Hyperspectral Estimation Of Rice And Wheat Growth Parameters Under High CO₂ Concentration

Due to the energy utilization,exhaust emissions and other human activities in various parts of the world,the concentration of CO₂in the atmosphere has approached 400?mol·mol^-1.If the current population growth and unreasonable exploitation and utilization of resources and energy are considered,the concentration of CO₂in the atmosphere is expected to reach 550?mol·mol^-1in the middle of this century and even exceed 700?mol·mol^-1by the end of this century.China is a developing country with a population of 1.4 billion,the importance of agriculture is self-evident.Rice and wheat are two staple foods in our daily life,it is of great theoretical significance to study the effect of elevated CO₂on rice and wheat growth parameters and the best estimation model of different growth parameters by hyperspectral spectroscopy for the growth of rice and wheat.It will have an important impact on the growth of rice and wheat,the management of farmland and the maintenance of long-term food security in China and even the world under the future global climate change.In this study,rice(Yangdao 6,Wuyunjing 30)and winter wheat(Ningmai 13)were selected as experimental materials,an open-top chamber experiment in 2019 was used to carry out the field experiment with ambient CO₂and elevated CO₂.The field spec 4 portable ground object spectrometer was used to measure the canopy spectral curve data of rice and winter wheat at key growing stages.Leaf area index(LAI),relative chlorophyll content(SPAD)and plant height of rice and winter wheat were also measured simultaneously.The red edge characteristic parameters of rice and winter wheat spectrum under ambient and elevated CO₂ concentration were analyzed and the correlation between LAI,SPAD value and plant height with original spectral reflectance and spectral characteristic parameters was analyzed to seek the best estimation model for LAI,SPAD and plant height.In 2019,there were two levels of CO₂ concentration in winter wheat and rice growing season,and the ambient CO₂(CK)and ambient CO₂+200?mol·mol^-1(T),CK and T treatments were repeated in 4 times.The main results are as follows:(1)The elevated CO₂ concentration increased the SPAD value of the booting-heading stage and flowering stage of winter wheat,decreased the SPAD value of the filling stage and the maturity stage,increased the SPAD value before the maturity stage of the low-response rice varieties,and decreased the SPAD value of the high-response rice varieties during the jointing stage,booting heading stage and filling stage;Elevated CO₂ concentration also increased LAI of winter wheat before filling stage,decreased LAI of winter at filling stage and maturity stage,decreased LAI of low-response rice varieties after jointing stage,and increased LAI at whole growing stage of high-response rice varieties;Elevated CO₂ concentration increased the plant height of winter wheat before maturity stage,decreased the plant height of winter wheat at maturity stage,increased the plant height of high-response rice varieties during the whole growing stages,decreased the plant height of low-response rice varieties at booting-heading stage,and increased the plant height of low-response rice varieties at other growing stages.(2)Elevated CO₂ concentration did not change the spectral curve of rice and wheat canopy at different growing stages,but only affected the reflectance at different growing stages.The difference of reflectance of winter wheat and rice was caused by the difference of growing stage caused by different CO₂ concentration.Elevated CO₂ concentration decreased the spectral reflectance of winter wheat in the whole growing stages,increased the spectral reflectance of low-response rice varieties before maturity stage,decreased the spectral reflectance of low-response rice varieties at maturity stage,and decreased the spectral reflectance of high-response rice varieties at vegetative growing stages.The first derivative of rice wheat spectrum was of"single peak","double peak"or"multi peak"type,and the"peak"phenomenon gradually weakens with the advance of growing stages.Elevated CO₂ concentration decreased the red edge amplitude and red edge area of winter wheat in the whole growing stages,increased the red edge amplitude and red edge area of low-response rice varieties before maturity stages,and decreased the red edge amplitude and red edge area of high-response rice varieties before grain filling stage.The influence amplitude varied with different growing stages of rice and wheat.(3)The best regression model for estimating LAI of winter wheat by hyperspectral first derivative R'755,the regression equation was y=-12910x²+488.5x+1.389.Red edge position?r was the best spectral parameter to estimate LAI of rice,the regression equation was y=3.44×10^-3x²-4.895x+1745.The best regression model for estimating SPAD value of winter wheat by hyperspectral first derivative R'755,the regression equation was y=-6.399×10⁵x²+7667x+36.67.Photochemical reflectance index(PRI)was the best spectral parameter to estimate SPAD value of rice,the regression equation was y=-1151x²-33.09x+45.24.It is the best to build the rice plant height estimation model based on the red edge position?r,the regression equation was y=-0.07282x²+109.4x-40950.