###### Abstract
Commonly used SIF is always at a far-red wavelength (e.g., the O\({}_{2}\)-A band of 760 nm), at which the \(f_{\text{esc}}\) can be approximated as a constant of 1 due to the low reabsorption by chlorophyll pigments of leaves [15]. Thus, the linear relationship between SIF and GPP is briefly determined by the ratio \(L\text{{LUE}}_{P}/L\text{{LUE}}_{F}(\lambda)\). A mass of evidence based on experimental measurements has shown that the relationship between \(L\text{{LUE}}_{P}\) and \(L\text{{LUE}}_{F}(\lambda)\) (i.e., the SIF yield at 760 nm) is linear [17; 18], which means that the value of \(L\text{{LUE}}_{P}/L\text{{LUE}}_{F}(\lambda)\) remains relatively constant and GPP is linearly related to SIF in a specific ecosystem. However, photosynthesis is a finely regulated process, whereby plants seek to assimilate maximum energy under optimum conditions and minimize short- and long-term photochemical damage under adverse conditions, such as high light or temperature [19]. Under favorable conditions, the linear relationship between SIF and GPP is robust, since the bulk of APAR is used for carbon assimilation by photochemistry [20]. In contrast, a suboptimal environmental and biotic conditions might reduce the photosynthetic activity and trigger photoprotective mechanisms by dissipating excess solar energy as heat [20]. The linear relationship between SIF and GPP is thus sensitive to the dynamics of physiological stress and photoprotective mechanisms induced by environmental factors [1]. Comparing the satellite results, it can be found that drought stress lowers the linear relationship between SIF and GPP (e.g., for normal and drought vegetation, R\({}^{2}\) = 0.87 and 0.53, respectively) [5; 15]. At the leaf level, the results based on cotton and tobacco measurements revealed a clearly nonlinear relationship between SIF and GPP under different environmental conditions due to the nonlinear relationship between the SIF yield and photosynthesis efficiency [21]. Despite an exponential rise in the number of studies exploring the linear relationship between SIF and GPP associated with environmental conditions, very little is known about SIF and its relationship with GPP in senescent crops. In annual crop plants, senescence is a developmental process that overlaps with the reproductive phase [22]. It involves a well-orchestrated degradation and remobilization and thus affects the productivity of crop plants. It is well documented that delayed senescence should yield a higher crop production [23]. However, crop plants frequently encounter varying degrees of environmental abiotic and biotic stress, which might induce the premature senescence of leaves [24]. The premature senescence may lead to a shortage of assimilates, ultimately causing an acceleration in the whole-plant maturation process and strongly lowering crop productivity in the end [22]. In the context of climate change, particularly of warming, the growth period is shortened and senescence is advanced [25; 26]. As a new tool for remote sensing, SIF has been used for estimating crop production for a whole growth period based on its linear relationship with GPP [27]. Their relationship in the phase of senescence is always omitted or considered as linear, like that of other growth phases. However, their relationship in senescence maybe not linear due to the rapid down-regulation of photosynthesis and other complicated physiological processes. Thus, to get a more accurate estimation of crop production from SIF, it is necessary to validate whether their relationship is linear or not. As we described above, the relationship between SIF at a far-red wavelength and GPP is mainly determined by the relationship between \(LUE_{P}\) and \(LUE_{F}(\lambda)\). Therefore, to determine their relationship at senescence, we sought to explore the relationship between the SIF yield and photosynthetic efficiency and how this relationship is mediated by photosynthesis processes.