## 4 Discussion  
### PRI and R690/R630 Tracked the Photosynthetic Changes under Enhanced Drought Levels  
The enhanced levels of drought caused reductions in CO2 assimilation rates (\(A\)) and photochemical efficiency of photosystem II (\(\Phi\)PSII) (F\({}_{\mathrm{V}}\)/F\({}_{\mathrm{M}}\) and Yield) in Mediterranean evergreen leaves (Figure 1). Similar trends were also recorded for the photochemical reflectance index (PRI, Figure 1a) and reflectance ratio based on the red-edge region of the spectrum (R690/R630, Figure 1b), indicating the sensitivity of PRI and R690/R630 to gradually increased drought stress. The close correspondence of PRI and R690/R630 with photosynthetic activity (Figure 3a) and \(\Phi\)PSII (Figure 3b) demonstrated the promise of pigment- or red-edge reflectance-based approaches to remote monitoring of evergreen photosynthetic activity under gradually enhanced drought. PRI is used to reveal the facultative xanthophyll cycle activity at short-term scale (hours to few days) [21; 22; 28], but also to estimate the changes in the constitutive pigment pool size at long-term scale (weeks to months) [27; 29; 58; 59]. In our study, PRI showed promise as an index of photosynthetic down-regulation and illustrated the activation of photo-protective carotenoid pigments under photosynthetic down-regulation in response to gradually enhanced drought stress. During the summer  
Figure 5: Relationships of differential \(A\) (\(\Delta\)Arecovery) with \(\Delta\)PRIrecovery (**a**) and \(\Delta\)R690/R630 recovery (**b**), and photochemical efficiency of PSII (\(\Delta\)\(\Phi\)PSII) (F\({}_{\mathrm{V}}\)/F\({}_{\mathrm{M}}\) and Yield) with \(\Delta\)PRIrecovery (**c**) and \(\Delta\)R690/R630 recovery (**d**) for _Quercus flex._ All the differential values are from Figure 4.  
drying period, leaves in evergreen vegetation of semi-arid Mediterranean region experience decreases in pigment contents and increases in carotenoids/chlorophyll (Car/Chl) ratio [60]. This change in Car/Chl ratios plays a key role in photosynthetic down-regulation, and is associated with variability in PRI [27; 58]. Decreases in PRI and photosynthesis presented in this study thus indirectly suggested that changes in Car/Chl had a significant effect on photosynthetic down-regulation. Therefore, PRI detected the effect of the progressive drought on photosynthesis, consistent with many previous studies that have demonstrated that PRI is correlated with photosynthesis and Yield under water stress conditions [14; 34]. Additionally, PRI was quite stable in the last three extreme drought level treatments (plants without water for 16-21 days), unlike the slight decrease of \(A\), \(\mathrm{F_{V}/F_{M}}\), Yield, and R690/R630, reflecting the insensitivity of PRI to severe drought, at intensities where there is observable leaf fall. Previous leaf-level studies have reported that PRI was unable to assess the negative light use efficiency (LUE) of severely damaged plants [34]. Under severe stress conditions, evergreen leaves could retain zeaxanthin pigments to protect the photosystem from damage, which might contribute to such stability in PRI [61]. At canopy level, changes in structure, heterogeneity in irradiance, and differences in sun angles within the crown have hindered PRI interpretation and assessment of LUE [24]. At larger spatial scales, satellite-based (Moderate Resolution Imaging Spectroradiometer, MODIS) PRI has tracked ecosystem LUE, even during severe water limitation of summer for _Q. liex_[62]. The normalized PRI by absorbed light detected the drought effect on gross primary production (GPP) in a deciduous forest and an evergreen broadleaf forest in France, but did not capture the reduction of GPP in a semi-arid grassland in Hungary [8]. [PERSON] et al. [36] demonstrated that MODIS-based PRI should be used with care under severe water stress, because the disturbances from canopy structure, illumination and viewing angles, and so forth, could increase the uncertainty of PRI in tracking LUE.