Effects Of Topping Time And Available Leaf Number On Growth,Development And Quality Of Upper Leaves Of Flue-cured Tobacco

Due to unreasonable cultivation measures and poor harvest maturity,the industrial availability of upper leaves of flue-cured tobacco is limited,which is difficult to meet the requirements of raw materials for the improvement of cigarette structure in China.How to improve the industrial usability of upper leaves is an important means to promote the high-quality development of tobacco leaves.In order to explore the effects of different topping times and available leaf number on the growth,development and quality of upper leaves,Zhongyan 100 was used as the test material in the field experiment at Juxian Tobacco Experimental Station in Shandong Province.The effects of topping times and available leaf number on agronomic traits,photosynthetic characteristics,senescence characteristics,carbon and nitrogen metabolism,internal chemical composition,yield and quality of tobacco leaves were analyzed to further reveal the physiological and biochemical mechanism that affects the formation of upper leaves quality,and provide a theoretical basis for improving the availability of upper leaves industry and promoting the high-quality development of tobacco leaves in Shandong Province.The main research results are as follows:（1）In terms of agronomic traits,appropriate topping time and available leaf number retention could significantly improve plant height,leaf length,leaf width and leaf thickness of tobacco.Increasing the number of leaves retained and delaying the topping time will lead to the increase of plant height;the leaf length and leaf width showed a decreasing trend with the increase of leaf retention and the delay of topping time,and the decrease of leaf width was greater than that of leaf length,which led to the increase of length-width ratio of tobacco leaves and affected the opening of tobacco leaves.The length-width ratio of 20 treatments with full flowering topping and leaf retention was the largest,and the treatment with early flowering topping and leaf retention was the smallest,which was 12.95%lower than that of the former,and the opening of tobacco leaves was better.Leaf thickness was negatively correlated with leaf number,and decreased with the delay of topping period.（2）In terms of photosynthetic and senescence characteristics,the chlorophyll content in the upper leaves of flue-cured tobacco showed a decreasing trend with the increase in the number of leaves retained,and the net photosynthetic rate was the lowest in the treatment of 20 leaves retained at the late maturity stage,with not significant difference in different topping times.From the topping time,SOD activity in early flowering topping was higher than that in full flowering topping,with a decrease of 1.06%,and POD activity in full flowering topping was higher than that in early flowering topping,with a decrease of 2.49%.From the number of leaves retained,antioxidant enzyme activity was negatively correlated with the number of leaves retained,MDA content increased with the increase of leaves retained,and the effect of topping time on each treatment was not obvious.Appropriate leaves retained can appropriately reduce the photosynthetic capacity of the upper leaves in the late mature stage and promote yellowing and maturity.In general,the activities of antioxidant enzymes in the upper leaves at the late mature stage were low and the photosynthetic capacity was suitable under the treatment of 20 pieces of leaves with first flowers topping and leaves remaining,which could better yellow and senescence and improve maturity.（3）In terms of carbon and nitrogen metabolism and chemical components,delaying topping can effectively reduce the nicotine content in the upper leaves of flue-cured tobacco.Under the low leaf retention level,the NR activity of different topping times is different,and the strength of nitrogen metabolism is different,which leads to the significant difference of nicotine content.However,delayed topping reduced the activity of enzymes related to carbon metabolism,resulting in low content of two sugars in upper leaves and uncoordinated carbon and nitrogen metabolism,which is not conducive to the coordination of chemical components and the improvement of tobacco quality.Topping at the beginning of flower is conducive to the accumulation of soluble sugar.With the increase of the number of leaves left,NR activity decreases,NI and amylase activities are higher,resulting in an upward trend of reducing sugar content and a decrease of nicotine content,which can improve the sugar alkali ratio.In general,the treatment of topping at the beginning of flower and leaving 20 leaves can improve the carbon and nitrogen metabolism process of upper leaves of flue-cured tobacco,improve the coordination of chemical components of tobacco leaves,and then improve the usability.（4）In terms of economic traits,suitable topping period can improve the yield of flue-cured tobacco and the proportion of upper and middle tobacco,and the yield and the proportion of upper and middle tobacco in the early flowering topping treatment were higher,which were 2.32%and 2.13%higher than those in the full flowering topping treatment,respectively.With the increase of leaf number,the proportion of upper and middle tobacco increased in the first flowering topping,and increased first and then decreased in the full flowering topping.The number of leaves retained had a certain impact on the proportion and output value of superior tobacco.The proportion and output value of superior tobacco in the treatment of 18 and 20 leaves retained were higher.In general,the proportion and average price of superior and middle tobacco in the treatment of first flowering topping and 20 leaves retained were the highest,and the output value was higher.