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Abstract Xuzishu 8 is a new cultivated sweetpotato with high anthocyanidin content. The growth period affects its tuber traits and quality. In this study， the changes in sweetpotato shape， number of tubers per plant， rate of big and medium tubers， yield， anthocyanin content and starch content of Xuzishu 8 were investigated under 12 growth periods from 78 to 188 d. The results showed that various traits of Xuzishu 8 were significantly different between different growth periods. The rate of large and medium tubers， the fresh sweetpotato yield， the starch content and the anthocyanin yield increased gradually with the extension of the growth period. The length/width ratio of sweetpotato tubers， the rate of small tubers and the contents of reducing sugar and soluble sugar decreased with the prolongation of the growth period. The correlation analysis showed that the fresh sweetpotato yield was in significant or extremely significant positive correlation with the number of tubers per plant， rate of big and medium tubers， dry matter content， dry yield and starch content. The 148day growth period treatment exhibited the highest number of tubers per plant， the highest fresh sweetpotato yield and anthocyanin yield reaching 3 927 and 4.25 kg/667 m2， respectively， as well as the rate of large and medium tubers up to 90%. Therefore， the tubers had good marketability and eating quality.
Key words Xuzishu 8; Growth period; Anthocyanin; Tuber trait; Quality trait; Yield
Purple sweetpotato is rich in natural watersoluble pigment anthocyanins， which have healthcare functions such as antiaging，anticancer， anticancer and slimming functions， and are widely used as food additive and health care products[1-3]. In recent years， purple sweetpotato has been widely planted and utilized. Xuzishu 8 was crossbred through controlled pollination by Xuzhou Institute of Agricultural Sciences in Xuhuai District， Jiangsu Province with Xuzishu 3 as the female parent and Wanzi 56 as the male parent. Xuzishu 8 is a sweetpotato variety for both processing and fresh eating. The sweetpotato planted in spring has a high rate of large tubers， and the dry matter content is about 28%. Xuzishu 8 is suitable for the processing of whole powder and instant whole powder. The sweetpotato planted in summer has a dry matter content of about 26% with fragrant and glutinous taste after cooking， and is thus suitable for fresh eating. The variety has good germination， and medium and short vines; the tubers are spindleshaped， with purple skin and flesh， neatly dispersed in the soil before harvesting; and the variety has high soluble sugar content， good taste and good storability. In this study， sweetpotato shape， number of tubers per plant， rate of big and medium tubers， yield， anthocyanin content and starch content of Xuzishu 8 were investigated under different growth periods， so as to select a reasonable growth period. This study will provide a theoretical basis for the promotion， production demonstration and processing of Xuzishu 8. Materials and Methods
Experimental materials
The experiment was carried out in the breeding field of the Xuzhou Sweetpotato Research Center in Jiangsu Province in 2017. The soil was typical loam with moderate fertility. The test materialXuzishu 8 was with highin quality and anthocyanin.
Experimental methods and determination indexes
Staged planting was performed from May 2 to August 22， 2017， with a total of 12 planting dates. The tubers were harvested on November 8 （Table 1）. The experiment adopted random arrangement. In each treatment， 5 rows were planted in each plot， with 3 replicates. The plot area was 20 m2. The rows had spacing of 0.85 m and a row length of 4.5 m， and the aisle was 0.9 m. Management measures were unified according to regular field requirements.
Referring to Zhang et al.[4]， the length and maximum diameter of tubers were measured with ruler and vernier caliper， and the aspect ratio （length/width， L/W） was calculated to determine the sweetpotato shape： 1.5≤L/W<2.0： short spindleshaped， 2.0≤L/W<3.0： spindleshaped， and L/W≥3.0： long spindleshaped. During the harvesting period， the number of big tubers， the number of medium tubers， the number of small tubers and the number of tubers per plant were recorded， and after weighing， the rate of big tubers， the rate of medium tubers， the rate of small tubers， and the rate of large and medium tubers were calculated， respectively[4]; the extraction of fresh sweetpotato anthocyanins was carried out by the citric aciddisodium hydrogen phosphate buffer method[5]; the tubers were shred and dried to constant weight at 70 ℃， for the measurement of dry matter content; and the contents of starch， protein， reducing sugar and soluble sugar in potato were determined using VECTOR22/N type Fourier transform nearinfrared reflectance spectrometer （BRUKER Spectrometer Company， Germany）[6-7].
