Physiological foundations of spectral imaging-based monitoring of apple fruit ripening
An important aspect of precision horticulture methodology is comprised by online monitoring of apple crop load, gauging fruit expansion, and estimating the extent and the rate of ripening. Remote monitoring based on multi- and hyperspectral imaging is a powerful tool for solving this problem. Still, its potential is often limited by insufficient understanding of the relationships between the observed changes in fruit optical properties and the evolution of the fruit biochemistry and other properties in the course of ripening. Non-invasive estimations of apple ripening are based predominantly on the chlorophyll degradation kinetics derived from the spectral reflectance data calibrated against biochemical and rheological assays. Nevertheless, the fruit skin chlorophyll displays irregularities in response to fluctuation in environmental (sunlight, temperature) and other stimuli obscuring the changes caused by ripening itself. Another major pigment group, carotenoids display the opposite pattern of changes: these pigments are often retained or even accumulated during apple ripening. Consequently, the ratio of the contents of carotenoids and chlorophylls displays more robust trends in apple fruit ripening reflecting the variation in on-tree and off-tree ripening rate as affected by environmental stimuli as compared to assessment of degradation of the chlorophylls alone. We propose to use the previously developed in our group plant senescence reflectance index (PSRI), which is closely related with carotenoid-to-chlorophyll contents ratio in fruit and hence with their ripening. The PSRI values plotted vs. chlorophylls (also derived from fruit reflectance) provide for a robust recording of apple ripening both on tree and in storage yielding data sets comparable across orchards and growing seasons. We demonstrate the applicability of this approach to reveal the effect of apple fruit picking date on their ripening rate in storage. We also test the 'PSRI vs. chlorophylls' approach for visualization of apple ripening on hyperspectral images and discuss its limitations and possible workarounds.
Solovchenko, A., Lukyanov, A., Nikolenko, A., Shurygin, B., Akimov, M. and Gitelson, A. (2021). Physiological foundations of spectral imaging-based monitoring of apple fruit ripening. Acta Hortic. 1314, 419-428
hyperspectrometer, proximal sensing, chlorophyll, carotenoids, postharvest monitoring, reflectance