Detecting Wheat Canopy Chlorophyll Content with SUN-Induced Chlorophyll Fluorescence through in-Filling of the O2-a Band.

The sun-induced chlorophyll fluorescence (SIF) signal is believed as a probe due to it is the core theory of the photosynthetic machinery that directly provides instantaneous information based on plant physiology functioning. The canopy chlorophyll content is also believed as a crucial parameter to monitor the capability of plants to assimilate CO₂. Due to saturation phenomenon is often occurred during estimating chlorophyll content, the SIF in-filling with the O₂-A and O₂-B bands can indicate the process and efficiency of plant. Besides, canopy chlorophyll content is significant important to precision agriculture. Therefore, this research will dedicate to determine whether the chlorophyll fluorescence will estimate the total chlorophyll content of a canopy. The ASD spectrometer (Fieldspec Pro FR) is used to measure solar irradiance and radiance spectra above wheat canopy under different nitrogen treatments. The spectral measurements of irradiance with a cosine corrector and wheat canopy radiance with bare fiber are adopted from a pole 1 m in height to collect nadir radiance from the top of wheat canopy. Through analyzing the spectral band range of 680-770 nm, the study indicates the chlorophyll fluorescence in-filling in the O₂-A band at 760 nm. The wheat leaf chlorophyll fluorescence is collected from the canopy leaf randomly using the SPAD 502. The chlorophyll content at the canopy level is defined as the product of leaf CH (per unit area of the leaf) and the LAI. This research analysis adopts Fraunhofer Line Discrimination (FLD) principle to extract canopy chlorophyll fluorescence from the reflection radiance. The statistical correlative coefficients (R²) between SIF and canopy chlorophyll content reach 0.89 which is significantly higher than the correlation of the canopy chlorophyll density index and canopy chlorophyll content. Based on the result, canopy chlorophyll fluorescence can be used to assess the canopy chlorophyll content more efficiently and accurately.