Data processing and charting were performed with Microsoft Excel 2007. Correlation analysis was performed using DPS V15.10processing software[8]. Multiple comparisons were performed by Duncans new complex range method. The variance analysiswas performed using a stochastic model.
Results and Analysis
Effects of different growth periods on sweetpotato shape of Xuzishu 8
The L/W ratio of Xuzishu 8 was significantly different between different growth periods （Fig. 1A）. The L/W ratio gradually decreased with the prolongation of growth period. It had the highest value of 10.71 in S1， and the tubers were long spindleshaped. The L/W ratio was the smallest in S12， being 1.96， and the tubers were short spindleshaped. It indicated that the potato tubers first elongated in the early development and then gradually enlarged with the growth period （Fig. 1B）.
af indicated significance at 95% confidence interval; A. difference analysis of L/W ratio under different growth periods; B. sweetpotato shape of Xuzishu8.
Uniform sweetpotato shape is the main reference index for measuring the marketability of sweetpotato. The marketability of sweetpotato was evaluated in this study under different growth periods. The differences in the rate of big tubers， the rate of medium potatoes， the rate of small tubers and the rate of large and medium potatoes were all extremely significant between different growth periods. With the prolongation of the growth period， the rate of large and mediumsized tubers was gradually increased， and the rate of small tubers was gradually decreased. There were no big tubers when the growth periods were less than 118 d （S1S5） （Fig. 2A）. The highest rate of medium tubers was 66.88 % in the growth period of 128 d （S6） （Fig. 2B）. All the tubers in the growth period of 78 d（S1） were small （Fig. 2C）. There were no significant differences in the rate of large and mediumsized tubers between the growth periods of 128-188 d （S6S12）， and the rates of large and mediumsized tubers were all over 90%， with the highest value of 94.67% in the growth period of 138 d （Fig. 2D）.
Based on the significant effects of different growth periods on the habits of tuberization， a reasonable growth period can be selected according to the use of sweetpotato. If it is mainly used for processing， the rate of large and medium tubers should be improved as much as possible， and the growth period should be appropriately extended in cultivation; and for the production of fresheating mini sweetpotatoes， we can choose a shorter growth period to ensure the marketability of the fresheating sweetpotatoes.
Effects of different growth periods on yield traits of Xuzishu 8
The dry matter content of Xuzishu 8 was significantly different between different growth periods. The dry matter content increased gradually at first and then decreased slowly with the prolongation of the growth period. The average dry matter contents of Xuzishu 8 were in the range of 23.69%-28.06%， and the highest dry matter content was 28.0% when the growth period was 168 d （S10） （Fig. 3A）.
There were significant differences in the fresh yield， the dry yield and the number of tubers per plant between different growth periods. The three traits all showed a consistent trend of increasing first and then decreasing. For the growth period of 148 d （S8）， the fresh potato yield and dried potato yield reached the highest values， respectively， of 3 928 and 1 019 kg/667 m2 （Fig. 3B， Fig. 3C）. The number of tubers per plant varied from 1.53 to 5.63， and reached the highest when the growth period was 148 d （Fig. 3D）. Effects of different growth periods on accumulation of anthocyanins in Xuzishu 8
The anthocyanin accumulation increased first and then decreased， and the contents ranged from 84.03 to 140.86 mg/100 g fresh sweetpotato. The anthocyanin content was the lowest with 84.03mg/100 g in the growth period of 188 d （S12）， and the highest at 140.86 mg/100 g in the growth period of 138 d （S7） （Fig. 4A）. According to the anthocyanin content and the calculated anthocyanin yield， the anthocyanin yield was significantly different between different growth periods. The anthocyanin yield increased significantly and then decreased slowly with the prolongation of the growth period. It was the highest in the growth period of 148 d， at 4.36 kg/667 m2 （Fig. 4B）.
Effects of different growth periods on quality traits of Xuzishu 8
The starch content increased gradually with the prolongation of the growth period. The starch content was the lowest in the growth period of 78 d， at 43.77%.The starch content was the highest at about 60% in the growth period of 188 d （Table 2）. The soluble sugar and reducing sugar contents were the highest in the growth period of 78 d， at 8.55% and 9.23%， respectively， and then gradually decreased with the prolongation of the growth period. The protein content was significant between different growth periods. The protein content was the lowest at 5.34% in the growth period of 188 d， and the highest at 8.28% in the growth period of 118 d.
