###### Abstract  
Chlorophyll fluorescence (ChlF) is a useful indicator of plant photosynthesis and stress conditions. ChlF spectra can be simulated with the Fluspect model, which is a radiative transfer model that simulates leaf reflectance, transmittance, and fluorescence; however, it has never been used or validated under natural conditions. In this paper, a new fluorescence quantum yield efficiency of photosystem (FQE) retrieval method based on the Fluspect model is proposed for use in simulating ChlF in two healthy varieties of soybeans grown under natural conditions. The parameters, Chlorophyll \(a+b\) content (\(C_{ab}\)), carotenoid (\(C_{ca}\)), dry matter content (\(C_{dm}\)), indicator of leaf water content (\(C_{w}\)) and leaf mesophyll structure (\(N\)) and the simulated fluorescence from the experiment were compared with the measured values to validate the model under natural conditions. The results show a good correlation (coefficient of determination \(R^{2}\) = 0.7-0.9) with the measured data at wavelengths of 650-880 nm. However, there is a large relative error (RE) that extends up to 150% at the peak of the fluorescence curve. To improve the accuracy of the simulation, an inversion code containing the emission efficiency parameters for photosystems I and II was added, which retrieves FQE I and II from the measured fluorescence spectra. The evaluation results for all wavelengths and two peaks demonstrated a significant reduction in the error at the peak of the curve by the Fluspect model with the FQE inversion code. This new method reduced the overestimation of fluorescence from 150% to 20% for the RE, and the \(R^{2}\) value was higher than 0.9 at the spectra peaks. Additionally, the original plant parameter information remained mostly unchanged upon the addition of the inversion code. sun-induced fluorescence; fluorescence quantum yield efficiency; SCOPE; Fluspect; fluorescence simulation; transmittance; reflectance +
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0000-0002-430X/00/0000-0002-430X/0000-0000-0000-0000-0000-0000-0000-0000-0000-00000-00000-00000-00000-00000-00000-00000-00000-00000-00000-00000-000000-00000-00000-00000-0  
the terrestrial vegetation's carbon budget and GPP require information on physiological parameters [2]. These parameters can be estimated by the use of fluorescence emissions [3; 4; 5], and several studies have successfully simulated fluorescence to evaluate the in vivo status of plant growth [6; 7; 8; 9; 10]. Chlorophyll fluorescence (Chlf), as a probe of photosynthesis, can reflect the primary reaction process of photosynthesis, such as light energy absorption, excitation energy transfer, and photochemical reactions. Normally, it is much weaker than the reflected solar radiation (approximately 2-5% in the near-infrared range). Both fluorescence and reflectance are properties in the red/far-red region (650-800 nm) of the vegetative reflectance spectrum [11]. Vegetation fluorescence is emitted by foliage through the photosystems. In the visible region of the spectrum, fluorescence peaks occur at 685 nm and 740 nm. The authors of [11; 12] first discovered the concepts of photosystems I and II (hereinafter, PS I and PS II) in 1960; however, the significance of this discovery is still gradually deepening [13; 14; 15; 16]. Photosystems I and II are the functional and structural units of the protein complex in photosynthesis that produces fluorescence. The absorption of light and the transfer of energy and electrons are light-dependent reactions. PS II is responsible for the emission of fluorescence at around 680 nm, while the fluorescence of PS I peaks at around 700 nm. The contributions of the two photosystems are shown as peaks in the fluorescence spectra [14]. Several researchers have explored the quantum efficiencies of PS I and PS II [17; 18; 19; 20; 21]. The emission efficiencies of these photosystems (fluorescence quantum yield efficiency of photosystem, FQE) influence the simulated fluorescence.