## 4 Discussion  
The relationship between isoprene emissions and PRI supported our hypothesis that estimates of isoprenoid emission could be improved by using remotely sensed PRI at the ecosystem level, under conditions of both moderate and extreme drought stress. This capacity of PRI, a good estimator of LUE at foliar, canopy and ecosystem levels [28; 29; 30; 31; 54], is based on the inverse relationship between isoprenoid emissions and LUE due to the higher availability of photosynthetic reducing power for isoprenoid production under lower LUEs [25; 26; 27] and to the positive relationship between PRI (via changes in the xanthophyll cycle and the carotenoid synthesis) and isoprene biosynthesis [25]. Additionally, PRI follows the seasonal change in carotenoid pigment pool sizes (relative to chlorophyll), and these carotenoid pigments are possibly related to isoprene biosynthesis [38; 39]. The emission peak of isoprene coincided with an inflection point in the decreasing trend of the LUEs of vegetation. PRI was sensitive to the isoprene-emission peak coinciding with the drought in 2011, a peak that  
Figure 8: Measured versus estimated isoprenoid emissions using the MEGAN model and MEGAN model+PRI with data from the increasing phase. The black line is the 1:1 line, and the shading represents the 95% confidence intervals. MEGAN underestimated. PRI was also sensitive to the advance of the emission peak due to severe stress in 2012. The highest isoprene emissions coinciding with the peak of the drought in 2011 in our study (a decrease in SWC, predrawn foliar water potential and net ecosystem production; [13]) were higher than expected due solely to the conditions of light and temperature. This increase in isoprene emissions could be partly due to an increase in foliar temperature caused by drought-induced reductions in stomatal conductance [13; 55], but it may also be due to the excess reducing power (Figure 1, DOY 203-215). Isoprene emissions are not well explained by light and temperature under high stress [56; 57]. The stimulation of ecosystem isoprene emissions under various abiotic stresses reported in some studies (see references in [11; 26; 57]) was likely associated with the excess reducing power under stress conditions [17; 25]. Residual reducing power not used by carbon assimilation can support increased isoprene emission under drought [58]. LUE explained part of the variance in isoprene emissions (Figure 2), and water stress and LUE were strongly correlated. Increased emission under drought is sustained if the intensity of drought remains within a species-specific tolerance threshold [59]. Under extreme stress conditions, isoprene emission starts declining for a length of time, which depends on the species studied, until emissions are fully inhibited. Isoprene emission in our study was mostly higher than predicted when SWC was \(<\)0.26 m\({}^{3}\) m\({}^{-3}\), and emission began to decrease when SWC reached the wilting point for this ecosystem (0.23 m\({}^{3}\) m\({}^{-3}\); [14]). This seasonal decrease occurred earlier in 2012 than 2011 and earlier than modeled. Carbon availability may have critically limited emission under the severe drought at the site in 2012. Water limitation can override the physiological effects of high temperature on isoprene emission during severe stress [60], perhaps due to a very limited amount of recently produced dimethylallyl pyrophosphate (DMAPP), an isoprene substrate [56]. Isoprene is produced in the chloroplast via the methylerythnitol phosphate (MEP) pathway [61], which requires both reducing power and carbon skeletons provided by photosynthesis [62]. [PERSON] and [PERSON] [63] attributed the decrease in isoprene emission during drought to a limitation of the carbon supply rather than to a downregulation of isoprene synthase, an enzyme used during isoprene production. PRI was, thus, not as good as an estimator of isoprene emissions later in the season likely because isoprene emissions were decoupled from excess reducing power.