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Abstract With Yunyan 87 as an experimental material， the effects of different transplanting dates on the growth and development of tobacco plants and the yield and quality of cured tobacco leaves were studied in Lingbao City， Sanmenxia City. With the postponement of transplanting date， the growth period in field was prolonged， and the plant height and leaf area both increased at first and decreased then. TMV， weather fleck and brown spot all showed an incidence and a disease index decreased with the postponement of transplanting date. The total sugar and reducing sugar contents of the cured tobacco leaves from the upper and middle part increased at first and decreased then with the postponement of transplanting date. The main nitrogenous compounds decreased at first and increased then. The economic traits and leaf class were on the decrease.
Key words Transplanting date; Growth and development; Chemical components; Quality
Materials and Methods
General information of the test site
This experiment was carried out in Zhuyang Village， Zhuyang Town， Lingbao City from 2016 to 2017. The test site is located in the western part of Henan Province. It has a temperate continental monsoon climate with concentrated light， heat and rainfall. The average annual temperature is 13.8， the annual average sunshine is 2 261.7 h， and the average annual precipitation is 580-680 mm. The test site has medium fertility， with flat terrain， convenient drainage and irrigation and sufficient sunlight， and the previous crop was tobacco.
The test material was Yunyan 87， the main local fluecured tobacco cultivar. The tested soil was cinnamon soil， and the basic physical and chemical properties of the soil were as follows： pH 7.87， organic matter 13.38 g/kg， alkalihydrolyzale nitrogen 68.40 mg/kg， available phosphorus 14.53 mg/kg and available potassium 175.13 mg/kg.
Experimental design
The experiment was designed with three treatments according to the transplanting date， with three replicates， in random block arrangement， and each plot had an area of 100 m2. The treatment were as follows： T1： transplanted on April 25， T2： transplanted on May 5， and T3： transplanted on May 15. As to fertilization， the nitrogen application rate was 90 kg/hm2， and the N≥P≥K ratio was 1≥1≥3. The tested fertilizers were compound fertilizer special for tobacco （with a N≥P≥K ratio of 10≥10≥20）， P2O5 44% and potassium sulfate （K2O 50%）， and the organic fertilizer used bean cake. In the various treatments， organic nitrogen accounted for 20% of the total nitrogen. The ratio of basic to dressing application was 7≥3. The base fertilizer was applied before transplanting， and topdressing was performed at 30 d after transplanting by hole application. The row and plant spacing was 1.2 and 0.5 m， respectively. Conventional transplanting was adopted. Except the transplanting date， the field cultivation management was carried out according to the standardized operation of tobacco planting in Lingbao City. Results and Analysis
Effect of transplanting date on the growth period of tobacco plants
It could be seen from Table 1 that the growth period was prolonged with the postponement of the transplanting date. Treatment T2 developed with the shortest time of 28 d from transplanting to rosette stage， and both treatments T1 and T3 took 32 d. However， from the rosette stage to flower budding， i.e.， in the vigorous growing period of tobacco plants， the differences between the three treatments were large. Specifically， treatment T3 took the longest time， 32 d， with the best field growth; treatment T2 was the second， taking 29 d; and treatment T1 was the shortest. The three treatments took close time to develop from flower budding to topping stage， from topping stage to the first curing date， and from the first curing to the last curing date. Overall， the transplanting date had a greater impact on the growth period， and the three treatments had the growth periods of 134， 140 and 147 d， respectively.
Effects of transplanting date on agronomic traits of tobacco plants
It could be seen from Table 2 that the plant height， stem girth， number of effective leaves and total leaf area per plant increased from transplanting to various growth stages. Except for the nonsignificant differences between the various treatments at rosette stage （30 d after transplanting）， there were significant differences between the various treatments at vigorous growing stage and mature stage. There were significant differences in total leaf area per plant between the various treatments at all stages， and there was a consistent changing law， i.e.， the total leaf area per plant was the highest in treatment T2， followed by treatment T3， and the lowest in treatment T1. There were basically no significant differences in stem girth and number of effective leaves at different growth stages. The results indicated that the transplanting date had a great effect on the plant height and leaf area of tobacco plants， but a little effect on stem girth and number of effective leaves.
