Airborne Subnanometer Hyperspectral Imaging Reveals Early Biotic-Stress Symptoms via Fraunhofer Lines

Scalable plant monitoring methods are needed for the early detection of physiological changes induced by pathogens and abiotic stressors. Recent advances in sub-nanometer hyperspectral imaging enable the detection of Fraunhofer lines (FLs), which represent distinct narrow absorption lines in the solar spectrum. This study explored the use of FLs to detect subtle physiological changes associated with early stages of biotic stress. Airborne sensors comprising a sub-nanometer hyperspectral imager, a narrow-band hyperspectral imager, and a thermal camera were flown over a commercial olive orchard infected with the vascular pathogenVerticillium dahliae. Thermal imagery screened early-stage symptomatic trees under fixed water stress levels so that canopy temperature would remain insensitive to transpiration changes caused by pathogen colonizing the vascular system. We then assessed the sensitivity of FLs and oxygen absorption features (O2-A and O2-B) by comparing absorption depths between asymptomatic and symptomatic trees. The sub-nanometer O2-A feature and six FLs differentiated early-stage symptomatic trees under constant (i.e., insensitive) water-stress levels, which the canopy temperature assessment failed to detect. A classification model integrating the O2-A band depth with two sensitive FL depths (FL680.98-nmand FL744.581-nm) achieved a mean overall accuracy of 0.77 and a mean kappa value of 0.55. These findings underscore the potential of FLs derived from airborne sub-nanometer imagery for early detection of vascular diseases when canopy temperature is insensitive to subtle transpiration depletion. This exploratory work provides a foundation for future research towards the role of sub-nanometer imaging-derived fluorescence emission in monitoring plant biotic-induced physiological responses.