The Contribution of the Fruit Component to the Hyperspectral Citrus Canopy Signal

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Very few attempts have been made to qualify and quantify the contribution of fruits to the hyperspectral canopy signal. The interference of fruit may influence the effectiveness of spectral indices and radiative transfer models used to monitor leaf-related biochemical and biophysical parameters. In a series of experiments, the spectral characteristics (350 to 2,500 nm range) of fruit and its contributions to the canopy signature were investigated. Leaf, fruit, and canopy spectra were collected in a Midknight Valencia orange orchard, evaluated, and cross-referenced against biophysical and biochemical tree characteristics in order to gain a better insight in the contribution of citrus fruits to canopy spectral reflectance. Results indicate that the presence of fruits leads to major reflectance decreases in the infrared regions (700 to 2,500 nm) of the electromagnetic spectrum. This may be explained by, on the one hand a partial obstruction of the volume scattering in the canopy, and on the other hand the significant contribution of the orange fruits to the overall equivalent canopy water thickness. In the visible (VIS: 350 to 700 nm) domain, the impact of the presence of fruit is much less evident. The occurrence of medium and heavy crop loads did not affect the visible reflectance signature significantly but was indirectly evidenced by leaf chlorosis mainly due to competition for nitrogen between the canopy elements. The knowledge gained from this study will contribute to better hyperspectral vegetation index design and improved radiative transfer models for fruit bearing perennial woody crops. What is more, the direct biochemical linkages between water, nitrogen, fruit biomass, and spectral characteristics are an important precursor for the development and implementation of an air- or space-borne crop load monitoring system for citrus orchards.