Abstract
Monitoring the components of crop canopies with remote sensing can help us understand the within-canopy variation in spectral properties and resolve the sources of uncertainties in the spectroscopic estimation of crop foliar chemistry. To date, the spectral properties of leaves and panicles in crop canopies and the shadow effects on their spectral variation remain poorly understood due to the insufficient spatial resolution of traditional spectroscopy data. To address this issue, we used a near-ground imaging spectroscopy system with high spatial and spectral resolutions to examine the spectral properties of rice leaves and panicles in sunlit and shaded portions of canopies and evaluate the effect of shadows on the relationships between spectral indices of leaves and foliar chlorophyll content. The results demonstrated that the shaded components exhibited lower reflectance amplitude but stronger absorption features than their sunlit counterparts. Specifically, the reflectance spectra of panicles had unique double-peak absorption features in the blue region. Among the examined vegetation indices (VIs), significant differences were found in the photochemical reflectance index (PRI) between leaves and panicles and further differences in the transformed chlorophyll absorption reflectance index (TCARI) between sunlit and shaded components. After an image-level separation of canopy components with these two indices, statistical analyses revealed much higher correlations between canopy chlorophyll content and both PRI and TCARI of shaded leaves than for those of sunlit leaves. In contrast, the red edge chlorophyll index (CIRed-edge) exhibited the strongest correlations with canopy chlorophyll content among all vegetation indices examined regardless of shadows on leaves. These findings represent significant advances in the understanding of rice leaf and panicle spectral properties under natural light conditions and demonstrate the significance of commonly overlooked shaded leaves in the canopy when correlated to canopy chlorophyll content.
Highlights
A crop canopy under natural light conditions is composed of sunlit and shaded parts, as shadows arise from blocking a fraction of direct light from solar illumination [1]
We investigated the four vegetation indices (VIs) including normalized difference vegetation index (NDVI) [30], index (PRI) [32] and red edge chlorophyll index (CIRed-edge) [33], which are related to transformed absorption reflectance of normalization, called continuum removal [34], to the carotenoid and chlorophyll absorption features in the blue (400–550 nm) and red (550–750 nm) domains, respectively [35,36]
Regardless of sunlit or the reflectance panicles was higher than was thathigher of leaves in the visible region before 450 nm
Summary
A crop canopy under natural light conditions is composed of sunlit and shaded parts, as shadows arise from blocking a fraction of direct light from solar illumination [1]. Others assume that the reflectance values of shadows equal zero or are constant for the purposes of spectral mixture analysis [2,12] and estimation of canopy parameters with radiative transfer [13,14] or geometrical optical models [15,16,17]. These studies suggest that the potential spectral information in shaded portions of the canopy have not been fully exploited. These relevant studies were devoted to such crops as sugar beet, barley, corn and wheat and focused on observations of leaves only
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