## 4 Discussion
### Seasonal Drivers of Solar Induced Fluorescence  
Seasonal relationships between SIF and other measured parameters were relatively consistent between raw and yield values of SIF. Previous studies suggested that SIF\({}_{\text{FR}}\) correlates with Chl content [38], NDVI [39], crop developmental stage [8], canopy and air temperatures [21], PAR [38] and F\({}^{\prime}\)[7] over seasonal time scales. We found significant associations between changes in SIF\({}_{\text{FR}}\), SIF\({}_{\text{red}}\) and YSIF\({}_{\text{FR}}\) with changes in leaf PAR, ETR and YNO. The relationship between SIF and ETR has been reported at global scales [17], but has not previously been reported at the leaf-level. Additionally, to the authors' knowledge, no study has indicated the relationship between SIF and YNO. YNO is thought to represent energy losses due to both fluorescence and constitutive heat dissipation [20]. However, we identified decreases in SIF with increases in YNO. This suggests that the dominant driver of YNO change was increases in constitutive heat loss, which may lead to reductions of SIF at seasonal time scales.  
Our results also indicate potential relationships between increases in the SIF\({}_{\text{FR}}\), YSIF\({}_{\text{FR}}\) and SIF\({}_{\text{red}}\) and increase in leaf pigment parameters, i.e., total Chl, Chl a/b, \(\Sigma\)VAZ and \(\sum\)LLx. Although the vector changes for leaf pigments were statistically insignificant (\(p>0.16\)), with a larger pigment sample set it is likely that vectors for total Chl content and Chl a/b ratio may become significant. Unlike other pigment vectors the vector for car \(\alpha/\beta\) ratio appeared to be associated with changes in leaf thickness and measurements of leaf stress and photosynthetic efficiency (LUE, YPSII and PRI). The further association of the stress and photosynthetic efficiency vectors with day length suggests that the relationship may be linked to seasonal changes in leaf longevity. During longer days (spring and summer) avocado plants produce new young leaves, which typically have a lower leaf thickness and a lower car \(\alpha/\beta\) ratio, but greater capacity for photo-protection (YNPQ) [40].  
Additionally, we also found a significant association between increases in SIF\({}_{\text{FR}}\), YSIF\({}_{\text{FR}}\) and SIF\({}_{\text{red}}\) and decreases in NDVI. Interestingly, the direction of the vector for NDVI is opposing the vector direction for leaf total Chl content and it is unrelated to changes in leaf thickness, both of which are known causes of leaf NDVI changes [16]. This may suggest that the variability in NDVI values caused by seasonal changes in diffuse vs. specular irradiance, and/or other confounding factors, may be greater than the measured NDVI changes due to phenological fluctuations in Chl and leaf thickness. Contrastingly, seasonal association of low PRI values with high LUE and PSII yields is congruent with previously published findings [18; 41; 42]. Moreover, the vectors for LUE, PRI and YPSII also showed the expected relationship with YNPQ, where high values of NPQ correspond with lower LUE, YPSII and increasing (positive) PRI values (lower DEPS) [18].