## 1 Introduction
Nitrogen is a fundamental constituent of leaves that is typically associated with higher LAI, and positively correlated with the amount of chlorophyll a and b (Cab) [22]. Higher LAI and Cab increase APAR, but at the same time should reduce Fesc due to higher absorption and scattering of emitted fluorescence [14]. Nitrogen is also positively related to the amount of ribulose-l,5-bisphosphate carboxylase and oxygenase (Rubisco) protein content [23; 24], and thus the maximum carboxylation rates (Vcmax), which is a key determinant of the maximum photosynthetic rates, and therefore GPP [25]. Therefore, nitrogen can influence the partitioning of APAR into PQ, NPQ, and fluorescence emission [15], but different studies, mainly at leaf level, showed contrasting results [14; 26]. Moreover, there is a lack of studies that investigate at canopy scale how \(\mathrm{LUE_{P}}\), \(\mathrm{LUE_{f}}\), and Fesc are modulated under varying nitrogen availability [14]. Canopy phosphorous concentration (hereafter P%) is another critical element for photosynthesis, being involved in the synthesis of Adenosine triphosphate (ATP) [27]. Leaf-level studies with active fluorescence measurements showed that P% deficient plants have lower chlorophyll fluorescence emission efficiency [28]. However, we are not aware of canopy level studies showing the effect of P% on \(\mathrm{F_{760}}\) and \(\mathrm{LUE_{f}}\). Canopy structural variables, such as LAI and LAD, influence the radiative transfer of incoming radiance and emitted SIF within the canopy [19]. LAD can vary on daily and seasonal bases and is strongly influenced by species composition and plant functional forms [29]. LAI and LAD can have a major influence on the sun/shaded leaf ratio through the canopy. This ratio has the potential to directly influence the level of NPQ in the canopy [30] (higher in sunlit, lower in shaded leaves) and therefore could indirectly influence the \(\mathrm{LUE_{f}}\). Canopy structure, through absorption and scattering of the fluorescence emitted by the leaves, has a significant influence on observed \(\mathrm{F_{760}}\), determining Fesc, the probability of \(\mathrm{F_{760}}\) to escape the canopy [31]. Absorption by chlorophyll is higher in the red region, whereas multiple scattering in the far-red region increases the probability of absorption by soil and woody elements. It has been shown recently with modeling studies that TOC observed \(\mathrm{F_{760}}\) (canopy scale) is only a fraction of the \(\mathrm{F_{760}}\) emitted at leaf scale (\(\mathrm{F_{760}}\)) [32]. The decoupling between \(\mathrm{F_{760}}\)leaf and \(\mathrm{F_{760}}\), mainly mediated by Fesc, can have implications for the GPP-\(\mathrm{F_{760}}\) relationships. Recently, new methods to estimate Fesc are being developed, potentially allowing to downscaling the \(\mathrm{F_{760}}\) signal at the leaf level [31; 33]. Finally, other variables such as soil moisture or surface temperature (Ts) also have the potential to impact the GPP-\(\mathrm{F_{760}}\) relationship. Heat and water stress have been proven to affect photorespiration, but not the PQ in Mediterranean species [34], thus decoupling photochemistry from \(\mathrm{F_{760}}\)[18]. Ts, in particular, contains information on both the activation of NPQ mechanisms and other processes related to stomatal closure and sensible heat losses [35]. Therefore, surface temperature might also help to better characterize the seasonal variations of \(\mathrm{LUE_{P}}\) and therefore to better predict GPP, in particular under stress conditions [35; 36]. Figure 1 illustrates a theoretical framework that sums up current knowledge and our hypothesis regarding the interlinks between GPP and \(\mathrm{F_{760}}\) and their relationship with canopy structural parameters and leaf traits of vegetation. In Figure 1, solid colored lines represent the energy partitioning at both leaf and canopy level and dotted lines represent the hypothesized relationships.