Correlation analysis of main traits in Xuzishu 8
Correlation analysis was performed on the averages of the main economic traits of Xuzishu 8 to further understand the interaction between the traits. The fresh yield is an important index to measure the economic value of sweet potato varieties. It can be seen from Table 3 that the fresh sweetpotato yield was in significant or extremely significant positive correlation with the number of tubers per plant， rate of big and medium tubers， dry matter content， dry yield and starch content， but significantly negatively correlated with L/W， rate of small tubers， reducing sugar content， soluble sugar content and protein content. The correlation analysis showed that the higher the number of tubers per plant， the rate of big tubers， the dry matter content and the starch content， the higher the fresh sweetpotato yield， while the higher the L/W ratio， the rate of small tubers， the reducing sugar content， the soluble sugar content and the protein content， the lower the fresh sweetpotato yield. Therefore， in the selection of sweetpotato varieties， the sweetpotato habits and quality traits can be used as reference indexes for sweetpotato breeding to indirectly evaluate the fresh sweetpotato yield and provide reference for the selection of sweetpotato varieties. The L/W ratio of potato tubers is a quantitative index for evaluatingpotato shape and reflecting the duration of growth period. The correlation analysis showed that the L/W ratio was in significantly negative correlation with the rate of big tubers， rate of large and medium tubers， dry matter content and starch content， indicating that a shorter growth period， a higher L/W ratio of tubers and longer tuber shape produced a lower fresh potato yield. Therefore， in sweetpotato cultivation， if high fresh potato yield and high starch yield are the production targets， the growth period should be appropriately extended to obtain a higher rate of large and medium tubers. The L/W ratio was significantly positively correlated with the rate of small tubers and reducing sugar and soluble sugar contents. According to this correlation， the growth period of sweetpotato can be appropriately shortened in production to increase the L/W ratio and the rate of small tubers and improve the reducing sugar and soluble sugar contents in practical quality.
Xuzishu 8 is a highanthocyanin sweetpotato variety. The correlation analysis showed that the anthocyanin content was in significant positive correlation with the L/W ratio and reducing sugar content， and in significant negative correlation with the rate of big tubers， dry matter content and starch content. It indicated that the higher the rate of large tubers， dry matter content and starch content， the lower the anthocyanin content， while the higher the L/W ratio and reducing sugar content， the higher the anthocyanin content. Therefore， in the breeding of highanthocyanin sweetpotato varieties， the quality traits such as the reducing sugar content and the yield traits such as the rate of big tubers can serve as the reference indexes for breeding.
Conclusions and Discussion
Root formation and expansion are not only regulated by the sourcesink regulation of the aboveground and underground parts， but also susceptible to environmental conditions. Therefore， it is possible to adopt appropriate cultivation measures for a certain variety to promote early sweetpotato growth and rapid expansion of sweetpotato and coordinate the relation between vine growth and tuber expansion， so as to achieve the purpose of increasing yield. Different climatic conditions and soil moisture directly or indirectly affect the formation and expansion of the roots. Different planting periods mean that the formation and expansion of the roots are affected by different environments. The soil temperature between 21-29 ℃is conducive to the division of cambial cells and the formation of tubers， and thus promotes early tuberization and improves number of tubers per plants[9]. In this study， for the treatment of planting on June 12 with the growth period of 148 d， because the soil temperature was higher and was conducive to the expansion of tubers， the number of tubers per plant and the yield were the highest， and the rate of large and medium tubers was over 90%. Although the growth periods of S9S12were longer than that of S8， the fresh yields were significantly lower than that of S8. The correlation analysis showed that the number of tubers per plant and the rate of large and medium tubers were significantly positively correlated with fresh sweetpotato yield; and S9S12had no significant differences in the rate of big and medium tubers， but showed the number of tubers per plant significantly lower than S8， indicating that the number of tubers per plant was the main factor limiting the yields of S9S12. The temperature in May was lower， which affected the formation of tuber. The rate of big and medium tubers and fresh potato yield were lower in the growth periods of S1S6， which might be because that the growth period was too short and the tubers were not fully expanded. The above results indicate that in sweet potato cultivation， a longer growth period does not always lead to a higher yield. The selection of the planting date is directly related to the number of tubers per plant， which in turn affects the yield of sweetpotato. Xuzishu 8 is a highanthocyanin sweetpotato variety for both processing and fresh eating. The results showed that the anthocyanin content increased first and then decreased with the extension of growth period. The anthocyanin content was the highest at 140.86 mg/100 g fresh potato in the 128day growth period， which was equivalentto the anthocyanin yield of 2.34 kg/667 m2; and the 148day growth period treatment exhibited slightly lower anthocyanin content， at 110.92 mg/100 g fresh potato， but due to high yield of fresh potato， the yield of extractable anthocyanins was the highest， up to 4.25 kg/667 m2. In addition， the 148day growth period had the fresh potato yield up to 3 927 kg/667 m2， the rate of large and medium tubers of 90%， as well as a higher starch content， and the tubers were sweet and glutinous in eating quality. Therefore， in production， whether it is for processing or fresh eating， in Xuzhou and surrounding areas， planting in midJune， about150 d of growth period can provide better quality and economic benefits.
