Study On Variation Of Spectrally Sensitive Growth Stage And Pollution Element Identification Of Maize Under Copper And Lead Stress

Agriculture security is the lifeblood and fundamental guarantee of the development of people’s livelihood,and the detection and control of agricultural pollution in our country has been raised to a strategic level.This thesis designed three phases of laboratory maize cultivation experiments for two heavy metals with different pollution mechanisms,copper and lead,using different concentrations of copper and lead to pollute the soil to simulate the growth environment of maize under the stress of copper and lead.Collecting the reflectance data of the 350～2500nm spectral range of the maize leaf during the key growth period under copper and lead pollution,and the copper and lead content in the maize leaf,and the Soil Plant Analyzer Development（SPAD）value of the leaf,respectively.In the process of obtaining experimental data,the leaf layer is the abbreviation of the position of the leaf layer.The position of the leaf layer not only includes the growth position of the maize leaf,but also includes the growth degree of the leaf at the same leaf layer position,and the difference in the size of the leaf is referred to as leaf.Among them,the top leaf（Top,T）refers to the top leaf layer and leaf position of the maize plant.Middle leaf（Middle,M）refers to the middle leaf layer and leaf position of the maize plant.Bottom（Bottom,B）refers to the lower leaf layer and leaf position of the maize plant.Study began from from three aspects below:First,the copper and lead pollution monitoring during the key growth period of maize is studied based on the area-based quotient variability algorithm,and the most suitable leaf layer in seedling stage and heading stage for copper and lead pollution monitoring is selected from three leaf types:top leaf,the middle leaf and the bottom leaf.In the full spectral range of surface reflectance of maize leaves,eight characteristic extreme points were selected to construct four typical characteristic regions by using feature extraction methods such as spectral differentiation.The variation radar charts of area-based quotient variability mean value,and SPAD value,and heavy metal content in leaves were drawn.According to the comparison of the hexagonal area of the radar chart,it is concluded that the bottom leaf of maize is the best choice of leaf layer in the monitoring of copper and lead pollution at the heading stage.It can be concluded that when maize at the seedling stage for the optimal selection of leaf type,the variation radar charts of the area quotient and the SPAD value under lead stress is more obvious than that of copper stress.The comparison of the hexagonal area of the radar of area quotient and SPAD value under copper stress was not clear.In the process of leaf layer selection of top,middle and bottom leaves at the heading stage of maize,the difference in the hexagonal area of the mean area quotient,SPAD value and leaf heavy metal content in the radar chart under lead stress was more obvious than that under copper stress,and both copper and lead stress indicated that the bottom leaf of maize is the optimal leaf layer selection for monitoring.Second,based on the obtained data,the frequency spectrum and amplitude spectrum of spectra were analyzed,and the spectra was decomposed and transformed by discrete wavelet to find the characteristics of minor variation.The results showed that the spectral phase of maize at seedling stage and jointing stage was horizontal and there were some differences,but the spectral phase at heading stage was inclined distribution.It’s showed that the most spectrally sensitive key growth stage of maize is heading stage.The introduction of the concept of zero-crossing rate made the results more quantitative and accurate.Therefore,the growth period of maize could be effectively determined according to the phase of leaf spectrum under copper stress treatment.Through further analysis of spectral transformation of maize leaves at heading stage by discrete wavelet,approximate coefficient‘ca’and detail coefficient‘cd₁’were obtained respectively.It was found that the wavelength corresponding to the peak value of approximate coefficient‘ca’in the range of 690～710nm was moving in the direction of shortwave.In addition,there was a peak offset boundary because of a jump in wavelength between CK to Cu（200）and Cu（800）to Cu（1000）.In this way the stress gradient interval of maize could be divided according to the peak deviation.The detail coefficient‘cd₁’fluctuated greatly in the total five bands of 680 to770nm,900 to 1000nm,1360 to1440nm,1600 to 1700nm and 1840 to 1920nm.The original spectrum also had some distortion in the corresponding band region.Third,the methods such as Minimum Entropy Deconvolution（MED）and Empirical Mode Decomposition（EMD）are combined with Auto-Regressive Moving Average（ARMA）bispectrum estimation to distinguish the elements of copper and lead pollution in maize leaves,and using Multiple Linear Regression（MLR）to construct the MED-EMD-MLR model to predict the copper and lead content of maize leaves.An identification model of copper and lead pollution elements in maize leaves were established based on the spectral reflectance data processing by spectral differentiation technology,MED and EMD and other spectral analysis methods,and relying on the advantages of the ARMA bispectral estimation visualization in high-order spectra,a three-dimensional visual graphy of copper and lead in maize leaves was constructed.According to the ARMA bispectral three-dimensional graph,the spectral reflectance curves of copper and lead under different stress gradients can be distinguished intuitively;and the difference of the spectral curves under copper and lead pollution can be distinguished.Meanwhile,a robust and reliable estimation model for the content of copper ions and lead ions in maize leaves is proposed,which is a MED-EMD-MLR model based on the sensitive band of IMF3 component.The test results show that the model can not only distinguish the types of copper and lead pollution elements in maize leaves,but also accurately predict the copper ions and lead ions contents in maize leaves while obtaining ideal model test results.The current research status of monitoring of vegetation heavy metal pollution,and monitoring of vegetation key growth period,and identification of vegetation heavy metal pollution element and so on are studied at home and abroad.In order to provide scientific reference and theoretical basis for the research on difference characteristics of maize leaf spectral information under the stress of copper and lead.It is explored from three aspects:the optimal selection of maize leaf layer for heavy metal pollution monitoring,the distinction of spectrally sensitive key growth stage of maize and the qualitative monitoring of heavy metal pollution,the discrimination of pollution elements of maize and the estimation of heavy metal content.