## 1 Introduction  
The increasing occurrence of drought in the Mediterranean region is widely reported [1; 2; 3] and during the last 20 years observations show that it has affected ecosystem functioning and structure [4; 5], decreased plant growth [6] and primary production [7; 8], and triggered vegetation mortality [9]. The increase of drought has also enhanced water scarcity [10], and elicited ecological damage [11] and crop failures [12]. Faced with these negative impacts, monitoring the timing of drought onset and the extent of the effects on vegetation ecosystems is being increasingly warranted. Summer drought, one of the key constraints of Mediterranean ecosystems [4], elicits a water deficit in the leaf tissue that can down-regulate photosynthesis [13]. This reduction in photosynthesis for Mediterranean species such as _Quercus__i__lex_ L. is generally caused by stomatal closure in response to drought stress [14; 15; 16]. _Quercus__i__lex_ is a broadleaved evergreen tree or shrub that is widely distributed from semi-arid to humid areas of the Mediterranean region [17]. The sclerophyllous characteristics of \(Q\). _ilex_ make it possible to reduce its transpiration and to withstand the effect of long summer drought events. _Quercus__ilex_ also has the ability to reactivate its photosynthetic machinery and increase photosynthesis rates when the soil water content (SWC) is high enough to again support water transport through the plant [18]. When plants suffer water stress, the reducing power from photosynthesis becomes excessive, but then the increases in zeaxanthin pigments from xanthophyll de-epoxidation can safely dissipate excessive energy and prevent the photosystems from potential damage by accumulating excitation energy [19; 20]. The photochemical reflectance index (PRI) was originally defined based on changes in zeaxanthin at the wavelength of 531 nm to assess the efficiency of absorbed photosynthetic active radiation (APAR) for photosynthesis (light use efficiency; LUE) [21; 22]. A great number of studies have found that PRI is able to track LUE changes at diurnal and seasonal scales from leaf to canopy to ecosystem levels [23; 24; 25], because apart from xanthophyll pigments, PRI was also associated with changes of carotenoid/chlorophyll ratios [26; 27; 28; 29], which play a key role in long-term dynamics of photosynthesis [20; 30]. Increasing attention has been focused on using PRI to detect the effect of environmental stress (e.g., intense irradiance, water shortage, nutrition deficit, and high and low temperature stresses) on photosynthetic changes [8; 14; 31; 32; 33; 34; 35; 36]. The changes in photosynthesis are synchronously accompanied by the emission of chlorophyll \(a\) fluorescence (ChIF), which is regulated by the photochemical conversion and heat dissipation of excitation energy [37]. The contribution of the ChIF emission is only a small part of the total radiation reflected from vegetation, however, ChIF is highly sensitive to the variability of plant physiological processes in response to various environmental conditions, and can provide a direct approach to detect the functional status of photosynthetic machinery [38; 39]. The range of the ChIF emission spectrum is from the red to near-infrared regions, with two peaks at around 690 and 740 nm [38]. The active ChIF, mainly obtained from a pulse-amplitude modulated (PAM) fluorometer, has been extensively used for monitoring foliar photosynthetic apparatus [40]. Recently, the passive ChIF (solar-induced fluorescence, SIF), derived from such methods as the Fraunhofer Line Depth technique or laser induction, has scaled up the measurements and the applications for detecting photosynthesis from leaf level to canopy, ecosystem and regional scales [31; 41; 42; 43; 44]. Some studies have illustrated that reflectance ratios such as R690/R600, R690/R630, R690/R655, and R740/R800 were sensitive to the changes of ChIF, because the ChIF signal is superimposed on the red-edge of leaf reflectance [38; 45; 46].