6 had the lowest first female flower node， the 16.32 node averagely， and the difference was very significant. Different bitter gourd germplasms differed in female flower ratio. T6 had the largest female flower ratio of 45.35%， T3 had the lowest female flower ratio of 25.79%， and there were significant differences between T6 and any one of T1， T2， T3， T4 and T5. The single fruit weight differed between germplasms. T4 had the highest single fruit weight of 480.17 g， and T5 showed the lowest single fruit weight of 386.63 g. Different bitter gourd germplasms differed in the fruit set percentage. T1 showed the highest fruit set percentage of 92.38%， while T5 had the lowest fruit set percentage of 72.46%， and the difference was significant. The commodity rate was also different between the various bitter gourd germplasms. T1 had the highest commodity rate of 95.16%， while T5 showed the lowest commodity rate of 70.41%， and the difference was significant. Different bitter gourd germplasms had different yields. T4 had the highest yield of 26 361.75 kg/hm2， while T5 had the lowest yield of 17 118.60 kg/hm2， and the difference was very significant. As to fruit color， T1??T4 and T5 had green fruit， T2 and T3 showed light green fruit， and T6 showed white fruit. The fruit of T1， T3 and T4 was in the shape of a long rod， that of T2 was in the shape of a short rod， and T5 and T6 exhibited spindle??shaped fruit. Correlation analysis between cold injury index and main agronomic traits of bitter gourd
Correlation analysis was performed between the cold injury index and main agronomic traits of bitter gourd （Table 5）. The results showed that the cold injury index of bitter gourd was in very significant negative correlation with the flowering node and commodity rate， but in significant positive correlation with the female flower ratio. It indicates that flowering node， commodity rate and female flower ratio could serve as the auxiliary indices for the identification of cold tolerance. Generally， germplasms with high flowering node， low female flower ratio and late mature property have better cold tolerance.
Through the statistical analysis of cold injury index of bitter gourd at seedling stage， detection of SOD activity， POD activity， CAT activity， MDA content， H2O2 content and PRO content and measurement of main agronomic traits of germplasms， it could be concluded that the cold injury index was in very significant negative correlation with SOD activity， POD activity and PRO content （P
Discussion
The planting area of bitter gourd is enlarging gradually year by year， and it has become a main vegetable planted in winter in South China. Due to the lack of cold tolerance varieties， the identification of cold tolerance in bitter gourd appears to be particularly important. In recent years， scholars have identified cold tolerance in bitter gourd varieties and materials. Some physiological and biochemical indices were selected， but the changes of physiological and biochemical indices are different. Sun et al.[4] studied nine physiological and biochemical indices with six bitter gourd varieties （lines） as materials， and found that PRO content was in significant negative correlation with low temperature??resistant factors， and MDA content was in significant correlation with low temperature??resistant factors. Huang et al.[5] determined the changes in POD activity， SOD activity and MDA content of bitter gourd at seedling stage under different temperature conditions， and the results showed that the increase of MDA was in significant positive correlation with cold injury index， and POD activity， SOD activity and soluble sugar content were in significant negative correlation with cold injury index. Zhang et al.[6] studied the changes in physiological and biochemical indices of eight bitter gourd inbred lines under low temperature and weak light， and the results showed that the SOD activity and CAT activity increased， POD activity decreased， and MDA and PRO content increased. Chen et al.[7] studied the change laws of POD activity， SOD activity， CAT activity and PRO content under low temperature stress with two bitter gourd inbred lines as materials， and the results showed that under low temperature stress， SOD and POD activity and PRO content increased， and CAT activity decreased. In this study， it was found that the cold injury index of bitter gourd was in very significant negative correlation with SOD activity， POD activity and PRO content， and in significant negative correlation with CAT activity. It might be because of the differences in tested bitter gourd varieties （lines）， during the determination of physiological and biochemical indices， the differences in temperature and time cause different physiological changes. In this study， it was concluded that the SOD activity， POD activity， CAT activity and PRO content could serve as the physiological and biochemical indices for bitter gourd cold tolerance identification at seedling stage， which accords with above research. During the identification of cold tolerance in bitter gourd， single morphological index and physiological index would reduce the accuracy and reliability of identification due to the instability of the index， and scholars have discussed the comprehensive evaluation of cold tolerance index. Gao et al.