(13)C discrimination in organic matter with respect to atmospheric CO(2) (Delta(13)C) is under tight genetic control in many plant species, including the pedunculate oak (Quercus robur L.) full-sib progeny used in this study. Delta(13)C is expected to reflect intrinsic water use efficiency, but this assumption requires confirmation due to potential interferences with mesophyll conductance to CO(2), or post-photosynthetic discrimination. In order to dissect the observed Delta(13)C variability in this progeny, six genotypes that have previously been found to display extreme phenotypic values of Delta(13)C [either very high ('high Delta') or low ('low Delta') phenotype] were selected, and transpiration efficiency (TE; accumulated biomass/transpired water), net CO(2) assimilation rate (A), stomatal conductance for water vapour (g(s)), and intrinsic water use efficiency (W(i)=A/g(s)) were compared with Delta(13)C in bulk leaf matter, wood, and cellulose in wood. As expected, 'high Delta' displayed higher values of Delta(13)C not only in bulk leaf matter, but also in wood and cellulose. This confirmed the stability of the genotypic differences in Delta(13)C recorded earlier. 'High Delta' also displayed lower TE, lower W(i), and higher g(s). A small difference was detected in photosynthetic capacity but none in mesophyll conductance to CO(2). 'High Delta' and 'low Delta' displayed very similar leaf anatomy, except for higher stomatal density in 'high Delta'. Finally, diurnal courses of leaf gas exchange revealed a higher g(s) in 'high Delta' in the morning than in the afternoon when the difference decreased. The gene ERECTA, involved in the control of water use efficiency, leaf differentiation, and stomatal density, displayed higher expression levels in 'low Delta'. In this progeny, the variability of Delta(13)C correlated closely with that of W(i) and TE. Genetic differences of Delta(13)C and W(i) can be ascribed to differences in stomatal conductance and stomatal density but not in photosynthetic capacity.

Keywords:carbon isotope discrimination / intrinsic water use efficiency / pedunculate oak / stomatal conductance / stomatal density Abstract • Previously, a large intra-specific diversity and a tight genetic control have been shown for Δ 13 C (carbon isotope discrimination) in a pedunculate oak (Quercus robur L.) family, which is an estimator for intrinsic water use efficiency (W i ), a complex trait defined as the ratio of net CO 2 assimilation rate (A) to stomatal conductance for water vapour (g s ).• In the present study, twelve genotypes with extreme phenotypic values of Δ 13 C were selected within this family to (i) asses the stability of genotype differences across contrasting environments and for different measures W i ; (ii) quantify the relationship between Δ 13 C and W i within this family; (iii) identify which leaf traits drive the diversity in W i observed in this family.• Genetic variability of Δ 13 C and W i was largely independent from different temporal integration scales and their correlation was found to be strong (R 2 = 88% for leaf sugars) within this family.• Weak correlations between measures of W i with estimators of photosynthetic capacity, suggest a minor role of the latter in the diversity of W i .• However, the tight correlation between g s and Δ 13 C as well as W i , and the related genotypic variation in stomatal density, suggest that the genotypic diversity in W i within this pedunculate oak family might be due to differences in g s . Mots-clés :discrimination isotopique du carbone / efficience intrinsèque de l'ullisation de l'eau / clêne pedonculé / conductance stomatique / densité stomatique Résumé -Diversité de l'efficience d'utilisation de l'eau entre différents génotypes de Quercus robur : contributions des traits foliaires.• Une large diversité intra-spécifique et un fort contrôle génétique ont été mis en évidence pour Δ 13 C (discrimination isotopique du carbone) dans une famille de chêne pédonculé (Quercus robur L.). Δ 13 C est un estimateur de l'efficience intrinsèque d'utilisation de l'eau (W i ), un caractère complexe défini comme le rapport entre l'assimilation nette de CO 2 (A) et la conductance stomatique pour la vapeur d'eau (g s ).• Douze génotypes présentant des valeurs phénotypiques extrêmes de Δ 13 C ont été sélectionnés dans cette famille pour (i) évaluer la stabilité des différences génotypiques dans des environnements contrastés et pour différents estimateurs de l'efficience d'utilisation de l'eau ; (ii) quantifier la relation entre Δ 13 C et W i dans cette famille ; (iii) identifier quels caractères foliaires sont impliqués dans la diversité de W i observée dans cette famille.• La variabilité génétique de Δ 13 C et W i était largement indépendante des différentes échelles d'intégration temporelles et leur corrélation était forte dans cette famille (R 2 = 88 % lorsque Δ 13 C était mesuré dans les sucres foliaires).• Les faibles corrélations entre W i (ou Δ 13 C) et des estimateurs de la capacité photosynthétique suggèrent un rôle mineur de celle-ci dans la dive...