Calculation method of wilting index for the soybean canopy based on the three-dimensional Fourier transform

Abstract

Drought stress significantly affecting soybean yield, thus, rapid detection of phenotypic traits for the soybean canopy is crucial for the breeding and cultivation. Traditional methods in the determination of soybean wilting index have the problems of cumbersome and time consuming, thus, a calculation method of wilting index was proposed for the soybean canopy based on three-dimensional (3D) Fourier transform. First, the Kinect 2.0 sensor was used to acquire the 3D point cloud data of the soybean canopy, and the filter integration algorithm was applied to remove the invalid point cloud. Second, a height-based slicing algorithm was used to convert the 3D spatial structure of the canopy into a 2D image sequences. Third, a constraint function based on spectral features was constructed to select the optimal sequence position as 1/2 of the canopy, which realized a 3D Fourier transform algorithm of the soybean canopy. Finally, a 3D Fourier transform-based soybean wilting index was proposed based on the proportion difference of the direct component (DC) and the spectral radius energy to the total energy in the 3D canopy based on the normal water treatment (NWT) and the drought stress treatment (DST). The result showed that the test coefficient $p$ of the significant difference reached 1.12 × 10⁻⁷ under NWT and DST, and the determination coefficients $R^2$ between the calculated wilting index and the corresponding mean leaf inclination were all above 0.83. The research result can accurately characterize the wilting degree, and provide technical support for irrigation, seed examination of the drought soybean plant.