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Abstract With 16 Yunnan tea tree varieties and 5 Kenya tea tree varieties as test materials， the differences in biochemical components between Yunnan and Kenya tea tree varieties were compared and analyzed. The results showed that the coefficients of variation of tea polyphenols， amino acids， caffeine， water extract， gallic acid （GA）， catechin （C）， epicatechin （EC）， epicatechin gallate （ECG）， epigallocatechin （EGC）， epigallocatechin gallate （EGCG） and total catechins in Yunnan tea tree varieties were greater than those in Kenyan tea trees. The contents of tea polyphenols， amino acids， caffeine， water extract， C， EC， EGC， EGCG and total catechins in Yunnan tea tree varieties had no significant differences from those in Kenyan tea trees varieties （P>0.05）， while there were significant differences in the contents of GA and ECG between Yunnan tea tree varieties and Kenya tea tree varieties （P<0.05）. Therefore， it could be predicted that GA and ECG might be one of the main characteristics of the differences in biochemical components between Yunnan tea tree varieties and Kenyan tea tree varieties. The cluster analysis results showed that when the genetic distance was 15， the 21 tested tea varieties could be divided into three groups with obvious biochemical differences.
Key words Yunnan; Kenya; Tea varieties; Biochemical components; Cluster analysis; Genetic distance
Tea varieties are one of the most important means of production in tea production， and they are closely related to tea yield， quality and tea tree resistance[1-2]. The quality of tea is determined by the types and contents of biochemical components of tea varieties. The differences in the content of biochemical components of tea varieties affect the formation of tea quality[3]. Biochemical composition analysis has always been an important research content in tea breeding. Scholars in China have analyzed and studied the diversity of biochemical components of tea varieties resources in different tea areas including Yunnan[4]， Sichuan[5]， Guangxi[6]， Fujian[7]， Guizhou[8] and Zhejiang[9]. Due to the low collection and preservation of foreign tea tree varieties resources in China， there are few reports on the analysis of the differences in biochemical components of tea varieties at home and abroad， most of which are focused on the sensory evaluation of domestic and foreign tea products and comparative studies on quality components.
Yunnan is center of origin of tea trees. The unique climatic conditions have bred abundant tea tree resources[14]. Yunnan big-leaf species is an excellent variety for processing broken black tea. The broken black tea processed with fresh leaves of Yunnan big-leaf species has the characteristics of tight heavy particles， bright red soup， prominent golden circle， rich aroma and rich flavor and strong freshness[15]. Kenya is an important producer and exporter of broken black tea in the world， and the produced broken black tea has excellent quality reflected by red and bright soup with strong flavor and long-lasting aroma， thus having strong competitiveness in the international market[16]. In order to understand the differences in biochemical composition between Yunnan tea tree varieties and Kenyan tea tree varieties， we compared and analyzed the biochemical components of 16 Yunnan tea tree varieties and 5 Kenyan tea tree varieties stored in the national germplasm large-leaf tea tree resource nursery， and discussed the differences in biochemical composition between Yunnan tea tree varieties and Kenyan tea tree varieties， providing a theoretical basis for promoting the improvement of Yunnan black tea varieties and the selection of high-quality black tea varieties.
Materials and Methods
Materials
16 Yunnan tea tree varieties （planted in 1996） and 5 introduced Kenyan varieties （planted in 2003） were stored in the "China National Germplasm Big-leaf Tea Trees Repository" of Tea Research Institute of Yunnan Academy of Agricultural Sciences were selected， as shown in Table 1.
Preparation of samples for biochemical analysis
Referring to Chen et al.[17]， in April 2018， about 100 g of fresh leaves with 1 bud and 2 leaves were picked from new shoots. The fresh leaves were steamed in a steamer for 2 min after the water was boiled. The leaves were then taken out and blown for 5 min to remove surface moisture， and then dried in an oven at 90 ℃ for biochemical analysis.
Analysis of biochemical components
The content of tea polyphenols was determined according to GB/T8313-2008 Determination of Total Polyphenols and Catechins Content in Tea. The content of amino acids was determined according to GB/T8314-2013 Tea-Determination of Free Amino Acids Content. The content of water extract was determined referring GB/T8305-2013 Tea―Determination of Water Extracts Content. The contents of catechins[catechin （C）， epicatechin （EC）， epicatechin gallate （ECG）， epigallocatechin （EGC）， epigallocatechin gallate （EGCG）]and caffeine were determined by high performance liquid chromatography （HLPC method）. The determination of the contents of biochemical components was repeated 3 times. Statistical analysis of data
Statistical analysis of the average， maximum， minimum， standard deviation and coefficient of variation of the biochemical components in fresh leaves of tea varieties from Yunnan and Kenya was performed using Excel. SPSS22 software was used for cluster analysis of the test data by Wards method under the genetic distance of Euclidean distance. An independent-sample t test was used to analyze the differences in biochemical components between Yunnan tea varieties and Kenyan tea varieties.
