## 4 Discussion
To our knowledge, the change (i.e., inversion) in the PRI diurnal profiles has not been reported previously for corn canopies, and further research is warranted to investigate if it also applies to other species. In addition to the metabolic changes advancing with phenology, major drivers of the seasonal dynamics in photosynthesis and the associated fluorescence and reflectance properties include variation in light and the seasonal dynamics in canopy chlorophyll. The seasonal changes in midday values for PRI appeared driven mostly by the changes in canopy chlorophyll, as indicated by their strong correlations with CLred (Figure 8). Similar relationships were also observed for both SIF\({}_{\text{A}}\) and SIF\({}_{\text{B}}\) in relation to CLred. These findings are corroborated by previous research evaluating the relationship of canopy PRI and a chlorophyll index (e.g., MTCI) for loblolly stands of various ages [30]. Our findings confirm the capability of SIF\({}_{\text{A}}\), SIF\({}_{\text{B}}\), SIF\({}_{\text{A+B}}\), and their yields to enable more consistent interpretation and estimation of GPP and LUE, and strongly suggest that their diurnal and seasonal dynamics should be considered for scaling of photosynthetic function from leaf to canopy levels. Although the spectral data used in this study were acquired from a nadir orientation, it is widely documented that the reflectance measured from vegetation canopies varies with solar illumination angle due to variations in shadows, understory background, and canopy parameters such as biomass and leaf area index, which contribute to the variation in the results to a different degree [12; 13; 58; 59]. A full analysis of the sensitivity of the relationships between GPP and the SIF metrics, focusing on these effects, is warranted. An improved understanding of the drivers of variation with phenology, and the implementation of a strategy for normalisation of their effects, would be a logical follow up research effort. Since the present dataset was collected at a close proximity to the canopy, future efforts would be warranted, using modeling scenarios [67; 68; 69] to determine whether our present findings can scale successfully to airborne and satellite levels. Incorporating the consistently collected spectral and field datasets into the Automated Radiative Transfer Models Operator (ARTMO) framework will facilitate the data interpretation, comparisons, and scaling from the leaf to the canopy and above [69] of vegetation function and the associated biophysical variables, which is of importance for the forthcoming spectroscopy missions such as NASA's SBG [70], EnMAP [71], and ESA FLEX [52; 72]. Figure 8: Association of midday SIF\({}_{\text{A+B}}\) (red, left axis, PAR \(\geq\) 1150 \(\upmu\)mol/m\({}^{2}\)/s) and PRI (blue, right axis) to canopy chlorophyll, as captured by CLred. The midday values are calculated from the observations collected within \(\pm 1\) h of local solar noon, and include 58 days from four growth stages of corn. Ultimately, we would like to trace the changes in photosynthetic function with phenology and not chlorophyll content or canopy biomass, the effects of which can be domineering and can obscure the relationships of PRI and SIF to GPP.