References
[1] MA DF， LIU QC. Chinese sweetpotato breeding and industrialization[M]. Beijing： China Agricultural University Press， 2005. （in Chinese）
[2] PHILPOTT M， GOULD KS， LIM C， et al. In situ and in vitro antioxidant activity of sweetpotato anthocyanins[J]. Journal of Agricultural and Food Chemistry， 2004， 52： 1511-1513.
[3] WANG GL， YUE J， SU DX， et al. Study on the antioxidant activity of sweet potato anthocyanin and its inhibiting effect on growth of cancer S180[J]. Acta Nutrimenta Sinica， 2006， 28（1）： 71-74. （in Chinese）
[4] ZHANG YG， FANG BP. Sweetpotato germplasm resource description and data analysis [M]. Beijing： China Agriculture Press， 2006： 27. （in Chinese）
[5] LI Q， WANG X， ZHANG YG， et al. Innovation and Identification of a new sweetpotato cultivar ‘Xuzishu No.3’ with high anthocyanin content and high starch content[J]. Southwest China Journal of Agricultural Sciences， 2014， 27（4）： 1409-1413. （in Chinese）
[6] MA DF， LI Q， LI XY， et al. Selection of parents for breeding edible varieties of sweetpotato with high carotene content[J]. Agricultural Science in China， 2009， 8（10）： 1166-1173.
[7] TANG W， LI Q， ZHANG YG， et al. Dynamic analysis of essential characters and biological yield change of purplefleshed sweetpotato ‘Xuzishu3’[J]. Southwest China Journal of Agricultural Sciences， 2014， 27（1）： 47-52. （in Chinese）
[8] TANG QY， FENG MG. Practical statistical analysis and its DPS data processing system[M]. Beijing： Science Press， 2002： 43-80. （in Chinese）
[9] LU SY， LIU QC， LI WJ. Sweetpotato breeding science[M]. Beijing： China Agriculture Press， 1998. （in Chinese）
Editor： Yingzhi GUANG Proofreader： Xinxiu ZHU
Key words Xuzishu 8; Growth period; Anthocyanin; Tuber trait; Quality trait; Yield
Purple sweetpotato is rich in natural watersoluble pigment anthocyanins， which have healthcare functions such as antiaging，anticancer， anticancer and slimming functions， and are widely used as food additive and health care products[1-3]. In recent years， purple sweetpotato has been widely planted and utilized. Xuzishu 8 was crossbred through controlled pollination by Xuzhou Institute of Agricultural Sciences in Xuhuai District， Jiangsu Province with Xuzishu 3 as the female parent and Wanzi 56 as the male parent. Xuzishu 8 is a sweetpotato variety for both processing and fresh eating. The sweetpotato planted in spring has a high rate of large tubers， and the dry matter content is about 28%. Xuzishu 8 is suitable for the processing of whole powder and instant whole powder. The sweetpotato planted in summer has a dry matter content of about 26% with fragrant and glutinous taste after cooking， and is thus suitable for fresh eating. The variety has good germination， and medium and short vines; the tubers are spindleshaped， with purple skin and flesh， neatly dispersed in the soil before harvesting; and the variety has high soluble sugar content， good taste and good storability. In this study， sweetpotato shape， number of tubers per plant， rate of big and medium tubers， yield， anthocyanin content and starch content of Xuzishu 8 were investigated under different growth periods， so as to select a reasonable growth period. This study will provide a theoretical basis for the promotion， production demonstration and processing of Xuzishu 8. Materials and Methods
Experimental materials
The experiment was carried out in the breeding field of the Xuzhou Sweetpotato Research Center in Jiangsu Province in 2017. The soil was typical loam with moderate fertility. The test materialXuzishu 8 was with highin quality and anthocyanin.
