## 2 Methodology
### Data Acquisition  
To provide data and verify the fluorescence simulated by the Fluspect model, the bidirectional reflectance (\(R\)), transmittance (\(T\)), and chlorophyll fluorescence (\(F\)) of the leaves were measured using a spectral radiometer (Field Spec 4, ASD Inc., Boulder, CO, USA) connected to a FluoWat leaf clip [(47)] via optical fiber. FluoWat is utilized with a hyperspectral spectroradiometer to measure the R-T-F in the upward and downward positions [(48)]. To measure the ChIP, the clip has a 45\({}^{\circ}\) optical inlet, a light source, and a short-range filter (TechSpec OD4, Edmund Optics GmbH, Mainz, Germany) to block any irradiance above 650 nm. The filter in the instrument cuts off incident light between 650 nm and 880 nm. There are two vertical optical fiber openings at the top and bottom of the clip [(46)]. FluoWat is a leaf clip attached to a device for measuring upward and downward leaf emissions under both natural and artificial light conditions. After the leaves had adapted to the light, the measurements were carried out at various times throughout the day. FluoWat has one spectroradiometer, which was used in two positions--one above and one below the leaf. In both positions, the fiber optics pointed to the leaf surface vertically. An open aperture was designed to allow solar beams to enter at a relative position of 45\({}^{\circ}\). Therefore, \(R\) and \(T\) were obtained with upward and downward fiber optics. Then, a short-pass filter restricted the light entering the open aperture and cut off the light wavelength above 650 nm. The upward and downward fluorescence values were measured by two probes. Thus, the \(R\), \(T\), and \(F\) values can be measured, and then the \(T\) of the filter, the true \(R\) and \(T\) from the apparent \(R\) and \(T\), and the fluorescence spectra were calculated. The top, middle, and bottom of the plant were measured. Measurements at the bottom of the canopy were difficult to obtain due to the lack of solar light. The leaf was plucked out in the sunshine to perform the measurement; however, under natural light conditions, the leaves on the bottom always grow in dark conditions. When the leaves on the bottom are exposed to sunlight, they receive excess light; thus, the measurements for these leaves were not accurate. In this study, we chose leaves on the top and bottom of the plants that are normally exposed to sunlight. A total of 102 soybean leaf spectra covering a large range of chlorophyll concentrations were collected, with 54 and 48 samples of WT and MG, respectively. Eight measurements were carried out for each leaf. Each measurement calibrated a type of radiance, and the measurement sequence was as follows:  
1. The unfiltered radiance (Figure 2A1) is the radiance reflected by a standard white reference board; the reference board can reflect the total light without absorbing energy. The unfiltered radiance is the total incident light. 2. The filtered (cut off at \(\sim\)650 nm) radiance (Figure 2A2) is the light that is cut off from the incident light between 650 nm and 800 nm. 3. Leaf up (Figure 2B1) is the radiance from the sun on the ventral side of the leaf (adaxial side up). The fiber optics focus on the top first and contain the reflectance and emissions of the leaf. 4. Leaf up with filter (Figure 2B2) is the radiance from the leaf in the upward direction, with filtered upwelling radiance. From 650 nm to 800 nm, this radiance only includes the emissions from the leaf; in other words, the incident light in the filter wavelength is cut off. 5. Leaf down (Figure 2C1) is the radiance in the downward direction. It contains the transmittance of light and the emissions from the leaf. 6. Leaf down with filter (Figure 2C2) is the radiance in the downward direction with filtered downwelling radiance (dorsal side of the leaf), and the fiber optics are focused at the bottom. The transmittance from the incident light is cut off by the filter from 650 nm to 800 nm. 7. The unfiltered radiance is the same as in Figure 2A1.