Effect of transplanting date on dry matter accumulation of tobacco plants
It could be seen from Fig. 1 that the dry matter accumulation of each treatment increased with the progress of the growth period. Among them， the dry matter accumulation of treatment T2 was the most， and that of treatments T1 and T3 was slightly lower， and had no big difference. From the perspective of growth period， the three treatments showed the largest increase in dry matter accumulation at vigorous growing stage， when the dry matter accumulation was 6.11， 3.51 and 5.61 times higher than that at rosette stage; and compared with the vigorous growing stage， the dry matter accumulation in the three treatments increased by 39.83%， 37.82% and 33.05% at mature stage， respectively. It could be seen that vigorous growing stage is the period when the vegetative growth of tobacco plants is the fastest， with strong metabolism and the fastest dry matter accumulation， and the dry matter accumulation slows down at mature stage when nitrogen metabolism is converted to carbon metabolism and vegetative growth is converted to reproductive growth. Effects of transplanting date on tobacco diseases
It could be seen from Fig. 3 that the diseased plant rate and disease index of each disease were different between different treatments according to different transplanting dates， with significant differences. The tobacco common mosaic disease （TMV） had the highest diseased plant rate and was the most serious in T1 treatment， and treatment T2 showed the diseased plant rate lower than treatment T1 and higher than treatment T3， but had the lowest disease index. In the case of weather fleck， the diseased plant rate was the highest in treatment T1， of 12.3%， followed by treatments T3 and T2 which had no significant difference， and the disease indexes of the three treatments were significantly different， and ranked as T1>T3>T2. As to the brown spot disease， there were significant differences in diseased plant rate and disease index between the various treatments， and the orders were both T1>T3>T2. Overall， the disease condition of treatment T1 was the most serious， followed by treatment T3， and the lightest in treatment T2. It might be due to that when transplanted too early， tobacco seedlings have weaker growth at early stages because of low air temperature， resulting in weak disease resistance; and later transplanting makes the vigorous growing stage encounter drought， which is not conducive to the growth of tobacco plants. Among the three main tobacco diseases， weather fleck occurred in the largest range， followed by brown spot disease， and TMV was the lightest.
Effect of transplanting date on tobacco quality
Main carbonaceous compounds
Carbohydrates are the products of tobacco photosynthesis and play a very important role in the growth and development of tobacco. They are not only the basic raw materials for the synthesis of tobacco components such as proteins， ribose and lipids， but also the main source of energy required for life activities and the skeleton substance of tobacco. The quality of the tobacco leaves after preparation is closely related to carbohydrates. Generally， a high watersoluble sugar content leads to fresh color of tobacco leaves， sufficient oil and more mild taste. As shown in Fig. 2-Fig. 3， the sugar content in tobacco leaves differed according to different transplanting dates. The starch contents in the upper and middle leaves were both the lowest in treatment T2， and the difference between treatments T1 and T3 was not significant. The total sugar and reducing sugar contents were opposite to that of starch， and were the highest in treatment T2， followed by treatments T3 and T1. From the point of position， the starch content of tobacco leaves had no big differences， and the contents of total sugar and reducing sugar in the middle leaves were higher than those in the upper leaves. Main nitrogenous compounds
Tobacco nitrogenous compounds have important effects on the physical properties of tobacco leaves， determine the yield and output value of tobacco leaves to a large extent， and also affect the smoking quality and safety of tobacco leaves. It could be seen from Fig. 4-Fig. 5 that the protein， total nitrogen and nicotine contents in tobacco leaves were different according to different transplanting dates. For the upper leaves， the protein， total nitrogen and nicotine contents all showed an order of T1>T3>T2， indicating that slightly early or later transplanting increased the contents of nitrogenous compounds in cured tobacco leaves. The middle leaves also showed similar laws， the main nitrogenous compounds showed the highest contents in treatment T2， while the nicotine content was basically the same. It indicated that transplanting date had little effect on nicotine content of tobacco leaves.
