## 3 Results
#### 3.1.4 Variation in Canopy SIF Metrics  
SIF\({}_{\rm A}\), SIF\({}_{\rm B}\), and SIF\({}_{\rm A+B}\) exhibited a pronounced diurnal pattern, with highest average values measured close to midday (Figure 5D-F, Table 3). During the young stage, in the morning, SIF\({}_{\rm A}\) and SIF\({}_{\rm A+B}\) increased relatively quickly, reaching a highest value at approximately 10:45 (Figure 5D). In comparison, SIF\({}_{\rm B}\) also increased fast until approximately 10:45, but afterwards it continued to rise more gradually until the early afternoon, declining afterward. During the young stage, the diurnal patterns of SIF\({}_{\rm A}\) and SIF\({}_{\rm A+B}\) (Figure 5D, F) closely followed the patterns of the PAR and GPP diurnals (Figure 4A, D). The highest SIF\({}_{\rm B}\) was measured during the young stage, while higher levels of SIF\({}_{\rm A}\) and SIF\({}_{\rm A+B}\) were measured during the mature stage (Figure 5, Table 3). During senescence, SIF yields (SIF\({}_{\rm By}\), SIF\({}_{\rm Ay}\) and SIF\({}_{\rm A+By}\)) were significantly higher in the morning, declining until midday, and remained relatively unchanged during the remainder of the day (Table 3). The highest SIF\({}_{\rm Ay}\) average was observed during the mature stage, when Cl\({}_{\rm red}\) also reached significantly higher levels (Table 3). During the mature stage, the magnitude of SIF\({}_{\rm Ay}\) increased in the morning, reaching a daily maximum at noon, and declined in the afternoon (Figure 5G). SIF\({}_{\rm Ay}\) separated the young and mature from the senescent growth stages during most of the day (Figure 5G, Table 3). Figure 5: Diurnal and seasonal variations in leaf-level light-adapted PAM fluorescence metrics (Fs, YII and ETR, **A–C**); canopy solar induced fluorescence (SIF, **D**,**E**), and SIF yields (SIFy, **G–I**). The growth stages are indicated in different colors (blue—young, green—mature, red—early senescence, yellow—late senescence). Each value represents a mean (\(\pm\)SE of the mean) of the measurements collected at this time on three consecutive clear days representing each growth stage. \begin{table}
\begin{tabular}{l c c c c c c c c c c c c c c c c c c c c c c c c c c c c c c c c c} \hline \hline
**Component size** & \multicolumn{4}{c}{**Young Data**} & \multicolumn{4}{c}{**Mature (Q00)**} & \multicolumn{4}{c}{**Early Sequence (S01)**} & \multicolumn{4}{c}{**Advanced Sequence (S02)**} & \multicolumn{4}{c}{**ADOVA GLM**} \\ \hline
**Time period** & **a** & **mean** & **mean** & **p** & **a** & **m** &  
The diurnal profiles of \(\mathrm{SIF_{By}}\) significantly differed with growth stage (Figure 5H, Table 3 AVOVA GLM). During the young stage, the magnitude of \(\mathrm{SIF_{By}}\) was distinctly higher, as compared to the remaining growth stages, and the dynamics of the diurnal variations were more complex (Figure 5H). During the young stage, \(\mathrm{SIF_{By}}\) decreased until mid-morning (Figure 5H, 9:45), which corresponds inversely with the increase in APAR (Figure 4C) at the same time. \(\mathrm{SIF_{By}}\) increased to a mean maximum at midday (Figure 5H), and gradually declined afterwards. The diurnal profile of \(\mathrm{SIF_{By}}\) depicted a clear separation between the observations of the young and the observations of the mature and senescent stages at midday (Figure 5H, Table 3), which resembled the pattern established for LUE (Figure 4E). The diurnal variations in \(\mathrm{SIF_{A+By}}\) during the young and mature stages did not significantly differ (Figure 5I), with highest mean \(\mathrm{SIF_{A+By}}\) values observed at midday for both (Table 3). During the senescent stages, the SIF yield metrics were higher in the morning, steadily declined throughout the day, and reached lowest mean values in the late afternoon, which distinctly differed from the patterns observed during the young and mature stages.