Several crops bear reproductive organs (RO) at the top of the canopy after the flowering stage, such as ears for wheat, tassels for maize, and heads for sunflowers. RO present specific architecture and optical properties as compared to leaves and stems, which may impact canopy reflectance. This study aims to understand and quantify the influence of RO on the bi-directional variation of canopy reflectance and NDVI.
Multispectral camera observations from a UAV were completed over wheat, maize, and sunflower just after flowering when the RO are fully developed and the leaf layer with only marginal senescence. The flights were designed to sample the BRDF with view zenith angles spanning from nadir to 60°and many compass directions. Three flights corresponding to three sun positions were completed under clear sly conditions. The camera was always pointing to two adjacent plots of few tenths of square meters: the RO were manually removed on one plot, while the other plot was kept undisturbed.
Results showed that the three visible bands (450 nm, 570 nm, 675 nm), and in a lesser way the red edge band (730 nm) were strongly correlated. We, therefore, focused on the 675 nm and 850 nm bands. The Bi-Directional Reflectance (BRF) of the canopy without RO shows that the BRF values were almost symmetrical across the principal plane, even for maize and sunflower canopies with a strong row structure. Examination of the BRF difference between the canopy with and without RO indicate that the RO impact canopy BRDF for the three crops. The magnitude of the impacts depends on crop, wavelength and observational geometry. These observations are generally consistent with realistic 3D reflectance simulations. However, some discrepancies were noticed, mainly explained by the small magnitude of the RO effect on canopy BRF, and the approximations made when simulating the RO layer and its coupling with the bottom canopy layer. We finally demonstrated that the RO layer impact the estimates of canopy traits such as GAI as derived from the multispectral observations.
