## 1 Introduction
This remote-sensing capacity is based on the inverse relationship between isoprenoid emission and LUE and because PRI has already been widely tested as a good estimator of LUE at the foliar, canopy and ecosystem levels at different temporal scales [28; 29], PRI is also related to isoprene emissions because the pathways of both isoprene biosynthesis and xanthophyll-cycle dissipation co-vary, both increasing when more reducing power is available [25], and on the diurnal time scale, PRI measures reflectance changes caused by the interconversion and dissipation of xanthophylls [30]. The next step is to test the ability of PRI to estimate isoprenoid emissions at larger scales, e.g., canopy or ecosystem levels, and throughout the season. Adding the temporal scale constitutes an additional challenge because the relationships of both PRI and isoprene with LUE can vary throughout the season. Diurnal PRI variation is mainly driven by changes in the xanthophyll cycle [30; 31], which are considered facultative changes in pigments. On seasonal time scales (over weeks and months), PRI variation can be a combined function of the xanthophyll cycle and changes in the poolsof carotenoids and chlorophylls [32; 33; 34; 35; 36], which are considered constitutive changes in pigments. Recent studies have indicated these constitutive changes in pigment pool size have a dominant influence on the Chlorophyll/Carotenoid Index (CCI, a modified PRI calculated using chlorophyll absorption band as reference) over long time spans [36; 37]. Similarly, the relationship between PRI and isoprene emission is strongly influenced at longer timescales by constitutive changes in the size of carotenoid pigment pools [38] presenting the interesting hypothesis that carotenoid pigments are causally related to isoprene emissions over seasonal time spans. This relationship may be due to the link (either direct, e.g., via substrate availability, or indirect, e.g., via complementary functionality) between potential isoprenoid emission and carotenoid synthesis at timescales of days to weeks [39]. In addition, the ability of PRI to predict seasonal variations in isoprene emission due to environmental factors other than light and temperature [40; 41; 42] and under stress conditions [13; 14] remains unknown. It is likely that over long time scales, PRI (or CCI) provides a general \"stress indicator\" that integrates the influence of a number of environmental factors on plant biochemistry, photosynthetic physiology, and isoprene emission. The main aim of this study was to test the utility of PRI as an estimator of isoprene emissions at the ecosystem level. To do this, we surveyed the isoprene emissions at the ecosystem level measured by eddy covariance throughout two consecutive growing seasons in a temperate deciduous forest in central Missouri, USA [13; 14]. The site experienced a mild drought in 2011 and an extreme drought in 2012. Both droughts were concurrent with high isoprene fluxes. The occurrence of different drought intensities during the measurement period added another important test for PRI. Accordingly, in this work we had two additional aims, to study the utility of PRI as an estimator of seasonal isoprene emissions and the sensitivity of PRI to the changes in isoprene emission caused by moderate and extreme drought.