et al.. The diversity of 13C isotope discrimination in a Quercus robur full-sib family is associated with differences in intrinsic water use efficiency, transpiration efficiency, and stomatal conductance. Abstract 13 C discrimination in organic matter with respect to atmospheric CO 2 (D 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. D 13 C is expected to reflect intrinsic water use efficiency, but

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... this assumption requires confirmation due to potential interferences with mesophyll conductance to CO 2 , or post-photosynthetic discrimination. In order to dissect the observed D 13 C variability in this progeny, six genotypes that have previously been found to display extreme phenotypic values of D 13 C [either very high ('high D') or low ('low D') 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 ¼A/g s ) were compared with D 13 C in bulk leaf matter, wood, and cellulose in wood. As expected, 'high D' displayed higher values of D 13 C not only in bulk leaf matter, but also in wood and cellulose. This confirmed the stability of the genotypic differences in D 13 C recorded earlier. 'High D' 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 D' and 'low D' displayed very similar leaf anatomy, except for higher stomatal density in 'high D'. Finally, diurnal courses of leaf gas exchange revealed a higher g s in 'high D' 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 D'. In this progeny, the variability of D 13 C correlated closely with that of W i and TE. Genetic differences of D 13 C and W i can be ascribed to differences in stomatal conductance and stomatal density but not in photosynthetic capacity. Abbreviations: A, net CO 2 assimilation rate; A max , maximum A measured during daily course of gas exchange (lmol m À2 s À1 ); A mean , average A calculated from daily course of gas exchange (lmol m À2 s À1 ); A sat , light-saturated A at ambient CO 2 concentration (lmol m À2 s À1 ); C a , atmospheric CO 2 mole fraction (lmol mol À1 ); Chl, chlorophyll content (g m À2 ); C i , CO 2 mole fraction in intercellular air spaces (lmol mol À1 ); DM, dry biomass; g i , mesophyll conductance for CO 2 (mol m À2 s À1 ); g s , stomatal conductance for water vapour; g max , maximum g s measured during daily course of gas exchange (mol m À2 s À1 ); g mean , average g s calculated from daily course of gas exchange (mol m À2 s À1 ); g sat , g s at ambient CO 2 concentration and light saturation (mol m À2 s À1 ); J max , maximal electron transport rate (lmol m À2 s À1 ); LET, lower epidermis thickness (lm); LMA, leaf mass-to-area ratio (g m À2 ); LA, leaf area estimated from dry biomass (m 2 ); N area , nitrogen content on an area basis (g m À2 ); N mass , nitrogen content on a mass basis (mg g À1 ); PMT, palisade mesophyll thickness (lm); PT, plant transpiration (kg H 2 O m À2 leaf area); qPCR, quantitative polymerase chain reaction; QTL, quantitative trait loci; SD, stomatal density (mm À2 ); SMT, spongy mesophyll thickness (lm); TE, transpiration efficiency (mg DM g À1 H 2 O); TLT, total leaf thickness (lm); UET, upper epidermis thickness (lm); V cmax , maximum carboxylation rate of Rubisco (lmol m À2 s À1 ); W i , intrinsic water use efficiency; W sat , W i at ambient CO 2 concentration and light saturation (lmol mol À1 ); W mean , average W i calculated from daily course of gas exchange (lmol mol À1 ); a, slope of the response of g s to light determined with daily course of gas exchange (10 4 mol m À2 s À1 /unit of global irradiance); DBA, basal area increment during the experiment (mm 2 ); DBM, biomass accumulation (g);