[8] studied the effects of low temperature and weak light on physiological and biochemical indices of bitter gourd， and the tolerance to low temperature and weak light in bitter gourd was comprehensively evaluated by principal factor analysis， hierarchical cluster analysis and subordinate function method. The results showed that there were significant differences in 12 physiological and biochemical indices between 23 tested bitter gourd inbred lines. The 23 lines of different genotypes were divided into four groups， two classes， according to the tolerance to low temperature and weak light by clustering analysis and subordinate function method， and six cold tolerance identification indices were screened. Lu et al.[9] divided 17 tested bitter gourd germplasm resources into highly low temperature sensitive type， medium low temperature sensitive type， low temperature sensitive type， cold tolerant type， medium cold tolerant type and highly cold tolerant type according to cold tolerance base on principal component analysis and fuzzy evaluation method. In this study， the cold injury index of bitter gourd at seedling stage was statistically analyzed， the changes in physiological indices were detected， and the main agronomic traits of germplasms were measured， followed by correlation analysis was performed. The results showed that cold injury index， SOD activity， POD activity， CAT activity and PRO content could serve as the cold tolerance identification indices for bitter gourd at seedling stage， and flowering node and commodity rate could be used as the auxiliary indices for field cold tolerance identification. The introduction of main agronomic traits into seedling??stage cold tolerance identification could improve the accuracy and reliability of cold tolerance identification of bitter gourd. In order to construct an accurate reliable cold tolerance system， more rational indices should be introduced to the evaluation， and more bitter gourd germplasms of different genotypes are also needed， so as to improve the evaluation system under complicated environment and provide scientific basis for the breeding of bitter gourd for cold tolerance.
References
[1] BEHERA TK， BEHERA S， BHARATHI LK， et al. Bitter Gourd： Botany， Horticulture， Breeding[J]. Hort Rev， 2010， 37：101-141. [2] CHEN XF， HUANG RK， HUANG YH， et al. Advances in bitter gourd （Momordica charantia L.） breeding and related basic research[J]. Guangxi Agricultural Sciences， 2011， 42（3）： 246-249.
[3] NIU Y， LIU ZJ， LIU ZH， et al. Identification of cold tolerance in bitter gourd germplasm resources[J]. Northern Horticulture， 2017（24）： 1-6.
[4] SUN FL. Studies on chilling??resistance indices appraisal and selecting on bitter gourd varieties[D]. Wuhan： Huazhong Agricultural University， 2003.
[5] HUANG YH， LUO HL， WEI LP， et al. Physiological criteria for cold tolerance evaluation in balsam pear[J]. Guangxi Agricultural Sciences， 2008， 39（6）： 805-807.
[6] ZHANG HM， JIN HJ， DING XT， et al. The growth and physiological characteristics of inbred lines of balsam pear under low temperature and low light intensity[J]. Acta Agriculturae Shanghai， 2011， 27（3）： 21-25.
[7] CHEN SQ， WU ZM， ZENG J， et al. Effects of low temperature stress on seed germination and seedling physiological and biochemical characteristics of Momordica charantia[J]. Chinese Journal of Tropical Crops， 2011， 32（11）： 2099-210.
[8] GAO S， ZHONG KQ， ZU DX， et al. Analysis of tolerance to low temperature and weak light and determination of evaluation parameters of seedling in bitter gourd[J]. Chinese Journal of Tropical Crops， 2014， 35（11）： 2191-2198.
[9] CHEN XF， HUANG RK， LIANG JZ， et al. Identification and evaluation of bitter gourd for chilling??tolerance based on principal components analysis and fuzzy evaluation method[J]. Guangxi Agricultural Sciences， 2016， 47（5）： 677-681.
Key words Bitter gourd； Germplasm； Low temperature； Physiological index； Agronomic trait； Cold tolerance identification
Bitter gourd （Momordica charantia L.， 2n=2x=22） is an annual vine climbing plant in Momordica of Cucurbitaceae， also known as bitter melon[1]. It is probably to suffer from seasonal low??temperature damage during plantation in winter and spring， leading to problems including quality reduction， yield decrease and lagged marketing[2]. The breeding of cold??tolerant bitter gourd varieties through the identification of cold tolerance is considered to be the most effective way of reducing low??temperature damage[3]. The identification of cold tolerance in bitter gourd mostly adopts morphological index or physiological index[4-7]. However， comprehensively identifying the cold tolerance in germplasms through the comparative analysis of morphological indices， physiological indices and main agronomic traits is more accurate and reliable. Therefore， the cold injury index of bitter gourd at seedling stage was statistically analyzed， the changes in physiological indices were detected， and the main agronomic traits of germplasms were measured， followed by correlation analysis. The screening of more indices suitable for cold tolerance identification of bitter gourd is of great significance to the breeding of bitter gourd for cold tolerance.