Potassium and chlorine contents and coordination of chemical components in cured tobacco
The quality of tobacco leaves is largely determined by the internal chemical composition， and the coordination degree among chemical components is crucial. It could be seen from Table 5 that the potassium ion contents in the upper leaves were in order of T1， T3 and T2 from large to small， while the chloride ion content was the smallest in treatment T1， and other two treatments have no difference; and the ratio of potassium to chlorine was in the range of 4.50%-5.52%， falling into the category of high quality tobacco， with T1 being the largest， followed by T3， and T2 being the smallest. The effect of transplanting date on potassium ions in the middle leaves differed， and the potassium ion content was the highest in treatment T3， followed by treatment T1， and the lowest in treatment T2. The chloride ion content decreased with the postponement of transplanting date， while the ratio of potassium to chlorine gradually increased. Furthermore， the ratio of potassium to chlorine was much larger in the middle leaves than that in the upper leaves. Because cigarette products of a higher class have higher watersoluble sugar and less protein， and thus larger ratio of sugar to protein， the best transplanting date was selected to be treatment T2， followed by treatment T3， and treatment T1 had a lower ratio of sugar to protein. The ratio of sugar to nicotine actually reflects the acidbase balance of tobacco leaves， and it is generally believed that the ratio of sugar to nicotine in fluecured tobacco is 6-10. In this study， the upper and middle leaves had a ratio of sugar to nicotine greater than 10， and in both of the two parts， the ratio of sugar to nicotine was the highest in treatment T2 and the lowest in treatment T1. The ratio of nitrogen to nicotine is generally 0.5-0.9， and in the upper leaves， the value of treatment T3 was close to that of treatment T1， and treatment T2 showed a relatively higher value. Economic traits
It could be seen from Table 6 that the final yield， output value， and middle and upperclassleaf percentages differed according to different transplanting dates. The yield was the highest in treatment T1 which might be related to the early transplanting date and long growth period， followed by treatments T2 and T3， and only the difference between T1 and T3 reached the significant level. The differences in the output value and upperclassleaf percentage between the three treatments were both significant， and both showed an order of T2>T1>T3; and the average prices of the three treatments exhibited a trend the same as the differences， but the differences in average price were smaller and did not reach a significant level. The middleclassleaf percentage was the highest in the treatment T3， which was significantly different from other two treatments. It indicated that the transplanting date had a greater impact on the output value of tobacco and the highclassleaf percentage， but had no significant effect on the average price. Taken together， the transplanting date was selected at the time of treatment T2， which showed the best output value， average price and highclassleaf percentage of cured tobacco.
Discussion and Conclusions
When the transplanting date of tobacco plants is different， the weather conditions such as light， temperature， rainfall and humidity at different growth and development stages will be greatly different， which will affect the growth and development of fluecured tobacco and ultimately affect the yield and quality of tobacco. If transplanted too early， the tobacco plants will be threatened by freezing damage， pests and diseases， but if transplanted too late， the temperature will be too high for the plants to grow from rosette stage to vigorous growing stage， and the tobacco plants tend to grow too fast， resulting in less dry matter accumulation， toothin leaves， and even difficult falling of yellow leaves.