Results and Analysis
Analysis of differences in contents of tea polyphenols， amino acids， caffeine and water extract
It could be seen from Table 2 and Table 3 that the tea polyphenol contents of 16 Yunnan tea tree varieties were in the range of 22.38%-25.80%， with an average value of 24.25%. Among them， Yunkang No.48 was the highest， reaching 25.80%， and No.73-11 was the lowest， being 22.38%. The amino acid contents were 0.77%-2.03%， with an average value of 1.36%. Among them， Aifeng was the highest， reaching 2.03%， and Yunkang No.50 was the lowest， 0.77%. The caffeine contents were in the range of 2.68%-4.68%， with an average value of 3.69%. Among them， Yunkang No.37 had the highest value of 4.68%， and Yunkang No.50 had the lowest value of 2.68%. The contents of water extract were in the range of 42.38%-51.67%， with an average value of 47.10%. Yunkang No.37 had the highest value， reaching 51.67%， and Foxiang No.2 was the lowest at 42.38%. The tea polyphenol contents of the 5 Kenyan tea tree varieties were in the range of 23.07%-25.09%， with an average value of 24.35%. TRFK303/231 was the highest， reaching 25.09%， and TRFK301/1 was the lowest， being 23.07%. The amino acid contents ranged from 1.53% to 2.78%， with an average value of 2.16%. TRFK6/8 was the highest， reaching 2.78%， and TRFK303/231 was the lowest， being 1.53%. The contents of caffeine were in the range of 3.85%-4.39%， with an average value of 4.07%. TRFK301/3 showed the highest value， reaching 4.39%， and TRFK6/8 was the lowest， 3.85%. The contents of water extract ranged from 48.67% to 52.89%， with an average value of 50.66%. TRFK6/8 was the highest at 52.89%， and TRFK303/231 was the lowest at 48.67%.
It could be seen from Table 3 that the coefficients of variation of tea polyphenols， amino acids， caffeine， and water extracts of Yunnan tea tree varieties were 4.25%， 27.21%， 14.09% and 5.24%， respectively. The coefficients of variation of tea polyphenols， amino acids， caffeine and water extracts of Kenya tea tree varieties were 3.24%， 20.83%， 5.41% and 3.40%， respectively. However， the results of statistical analysis showed that there were no significant differences in the contents of tea polyphenols， amino acids， caffeine and water extract between Yunnan tea varieties and Kenyan tea varieties. The ratio of tea ployphenols to amino acids can be used as a biochemical index for the processing property of tea varieties[18]. It is generally believed that the material with tea polyphenol-to-amino acid ratio less than 8 is suitable for making green tea， that with a value between 8 and 15 is suitable for making both black tea and green tea， and a value greater than 15 means that the material is suitable for making black tea[18]. The tea polyphenol-to-amino acid ratios of Yunnan tea tree varieties ranged from 11.15 to 33.17， with an average value of 19.37. The tea polyphenol-to-amino acid ratios of Kenyan tea tree varieties were in the range of 8.75-16.40， with an average value of 11.71. From the perspective of tea polyphenol-to-amino acid ratio， the average tea polyphenol-to-amino acid ratio of Yunnan tea varieties was significantly higher than that of Kenya tea varieties. Analysis on the differences in catechins and gallic acid
It could be seen from Table 2 and Table 3 that the total amounts of catechins in Yunnan tea varieties were in the range of 135.04-206.96 mg/g， with an average content of 165.00 mg/g. Yunkang No.50 was the highest， reaching 206.96 mg/g， Yunkang 48 was the lowest at 135.04 mg/g. The total amounts of catechins in Kenyan tea tree varieties ranged from 163.03 to 183.86 mg/g， with an average content of 174.79 mg/g. TRFK301/1 was the highest， reaching 183.86 mg/g， and TRKF301/3 was the lowest， at 163.03 mg/g. The contents of gallic acid （GA） in Yunnan tea varieties ranged from 4.71 to 19.02 mg/g， with an average content of 10.44 mg/g; and the contents of GA in Kenya tea varieties were in the range of 6.34-8.78 mg/g， with an average content of 6.90 mg/g. Among the Yunnan tea varieties， Zhongxiang No.1， No.73-8， Yungui， Aifeng， Foxiang No.2 and Foxiang No.3 had GA contents close to Kenyan tea tree varieties， and the rest had significantly higher GA contents than Kenyan tea tree varieties.