Experimental methods and determination indexes
Staged planting was performed from May 2 to August 22， 2017， with a total of 12 planting dates. The tubers were harvested on November 8 （Table 1）. The experiment adopted random arrangement. In each treatment， 5 rows were planted in each plot， with 3 replicates. The plot area was 20 m2. The rows had spacing of 0.85 m and a row length of 4.5 m， and the aisle was 0.9 m. Management measures were unified according to regular field requirements.
Referring to Zhang et al.[4]， the length and maximum diameter of tubers were measured with ruler and vernier caliper， and the aspect ratio （length/width， L/W） was calculated to determine the sweetpotato shape： 1.5≤L/W<2.0： short spindleshaped， 2.0≤L/W<3.0： spindleshaped， and L/W≥3.0： long spindleshaped. During the harvesting period， the number of big tubers， the number of medium tubers， the number of small tubers and the number of tubers per plant were recorded， and after weighing， the rate of big tubers， the rate of medium tubers， the rate of small tubers， and the rate of large and medium tubers were calculated， respectively[4]; the extraction of fresh sweetpotato anthocyanins was carried out by the citric aciddisodium hydrogen phosphate buffer method[5]; the tubers were shred and dried to constant weight at 70 ℃， for the measurement of dry matter content; and the contents of starch， protein， reducing sugar and soluble sugar in potato were determined using VECTOR22/N type Fourier transform nearinfrared reflectance spectrometer （BRUKER Spectrometer Company， Germany）[6-7].
Data processing and charting were performed with Microsoft Excel 2007. Correlation analysis was performed using DPS V15.10processing software[8]. Multiple comparisons were performed by Duncans new complex range method. The variance analysiswas performed using a stochastic model.
Results and Analysis
Effects of different growth periods on sweetpotato shape of Xuzishu 8
The L/W ratio of Xuzishu 8 was significantly different between different growth periods （Fig. 1A）. The L/W ratio gradually decreased with the prolongation of growth period. It had the highest value of 10.71 in S1， and the tubers were long spindleshaped. The L/W ratio was the smallest in S12， being 1.96， and the tubers were short spindleshaped. It indicated that the potato tubers first elongated in the early development and then gradually enlarged with the growth period （Fig. 1B）.
af indicated significance at 95% confidence interval; A. difference analysis of L/W ratio under different growth periods; B. sweetpotato shape of Xuzishu8.
Uniform sweetpotato shape is the main reference index for measuring the marketability of sweetpotato. The marketability of sweetpotato was evaluated in this study under different growth periods. The differences in the rate of big tubers， the rate of medium potatoes， the rate of small tubers and the rate of large and medium potatoes were all extremely significant between different growth periods. With the prolongation of the growth period， the rate of large and mediumsized tubers was gradually increased， and the rate of small tubers was gradually decreased. There were no big tubers when the growth periods were less than 118 d （S1S5） （Fig. 2A）. The highest rate of medium tubers was 66.88 % in the growth period of 128 d （S6） （Fig. 2B）. All the tubers in the growth period of 78 d（S1） were small （Fig. 2C）. There were no significant differences in the rate of large and mediumsized tubers between the growth periods of 128-188 d （S6S12）， and the rates of large and mediumsized tubers were all over 90%， with the highest value of 94.67% in the growth period of 138 d （Fig. 2D）.
Based on the significant effects of different growth periods on the habits of tuberization， a reasonable growth period can be selected according to the use of sweetpotato. If it is mainly used for processing， the rate of large and medium tubers should be improved as much as possible， and the growth period should be appropriately extended in cultivation; and for the production of fresheating mini sweetpotatoes， we can choose a shorter growth period to ensure the marketability of the fresheating sweetpotatoes.
Effects of different growth periods on yield traits of Xuzishu 8
The dry matter content of Xuzishu 8 was significantly different between different growth periods. The dry matter content increased gradually at first and then decreased slowly with the prolongation of the growth period. The average dry matter contents of Xuzishu 8 were in the range of 23.69%-28.06%， and the highest dry matter content was 28.0% when the growth period was 168 d （S10） （Fig. 3A）.