Effect of transplanting date on growth and development of tobacco plants
The postponement of transplanting date had a little effect on the number of effective leaves and stem girth of tobacco plants， while the plant height and total leaf area per plant were the largest in treatment T2， followed by treatment T3， and the smallest in treatment T1， that is， they had a trend of increasing at first and decreasing then with the postponement of transplanting date. In this study， the growth period of tobacco plants in field was prolonged with the postponement of transplanting date， and lasted for 134， 140 and 147 d， respectively， and the effect was the greatest on the vigorous growing stage. TMV， weather fleck and brown spot disease showed a consistent law with the postponement of transplanting date， that is， the disease incidence and incidence index decreased with the postponement of transplanting date， except that the incidence of TMV was the smallest in treatment T2， followed by treatment T3， with a small difference therebetween. At rosette， vigorous growing and mature stages， the dry matter accumulation was the largest in treatment T2， followed by treatment T3， and the lowest in treatment T1， and the difference between T3 and T1 was smaller. In general， proper postponement of transplanting date can effectively promote the growth， dry matter accumulation and disease resistance of tobacco plants. Effect of transplanting date on quality of cured tobacco
Transplanting date has an effect on both the internal chemical component contents and their coordination in cured tobacco leaves. The results of this study showed that the upper and middle leaves showed the same performance： with the postponement of transplanting date of tobacco plants， the starch content in leaves decreased， especially in treatment T2， while the total sugar and reducing sugar contents increased at first and decreased then with the delay of transplanting date， indicating that the transplanting time of tobacco plants should be postponed appropriately， which provides higher temperature for late growth and development of tobacco plants， which can promote the carbon metabolism in tobacco leaves to be more sufficient and increase sugar content. However， the main nitrogenous compounds in the upper and middle leaves were the lowest in treatment T2 and slightly higher in treatments T1 and T3. In terms of the coordination of tobacco leaves， the ratio of potassium to chlorine in the middle leaves increased with the postponement of transplanting date， while the differences in the upper leaves were not significant. The ratio of sugar to protein and the ratio of sugar to nicotine ranked as T2>T3>T1， while the ratio of nitrogen to nicotine was not much different. In summary， proper postponement of transplanting date can make the chemical composition of tobacco leaves more coordinated， and treatment T2 performed the best， followed by treatment T3， and they were both better than treatment T1. The yield， output value， average price， and high and middleclassleaf percentages were all the best in treatment T2， and with further delay of the transplanting date， the economic indicators showed a decrease instead. Among them， the differences in the output value and highclassleaf percentage between the three treatments both reached the significant level， indicating that the output value and highclassleaf percentage were more affected by transplanting date.
References
[1]RAN FF， WANG HT， XU ZC. Effects of different transplanting dates and soil types on quality of fluecured tobacco variety NC89[J]. Acta Agriculturae Jiangxi， 2009， 21： 24-26. （in Chinese）
[2]WU L. Matching technology for highquality fluecured tobacco production[M]. Hefei： Press of University of Science and Technology of China， 1994. （in Chinese）
[3]WANG KZ， SUN WQ， WANG YJ. Effects of different transplanting time on growth， yield and quality of tobacco[J]. Shandong Agricultural Sciences， 2009， 2： 48-49， 61. （in Chinese） [4]XU Q， ZHOU ZQ， WU CB. Influences on the growth， yield and quality by the different transplanting times of cigarette seedlings[J]. Fujian Science & Technology of Tropical Crops， 2003， 3：8-10. （in Chinese）
[5]WANG GZ， SUN WQ， WANG YJ. Effects of different transplanting time on growth， yield and quality of tobacco[J]. Shandong Agricultural Sciences， 2009， 2： 48-49， 61. （in Chinese）
[6]GUO FJ， YE XQ， ZHANG JZ. Effects of sowing and transplanting dates on growth， yield and quality of fluecured tobacco[J]. Subtropical Agriculture Research， 2017， 2： 93-98. （in Chinese）
[7]GAO WK， CHEN J， LUO HH， et al. Effects of transplanting time on growth， quality and style of fluecured tobacco[J]. Journal of Anhui Agricultural Sciences， 2015， 33： 48-50， 53. （in Chinese）