Statistical analysis results （Table 3） showed that there were no significant differences in C， EGC， EC， EGCG contents and total catechin content between Yunnan tea varieties and Kenyan tea varieties （P>0.05）， while there were significant differences in GA and ECG contents （P<0.05）.
Cluster analysis of biochemical components
With the contents of tea polyphenols， amino acids， caffeine， water extract， total catechins， C， EC， ECG， EGC， EGCG and GA as variables， the biochemical components of Yunnan and Kenya tea tree varieties were clustered into groups， and the results of cluster analysis are shown in Fig. 1.
It could be seen from Fig. 1 that at the genetic distance of 15， the 21 tested tea varieties were divided into 3 groups with obvious differences in biochemical composition. The first group consisted of Aifeng， TRFK301/5， TRFK301/3， Yungui， Yunkang No.43， Foxiang No.1， TRFK301/1， TRFK303/231， TRFK6/8 and No.73-8. The second group comprised Foxiang No.2， Foxiang No.3， Yunkang No.27， Yunkang No.48， Changye Baihao， Yunkang No.10 and Yunkang No.37. The third group included No.73-11， No.76-38， Yunkang No.14 and Yunkang No.50.
Discussion and Conclusions
Tea polyphenols， amino acids， caffeine， and water extract are important material foundations for the formation of tea color， aroma， and taste， and especially the content of tea polyphenols has an important impact on the quality of black tea[19]. This study showed that the coefficients of variation of tea polyphenols， amino acids， caffeine， and water extract of Yunnan tea tree varieties were all greater than Kenyan tea tree varieties， indicating that Yunnan tea tree varieties are rich in biochemical components and the differences between the varieties are large， which is in line with the goal of tea tree variety breeding in Yunnan Province in the past 20 years for diversified black tea， green tea and Puer tea varieties. However， to a certain extent， it restricts the promotion and application of black tea varieties with strong specificity and high quality. The selection of Kenyan tea tree varieties is based on the processing of high-quality black tea. The variation coefficients of tea polyphenols， amino acids， caffeine and water extract of Kenyan tea tree varieties were small， the differences were not obvious， and the quality of fresh leaves was relatively stable. All these lay a material foundation for the processing of high-quality black tea. Therefore， when processing black tea， choosing a relatively suitable black tea specific variety is beneficial to further enhance the overall quality of Yunnan black tea. The catechins in the fresh tea polyphenols are important chemical components for the formation of broken black tea[20]. The analysis results showed that the average content of total catechins in Yunnan and Kenya tea varieties were 165.00 and 174.79 mg/g， respectively， and the coefficients of variation were 11.07% and 4.85%， respectively. The coefficients of variation indicated that the total amount of catechins was relatively stable in Yunnan and Kenya tea varieties. Studies by Liang et al.[21] have shown that the high content of catechins EGC and EC can provide sufficient precursors for the synthesis of theaflavins （TF） in the fermentation process， and suggests high TF synthesis and quality potential of broken black tea. The analysis results showed that the total contents of EGC and EC in Yunnan tea varieties ranged from 5.86 to 72.05 mg/g， with an average content of 30.31 mg/g， and the sums of EGC and EC contents in Kenya tea varieties ranged from 35.64 to 52.66 mg/g， with an average content of 42.54 mg/g. It could be seen intuitively from Fig. 2 that Yunnan tea tree varieties showing the sum of EGC and EC contents close to or exceeds Kenya tea tree varieties included No.73-8， Yungui， Aifeng， Foxiang No.1 and Yunkang No.43. Therefore， they can be initially used as the selection target of high-flavin black tea varieties.
Gallic acid （GA） is relatively high in Yunnan Puer tea， which is a simple phenolic compound and a characteristic trait of Puer tea with physiological activity[21]. This study showed that the gallic acid contents of Yunnan tea varieties were in the range of 4.71-19.02 mg/g， with a variation coefficient of 39.27%， so the gallic acid content varied greatly; and the gallic acid contents of Kenyan tea tree varieties were in the range of 6.34-8.78 mg/g， with a variation coefficient of 15.22%， so the gallic acid content was relatively stable. The content of gallic acid in Yunnan tea varieties was significantly higher than that in Kenya tea varieties overall， which is one of the main characteristics of the differences in biochemical composition between Yunnan tea varieties and Kenya varieties.
