Intrinsic water use efficiency (Wi) is the ratio between net leaf CO2 assimilation rate (A) and stomatal conductance to water vapour (gs). QTL have been detected for A, gs or Wi, mainly for crop species (rice, what, maize), Arabidopsis and a few tree species (poplar, apple, maritime pine and pedonculate oak). However the large number of individuals to measure usual for QTL studies complicates the estimation of genotype-typical values by introducing response-variations during the measurement campaign. Repeated measurements and multi-environment QTL analyses are one approach to gain detection power. In this study, we investigated the impact of elevated atmospheric CO2 concentration on diversity of A, gs and Wi in a Quercus robur mapping population. The atmospheric CO2 concentration impacts both, A and gs. Short term responses to an increased atmospheric CO2 concentration are an increase in A and a decrease in gs, resulting in an increased Wi. However, plant co-regulate A and gs, resulting in a complex system. For long-term exposure to elevated CO2, acclimation can also play a role and impact A and gs differently, which would have a large impact on the temporal evolution of Wi. 183 three-year old cuttings from a Quercus robur full-sib family were exposed before bud flushing in two adjacent greenhouses to two different CO2 levels (ambient, controlled : 380 µmolmol-1 and elevated : 690 µmolmol-1). Gas exchange measurements were done on five dates from May until September. Combining 5 dates with two CO2 levels, resulted in 10 data sets that were combined in one QTL analysis similarly to a full-cross statistical analysis. The multi-environment QTL analysis (MultiQTL) was used to test for significant date and/or CO2 differences for the allelic effects of the detected QTL, where significance is an indication for a plasticity QTL. Plasticity QTL were also detected for the CO2 effect by using the difference between environments for A, gs and Wi as an estimator of plasticity. A significant CO2 or CO2 x date effect has been detected for two QTL for gs, whereas a significant date effect has been detected for two QTL for A and Wi, respectively. The results will be discussed in terms of allelic sensitivity or gene regulation plasticity theory. The detected QTL for Wi will play an important role in the ongoing French national project H2Oak (https://www6.inra.fr/anr-h2oak/) to select positional candidate genes for screening in natural populations.