Materials and Methods Experimental materials
There were six experimental materials， which were all homozygous inbred lines obtained by Cucurbits and Vegetables Laboratory， Tropical Crop Germplasm Research Institute through self??cross selective breeding： T1： 07??20 （Guangdong）， T2： 08??41（Guangdong）， T3： 11??11 （Fujian）， T4： 08??45 （Guangxi）， T5： 08??23 （Guangxi）， T6： T06 （Thailand）.
Experimental method
Determination of cold damage index and physiological indices at seedling stage The experiment was carried out in the solar greenhouse of the eight team of Cucurbits and Vegetables Laboratory， Tropical Crop Germplasm Research Institute. Full seeds of bitter gourd with uniform size were selected and subjected to pregermination. After the emergence of white buds， the seeds were sown into 50??hole trays filled with Peilei medium （Zhenjiang Peilei Organic Fertilizer Co.， Ltd.） for raising seedlings. At the stage of two leaves and one heart， uniform seedlings were selected and cultured in an illumination incubator for 2 d under the day/night temperature at 28 ??/18 ??， light period of 14 h and dark period of 10 h， and illumination intensity of 6 000 lx. The seedlings were then placed in an manual climatic box （Shanghai Yiheng Scientific Instrument Co.， Ltd.， MGC??450HP??2） for low temperature treatment at 6 ?? for 1 d， and during the period of treatment， the cold injury characteristics were investigated， and the cold injury index was investigated. The experiment had three replicates， each including 25 plants.
Determination of physiological indices： During the determination， 0.1 g of the second main leaf of bitter gourd was obtained and determined with the kit purchased by Suzhou Comin Biotechnology Co.， Ltd. for superoxide dismutase （SOD） activity， peroxidase （POD） activity， catalase （CAT） activity， malondialdehyde （MAD） content， hydrogen peroxide （H2O2） content and proline （PRO） content.
Cold injury index was calculated according to Cold injury index=??（Grade of cold injury??Number of plants of the grade）/4??Total number of investigated plants??100%， and the grading standards of cold tolerance at seedling stage are shown in Table 1.
Investigation of main agronomic traits of bitter gourd All the materials were subjected to pregermination on November 18， 2016. Field planting was performed on November 30. And harvesting was completed totally on March 14， 2017. Each plot had an area of 6.0 m??4.0 m=24.0 m2. The experiment had three replications in random arrangement. Each plot included 16 plants planted according to a plant spacing of 0.8 m. Flat frames were constructed to support vines. Field management was unified. Data processing and analysis Data analysis was performed in Excel， and analysis of significance and correlation coefficient was performed with SPSS software.
Results and Analysis
Comparative analysis of cold injury index and physiological indices at seedling stage
After treating at 6 ?? for 1 d， the different bitter gourd germplasms differed significantly in cold injury index （Table 2）. Among them， germplasms T1 and T2 had smaller cold injury index values， exhibiting stronger cold tolerance. T6 had the largest cold injury index of 84.38%， exhibiting weaker cold tolerance. Different bitter gourd germplasms differed in SOD activity， though the differences were not significant. T1 had the strongest SOD activity of 225.64 U/g FW， while T5 had the weakest activity of 219.18 U/g FW. The germplasms exhibiting cold tolerance overall had stronger SOD activity. Different bitter gourd germplasms had different POD activity. Among them， T1 and T2 had significant differences from T4， T5 and T6. The germplasms with stronger cold tolerance also had stronger POD activity. Germplasms with stronger cold tolerance had relatively higher CAT activity， though the differences were not significant. T2 had the strongest activity of 28.99 U/g FW， while T3 had the weakest activity of 26.36 U/g FW. There were no significant differences in H2O2 content between germplasms. Among them， T6 had the highest content of 2.48 mol/g， while T4 had the lowest content of 2.37 mol/g， and there were no obvious laws between germplasms. There were significant differences in PRO content between germplasms. Among them， T1 had the highest content of 51.16 mg/g， while T6 had the lowest content of 39.79 mg/g. The germplasms exhibiting stronger cold tolerance had higher PRO contents. There were significant differences in MDA content between germplasms. Among them， T4 had the highest content of 4.25 mol/g， while T3 had the lowest content of 3.53 mol/g.