[8]WANG XR， HUANG YL， LIN JQ， et al. Effects of different transplanting time on growth， development， yield and quality of fluecured tobacco variety CB1[J]. Journal of Anhui Agricultural Sciences， 2015， 36： 69-72， 80. （in Chinese）
[9]QI F. Effects of climate factors for different transplanting dates on quality and ripe tissue in fluecured tobacco[D]. Zhengzhou： Henan Agricultural University， 2011. （in Chinese）
[10]HUANG YL， LI WQ， CHEN SH， et al. Effect of transplanting stage on growth， leaves rates from various stalk positions， yields and quality of tobacco[J]. Tobacco Science & Technology， 2001， 11： 38-40. （in Chinese）
[11]LU H， ZHANG YY， LIU GQ， et al. Influences of different transplanting periods on quality of upper leaves and economic benefits of fluecured tobacco in tobacco planted area in Rizhao of Shandong[J]. Hunan Agricultural Sciences， 2012， 17： 40-43. （in Chinese）
[12]KONG YL， LIU QY. Utilization of climate resources in tobaccogrowing areas in plain region of central Henan and selection of best transplanting period of tobacco[J]， Tobacco Science & Technology， 1999（6）： 42-43. （in Chinese）
[13]ZHANG XF. Effects of transplanting date on growth， yield and quality of fluecured tobacco in southern Shaanxi and its biological mechanism[D]. Yangling： North West Agriculture and Forestry University， 2013. （in Chinese）
[14]HU ZS， YANG CJ， SHI X， et al. Comparison on meteorological conditions of growth periods， yield and quality of fluecured tobacco in different transplanting dates[J]. Journal of Meteorology and Environment， 2012， 2： 66-70. （in Chinese）
Key words Transplanting date; Growth and development; Chemical components; Quality
Materials and Methods
General information of the test site
This experiment was carried out in Zhuyang Village， Zhuyang Town， Lingbao City from 2016 to 2017. The test site is located in the western part of Henan Province. It has a temperate continental monsoon climate with concentrated light， heat and rainfall. The average annual temperature is 13.8， the annual average sunshine is 2 261.7 h， and the average annual precipitation is 580-680 mm. The test site has medium fertility， with flat terrain， convenient drainage and irrigation and sufficient sunlight， and the previous crop was tobacco.
The test material was Yunyan 87， the main local fluecured tobacco cultivar. The tested soil was cinnamon soil， and the basic physical and chemical properties of the soil were as follows： pH 7.87， organic matter 13.38 g/kg， alkalihydrolyzale nitrogen 68.40 mg/kg， available phosphorus 14.53 mg/kg and available potassium 175.13 mg/kg.
Experimental design
The experiment was designed with three treatments according to the transplanting date， with three replicates， in random block arrangement， and each plot had an area of 100 m2. The treatment were as follows： T1： transplanted on April 25， T2： transplanted on May 5， and T3： transplanted on May 15. As to fertilization， the nitrogen application rate was 90 kg/hm2， and the N≥P≥K ratio was 1≥1≥3. The tested fertilizers were compound fertilizer special for tobacco （with a N≥P≥K ratio of 10≥10≥20）， P2O5 44% and potassium sulfate （K2O 50%）， and the organic fertilizer used bean cake. In the various treatments， organic nitrogen accounted for 20% of the total nitrogen. The ratio of basic to dressing application was 7≥3. The base fertilizer was applied before transplanting， and topdressing was performed at 30 d after transplanting by hole application. The row and plant spacing was 1.2 and 0.5 m， respectively. Conventional transplanting was adopted. Except the transplanting date， the field cultivation management was carried out according to the standardized operation of tobacco planting in Lingbao City. Results and Analysis
Effect of transplanting date on the growth period of tobacco plants
It could be seen from Table 1 that the growth period was prolonged with the postponement of the transplanting date. Treatment T2 developed with the shortest time of 28 d from transplanting to rosette stage， and both treatments T1 and T3 took 32 d. However， from the rosette stage to flower budding， i.e.， in the vigorous growing period of tobacco plants， the differences between the three treatments were large. Specifically， treatment T3 took the longest time， 32 d， with the best field growth; treatment T2 was the second， taking 29 d; and treatment T1 was the shortest. The three treatments took close time to develop from flower budding to topping stage， from topping stage to the first curing date， and from the first curing to the last curing date. Overall， the transplanting date had a greater impact on the growth period， and the three treatments had the growth periods of 134， 140 and 147 d， respectively.
