## 2 Materials and Methods  
### Canopy Spectral Measurements  
The experiments were performed on autumn soybean and sweet potato, which were separately planted in a 3 \(\times\) 3 m\({}^{2}\) grid in the Huailai Comprehensive Experimental Station, Beijing Normal University. All of these plants were rainfed during their growth periods. The soil was loamy soil and was considered to have a high level of nutrient status, since it had a mass of 3.10-31.20 g organic matter/kg (mean 12.85 g/kg), a storage of 576.6-1542.8 mg N/kg (mean 956.08 mg/kg), 572-3101.5 mg P/kg (mean 1288.44 mg/kg), and 21.70- 31.0 g K\({}_{2}\)O/kg (mean 25.36 g/kg) [28]. Furthermore, differences in the nutrient content significantly affect the photochemical process of photosynthesis and play a crucial role in plant growth and development [29]. Therefore, considering such high nutritional status and to avoid the nutritional difference between the two plots, the same amount of \(\sim\)174 g/m\({}^{2}\) of Urea-fertilizer was applied to each crop as ground fertilizer.  
In this experiment, two diurnal courses of canopy solar-induced chlorophyll fluorescence and reflectance were acquired from the soybean and sweet potato on 22 and 23 September 2019, respectively. Since that harvest time for soybean and sweet potato is always in early October, we could consider that all these crops were at the period of senescence.  
The SIF at the canopy level was measured in the middle of each plot with a diameter of \(\sim\)0.65 m using a manual spectrum system. This system is similar to our previously described one, which is described in [PERSON] et al. (2018a, 2018b) [30, 31]. Specifically, a high-resolution spectrometer (HR4000, 720-810 nm, OceanOptics, Inc., Largo, FL, USA) was used to collect down-welling solar flux and up-welling canopy flux with two optical fibers, one attached to a cosine corrector (CC-3, OceanOptics, Inc., USA) and the other as a bare fiber (field of view, FOV, 25\({}^{\circ}\)). An electrical shutter (FOS-2 \(\times\) 2-TTL, OceanOptics, Inc., USA) was connected to these two fibers to collect the signal from either the canopy or sky at one time. The spectrometer first collected solar irradiance and then canopy radiance within 5 minutes. The mean values of \(\geq\)4 measurements of solar irradiance and \(\geq\)60 measurements of canopy radiance were recorded in each cycle. Dark currents corresponding to the solar irradiance and canopy radiance were collected afterward. However, without temperature control, wavelength correction and radiance correction were conducted before each measurement cycle. All of these processes were performed using OceanView software (OceanOptics, Inc., Largo, FL, USA).  
After the collection of SIF was finished, in the subsequent 5 minutes, canopy reflectance of the same vegetation target was also measured using another spectrometer (USB 2000, OceanOptics, Inc., USA) equipped with one optical fiber. This spectrometer measured the reflected radiance of the vegetation target and a Spectralon reference panel between 200 and 1000 nm, with a spectral sampling interval of 1.5 nm and a full width at half maximum (FWHM) of 3.0 nm. The reflected radiance was also measured using OceanView software. The mean values of \(\geq\)30 measurements of the canopy and Spectralon reference panel were separately recorded in each cycle.