## 4 Discussion
The decrease in emissions was likely caused by substrate limitation and lower temperatures in 2011 and by substrate limitation in 2012. Photosynthesis and isoprene emissions were decoupled seasonally, as previously described under drought conditions (e.g., [64; 65; 66; 67; 68; 69]). Isoprenoid emissions continued to increase concurrently with a decrease in photosynthetic CO2 fixation (GPP) during the increasing phase. Mild stress may decrease carbon assimilation, but isoprene emissions would not be affected, because the rate of electron transport would be maintained and reserve carbon in the form of sugars and starch would support the production of isoprenoids [55]. Isoprene might reduce cell oxidation and increase cell stabilization in plants [70; 71], so increasing the allocation of the recently assimilated carbon to isoprene would constitute an additional mechanism of drought resistance [58]. The relationship of PRI with isoprene differed between the two years, as indicated by the different intercepts in the fit (Figure 5). However, the PRI-LUE relationship followed the same regression line for the two years (Figure 4), indicating the good performance of PRI in estimating LUE. The relationship between isoprene emissions and LUE, however, was year-dependent even though the slopes were similar (Figure 3), suggesting that the proportion of available photosynthetic reducing power for isoprene formation differed between the two years. This difference might have been due to a higher competition between photorespiration and the MEP pathway under drought stress as the rate of photorespiration increases and reducing power and carbon supply for the MEP pathway decrease [55], because carbon availability can limit emission rates under severe drought and photorespiratory stresses [55] and isoprene emission may be regulated by substrate availability [67]. The residual reducing power not invested in carbon assimilation is shared by non-photosynthetic sinks such as photorespiration or the MEP pathway [55], and isoprene emission may be most strongly influenced by pathways co-located with the MEP pathway in the chloroplast, such as photorespiration, because they compete directly for both reducing power and photosynthetic carbon [59]. In summary, we hypothesize that seasonal isoprene emissions would be primarily driven by temperature but modulated by reducing power and substrate availability, based on observations and previous conceptual models [13; 59]. Photosynthesis, photorespiration and the MEP pathway directly depend on reducing power and substrate availability. The availability of reducing power and substrate for the MEP pathway is thus dependent on photosynthesis and photorespiration. Isoprene emission would thus be stimulated by an increase in available reducing power when drought conditions decrease photosynthesis but would begin to decrease later in the season, despite the high availability of reducing power, because temperature would begin to decrease and substrate would become limiting. Photorespiration increases under drought conditions, causing a decrease in isoprene emissions, and photosynthesis (GPP) becomes minimal under severe drought, so the carbon supply for the MEP pathway is exhausted earlier, advancing the seasonal decrease in isoprene emissions. We summarize the feedbacks involved in the proposed conceptual model in Figure 9. Accordingly, we also propose a model of the performance of PRI and isoprene emissions throughout the season including the final decoupling between them (Figure 10). PRI is highly correlated with isoprene emissions at the beginning of the season and can thus be used to estimate them. Emissions and LUE become decoupled at the end of the season, when LUE (GPP) is lowest and stabilizes; LUE remains low and isoprene emissions begin to decrease.