Effects of transplanting date on agronomic traits of tobacco plants
It could be seen from Table 2 that the plant height， stem girth， number of effective leaves and total leaf area per plant increased from transplanting to various growth stages. Except for the nonsignificant differences between the various treatments at rosette stage （30 d after transplanting）， there were significant differences between the various treatments at vigorous growing stage and mature stage. There were significant differences in total leaf area per plant between the various treatments at all stages， and there was a consistent changing law， i.e.， the total leaf area per plant was the highest in treatment T2， followed by treatment T3， and the lowest in treatment T1. There were basically no significant differences in stem girth and number of effective leaves at different growth stages. The results indicated that the transplanting date had a great effect on the plant height and leaf area of tobacco plants， but a little effect on stem girth and number of effective leaves.
Effect of transplanting date on dry matter accumulation of tobacco plants
It could be seen from Fig. 1 that the dry matter accumulation of each treatment increased with the progress of the growth period. Among them， the dry matter accumulation of treatment T2 was the most， and that of treatments T1 and T3 was slightly lower， and had no big difference. From the perspective of growth period， the three treatments showed the largest increase in dry matter accumulation at vigorous growing stage， when the dry matter accumulation was 6.11， 3.51 and 5.61 times higher than that at rosette stage; and compared with the vigorous growing stage， the dry matter accumulation in the three treatments increased by 39.83%， 37.82% and 33.05% at mature stage， respectively. It could be seen that vigorous growing stage is the period when the vegetative growth of tobacco plants is the fastest， with strong metabolism and the fastest dry matter accumulation， and the dry matter accumulation slows down at mature stage when nitrogen metabolism is converted to carbon metabolism and vegetative growth is converted to reproductive growth. Effects of transplanting date on tobacco diseases
It could be seen from Fig. 3 that the diseased plant rate and disease index of each disease were different between different treatments according to different transplanting dates， with significant differences. The tobacco common mosaic disease （TMV） had the highest diseased plant rate and was the most serious in T1 treatment， and treatment T2 showed the diseased plant rate lower than treatment T1 and higher than treatment T3， but had the lowest disease index. In the case of weather fleck， the diseased plant rate was the highest in treatment T1， of 12.3%， followed by treatments T3 and T2 which had no significant difference， and the disease indexes of the three treatments were significantly different， and ranked as T1>T3>T2. As to the brown spot disease， there were significant differences in diseased plant rate and disease index between the various treatments， and the orders were both T1>T3>T2. Overall， the disease condition of treatment T1 was the most serious， followed by treatment T3， and the lightest in treatment T2. It might be due to that when transplanted too early， tobacco seedlings have weaker growth at early stages because of low air temperature， resulting in weak disease resistance; and later transplanting makes the vigorous growing stage encounter drought， which is not conducive to the growth of tobacco plants. Among the three main tobacco diseases， weather fleck occurred in the largest range， followed by brown spot disease， and TMV was the lightest.
Effect of transplanting date on tobacco quality
Main carbonaceous compounds
Carbohydrates are the products of tobacco photosynthesis and play a very important role in the growth and development of tobacco. They are not only the basic raw materials for the synthesis of tobacco components such as proteins， ribose and lipids， but also the main source of energy required for life activities and the skeleton substance of tobacco. The quality of the tobacco leaves after preparation is closely related to carbohydrates. Generally， a high watersoluble sugar content leads to fresh color of tobacco leaves， sufficient oil and more mild taste. As shown in Fig. 2-Fig. 3， the sugar content in tobacco leaves differed according to different transplanting dates. The starch contents in the upper and middle leaves were both the lowest in treatment T2， and the difference between treatments T1 and T3 was not significant. The total sugar and reducing sugar contents were opposite to that of starch， and were the highest in treatment T2， followed by treatments T3 and T1. From the point of position， the starch content of tobacco leaves had no big differences， and the contents of total sugar and reducing sugar in the middle leaves were higher than those in the upper leaves. Main nitrogenous compounds
Tobacco nitrogenous compounds have important effects on the physical properties of tobacco leaves， determine the yield and output value of tobacco leaves to a large extent， and also affect the smoking quality and safety of tobacco leaves. It could be seen from Fig. 4-Fig. 5 that the protein， total nitrogen and nicotine contents in tobacco leaves were different according to different transplanting dates. For the upper leaves， the protein， total nitrogen and nicotine contents all showed an order of T1>T3>T2， indicating that slightly early or later transplanting increased the contents of nitrogenous compounds in cured tobacco leaves. The middle leaves also showed similar laws， the main nitrogenous compounds showed the highest contents in treatment T2， while the nicotine content was basically the same. It indicated that transplanting date had little effect on nicotine content of tobacco leaves.
