Effects Of Nitrogen Levels On Hyperspectral Characteristics And Chemical Composition Of Flue-Cured Tobacco Leaves

Hyperspectral analysis is playing an increasingly important role in the field of agriculture.It is widely used in the fields of information acquisition,crop growth judgment,crop yield quality estimation and so on.Nitrogen has an important effect on the growth,yield and quality of flue-cured tobacco,and tobacco is very sensitive to nitrogen nutrition.It is very necessary to clarify the spectral characteristics of tobacco and the basic characteristics of its spectral response to nitrogen,and use hyperspectral analysis technology to rapidly monitor the growth and nutrition of flue-cured tobacco and detect the content of each component.In this experiment,portable ground object spectrometer was used to measure the spectrum of fresh flue-cured tobacco leaves at different growth stages in the laboratory by a non-destructive method.Yunyan 87,K326 and Honghua Dajin Yuan were selected as the research objects,and field experiments with different nitrogen treatments,different parts,different leaf sites and different leaf water content were set up.The chemical composition changes of fresh tobacco leaves and hyperspectral reflection characteristics of flue-cured tobacco leaves were studied,and the relationship between the chemical composition of tobacco leaves and the corresponding hyperspectral parameters of leaves was analyzed.Stepwise regression method was used to establish an estimation model for various chemical components,and through testing the predicted and measured values of test samples,the estimation and prediction effects of the model were good.The results show that:(1)The change trend of spectral reflectance of leaves of different varieties of flue-cured tobacco under different nitrogen levels was basically the same.The hyperspectral reflectance curves of leaves of different varieties of flue-cured tobacco under different nitrogen levels had obvious rules,and the change rule was basically the same.The reflectance of leaves of flue-cured tobacco under different nitrogen treatments had significant differences.From the perspective of the whole growth period,the reflectance of leaves in the visible range(400-700 nm)changes as follows:With the increase of nitrogen content,the spectral reflectance of leaves is as follows:N1>N2>N3>N4,the spectral reflectance of flue-cured tobacco leaves was the lowest in the middle leaf maturity stage,increased in the prosperous period,and decreased in the mature period,but the overall change trend was not obvious.The reflectance of the three varieties in the three periods was basically maintained at about 0.2.In the near infrared region(700-1075 nm),the spectral reflectance of leaves was N4>N3>N2>N1 with the increase of nitrogen content.(2)The effects of different nitrogen treatments on the chemical constituents of the three varieties of fresh tobacco leaves were basically the same.During the whole growth period,the chemical components of total sugar,reducing sugar,total nitrogen,nicotine,total potassium,chloride ion,starch,protein,total polyphenols,chlorophyll a,chlorophyll b,total chlorophyll,lutein and water increased with the increase of nitrogen application level,as follows:N4>N3>N2>N1.During the whole growth period,the contents of main carbohydrate such as total sugar,reducing sugar and starch increased with the increase of nitrogen application level.However,the variation trend of starch content in each growth stage was different from that of total sugar and reducing sugar,which showed that the middle leaf maturity stage>maturity stage>prosperous stage.The contents of total nitrogen,nicotine,protein and other nitrogen compounds increased with the increase of nitrogen application rate.With the development of growth period,the content of total nitrogen and protein in tobacco decreased gradually,while the content of nicotine increased gradually.In the whole growth period,chlorophyll a,chlorophyll b,total chlorophyll and lutein contents of tobacco leaves treated with nitrogen showed an increasing trend.However,the variation trend of lutein content in each growth stage was different from that of chlorophyll a,chlorophyll b and total chlorophyll,which showed that the middle leaf maturity stage>maturity stage>flourishing stage.Water content,total polyphenol content,total potassium content and chloride ion content all increased with the increase of nitrogen application level.With the advance of growth period,the water content of tobacco leaf decreased gradually,while the total amount of polyphenols increased gradually.Total potassium content first increased and then decreased,chloride content first decreased and then increased.(3)By observing the reflectance of flue-cured tobacco leaves during the growing period of the field,yellow edge amplitude(Dy)and blue edge position(λy)were found to be the most correlated position variables with total sugar content,reducing sugar content,nicotine content and total potassium content.The position variables that had the strongest correlation with total nitrogen content,starch content,protein content,total polyphenol content,chlorophyll a content,chlorophyll b content and total chlorophyll content were red edge amplitude(Dr),blue edge amplitude(Db),red edge position(λr),green peak amplitude(Rg)and red light absorption valley amplitude(Rr).Blue edge amplitude(Db)and red edge position(λr)were the most correlated position variables with chloride ion content,lutein content and moisture content.The red side area(SDr)and the normalized ratio of red side area to yellow side area(SDr-SDy)/(SDr+SDy)have the strongest correlation with total sugar content and reducing sugar content.The area variables and vegetation variables that had the strongest correlation with nicotine content,chloride ion content,starch content,protein content,chlorophyll a content,chlorophyll b content,total chlorophyll,lutein content and water content were the ratio of green peak to red valley(Rg/Rr)and the normalized ratio of red border area to blue border area(SDr-SDb)/(SDr+SDb).The area variables and vegetation variables that were most correlated with total nitrogen content,total potassium content and protein content were red border area(SDr),yellow border area(SDy),and the normalized ratio of red border area to blue border area(SDr-SDb)/(SDr+SDb).Yellow border area(SDy)and green peak to red valley ratio(Rg/Rr)were the most correlated area variables and vegetation variables with total nitrogen content,chloride ion content,starch content,protein content and polyphenol content.(4)With 19 hyperspectral parameters as independent variables and chemical components as dependent variables,a stepwise regression prediction model for each chemical component was established.Through the significance test of the regression coefficient R~2of the independent variable of the model,and through the test of flue-cured tobacco leaf test samples of different parts,different leaf sites and different leaf water content,the results all reached significance.It is feasible to estimate the chemical composition of flue-cured tobacco by spectral parameters.To predict the chemical composition content of flue-cured tobacco leaves and obtain the change information of chemical composition content in the growth process of flue-cured tobacco in time can quickly and simply predict the growth and quality of flue-cured tobacco under different soil nitrogen content levels,and provide a theoretical basis for the development of portable tobacco quality testing equipment.