The Monitoring of the Pollution Degree of Maize Under Copper Stress

Abstract

This study proposed a new vegetation index ADVI to detect the pollution degree of different varieties of maize under copper stress, which provides a new idea for the detection of heavy metal pollution in vegetation. In order to ensure the outdoor growth environment of maize, we put all maize into outdoor greenhouse. The spectral reflectance interval of 450–850 nm of maize leaves was processed by the absorbance conversion (A) and the first-order differential (D), and the absorbance differential (AD) spectral curve was obtained. The Pearson correlation coefficient (r) was used to analyze the AD data and the biochemical data and select characteristic bands that are sensitive to heavy metal copper (Cu). The calculated Pearson correlation coefficients suggested that the AD value at 547–553 nm and 672–674 nm presented a linear positive correlation close to 1 with the Cu2+ contents in soil and leaves, and a linear negative correlation close to − 1 was present in the range of 496–506 nm and 677–679 nm. We selected the AD value of wavelengths 501 nm, 550 nm, 673 nm, and 678 nm to establish ADVI and compared it with conventional vegetation indices (VIs) by calculating Pearson correlation coefficient between them and Cu contents in soil and leaves, and vegetation indices include Water Band Index (WBI), Photochemical Reflectance Index (PRI), MERIS Terrestrial Chlorophyll Index (MTCI), Modified Red Edge Simple Ratio Index (mSR), Modified Chlorophyll Absorption Ratio Index (MCARI), Water Index (WI), Normalized Difference Water Index (NDWI). Maize leaf spectral data obtained from experiments in 2017 were used for verification, and ADVI was also compared with WBI, PRI, MTCI, mSR, MCARI, WI, and NDWI. The calculated Pearson correlation coefficient between ADVI and Cu2+ contents in soil and leaves is 0.9816 and 0.9460 (experimental data of 2016). The calculated Pearson correlation coefficient between ADVI and Cu2+ contents in soil and leaves is 0.9109 and 0.9639 (experimental data of 2017). The results suggested that ADVI showed a significant correlation with Cu2+ stress concentration, and the correlation of ADVI was much stronger than that of other vegetation indices. The proposed ADVI detects the pollution degree of maize with different varieties and in different periods under copper stress has advantages of straightforward calculation, robustness, and high effectiveness. This study focused on the laboratory leaf scale, so it is expected that future work extends it to a wide range of field scale and image scale.