Through cluster analysis of biochemical components， the 21 tea varieties were divided into 3 groups with differences in biochemical components. Most varieties with the same geographical location and close kinship tend to be grouped together， for example， five varieties in Kenya， TRFK301/1， TRFK303/231， TRFK301/5， TRFK301/3 and TRFK6/8， were clustered together; and Yunnan tea varieties， Aifeng， Yungui， Yunkang No.43， Foxiang No.1 and No.73-8 were clustered with five Kenyan tea tree varieties into the same group， indicating that their main biochemical components are similar to Kenyan tea tree varieties， and they have the potential to be processed into high-quality broken black tea. References
[1] CHEN WH. Tea varieties and tea quality[J]. China Tea， 1984（1）： 26-29. （in Chinese）
[2] LIANG YR， SHI M. Advances in genetics and breeding of tea plant in 2016[J]. Journal of Tea Science， 2015， 35（2）： 103-109. （in Chinese）
[3] LU JS， WEI FH， LI CH. A study on the relationship between main chemical components and quality of tea varieties[J]. Southwest China Journal of Agricultural Sciences， 1994， 7（1）： 1-4. （in Chinese）
[4] DU YK， QIN TF， WANG XH， et al. Diversity analysis of biochemical components of 35 tea germplasms in Yunnan[J]. Journal of Southwest University： Natural Science， 2013， 35（10）： 36-41. （in Chinese）
[5] ZHU MZ， QI GN， LI JH， et al. The main biochemical composition analysis of the Summer and Autumn leaves of 60 tea germplasms[J]. Journal of Yunnan University： Natural Sciences Edition， 2016， 38（3）： 494-500. （in Chinese）
[6] WANG XC， CHEN L， YANG YJ. Biochemical diversity analysis of tea germplasms in Guangxi[J]. Journal of Plant Genetic Resources， 2010， 11（3）： 309-314， 319. （in Chinese）
[7] WANG FQ， HUANG LC， LUO SC， et al. Analysis of biochemical composition diversity of 46 tea germplasm resources in summer tea[J]. Jiangsu Agricultural Sciences， 2017， 45（18）： 105-110. （in Chinese）
[8] ZHAO HF， GAO XB， LIU XX， et al. Analysis and evaluation of polyphenols quality of high polyphenols content tea varieties in Guizhou[J]. Chinese Agricultural Science Bulletin， 2016， 32（16）： 149-154. （in Chinese）
[9] HUANG HT， ZHANG W， AO C， et al. Diversity analysis of biochemical compositions in local tea germplasm resources and screening of excellent tea varieties in Hangzhou[J]. Acta Agriculturae Jiangxi， 2019， 31（1）： 21-25. （in Chinese）
[10] PAN B. Comparison of similarities and differences of sensory evaluation of the worlds major tea-producing countries[J]. China Tea， 2019， 41（2）： 38-42.
[11] LI Z， LIU ZQ， LIU ZY， et al. Aroma characteristics of the broken black tea abroad[J]. Journal of Anhui Agricultural University， 2015， 42（5）： 692-699. （in Chinese）
[12] HUANG JQ. Comparative study on the effects of different processing methods on quality components of black tea from Qihong， Kenya and Sri Lanka[J]. China Tea Processing， 2003（1）： 16-18. （in Chinese）
[13] LIANG YR， LA MS， XU YR， et al. A study on chemical compositions of Guangxi and Kenyan broken black teas[J]. Journal of Tea， 1998（3）： 131-133. （in Chinese）
[14] YU FL. On the origin and origin center of tea tree[J]. Journal of Tea Science， 1986， 6（1）： 1-8. [15] ZHANG YB， LIU ZH， RAO TT， et al. Analysis of the quality characteristics color， aroma and taste of Yunnan broken black tea[J]. Food & Machinery， 2019， 35（8）： 37-43. （in Chinese）
[16] MAMATI GE， LIANG YR. Tea （Camellia sinensis）： Comparison of the crop in Kenya and China[J]. Journal of Tea， 2004， 30（4）： 197-202. （in Chinese）
[17] CHEN L， YANG YJ， YU FL， et al. Descriptors for tea germplasm resources and data standards[M]. Beijing： China Agriculture Press， 2005. （in Chinese）
[18] ZHANG ZC. Some opinions on quality index and ployphenol-to-amino acid ratio of early identification in tea plants[J]. Journal of Tea Communication， 1991（3）： 22-25. （in Chinese）
[19] YANG YJ. Chemical evaluation on tea quality during early-stage of breeding program——I. Relationship between the biochemical component content in shoot and the quality of black tea[J]. Journal of Tea Science， 1990， 10（2）： 59-63. （in Chinese）
[20] LIANG YR， LIU ZR. Study on the relationship between the chemical composition and the quality of broken black tea of different tea varieties[J]. Journal of Zhejiang Agricultural University， 1994， 20（2）： 149-154. （in Chinese）
[21] ZHE GM， ZHANG XL， CHEN KK， et al. Content variation of theanine and gallic acid in Puer tea [J]. Acta Botanica Yunnanica， 2005， 27（5）： 572-576. （in Chinese）
