Systematic Investigation of Microstructural and Spectral Characteristics in Citrus Midrib for Huanglongbing Detection

Citrus Huanglongbing (HLB) disease is a devastating disease faced by the global citrus industry, for which there is currently no effective cure. This study systematically investigated the anatomical characteristics and infrared spectral properties of different microstructures (phloem, xylem, pith, and cortical tissues) in the midribs of healthy and HLB-infected citrus leaves. Scanning electron microscopy observations revealed obvious phloem breakage and massive starch granule accumulation in various tissues of HLB-infected samples. Using micro-Fourier transform infrared spectroscopy, the spectral acquisition parameters were optimized (slice thickness: 10 μm, spectral resolution: 8 cm^-1, spatial resolution: 10 μm × 10 μm, number of scans: 256), and in-situ spectral information from different tissues were obtained. The results showed significant changes in the intensity and position of absorption peaks in the fingerprint region (1,800 to 675 cm^-1) of all tissues after HLB infection, particularly enhanced carbohydrate absorption at bands such as 1,099, 1,060, and 1,033 cm^-1, indicating that abnormal carbohydrate accumulation is a typical symptom of HLB. A principal component analysis score plot based on spectral data from the phloem demonstrates a clear spatial separation trend between healthy and HLB-infected samples, providing a theoretical basis and methodological support for the fast, early, nondestructive detection of citrus HLB disease.