Correlation analysis between cold injury index and physiological indices
Correlation analysis was performed between the cold injury index and physiological indices of bitter gourd under low temperature stress. The results showed that the cold injury index of bitter gourd germplasms were in very significant negative correlation with SOD activity， POD activity and PRO content of leaves （P
Comparative analysis on main agronomic traits of bitter gourd germplasms
Different bitter gourd germplasms differed in the first female flower node. Among them， T1 had the highest first female flower node， the 24 node averagely， while T6 had the lowest first female flower node， the 16.32 node averagely， and the difference was very significant. Different bitter gourd germplasms differed in female flower ratio. T6 had the largest female flower ratio of 45.35%， T3 had the lowest female flower ratio of 25.79%， and there were significant differences between T6 and any one of T1， T2， T3， T4 and T5. The single fruit weight differed between germplasms. T4 had the highest single fruit weight of 480.17 g， and T5 showed the lowest single fruit weight of 386.63 g. Different bitter gourd germplasms differed in the fruit set percentage. T1 showed the highest fruit set percentage of 92.38%， while T5 had the lowest fruit set percentage of 72.46%， and the difference was significant. The commodity rate was also different between the various bitter gourd germplasms. T1 had the highest commodity rate of 95.16%， while T5 showed the lowest commodity rate of 70.41%， and the difference was significant. Different bitter gourd germplasms had different yields. T4 had the highest yield of 26 361.75 kg/hm2， while T5 had the lowest yield of 17 118.60 kg/hm2， and the difference was very significant. As to fruit color， T1??T4 and T5 had green fruit， T2 and T3 showed light green fruit， and T6 showed white fruit. The fruit of T1， T3 and T4 was in the shape of a long rod， that of T2 was in the shape of a short rod， and T5 and T6 exhibited spindle??shaped fruit. Correlation analysis between cold injury index and main agronomic traits of bitter gourd
Correlation analysis was performed between the cold injury index and main agronomic traits of bitter gourd （Table 5）. The results showed that the cold injury index of bitter gourd was in very significant negative correlation with the flowering node and commodity rate， but in significant positive correlation with the female flower ratio. It indicates that flowering node， commodity rate and female flower ratio could serve as the auxiliary indices for the identification of cold tolerance. Generally， germplasms with high flowering node， low female flower ratio and late mature property have better cold tolerance.
Through the statistical analysis of cold injury index of bitter gourd at seedling stage， detection of SOD activity， POD activity， CAT activity， MDA content， H2O2 content and PRO content and measurement of main agronomic traits of germplasms， it could be concluded that the cold injury index was in very significant negative correlation with SOD activity， POD activity and PRO content （P
Discussion
The planting area of bitter gourd is enlarging gradually year by year， and it has become a main vegetable planted in winter in South China. Due to the lack of cold tolerance varieties， the identification of cold tolerance in bitter gourd appears to be particularly important. In recent years， scholars have identified cold tolerance in bitter gourd varieties and materials. Some physiological and biochemical indices were selected， but the changes of physiological and biochemical indices are different. Sun et al.[4] studied nine physiological and biochemical indices with six bitter gourd varieties （lines） as materials， and found that PRO content was in significant negative correlation with low temperature??resistant factors， and MDA content was in significant correlation with low temperature??resistant factors. Huang et al.[5] determined the changes in POD activity， SOD activity and MDA content of bitter gourd at seedling stage under different temperature conditions， and the results showed that the increase of MDA was in significant positive correlation with cold injury index， and POD activity， SOD activity and soluble sugar content were in significant negative correlation with cold injury index. Zhang et al.[6] studied the changes in physiological and biochemical indices of eight bitter gourd inbred lines under low temperature and weak light， and the results showed that the SOD activity and CAT activity increased， POD activity decreased， and MDA and PRO content increased. Chen et al.