## 3 Results
#### 3.1.2 Variation in Reflectance VIs  
The reflectance indices NDVI and Clred (Figure 2C) were calculated to evaluate the changes in canopy biomass and chlorophyll content, respectively. The diurnal variation in NDVI and Clred exhibited similar patterns, with lowest values measured at midday.  
Figure 4: Diurnal and seasonal variations in photosynthetically active radiation (PAR, \(\upmu\)mol/m\({}^{2}\)/s), absorbed PAR (APAR), fraction of APAR (/APAR), gross primary production (GPP), light use efficiency (LUE), and photochemical reflectance index (PRI). Each value represents a mean (\(\pm\)SE of the mean, n = 12) of the measurements collected at this time on three consecutive clear days, representative of key growth stages for corn. The growth stages are indicated in different colors (blue—young, green—mature, red—early senescence, yellow—late senescence).  
The diurnal variation in NDVI significantly differed with growth stage (Table 3, ANOVA significant interaction between 'time of day' and 'growth stage'). The largest diurnal range in NDVI was measured on young corn (i.e., diurnal range from 0.6-0.9, Figure 2). The low midday values and larger diurnal variation during the young stage contributed to influence of the soil background at higher sun elevations, which was most apparent when the canopy LAI was lower. With an increase in foliage and canopy closure, the NDVI diurnal variation decreased and remained steady until the S2 stage. There were no statistically significant diurnal changes in NDVI during the mature stage, however, during this growth stage, significant differences were observed in GPP, Cl\({}_{\text{red}}\), PRI, and SIF (Figures 4 and 5, Table 3). Midday NDVI decreased from 0.9 to 0.4 by DOY 262 (Figure 3C, S2).  
As compared to NDVI, Cl\({}_{\text{red}}\) exhibited very different seasonal dynamics, gradually increasing during the young stage to reach maximum during the mature stage, declining gradually during the S1 and S2 stages (Figure 2C). The diurnal variation in Cl\({}_{\text{red}}\) was also largest during the early stage (Figure 2C, Yn), with a minimum observed at noon in all growth stages. The absolute daily maximum of Clred was measured during the mature stage (Figure 2), after which Cl\({}_{\text{red}}\) declined during senescence, and during S2, the diurnal variations were not statistically significant (Table 3).  
The diurnal variation in PRI significantly differed with growth stage (Table 3, ANOVA significant interaction between 'time of day' and 'growth stage'). During the young and mature stages, daily minimum PRI was measured at noon (i.e., a 'v' pattern), while during senescence the pattern gradually changed, and during advanced senescence, the daily maximum PRI values occurred at noon (i.e., '\(\Lambda\)' pattern). Similarly to NDVI and Cl\({}_{\text{red}}\), the highest magnitude of diurnal variation was also observed for the young canopy, when lower PRI values were measured at midday. In contrast, during S2 stage, PRI increased during the middle of the day (Figures 2E and 4F). Only in the early morning during the young stage did PRI have positive mean values (Table 3, Figure 4F). Highest midday values were measured during the young (Figure 4F, Table 3), followed by the mature and senescent stages. PRI reached lowest values during the senescent stage, which occurred in the early morning (\(-\)0.106, Table 3).