Response Mechanism Of Cadmium Stress Based On Rice Canopy Leaves Spectrum

Leaf is an important indicator of rice response to environmental stress,and also is an important source organ of rice and.The early rapid and nondestructive detection of cadmium(Cd)pollution in rice canopy leaves is the premise and the core of preventing and controlling the transfer of Cd from rice plants to grains during grain filling.This Nondestructive detection is based on hyperspectral remote sensing technology,it directly diagnose the degree of Cd Stress on rice plants and indirectly estimate the Cd pollution in rice grains.This hyperspectral remote sensing technology is great significance to improve and monitor the level of Cd pollution in crops and ensure food safety production.This study was based experiments on rice with different Cd stress levels and foliar spraying treatments,obtained information about leaf and canopy hyperspectral reflectance spectrum and the corresponding agronomic parameters in critical growing stage,and then analyzed impact of different biochemical and spectral of rice under different Cd stress levels,determined Cd sensitive bands and spectral characteristic parameters,established stable and reliable monitoring Cd model in rice.Firstly,we studied the Cd content and the migration law of different parts of rice under foliar spraying.The results showed that the Cd stress treatment not only significantly affected the Cd content of rice leaves,but also changed the ability of Cd transport from rice roots,stems,leaves,rachis and husk to brown rice in different degree.Foliar spraying treatment could significantly reduce the content of Cd in brown rice when the content of Cd in soil was 1.8 mg·kg-1 and 8.4 mg·kg-1.At the same time,the foliar spraying had the greatest effect on the transport of Cd to brown rice from leaves and rice husk.The Cd content of rice canopy leaves at jointing stage had the greatest impact on the Cd content of brown rice,while the correlation between Cd content of top 2 leaves and canopy leaves was the highest at jointing stage.Secondly,we studied the physiological,biochemical and spectral response of rice under the Cd stress.The results showed that under the Cd stress,the biomass and Leaf SPAD value of rice decreased significantly,the content of MDA(malondialdehyde)increased significantly,and the activities of POD(peroxide dismutase)and SOD(superoxide dismutase)increased significantly.At the same time,the reflection spectrum of rice showed an upward trend in the range of green peak(530-580 nm)and near infrared platform(750-850 nm).The foliar spraying treatment significantly changed the inhibitory effect of Cd Stress on the above physiological and biochemical parameters,but reduced the spectral reflectance of rice at green peak(530-580 nm)and near-infrared platform(750-850 nm).Finally,we studied the monitoring of Cd content in rice canopy leaves based on reflection spectrum.The results showed that the monitoring model based on the first derivative spectrum is better than that based on the original spectrum.In the band combination constructed by normalized spectral index(NDSI),ratio spectral index(RSI)and difference spectral index(DSI),the best monitoring band combination of top 1 leaf is761 nm and 772 nm,the best monitoring band combination of top 2 leaves is 811 nm and799 nm,the best monitoring band combination of top 3 leaves is 881 nm and 507 nm,the best monitoring band combination of top 4 leaves is 890 nm and 870 nm,and the best monitoring band combination of canopy leaves is 710 nm and 529 nm.Compared with the accuracy of modeling and verification on single leaf scale and canopy scale,the effect of single leaf scale is better than that on canopy scale.Among them,the best accuracy of prediction and verification of Cd content is the top 2 leaves’model,and the same R~2is0.658.In conclusion,the Cd content in the top 2 leaves was extremely significantly correlated with the canopy leaves,and the Cd content predict and test model based on the reflection spectrum of the top 2 leaves is the best of all others.This results will provide technical support for the monitoring and early warning of Cd stress in rice exposed to Cd pollution.