Potassium and chlorine contents and coordination of chemical components in cured tobacco
The quality of tobacco leaves is largely determined by the internal chemical composition， and the coordination degree among chemical components is crucial. It could be seen from Table 5 that the potassium ion contents in the upper leaves were in order of T1， T3 and T2 from large to small， while the chloride ion content was the smallest in treatment T1， and other two treatments have no difference; and the ratio of potassium to chlorine was in the range of 4.50%-5.52%， falling into the category of high quality tobacco， with T1 being the largest， followed by T3， and T2 being the smallest. The effect of transplanting date on potassium ions in the middle leaves differed， and the potassium ion content was the highest in treatment T3， followed by treatment T1， and the lowest in treatment T2. The chloride ion content decreased with the postponement of transplanting date， while the ratio of potassium to chlorine gradually increased. Furthermore， the ratio of potassium to chlorine was much larger in the middle leaves than that in the upper leaves. Because cigarette products of a higher class have higher watersoluble sugar and less protein， and thus larger ratio of sugar to protein， the best transplanting date was selected to be treatment T2， followed by treatment T3， and treatment T1 had a lower ratio of sugar to protein. The ratio of sugar to nicotine actually reflects the acidbase balance of tobacco leaves， and it is generally believed that the ratio of sugar to nicotine in fluecured tobacco is 6-10. In this study， the upper and middle leaves had a ratio of sugar to nicotine greater than 10， and in both of the two parts， the ratio of sugar to nicotine was the highest in treatment T2 and the lowest in treatment T1. The ratio of nitrogen to nicotine is generally 0.5-0.9， and in the upper leaves， the value of treatment T3 was close to that of treatment T1， and treatment T2 showed a relatively higher value. Economic traits
It could be seen from Table 6 that the final yield， output value， and middle and upperclassleaf percentages differed according to different transplanting dates. The yield was the highest in treatment T1 which might be related to the early transplanting date and long growth period， followed by treatments T2 and T3， and only the difference between T1 and T3 reached the significant level. The differences in the output value and upperclassleaf percentage between the three treatments were both significant， and both showed an order of T2>T1>T3; and the average prices of the three treatments exhibited a trend the same as the differences， but the differences in average price were smaller and did not reach a significant level. The middleclassleaf percentage was the highest in the treatment T3， which was significantly different from other two treatments. It indicated that the transplanting date had a greater impact on the output value of tobacco and the highclassleaf percentage， but had no significant effect on the average price. Taken together， the transplanting date was selected at the time of treatment T2， which showed the best output value， average price and highclassleaf percentage of cured tobacco.
Discussion and Conclusions
When the transplanting date of tobacco plants is different， the weather conditions such as light， temperature， rainfall and humidity at different growth and development stages will be greatly different， which will affect the growth and development of fluecured tobacco and ultimately affect the yield and quality of tobacco. If transplanted too early， the tobacco plants will be threatened by freezing damage， pests and diseases， but if transplanted too late， the temperature will be too high for the plants to grow from rosette stage to vigorous growing stage， and the tobacco plants tend to grow too fast， resulting in less dry matter accumulation， toothin leaves， and even difficult falling of yellow leaves.
