## 3 Results
#### 3.1.1 Variation in GPP, PAR, and LUE  
GPP, APAR, and LUE maintained significantly higher values during the young and mature stages and declined with senescence (Figures 2 and 4). During the summer of 2017, the corn emerged at approximately DOY 170, at which time midday GPP was 0.50 mg/m\({}^{2}\)/s (Figure 1). GPP increased after DOY 180, in response to applied nitrogen (N) treatment (140 kg N/ha), to an absolute maximum of approximately 2.35 mg/m\({}^{2}\)/s (Figures 1 and 2A). GPP varied significantly during the young period DOY 193-194 (Figures 2A and 4D). During the mature stage GPP reached highest value on DOY 222, when the canopy was still developing and new leaves were being produced (Figure 2A). The diurnal patterns of GPP through the entire growing season were consistent, with the highest GPP values always measured near midday (Figure 2A). Midday average GPP was highest during the young stage (Figure 3A, Figure 4C, Table 3), declining thereafter.  
The diurnal variations in PAR, _f_APAR, and APAR are compared across the growth stages (Figure 2B, Figure 3B, Figure 4A-C). PAR (Figure 4A) consistently increased in the morning to a maximum around midday, and declined in the afternoon at different rates. Mean midday PAR reached a highest value of 1275 mol/m\({}^{2}\)/s (Table 3) during early senescence (S1, Figures 1, 2B and 3A). The largest diurnal change in PAR was observed during the early senescent stage (i.e., increase until midday of \(\sim\)600 mol/m\({}^{2}\)/s, Figure 4A).  
During the young and early senescent stages (Figure 3B, 4B), _f_APAR declined in the morning to reach minimum in the early afternoon at approximately 14:15, increasing in the afternoon close to the levels measured in the morning. During the mature stage, the behavior of _f_APAR did not change significantly throughout the day (Figure 3B, green), indicating that, only during this stage, the use of either PAR or APAR could yield similar results and derivative parameters.  
The magnitude of midday average APAR was highest during the early senescence stage (Figure 3B, Table 3, S1), which coincided with the maximal PAR levels observed during this stage and the high canopy chlorophyll content of the established canopy, as captured by the Cl\({}_{\text{red}}\) and NDVI values(Figure 3E). Comparing the diurnal profiles of APAR across the growth stages, APAR was significantly lower during the S2 stage (Figure 4C). During the young stage, ANOVA did not identify statistically significant changes in APAR associated with the diurnal time periods (Table 3, am/noon/pm). However, during the young stage, the diurnal profile of APAR exhibited a complex dynamic behavior (Figure 4C) similarly to PAR, increasing quickly in the morning to reach highest magnitude in the late morning, and afterward declining by midday approximately in half to a level maintained until the end of the day. During the S1 stage, APAR remained relatively high in the late afternoon (Figure 4C), which was attributed to the high PAR observed at this time (Figure 4A). During late senescence (i.e., S2) when chlorophyll levels were significantly lower, APAR remained low during most of the day (Figure 4C, points in yellow), while PAR exhibited a distinct midday maximum (Figure 4A).  
LUE declined in the morning to reach minimum in the early afternoon during all stages (Figure 4E). Similarly to GPP, the highest LUE throughout the day was observed in the morning during the young stage. While the late morning LUE values separated the four growth stages, the midday and afternoon values grouped together the S1 and S2 stages, separating them from the young and mature stages (Figure 4E). The diurnal variation in LUE significantly differed with growth stage (Table 3, significant interaction between 'time of day' and 'growth stage'). During the young stage, LUE did not change significantly throughout the day (Table 3), while significant diurnal changes were observed during the remaining growth stages.