There were significant differences in the fresh yield， the dry yield and the number of tubers per plant between different growth periods. The three traits all showed a consistent trend of increasing first and then decreasing. For the growth period of 148 d （S8）， the fresh potato yield and dried potato yield reached the highest values， respectively， of 3 928 and 1 019 kg/667 m2 （Fig. 3B， Fig. 3C）. The number of tubers per plant varied from 1.53 to 5.63， and reached the highest when the growth period was 148 d （Fig. 3D）. Effects of different growth periods on accumulation of anthocyanins in Xuzishu 8
The anthocyanin accumulation increased first and then decreased， and the contents ranged from 84.03 to 140.86 mg/100 g fresh sweetpotato. The anthocyanin content was the lowest with 84.03mg/100 g in the growth period of 188 d （S12）， and the highest at 140.86 mg/100 g in the growth period of 138 d （S7） （Fig. 4A）. According to the anthocyanin content and the calculated anthocyanin yield， the anthocyanin yield was significantly different between different growth periods. The anthocyanin yield increased significantly and then decreased slowly with the prolongation of the growth period. It was the highest in the growth period of 148 d， at 4.36 kg/667 m2 （Fig. 4B）.
Effects of different growth periods on quality traits of Xuzishu 8
The starch content increased gradually with the prolongation of the growth period. The starch content was the lowest in the growth period of 78 d， at 43.77%.The starch content was the highest at about 60% in the growth period of 188 d （Table 2）. The soluble sugar and reducing sugar contents were the highest in the growth period of 78 d， at 8.55% and 9.23%， respectively， and then gradually decreased with the prolongation of the growth period. The protein content was significant between different growth periods. The protein content was the lowest at 5.34% in the growth period of 188 d， and the highest at 8.28% in the growth period of 118 d.
Correlation analysis of main traits in Xuzishu 8
Correlation analysis was performed on the averages of the main economic traits of Xuzishu 8 to further understand the interaction between the traits. The fresh yield is an important index to measure the economic value of sweet potato varieties. It can be seen from Table 3 that the fresh sweetpotato yield was in significant or extremely significant positive correlation with the number of tubers per plant， rate of big and medium tubers， dry matter content， dry yield and starch content， but significantly negatively correlated with L/W， rate of small tubers， reducing sugar content， soluble sugar content and protein content. The correlation analysis showed that the higher the number of tubers per plant， the rate of big tubers， the dry matter content and the starch content， the higher the fresh sweetpotato yield， while the higher the L/W ratio， the rate of small tubers， the reducing sugar content， the soluble sugar content and the protein content， the lower the fresh sweetpotato yield. Therefore， in the selection of sweetpotato varieties， the sweetpotato habits and quality traits can be used as reference indexes for sweetpotato breeding to indirectly evaluate the fresh sweetpotato yield and provide reference for the selection of sweetpotato varieties. The L/W ratio of potato tubers is a quantitative index for evaluatingpotato shape and reflecting the duration of growth period. The correlation analysis showed that the L/W ratio was in significantly negative correlation with the rate of big tubers， rate of large and medium tubers， dry matter content and starch content， indicating that a shorter growth period， a higher L/W ratio of tubers and longer tuber shape produced a lower fresh potato yield. Therefore， in sweetpotato cultivation， if high fresh potato yield and high starch yield are the production targets， the growth period should be appropriately extended to obtain a higher rate of large and medium tubers. The L/W ratio was significantly positively correlated with the rate of small tubers and reducing sugar and soluble sugar contents. According to this correlation， the growth period of sweetpotato can be appropriately shortened in production to increase the L/W ratio and the rate of small tubers and improve the reducing sugar and soluble sugar contents in practical quality.
Xuzishu 8 is a highanthocyanin sweetpotato variety. The correlation analysis showed that the anthocyanin content was in significant positive correlation with the L/W ratio and reducing sugar content， and in significant negative correlation with the rate of big tubers， dry matter content and starch content. It indicated that the higher the rate of large tubers， dry matter content and starch content， the lower the anthocyanin content， while the higher the L/W ratio and reducing sugar content， the higher the anthocyanin content. Therefore， in the breeding of highanthocyanin sweetpotato varieties， the quality traits such as the reducing sugar content and the yield traits such as the rate of big tubers can serve as the reference indexes for breeding.