Key words Yunnan; Kenya; Tea varieties; Biochemical components; Cluster analysis; Genetic distance
Tea varieties are one of the most important means of production in tea production， and they are closely related to tea yield， quality and tea tree resistance[1-2]. The quality of tea is determined by the types and contents of biochemical components of tea varieties. The differences in the content of biochemical components of tea varieties affect the formation of tea quality[3]. Biochemical composition analysis has always been an important research content in tea breeding. Scholars in China have analyzed and studied the diversity of biochemical components of tea varieties resources in different tea areas including Yunnan[4]， Sichuan[5]， Guangxi[6]， Fujian[7]， Guizhou[8] and Zhejiang[9]. Due to the low collection and preservation of foreign tea tree varieties resources in China， there are few reports on the analysis of the differences in biochemical components of tea varieties at home and abroad， most of which are focused on the sensory evaluation of domestic and foreign tea products and comparative studies on quality components.
Yunnan is center of origin of tea trees. The unique climatic conditions have bred abundant tea tree resources[14]. Yunnan big-leaf species is an excellent variety for processing broken black tea. The broken black tea processed with fresh leaves of Yunnan big-leaf species has the characteristics of tight heavy particles， bright red soup， prominent golden circle， rich aroma and rich flavor and strong freshness[15]. Kenya is an important producer and exporter of broken black tea in the world， and the produced broken black tea has excellent quality reflected by red and bright soup with strong flavor and long-lasting aroma， thus having strong competitiveness in the international market[16]. In order to understand the differences in biochemical composition between Yunnan tea tree varieties and Kenyan tea tree varieties， we compared and analyzed the biochemical components of 16 Yunnan tea tree varieties and 5 Kenyan tea tree varieties stored in the national germplasm large-leaf tea tree resource nursery， and discussed the differences in biochemical composition between Yunnan tea tree varieties and Kenyan tea tree varieties， providing a theoretical basis for promoting the improvement of Yunnan black tea varieties and the selection of high-quality black tea varieties.
Materials and Methods
Materials
16 Yunnan tea tree varieties （planted in 1996） and 5 introduced Kenyan varieties （planted in 2003） were stored in the "China National Germplasm Big-leaf Tea Trees Repository" of Tea Research Institute of Yunnan Academy of Agricultural Sciences were selected， as shown in Table 1.
Preparation of samples for biochemical analysis
Referring to Chen et al.[17]， in April 2018， about 100 g of fresh leaves with 1 bud and 2 leaves were picked from new shoots. The fresh leaves were steamed in a steamer for 2 min after the water was boiled. The leaves were then taken out and blown for 5 min to remove surface moisture， and then dried in an oven at 90 ℃ for biochemical analysis.
Analysis of biochemical components
The content of tea polyphenols was determined according to GB/T8313-2008 Determination of Total Polyphenols and Catechins Content in Tea. The content of amino acids was determined according to GB/T8314-2013 Tea-Determination of Free Amino Acids Content. The content of water extract was determined referring GB/T8305-2013 Tea―Determination of Water Extracts Content. The contents of catechins[catechin （C）， epicatechin （EC）， epicatechin gallate （ECG）， epigallocatechin （EGC）， epigallocatechin gallate （EGCG）]and caffeine were determined by high performance liquid chromatography （HLPC method）. The determination of the contents of biochemical components was repeated 3 times. Statistical analysis of data
Statistical analysis of the average， maximum， minimum， standard deviation and coefficient of variation of the biochemical components in fresh leaves of tea varieties from Yunnan and Kenya was performed using Excel. SPSS22 software was used for cluster analysis of the test data by Wards method under the genetic distance of Euclidean distance. An independent-sample t test was used to analyze the differences in biochemical components between Yunnan tea varieties and Kenyan tea varieties.
