## 2 Materials and Methods
### Automated Field Spectroscopy System and the Measurements of SIF  
An automated field spectroscopy system was designed using commercial-grade optoelectronic devices for continuous and simultaneous collection of canopy SIF over the four plots. This platform has been extensively described in [PERSON] et al. [32; 33]. The automated system consisted of four key components: one key component is a high-resolution spectrometer HR2000+ (Ocean Optics Inc., Largo, FL, USA), covering the spectral range of 680-775 nm with a spectral resolution of \(\approx\)0.13 nm (full width at half maximum, FWHM); the second one is an electronic shutter (FOS-2 x 2-TTL, Ocean Optics Inc.) to connect the spectrometer with a splitter fiber (Ocean Optics Inc.); the third one is two fiber optics that were fixed on a mechanical arm and attached with the electronic shutter; and the fourth key component was a step motor, which was used to drive a mechanical arm from one plot to another one. All devices embedded in the system were connected to a PC through the connection of Universal Serial Bus 2.0 (USB 2.0) or Recommended Standard 232 (RS232). The system was placed in an incubator, where the temperature was kept at 23 \({}^{\circ}\)C to reduce the influence of temperature on the dark current of the spectrometer. An instrument box was used to protect these devices from rain. Solar irradiance and canopy radiance were collected by a custom-developed software. This software was compiled into an executable file in Microsoft Visual Studio 2015 (Microsoft, Redmond, WA, USA) and run under the Microsoft Windows operating system (Windows 7). The SoftwareDevelopment Kit (SDK) in the driver OOIWinIP (OOIDrv32.lib, Ocean Optics Inc.) was used to control HR2000+ and the electronic shutter. Solar irradiance was collected by the up-looking fiber with a cc-3 cosine diffuser (Ocean Optics Inc.) with a field of view (FOV) of 180\({}^{\lx@math@degree}\), while the up-welling canopy flux was collected from nadir with a down-looking bare fiber with a FOV of 25\({}^{\lx@math@degree}\) (Figure 1a). The distance between the bare fiber and canopy was greater than 150 cm for wheat during the whole field campaign. However, such a distance for maize was greater than 100 cm before late August; after that, the distance became smaller. These two optical paths could be switched by the electronic shutter that allows only one signal enter the spectrometer at a one time. In our field campaigns, all the measurements of spectrum were collected during 6:00 to 19:00. In each plot, solar irradiance was first collected through the up-looking fiber, then canopy radiance was collected using the down-looking bare fiber. Solar irradiance and canopy radiance in each plot were collected in five minutes to reduce the impact of short-term changes in illumination conditions. The dark current with the same integration time as spectra collection (i.e., 2 s, 3 s, or 5 s in our study) was also collected as a digital number (DN) before filed campaigns at the fixed temperature of 23 \({}^{\circ}\)C. Dark-current correction was performed according to the method of \"raw spectra (DN) - dark current (DN)\" to eliminate the impact of the dark current on raw spectra. After that, a second degree polynomial Savitzky-Golay filtering method with a filter of 30 \(\times\) 30 spectral bands was used to reduce the noise of the raw spectra. We also performed radiometric and wavelength calibrations before filed campaigns two times in the platform of SpectraSuite (Ocean Optics Inc.) by using a radiometric calibration light source (HL-2000-CAL, Ocean Optics Inc.) and a wavelength calibration light source (HG-1, Ocean Optics Inc.). The radiance correction factor and absolute irradiance correction factor were collected using the field calibration. Based on the correction factors, the output raw data were then converted to irradiance (E, mW-m\({}^{-2}\)-nm\({}^{-2}\)) and radiance (L, mW-m\({}^{-2}\)-nm\({}^{-2}\)-sr\({}^{-1}\)).