## 1 Introduction
SIF signals can be measured as a proxy of photosynthesis from air-borne devices for canopy and larger scale applications [27; 34; 42; 43; 44; 45; 46; 47; 48; 49; 50; 51]. More recently, the development of space-borne SIF remote sensing capability of the GOSAT (Greenhouse gases Observing SATellite) has provided the opportunity for SIF measurement from space [26; 52] and for global mapping of SIF in the far-red region from satellites [53; 54; 55; 41; 56]. The application of SIF to assess the global terrestrial carbon cycle through the gross primary production (GPP) estimation has increasingly drawn attention and several research efforts have been made over the recent years to study the mechanisms that link SIF to GPP [57; 58; 59; 60] and photosynthesis [61; 62; 63]. A new FLEX (Flourescence Explorer) carrying the FLORIS (FLuORescence Imaging Spectrometer) has already been planned to be launched in 2022 to provide high spectral (0.3 to 3 nm) and fine spatial resolution (300 m) SIF imagery [64]. Different plant species have different SIF magnitudes in both the red (O\({}_{2}\)B) and the far-red (O\({}_{2}\)A) regions. SIF signals in these regions depend on variations in leaf characteristics such as Chl content and leaf area index (LAI). Therefore, the effects of Chl content and LAI on SIF signals need to be identified and removed [48; 49; 58; 65]. [PERSON] et al. (2015) argued that SIF emission is related to the total absorbed radiation by Chl, the functional status of photosynthesis, and stress in the plants [48]. [PERSON] et al. (2015) also confirmed that the SIF signal largely depends on apparent Chl content in both the O\({}_{2}\)A and O\({}_{2}\)B bands, especially the latter [65]. Mechanisms that control ChlF signals over the short-term are well known, however seasonal interactions of ChlF signals and photosynthesis are less understood. Still more research is required on both leaf and canopy scales to understand seasonal interplay of ChlF and photosynthesis using both active and passive methods especially when the aim is to relate the knowledge acquired using active techniques to passive techniques [6; 66]. Both the PAM and FLD methods provide information on ChlF, however their measurement principles are different in wavelength and intensity. The ordinary FLD method is a method for steady-state ChlF measurement, and therefore it depends on wavelength and Chl content. In the FLD method, ChlF parameters cannot be measured and this makes the comparison of active and passive methods difficult [34; 47]. Seasonal steady-state ChlF measurement comparison of PAM and FLD methods using outdoor wheat plants in three different treatments showed a weak but statistically significant relationship between active and passive methods, but the FLD measurement presented higher seasonal variability compared to PAM [66]. The authors suggest that these results indicate the complexity of measuring ChlF using passive techniques (FLD) as compared with active methods. In our previous research [65], a new technique was introduced to estimate \(\Phi_{\text{PSII}}\) by joining the FLD method and the laser induced saturation pulse method in order to directly retrieve information on photosynthesis from SIF signals. We have named this method the Fraunhofer line depth and laser-induced saturation pulse (FLD-LISP). The advantages of the suggested method are: (i) ChlF parameters can be estimated using the SIF method under sunlight by the combined application of the FLD technique and the saturation pulse laser illumination; (ii) FLD-LISP signals and PAM retrieved ChlF signals can be better compared; and (iii) unlike PAM, the suggested method does not require synchronization with the measuring pulse because the measuring pulse for fluorescence measurement is not needed. Therefore, there is good potential for the application of this method for longer distance measurements in a new type of portable instrument. In this paper, a more comprehensive study of the methodology suggested by [PERSON] et al. (2015) at the leaf level is presented.