Results and Analysis
Analysis of differences in contents of tea polyphenols， amino acids， caffeine and water extract
It could be seen from Table 2 and Table 3 that the tea polyphenol contents of 16 Yunnan tea tree varieties were in the range of 22.38%-25.80%， with an average value of 24.25%. Among them， Yunkang No.48 was the highest， reaching 25.80%， and No.73-11 was the lowest， being 22.38%. The amino acid contents were 0.77%-2.03%， with an average value of 1.36%. Among them， Aifeng was the highest， reaching 2.03%， and Yunkang No.50 was the lowest， 0.77%. The caffeine contents were in the range of 2.68%-4.68%， with an average value of 3.69%. Among them， Yunkang No.37 had the highest value of 4.68%， and Yunkang No.50 had the lowest value of 2.68%. The contents of water extract were in the range of 42.38%-51.67%， with an average value of 47.10%. Yunkang No.37 had the highest value， reaching 51.67%， and Foxiang No.2 was the lowest at 42.38%. The tea polyphenol contents of the 5 Kenyan tea tree varieties were in the range of 23.07%-25.09%， with an average value of 24.35%. TRFK303/231 was the highest， reaching 25.09%， and TRFK301/1 was the lowest， being 23.07%. The amino acid contents ranged from 1.53% to 2.78%， with an average value of 2.16%. TRFK6/8 was the highest， reaching 2.78%， and TRFK303/231 was the lowest， being 1.53%. The contents of caffeine were in the range of 3.85%-4.39%， with an average value of 4.07%. TRFK301/3 showed the highest value， reaching 4.39%， and TRFK6/8 was the lowest， 3.85%. The contents of water extract ranged from 48.67% to 52.89%， with an average value of 50.66%. TRFK6/8 was the highest at 52.89%， and TRFK303/231 was the lowest at 48.67%.
It could be seen from Table 3 that the coefficients of variation of tea polyphenols， amino acids， caffeine， and water extracts of Yunnan tea tree varieties were 4.25%， 27.21%， 14.09% and 5.24%， respectively. The coefficients of variation of tea polyphenols， amino acids， caffeine and water extracts of Kenya tea tree varieties were 3.24%， 20.83%， 5.41% and 3.40%， respectively. However， the results of statistical analysis showed that there were no significant differences in the contents of tea polyphenols， amino acids， caffeine and water extract between Yunnan tea varieties and Kenyan tea varieties. The ratio of tea ployphenols to amino acids can be used as a biochemical index for the processing property of tea varieties[18]. It is generally believed that the material with tea polyphenol-to-amino acid ratio less than 8 is suitable for making green tea， that with a value between 8 and 15 is suitable for making both black tea and green tea， and a value greater than 15 means that the material is suitable for making black tea[18]. The tea polyphenol-to-amino acid ratios of Yunnan tea tree varieties ranged from 11.15 to 33.17， with an average value of 19.37. The tea polyphenol-to-amino acid ratios of Kenyan tea tree varieties were in the range of 8.75-16.40， with an average value of 11.71. From the perspective of tea polyphenol-to-amino acid ratio， the average tea polyphenol-to-amino acid ratio of Yunnan tea varieties was significantly higher than that of Kenya tea varieties. Analysis on the differences in catechins and gallic acid
It could be seen from Table 2 and Table 3 that the total amounts of catechins in Yunnan tea varieties were in the range of 135.04-206.96 mg/g， with an average content of 165.00 mg/g. Yunkang No.50 was the highest， reaching 206.96 mg/g， Yunkang 48 was the lowest at 135.04 mg/g. The total amounts of catechins in Kenyan tea tree varieties ranged from 163.03 to 183.86 mg/g， with an average content of 174.79 mg/g. TRFK301/1 was the highest， reaching 183.86 mg/g， and TRKF301/3 was the lowest， at 163.03 mg/g. The contents of gallic acid （GA） in Yunnan tea varieties ranged from 4.71 to 19.02 mg/g， with an average content of 10.44 mg/g; and the contents of GA in Kenya tea varieties were in the range of 6.34-8.78 mg/g， with an average content of 6.90 mg/g. Among the Yunnan tea varieties， Zhongxiang No.1， No.73-8， Yungui， Aifeng， Foxiang No.2 and Foxiang No.3 had GA contents close to Kenyan tea tree varieties， and the rest had significantly higher GA contents than Kenyan tea tree varieties.
Statistical analysis results （Table 3） showed that there were no significant differences in C， EGC， EC， EGCG contents and total catechin content between Yunnan tea varieties and Kenyan tea varieties （P>0.05）， while there were significant differences in GA and ECG contents （P<0.05）.
