## 4 Discussion
Some of the variations observed in the morning and afternoon estimates of APAR and the associated \(\text{SIF}_{\text{Ay}}\), \(\text{SIF}_{\text{By}}\), and \(\text{SIF}_{\text{A+By}}\) could be attributed to the field data used for deriving the linear function between \(f\)APAR and NDVI, which were collected near solar noon. While the implemented equation for deriving \(f\)APAR describes the seasonal change in NDVI/\(f\)APAR, it may underperform under low PAR early in the morning and in the late afternoon. Prior studies characterizing the diurnal cycle in chlorophyll fluorescence during several consecutive days [38] reported that SIF (i.e., \(\text{SIF}_{\text{A}}\) and \(\text{SIF}_{\text{B}}\)) was mainly driven by PAR variation, but that fluorescence yield was severely reduced during the central hours of the day in association with stress factors (e.g., high light and temperature). Our results corroborate the importance of PAR as a driving factor for both GPP and SIF, as their yields (LUE and \(\text{SIF}_{\text{y}}\)) co-varied closely (Table 4). Under low PAR, \(\text{SIF}_{\text{A+B}}\) and \(\text{SIF}_{\text{A}}\) were correlated strongly with GPP, demonstrating higher sensitivity to GPP as compared to the sensitivity of APAR to GPP (Table 3). Under high PAR levels, \(\text{SIF}_{\text{A+By}}\) and \(\text{SIF}_{\text{Ay}}\) captured the variation in GPP optimally. Under a saturating PAR leaf level YII was also strongly correlated with GPP and indicative of canopy photosynthetic function. \(\text{SIF}_{\text{A+By}}\) and \(\text{SIF}_{\text{By}}\) offered sensitive capabilities for assessment of LUE under saturating PAR, which outperformed PRI, but were inferior as compared to the utility of YII, which explained the highest percentage of the variation in LUE. Under low PAR, the sensitivity of YII to LUE was significantly lower, and the correlations using SIF metrics varied strongly according to time of observation during the day. The use of multiple SIF metrics, depending on PAR condition, offers the potential to improve the sensitivity of statistical models for estimating GPP and LUE, and therefore needs to be considered. Our findings demonstrate that, at midday, under higher PAR levels (e.g., saturating for photosynthesis) YII was correlated more accurately with GPP and LUE (Table 3). Under lower PAR levels, ETR exhibited strong correlations to GPP (Table 3), and also to the canopy SIF metrics, which are strongly influenced by the changing PAR levels, and therefore may suffer inaccuracies. Further research, investigating the capability of SIF metrics for scaling and spatially extrapolating leaf YII and ETR under varying PAR conditions, is warranted for both C3 and C4 canopies. Recent studies [42; 44; 47] measuring C3 vegetation used supplemental dark-adapted measurements to augment PAM light-adapted observations and successfully estimate non-photochemical quenching during the day (NPQ) in a modeling framework, and thus enabled the partitioning of leaf PAR into YII, NPQ, and heat dissipation. Future efforts enhancing the current dataset with estimates of NPQ would yield a long term dataset for partitioning of APAR for C4 vegetation, which is not currently available. Currently SIF observations are available from field, airborne, and space-borne platforms, which are collected at different light levels and times during the day. Their sensitivity for evaluation of photosynthetic function and scaling between leaf and canopy levels would likely benefit from a separate consideration depending on PAR condition (i.e., PAR below or above saturation level for photosynthesis), which would likely improve their applicability for monitoring of GPP and LUE. GPP, APAR, NDVI, CLred and SIF maintained similar diurnal patterns across the growing season, while the diurnal pattern of PRI changed during the early senescence stage (Figures 2E and 4F).