Effect of transplanting date on growth and development of tobacco plants
The postponement of transplanting date had a little effect on the number of effective leaves and stem girth of tobacco plants， while the plant height and total leaf area per plant were the largest in treatment T2， followed by treatment T3， and the smallest in treatment T1， that is， they had a trend of increasing at first and decreasing then with the postponement of transplanting date. In this study， the growth period of tobacco plants in field was prolonged with the postponement of transplanting date， and lasted for 134， 140 and 147 d， respectively， and the effect was the greatest on the vigorous growing stage. TMV， weather fleck and brown spot disease showed a consistent law with the postponement of transplanting date， that is， the disease incidence and incidence index decreased with the postponement of transplanting date， except that the incidence of TMV was the smallest in treatment T2， followed by treatment T3， with a small difference therebetween. At rosette， vigorous growing and mature stages， the dry matter accumulation was the largest in treatment T2， followed by treatment T3， and the lowest in treatment T1， and the difference between T3 and T1 was smaller. In general， proper postponement of transplanting date can effectively promote the growth， dry matter accumulation and disease resistance of tobacco plants. Effect of transplanting date on quality of cured tobacco
Transplanting date has an effect on both the internal chemical component contents and their coordination in cured tobacco leaves. The results of this study showed that the upper and middle leaves showed the same performance： with the postponement of transplanting date of tobacco plants， the starch content in leaves decreased， especially in treatment T2， while the total sugar and reducing sugar contents increased at first and decreased then with the delay of transplanting date， indicating that the transplanting time of tobacco plants should be postponed appropriately， which provides higher temperature for late growth and development of tobacco plants， which can promote the carbon metabolism in tobacco leaves to be more sufficient and increase sugar content. However， the main nitrogenous compounds in the upper and middle leaves were the lowest in treatment T2 and slightly higher in treatments T1 and T3. In terms of the coordination of tobacco leaves， the ratio of potassium to chlorine in the middle leaves increased with the postponement of transplanting date， while the differences in the upper leaves were not significant. The ratio of sugar to protein and the ratio of sugar to nicotine ranked as T2>T3>T1， while the ratio of nitrogen to nicotine was not much different. In summary， proper postponement of transplanting date can make the chemical composition of tobacco leaves more coordinated， and treatment T2 performed the best， followed by treatment T3， and they were both better than treatment T1. The yield， output value， average price， and high and middleclassleaf percentages were all the best in treatment T2， and with further delay of the transplanting date， the economic indicators showed a decrease instead. Among them， the differences in the output value and highclassleaf percentage between the three treatments both reached the significant level， indicating that the output value and highclassleaf percentage were more affected by transplanting date.
References
[1]RAN FF， WANG HT， XU ZC. Effects of different transplanting dates and soil types on quality of fluecured tobacco variety NC89[J]. Acta Agriculturae Jiangxi， 2009， 21： 24-26. （in Chinese）
[2]WU L. Matching technology for highquality fluecured tobacco production[M]. Hefei： Press of University of Science and Technology of China， 1994. （in Chinese）
[3]WANG KZ， SUN WQ， WANG YJ. Effects of different transplanting time on growth， yield and quality of tobacco[J]. Shandong Agricultural Sciences， 2009， 2： 48-49， 61. （in Chinese） [4]XU Q， ZHOU ZQ， WU CB. Influences on the growth， yield and quality by the different transplanting times of cigarette seedlings[J]. Fujian Science & Technology of Tropical Crops， 2003， 3：8-10. （in Chinese）
[5]WANG GZ， SUN WQ， WANG YJ. Effects of different transplanting time on growth， yield and quality of tobacco[J]. Shandong Agricultural Sciences， 2009， 2： 48-49， 61. （in Chinese）
[6]GUO FJ， YE XQ， ZHANG JZ. Effects of sowing and transplanting dates on growth， yield and quality of fluecured tobacco[J]. Subtropical Agriculture Research， 2017， 2： 93-98. （in Chinese）
[7]GAO WK， CHEN J， LUO HH， et al. Effects of transplanting time on growth， quality and style of fluecured tobacco[J]. Journal of Anhui Agricultural Sciences， 2015， 33： 48-50， 53. （in Chinese）
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