## 4 Discussion
### PRI and R690/R630 Tracked the Photosynthetic Changes under Enhanced Drought Levels
These studies presented the complications of the applicability of PRI under severe drought conditions at different spatial scales, and other factors, such as soil backgrounds, vegetation functional types, atmospheric interference and instrument characteristics, can also complicate the applications of PRI in assessing LUE [24]. However, the strong correlations between PRI and \(A\) and \(\mathrm{\Phi PSII}\) (\(\mathrm{F_{V}/F_{M}}\) and Yield) under enhanced drought levels demonstrate that PRI can be applied to detect the effect of continuously increased drought on photosynthesis for Mediterranean evergreen sclerophylls, which generally experience summer drought. Interestingly, the reflectance ratio based on the red-edge region of the spectrum R690/R630 presented significant correlations with \(A\), \(\mathrm{F_{V}/F_{M}}\), and Yield, and efficiently tracked the photosynthetic changes to enhanced drought levels (Figure 2b,d). Previous studies have shown reflectance ratios such as R685/R630 and R690/R630 are sensitive to changes in foliar \(\mathrm{F_{V}/F_{M}}\). [PERSON] et al. [31] used similar reflectance ratios to quantify the chlorophyll fluorescence (ChLF) emissions because of their superimposition at the red reflectance region, with a maximum emission in the near red-edge region of 690 and 740 nm. The reference band (e.g., R630 or R655) was chosen due to its insensitivity to ChlF emission and sensitivity to chlorophyll pigments [47]. These simple reflectance ratios have tracked the foliar ChlF under both diurnal natural, stress and recovery conditions [31; 47; 49]. The links between R690/R630 and ChlF emissions probably contribute to interpreting the relationships between R690/R630 and photosynthesis in this study. In addition, ChlF can increase under severely low quantum yields [63], however, continuous decreases in R690/R630 during extreme drought periods suggested that, apart from ChlF emission at 690 nm, other plant physiological functions also control the changes in R690/R630. The reabsorption of red ChlF (particularly at ChlF emission peak near 690 nm) by chlorophyll pigments can also affect the variability of R690/R630 [45]. Further, the low reflectance in the red-edge of the spectrum generally resulted from the high absorption of chlorophyll pigments. [PERSON] et al. [49] showed the large effect of chlorophyll content on similar reflectance ratios such as R685/R630 and R680/R630. However, NDVI, which could indirectly indicate the chlorophyll pigment changes, was quite stable and did not track the photosynthetic activity during the study period. The chlorophyll content thus probably had low effects on R690/R630 and its correlation with photosynthesis for this Mediterranean evergreen species. Additionally, the down-regulation of maximum photochemical efficiency of PSII (\(\rm F_{V}/F_{M}\)) is associated with changes of non-photochemical quenching (NPQ) under environmental stress [61; 64; 65]. The significant correlation of R690/R630 with \(\rm F_{V}/F_{M}\) in this and previous diurnal studies [47; 48] indicated that NPQ probably plays a key role in linking R690/R630 with changes in photosynthetic activity. Although PRI was also highly correlated with \(\rm F_{V}/F_{M}\), the rather constant PRI in the extreme drought conditions (the last three drought treatments) was decoupled with the strong depression of \(\rm F_{V}/F_{M}\). Such decoupling of PRI with LUE and NPQ was also found in early spring when Scots pine needles demonstrated large down-regulation due to the effect of severe stress [27]. In contrast, in our study, R690/R630 presented consistent changes with \(\rm F_{V}/F_{M}\), Yield and \(A\) even under extreme water stress conditions.