Cluster analysis of biochemical components
With the contents of tea polyphenols， amino acids， caffeine， water extract， total catechins， C， EC， ECG， EGC， EGCG and GA as variables， the biochemical components of Yunnan and Kenya tea tree varieties were clustered into groups， and the results of cluster analysis are shown in Fig. 1.
It could be seen from Fig. 1 that at the genetic distance of 15， the 21 tested tea varieties were divided into 3 groups with obvious differences in biochemical composition. The first group consisted of Aifeng， TRFK301/5， TRFK301/3， Yungui， Yunkang No.43， Foxiang No.1， TRFK301/1， TRFK303/231， TRFK6/8 and No.73-8. The second group comprised Foxiang No.2， Foxiang No.3， Yunkang No.27， Yunkang No.48， Changye Baihao， Yunkang No.10 and Yunkang No.37. The third group included No.73-11， No.76-38， Yunkang No.14 and Yunkang No.50.
Discussion and Conclusions
Tea polyphenols， amino acids， caffeine， and water extract are important material foundations for the formation of tea color， aroma， and taste， and especially the content of tea polyphenols has an important impact on the quality of black tea[19]. This study showed that the coefficients of variation of tea polyphenols， amino acids， caffeine， and water extract of Yunnan tea tree varieties were all greater than Kenyan tea tree varieties， indicating that Yunnan tea tree varieties are rich in biochemical components and the differences between the varieties are large， which is in line with the goal of tea tree variety breeding in Yunnan Province in the past 20 years for diversified black tea， green tea and Puer tea varieties. However， to a certain extent， it restricts the promotion and application of black tea varieties with strong specificity and high quality. The selection of Kenyan tea tree varieties is based on the processing of high-quality black tea. The variation coefficients of tea polyphenols， amino acids， caffeine and water extract of Kenyan tea tree varieties were small， the differences were not obvious， and the quality of fresh leaves was relatively stable. All these lay a material foundation for the processing of high-quality black tea. Therefore， when processing black tea， choosing a relatively suitable black tea specific variety is beneficial to further enhance the overall quality of Yunnan black tea. The catechins in the fresh tea polyphenols are important chemical components for the formation of broken black tea[20]. The analysis results showed that the average content of total catechins in Yunnan and Kenya tea varieties were 165.00 and 174.79 mg/g， respectively， and the coefficients of variation were 11.07% and 4.85%， respectively. The coefficients of variation indicated that the total amount of catechins was relatively stable in Yunnan and Kenya tea varieties. Studies by Liang et al.[21] have shown that the high content of catechins EGC and EC can provide sufficient precursors for the synthesis of theaflavins （TF） in the fermentation process， and suggests high TF synthesis and quality potential of broken black tea. The analysis results showed that the total contents of EGC and EC in Yunnan tea varieties ranged from 5.86 to 72.05 mg/g， with an average content of 30.31 mg/g， and the sums of EGC and EC contents in Kenya tea varieties ranged from 35.64 to 52.66 mg/g， with an average content of 42.54 mg/g. It could be seen intuitively from Fig. 2 that Yunnan tea tree varieties showing the sum of EGC and EC contents close to or exceeds Kenya tea tree varieties included No.73-8， Yungui， Aifeng， Foxiang No.1 and Yunkang No.43. Therefore， they can be initially used as the selection target of high-flavin black tea varieties.
Gallic acid （GA） is relatively high in Yunnan Puer tea， which is a simple phenolic compound and a characteristic trait of Puer tea with physiological activity[21]. This study showed that the gallic acid contents of Yunnan tea varieties were in the range of 4.71-19.02 mg/g， with a variation coefficient of 39.27%， so the gallic acid content varied greatly; and the gallic acid contents of Kenyan tea tree varieties were in the range of 6.34-8.78 mg/g， with a variation coefficient of 15.22%， so the gallic acid content was relatively stable. The content of gallic acid in Yunnan tea varieties was significantly higher than that in Kenya tea varieties overall， which is one of the main characteristics of the differences in biochemical composition between Yunnan tea varieties and Kenya varieties.