[7] studied the change laws of POD activity， SOD activity， CAT activity and PRO content under low temperature stress with two bitter gourd inbred lines as materials， and the results showed that under low temperature stress， SOD and POD activity and PRO content increased， and CAT activity decreased. In this study， it was found that the cold injury index of bitter gourd was in very significant negative correlation with SOD activity， POD activity and PRO content， and in significant negative correlation with CAT activity. It might be because of the differences in tested bitter gourd varieties （lines）， during the determination of physiological and biochemical indices， the differences in temperature and time cause different physiological changes. In this study， it was concluded that the SOD activity， POD activity， CAT activity and PRO content could serve as the physiological and biochemical indices for bitter gourd cold tolerance identification at seedling stage， which accords with above research. During the identification of cold tolerance in bitter gourd， single morphological index and physiological index would reduce the accuracy and reliability of identification due to the instability of the index， and scholars have discussed the comprehensive evaluation of cold tolerance index. Gao et al.[8] studied the effects of low temperature and weak light on physiological and biochemical indices of bitter gourd， and the tolerance to low temperature and weak light in bitter gourd was comprehensively evaluated by principal factor analysis， hierarchical cluster analysis and subordinate function method. The results showed that there were significant differences in 12 physiological and biochemical indices between 23 tested bitter gourd inbred lines. The 23 lines of different genotypes were divided into four groups， two classes， according to the tolerance to low temperature and weak light by clustering analysis and subordinate function method， and six cold tolerance identification indices were screened. Lu et al.[9] divided 17 tested bitter gourd germplasm resources into highly low temperature sensitive type， medium low temperature sensitive type， low temperature sensitive type， cold tolerant type， medium cold tolerant type and highly cold tolerant type according to cold tolerance base on principal component analysis and fuzzy evaluation method. In this study， the cold injury index of bitter gourd at seedling stage was statistically analyzed， the changes in physiological indices were detected， and the main agronomic traits of germplasms were measured， followed by correlation analysis was performed. The results showed that cold injury index， SOD activity， POD activity， CAT activity and PRO content could serve as the cold tolerance identification indices for bitter gourd at seedling stage， and flowering node and commodity rate could be used as the auxiliary indices for field cold tolerance identification. The introduction of main agronomic traits into seedling??stage cold tolerance identification could improve the accuracy and reliability of cold tolerance identification of bitter gourd. In order to construct an accurate reliable cold tolerance system， more rational indices should be introduced to the evaluation， and more bitter gourd germplasms of different genotypes are also needed， so as to improve the evaluation system under complicated environment and provide scientific basis for the breeding of bitter gourd for cold tolerance.
References
[1] BEHERA TK， BEHERA S， BHARATHI LK， et al. Bitter Gourd： Botany， Horticulture， Breeding[J]. Hort Rev， 2010， 37：101-141. [2] CHEN XF， HUANG RK， HUANG YH， et al. Advances in bitter gourd （Momordica charantia L.） breeding and related basic research[J]. Guangxi Agricultural Sciences， 2011， 42（3）： 246-249.
[3] NIU Y， LIU ZJ， LIU ZH， et al. Identification of cold tolerance in bitter gourd germplasm resources[J]. Northern Horticulture， 2017（24）： 1-6.
[4] SUN FL. Studies on chilling??resistance indices appraisal and selecting on bitter gourd varieties[D]. Wuhan： Huazhong Agricultural University， 2003.
[5] HUANG YH， LUO HL， WEI LP， et al. Physiological criteria for cold tolerance evaluation in balsam pear[J]. Guangxi Agricultural Sciences， 2008， 39（6）： 805-807.
[6] ZHANG HM， JIN HJ， DING XT， et al. The growth and physiological characteristics of inbred lines of balsam pear under low temperature and low light intensity[J]. Acta Agriculturae Shanghai， 2011， 27（3）： 21-25.
[7] CHEN SQ， WU ZM， ZENG J， et al. Effects of low temperature stress on seed germination and seedling physiological and biochemical characteristics of Momordica charantia[J]. Chinese Journal of Tropical Crops， 2011， 32（11）： 2099-210.
[8] GAO S， ZHONG KQ， ZU DX， et al. Analysis of tolerance to low temperature and weak light and determination of evaluation parameters of seedling in bitter gourd[J]. Chinese Journal of Tropical Crops， 2014， 35（11）： 2191-2198.
[9] CHEN XF， HUANG RK， LIANG JZ， et al. Identification and evaluation of bitter gourd for chilling??tolerance based on principal components analysis and fuzzy evaluation method[J]. Guangxi Agricultural Sciences， 2016， 47（5）： 677-681.