Inversion of Chlorophyll Content and SPAD Value of Vegetable Leaves Based on PROSPECT Model
Chlorophyll content is an important indicator for evaluation of plant nutrition and the occurrence of the pests .T he tra-ditional spectrophotometry causes damage to plant leaves and can not be used to obtain the chlorophyll content in a real time ,fast and non-destructive way .T he chlorophyll meter is recently developed to measure the relative content of chlorophyll (referred to as SPAD value).But this method cannot be used to quantitatively obtain the actual content .T he optical radiation transmission model PROSPECT can quantitatively describe the effects of leaf pigment ,moisture and structural parameters on the reflection spectrum of the leaves with careful consideration of the biophysics ,chemistry and the process of energy transfer in the leaves .In the paper ,therefore ,this model is used to simultaneously inverse the chlorophyll content and SPAD value of vegetable leaves , and obtain the chlorophyll content of plant leaves real-timely ,quickly ,non-destructively and quantitatively .First ,the reflection spectra of the leaves for three vegetables were measured several times ,and the SPAD values of these leaves were measured with a chlorophyll meter .T hen ,the spectral data were preprocessed to obtain the average reflectance spectrum .Second ,the averaged reflectance spectra were fitted by the PROSPECT model with the Euclidean distance as the evaluation function .T he maximum distance of the three vegetables in the fitting process was 0.008 9 ,the minimum was 0.006 4 ,and the average was 0.007 5 . Such a low Euclidean distance indicated that the model could well fit the reflectance spectrum of vegetable leaves .T hirdly ,ac-cording to the fitting results ,the chlorophyll content and the transmittance spectrum were inversed ,and the inversed SPAD val-ue of the leaves was calculated with the light transmittance of the leaves at 940 and 650 nm as input parameters .Fourth ,this pa-per established a relationship model between the inversed chlorophyll content ,the inversed SPAD value and the measured SPAD value.T wo main results were obtained :(1) the chlorophyll content obtained by the model has a good linear relationship with the measured SPAD value and the relationship model is y＝ 1.463 3 x+ 16.374 3 .T he correlation coefficient between them is 0.927 1 .T he coefficient of determination of the model is 0. 862 and the root mean square error is 2.11 .(2) A good linear rela-tionship of y＝0. 986 9 x－0.668 3 is obtained for the inversed SPAD value and the measured SPAD value .T he correlation coeffi-cient between them is 0.845 1 .T he coefficient of determination of the model is 0. 714 3 and the root mean square error is 3.380 2 .T he research show s that the PROSPECT model can be used to obtain the chlorophyll content and SPAD value of vege-table leaves nondestructively and quantitatively with the measured reflectance spectrum of plant leaves as input parameters .T his method can be extended to other plants for chlorophyll measurement and real-time monitoring and can provide reliable data sup-port for variable rate fertilization and precision planting .T he results presented in this paper can be applied to monitor the grow th of the vegetables nondestructively .
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