Through cluster analysis of biochemical components， the 21 tea varieties were divided into 3 groups with differences in biochemical components. Most varieties with the same geographical location and close kinship tend to be grouped together， for example， five varieties in Kenya， TRFK301/1， TRFK303/231， TRFK301/5， TRFK301/3 and TRFK6/8， were clustered together; and Yunnan tea varieties， Aifeng， Yungui， Yunkang No.43， Foxiang No.1 and No.73-8 were clustered with five Kenyan tea tree varieties into the same group， indicating that their main biochemical components are similar to Kenyan tea tree varieties， and they have the potential to be processed into high-quality broken black tea. References
[1] CHEN WH. Tea varieties and tea quality[J]. China Tea， 1984（1）： 26-29. （in Chinese）
[2] LIANG YR， SHI M. Advances in genetics and breeding of tea plant in 2016[J]. Journal of Tea Science， 2015， 35（2）： 103-109. （in Chinese）
[3] LU JS， WEI FH， LI CH. A study on the relationship between main chemical components and quality of tea varieties[J]. Southwest China Journal of Agricultural Sciences， 1994， 7（1）： 1-4. （in Chinese）
[4] DU YK， QIN TF， WANG XH， et al. Diversity analysis of biochemical components of 35 tea germplasms in Yunnan[J]. Journal of Southwest University： Natural Science， 2013， 35（10）： 36-41. （in Chinese）
[5] ZHU MZ， QI GN， LI JH， et al. The main biochemical composition analysis of the Summer and Autumn leaves of 60 tea germplasms[J]. Journal of Yunnan University： Natural Sciences Edition， 2016， 38（3）： 494-500. （in Chinese）
[6] WANG XC， CHEN L， YANG YJ. Biochemical diversity analysis of tea germplasms in Guangxi[J]. Journal of Plant Genetic Resources， 2010， 11（3）： 309-314， 319. （in Chinese）
[7] WANG FQ， HUANG LC， LUO SC， et al. Analysis of biochemical composition diversity of 46 tea germplasm resources in summer tea[J]. Jiangsu Agricultural Sciences， 2017， 45（18）： 105-110. （in Chinese）
[8] ZHAO HF， GAO XB， LIU XX， et al. Analysis and evaluation of polyphenols quality of high polyphenols content tea varieties in Guizhou[J]. Chinese Agricultural Science Bulletin， 2016， 32（16）： 149-154. （in Chinese）
[9] HUANG HT， ZHANG W， AO C， et al. Diversity analysis of biochemical compositions in local tea germplasm resources and screening of excellent tea varieties in Hangzhou[J]. Acta Agriculturae Jiangxi， 2019， 31（1）： 21-25. （in Chinese）
[10] PAN B. Comparison of similarities and differences of sensory evaluation of the worlds major tea-producing countries[J]. China Tea， 2019， 41（2）： 38-42.
[11] LI Z， LIU ZQ， LIU ZY， et al. Aroma characteristics of the broken black tea abroad[J]. Journal of Anhui Agricultural University， 2015， 42（5）： 692-699. （in Chinese）
[12] HUANG JQ. Comparative study on the effects of different processing methods on quality components of black tea from Qihong， Kenya and Sri Lanka[J]. China Tea Processing， 2003（1）： 16-18. （in Chinese）
[13] LIANG YR， LA MS， XU YR， et al. A study on chemical compositions of Guangxi and Kenyan broken black teas[J]. Journal of Tea， 1998（3）： 131-133. （in Chinese）
[14] YU FL. On the origin and origin center of tea tree[J]. Journal of Tea Science， 1986， 6（1）： 1-8. [15] ZHANG YB， LIU ZH， RAO TT， et al. Analysis of the quality characteristics color， aroma and taste of Yunnan broken black tea[J]. Food & Machinery， 2019， 35（8）： 37-43. （in Chinese）
[16] MAMATI GE， LIANG YR. Tea （Camellia sinensis）： Comparison of the crop in Kenya and China[J]. Journal of Tea， 2004， 30（4）： 197-202. （in Chinese）
[17] CHEN L， YANG YJ， YU FL， et al. Descriptors for tea germplasm resources and data standards[M]. Beijing： China Agriculture Press， 2005. （in Chinese）
[18] ZHANG ZC. Some opinions on quality index and ployphenol-to-amino acid ratio of early identification in tea plants[J]. Journal of Tea Communication， 1991（3）： 22-25. （in Chinese）
[19] YANG YJ. Chemical evaluation on tea quality during early-stage of breeding program——I. Relationship between the biochemical component content in shoot and the quality of black tea[J]. Journal of Tea Science， 1990， 10（2）： 59-63. （in Chinese）
[20] LIANG YR， LIU ZR. Study on the relationship between the chemical composition and the quality of broken black tea of different tea varieties[J]. Journal of Zhejiang Agricultural University， 1994， 20（2）： 149-154. （in Chinese）
[21] ZHE GM， ZHANG XL， CHEN KK， et al. Content variation of theanine and gallic acid in Puer tea [J]. Acta Botanica Yunnanica， 2005， 27（5）： 572-576. （in Chinese）