Abstract
Remote sensing approaches have several advantages over traditional methods in determining information on physical and chemical parameters, including timely data acquisition, low costs, and wide coverage. Thus, remote sensing is widely used in crop growth monitoring. Unlike vertical observations, multi-angular remote sensing technology can obtain the vertical distribution information of the central and lower leaves of a crop. Furthermore, applications of remote sensing on the vertical distribution of maize canopy components is complicated, and related research is limited. In the current paper, we employed multi-angular spectral data, measured by a self-designed multi-angular observation instrument at view zenith angles (VZAs) of 0°, 10°, 20°, 30°, 40°, 50°, and 60°, to explore the monitoring strategy and monitoring precision of the vertical distribution of chlorophyll content in the maize canopy. This was then used to determine the optimal monitoring method for the chlorophyll content (soil and plant analyzer development (SPAD) value) of each layer. The correlation between SPAD value and chlorophyll sensitivity indices at different growth stages was used as the basis for screening indices and VZAs. The correlation between the selected EPI (eucalyptus pigment index) and REIP (red edge inflection point) indices and chlorophyll content indicated view zenith angles (VZAs) of 0°, 30°, and 40° as optimal for the early growth stage monitoring of chlorophyll content in the 1st, 2nd, and 3rd layers, respectively. These values were associated with RMSEs of 4.14, 1.71, and 1.11 for EPI, respectively; and 4.61, 2.31, and 1.00 for REIP, respectively. In addition, a VZA of 50° was selected to monitor the chlorophyll content of the 1st, 2nd, 3rd, and 4th layers at the late growth stage, with RMSE values of 2.97, 3.50, 2.80, and 4.80 for EPI, respectively; and 3.16, 5.02, 4.55, and 7.85 for REIP, respectively. The results demonstrated the ability of canopy multi-angular spectral reflectance to accurately estimate the maize canopy chlorophyll content vertical distribution, with the VZAs of different vertical layers varying between the early and late growth stages.
Highlights
The chlorophyll content of the crop canopy is vertically distributed and exhibits a certain level of stratification
Despite the extensive research performed on crops such as wheat (Triticum aestivum L.), studies on the remote sensing monitoring of the vertical distribution of chlorophyll content in maize is lacking, perhaps due to the difficulties of this application [4,5,6,7,8,9]
The leaves of chlorophyll content; the content was less than that of the functional at the top of the canopy were typically less chlorophyll developed, with an increasing accumulation leaves at the center of the canopy
Summary
The chlorophyll content of the crop canopy is vertically distributed and exhibits a certain level of stratification. This vertical distribution is of great significance for accurate crop growth monitoring and management. Despite the extensive research performed on crops such as wheat (Triticum aestivum L.), studies on the remote sensing monitoring of the vertical distribution of chlorophyll content in maize is lacking, perhaps due to the difficulties of this application [4,5,6,7,8,9]. SPAD measurements are fast and can reflect the characteristics of the plant chlorophyll content, and are often used in crop health status evaluations and plant physiological research (e.g., with rice, vegetables, fruits) [1,10,11,12,13,14]. Chlorophyll absorption peaks appear in the blue region (400–500 nm) and red region (600–700 nm), but not in the near-infrared region
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