Conclusions and Discussion
Root formation and expansion are not only regulated by the sourcesink regulation of the aboveground and underground parts， but also susceptible to environmental conditions. Therefore， it is possible to adopt appropriate cultivation measures for a certain variety to promote early sweetpotato growth and rapid expansion of sweetpotato and coordinate the relation between vine growth and tuber expansion， so as to achieve the purpose of increasing yield. Different climatic conditions and soil moisture directly or indirectly affect the formation and expansion of the roots. Different planting periods mean that the formation and expansion of the roots are affected by different environments. The soil temperature between 21-29 ℃is conducive to the division of cambial cells and the formation of tubers， and thus promotes early tuberization and improves number of tubers per plants[9]. In this study， for the treatment of planting on June 12 with the growth period of 148 d， because the soil temperature was higher and was conducive to the expansion of tubers， the number of tubers per plant and the yield were the highest， and the rate of large and medium tubers was over 90%. Although the growth periods of S9S12were longer than that of S8， the fresh yields were significantly lower than that of S8. The correlation analysis showed that the number of tubers per plant and the rate of large and medium tubers were significantly positively correlated with fresh sweetpotato yield; and S9S12had no significant differences in the rate of big and medium tubers， but showed the number of tubers per plant significantly lower than S8， indicating that the number of tubers per plant was the main factor limiting the yields of S9S12. The temperature in May was lower， which affected the formation of tuber. The rate of big and medium tubers and fresh potato yield were lower in the growth periods of S1S6， which might be because that the growth period was too short and the tubers were not fully expanded. The above results indicate that in sweet potato cultivation， a longer growth period does not always lead to a higher yield. The selection of the planting date is directly related to the number of tubers per plant， which in turn affects the yield of sweetpotato. Xuzishu 8 is a highanthocyanin sweetpotato variety for both processing and fresh eating. The results showed that the anthocyanin content increased first and then decreased with the extension of growth period. The anthocyanin content was the highest at 140.86 mg/100 g fresh potato in the 128day growth period， which was equivalentto the anthocyanin yield of 2.34 kg/667 m2; and the 148day growth period treatment exhibited slightly lower anthocyanin content， at 110.92 mg/100 g fresh potato， but due to high yield of fresh potato， the yield of extractable anthocyanins was the highest， up to 4.25 kg/667 m2. In addition， the 148day growth period had the fresh potato yield up to 3 927 kg/667 m2， the rate of large and medium tubers of 90%， as well as a higher starch content， and the tubers were sweet and glutinous in eating quality. Therefore， in production， whether it is for processing or fresh eating， in Xuzhou and surrounding areas， planting in midJune， about150 d of growth period can provide better quality and economic benefits.
References
[1] MA DF， LIU QC. Chinese sweetpotato breeding and industrialization[M]. Beijing： China Agricultural University Press， 2005. （in Chinese）
[2] PHILPOTT M， GOULD KS， LIM C， et al. In situ and in vitro antioxidant activity of sweetpotato anthocyanins[J]. Journal of Agricultural and Food Chemistry， 2004， 52： 1511-1513.
[3] WANG GL， YUE J， SU DX， et al. Study on the antioxidant activity of sweet potato anthocyanin and its inhibiting effect on growth of cancer S180[J]. Acta Nutrimenta Sinica， 2006， 28（1）： 71-74. （in Chinese）
[4] ZHANG YG， FANG BP. Sweetpotato germplasm resource description and data analysis [M]. Beijing： China Agriculture Press， 2006： 27. （in Chinese）
[5] LI Q， WANG X， ZHANG YG， et al. Innovation and Identification of a new sweetpotato cultivar ‘Xuzishu No.3’ with high anthocyanin content and high starch content[J]. Southwest China Journal of Agricultural Sciences， 2014， 27（4）： 1409-1413. （in Chinese）
[6] MA DF， LI Q， LI XY， et al. Selection of parents for breeding edible varieties of sweetpotato with high carotene content[J]. Agricultural Science in China， 2009， 8（10）： 1166-1173.
[7] TANG W， LI Q， ZHANG YG， et al. Dynamic analysis of essential characters and biological yield change of purplefleshed sweetpotato ‘Xuzishu3’[J]. Southwest China Journal of Agricultural Sciences， 2014， 27（1）： 47-52. （in Chinese）
[8] TANG QY， FENG MG. Practical statistical analysis and its DPS data processing system[M]. Beijing： Science Press， 2002： 43-80. （in Chinese）
[9] LU SY， LIU QC， LI WJ. Sweetpotato breeding science[M]. Beijing： China Agriculture Press， 1998. （in Chinese）
Editor： Yingzhi GUANG Proofreader： Xinxiu ZHU