## 4 Discussion
### Daily Trends in Solar Induced Fluorescence  
The diurnal trends of both avocado and orange jasmine leaves showed the expected changes in photosynthetic parameters associated with changes in PAR. However, the two species differed in the magnitude of these changes. To our knowledge, there is no literature on active fluorescence measurement of orange jasmine available. As such, it is difficult to gauge if the lower YPSII and YNPQ of orange jasmine leaves in comparison to avocado leaves are triggered by physiological differences between the species. Moreover, differences in YPSII and YNPQ between the two species may be influenced by the mounting of orange jasmine leaves to a foam board target, which may have interfered with air flow and leaf temperature. For avocado, a few studies focusing on light adaptation and lutein cycle dynamics in avocado have reported PAM [25; 44] and LIFT [31] estimates of YPSII and ETR similar to those reported under the comparable light conditions in this study. Diurnal SIF changes of both plant species were found to occur in three discrete stages, as previously reported in canopy measurements of maize by [PERSON] et al. [8]. Morning SIF increases were often followed by a decrease, which was particularly evident in measurements of avocado leaves. No coincident changes in photosynthetic parameters were present during the SIF decrease. However, coincident changes in both NDVI and PRI, which were more pronounced in avocado leaves, were present. Since no changes in photosynthetic measurements or light conditions coincide with these decreases, we hypothesise that they correspond to solar geometries, where direct specular leaf reflections that are not present in measurements of the Lambertian white reference result in distortion of the optical indices and depth of O\({}_{2}\)-A and O\({}_{2}\)-B absorption features. It should, however, be noted that the behaviour of SIF\({}_{\text{red}}\) was found to be less reliable, because of bigger associated errors and occasionally erratic behaviour that can be attributed to the shallower depth of the O\({}_{2}\)-B absorption feature [6; 45]. The photosynthetic parameters (ETR, YNO, YNPQ and YPSII) behaved as would be expected, with changes in avocado measurements similar to measurements of YPSII, ETR and NPQ reported by [PERSON] and [PERSON] [31]. Maximum light utilization (as measured by ETR) in avocado leaves occurred under moderate light in the morning and afternoon. The high light periods were dominated by low YPSII, high YNPQ and lower ETR. In contrast, peak light utilization in leaves of orange jasmine occurred during the middle of the day under high light. The decrease in ETR observed under the high light in some avocado measurements is indicative of reversible acute water stress and increased photorespiration that can occur during stomatal closure [46]. This is also evident in maximum ratesof YNPQ identified in avocado, which responded faster and reached higher levels than in orange jasmine, indicating better low light utilization and an occurrence of light stress under high levels of illumination. Additionally, differences in constitutive heat dissipation (YNO) were recorded between the two examined species. In situations when YNPQ and YPSII can regulate disposal of excess APAR, YNO may remain constant [20]. In leaves of avocado YNO remained constant throughout the day, with the exception of the rapid onset of direct irradiance in the morning, which caused a transient decrease in YNO. This decrease in YNO is indicative of a light induction, where the fast increase in light exceeds the slower increasing quenching capacity of YNPQ up-regulation. In contrast, YNO in orange jasmine leaves dropped during morning increases in light and slowly recovered with decreasing light in the afternoon. These fluctuations in YNO may suggest that under high irradiances APAR exceeds the quenching capacity of YPSII and YNPQ with the excess energy regulated through YNO.