Three-dimensional fluorescence spectral characteristic of flavonoids for citrus Huanglongbing disease early detection

To explore the potential of using flavonoid fluorescence characteristics in citrus leaves and peels for detecting citrus Huanglongbing (HLB), this study focused on Citrus reticulata ‘Shiyue Ju’ and utilized three-dimensional fluorescence spectroscopy for data collection. Excitation-Emission matrices (EEM) of flavonoid standards and citrus leaves and peels were acquired and comparatively analyzed. The concentration of HLB pathogens in citrus tissues was also assessed using qPCR. In order to identify the range of fluorescence bands that are sensitive to HLB, different machine-learning algorithms were used to construct classification models. Support Vector Machine (SVM) was proved most precise, with 93.1 % accuracy for leaves and 86.67 % for peels. The sensitive bands for leaves were EX (Excitation) = 375–455 nm, EM (Emission) = 490–570 nm and for peels were EX = 300–340 nm, EM = 430–470 nm and EX = 330–370 nm, EM = 425–465 nm. These bands correspond closely with the fluorescence signatures of nobiletin, hesperidin, and narirutin. It was further verified that HLB stress affected the changes in the content of flavonoids in the leaves and peels of ‘Shiyue Ju’. This study provides a reference for using flavonoid fluorescence in citrus HLB detection and presents a novel approach for applying spectral imaging techniques in citrus HLB detection.