## 4 Discussion
The close link between fluorescence emission and photosynthesis is well known [53]. Indices derived from this process may track changes in photosynthetic activity. In fact, [PERSON] _et al._[54] evaluated the usefulness of chlorophyll fluorescence measurements at leaf level during grain filling to evaluate yield performance under Mediterranean conditions. Remote sensing of chlorophyll fluorescence emissions under natural sunlight conditions confirmed its usefulness for tracking gross primary productivity (GPP) at field [23] and even global level [55]. Fluorescence retrieval using the Fraunhofer Line Depth (FLD) has recently been linked to stomatal conductance [38] and net photosynthesis [23]. Our results confirmed these findings and scaled up the usefulness of fluorescence for phenotyping purposes. In this study, indices related to the xanthophyll pigments cycle (mainly the PRI) showed a close relationship with yield. However, previous studies have demonstrated a lower discrimination power for PRI compared to other indices at different locations [19]. Our study was limited to a single site, although the relationship was consistent for the two species considered. The weaker relationship with the structural-derived indices was evaluated by the stepwise multiple regression. The usefulness of these indicators (mainly the NDVI) as a criterion for yield selection has been well established [12, 56]. It has been shown to be effective under well-watered conditions [20, 57]. In high-yielding environments, the ability to intercept radiation can define final yield much more accurately than under arid Mediterranean conditions, where limitations on water availability during grain filling can overcome other constraints and defines yield. [PERSON] _et al._[58] recently observed a good relationship between yield and NDVI for a large number of spring bread wheat varieties under Mediterranean conditions. However, compared to our average temperatures, their climate was milder and, even under severe water stress conditions, the level of stress reached was probably lower than in our study. The sudden change in air temperature and thus atmospheric demand for transpiration would have substantially increased the use of water by the crop. Plots displaying more vigorous growth would therefore already have depleted the soil water reserves, and thus, would have been more sensitive to terminal stress. This is corroborated by the relationship observed between NDVI and assimilation rate (Figure 7f) and would impair the relationship between crop vigor and yield (Figure 8d). In agreement with this conclusion, [PERSON] _et al._[19] observed that NDVI was not closely correlated with grain yield at the milk stage for those experiments that received less water and produced lower yields. The early leaf senescence caused by the terminal stress was probably the origin of this impairment. Other indices have been proposed for screening yield performance under arid conditions. [PERSON] _et al._[21] observed that vegetation indices did not clearly correlate with yield, while other indices, related to water content (calculated from the near infrared spectral region) obtained better results under a wide range of environmental conditions.