White poplar (Populus alba L.) is native to Eurasia and is unexploited for its growth potential and stress-adaptive mechanisms. A better knowledge of its genome will allow for more effective protection and use of critical genetic resources. The main objective of this study was the construction of highly informative P. alba genetic maps. Two genotypes were selected from contrasting natural Italian populations and crossed to generate an F 1 mapping pedigree. Amplified fragment length polymorphism and simple sequence repeat markers were used to genotype 141 F 1 individuals. The pseudo-testcross strategy was applied for linkage analysis. The generated maps showed good overall colinearity to each other and allowed for a complete alignment with the 19 haploid chromosomes of the Populus genome sequence. The locus that determines sex as a morphological trait was positioned on a nonterminal position of LG XIX of the female parent map. Comparison among Populus species revealed differences in the location of the sex locus on LG XIX as well as inconsistencies in the heterogametic sex. The genetic analysis of the sex locus in P. alba provides insights into sex determination in the genus and is useful for the identification of sex-linked markers and the early assessment of plant gender. Furthermore, these genetic maps will greatly facilitate the study of the genomics of Populus and how it can be exploited in applied breeding programs. Communicated by S. González-Martínez Isabella Paolucci and Muriel Gaudet contributed equally to this research. Electronic supplementary material The online version of this article (

Leaf expansion in the fast-growing tree, Populus ϫ euramericana was stimulated by elevated [CO 2 ] in a closed-canopy forest plantation, exposed using a free air CO 2 enrichment technique enabling long-term experimentation in field conditions. The effects of elevated [CO 2 ] over time were characterized and related to the leaf plastochron index (LPI), and showed that leaf expansion was stimulated at very early (LPI, 0-3) and late (LPI, 6-8) stages in development. Early and late effects of elevated [CO 2 ] were largely the result of increased cell expansion and increased cell production, respectively. Spatial effects of elevated [CO 2 ] were also marked and increased final leaf size resulted from an effect on leaf area, but not leaf length, demonstrating changed leaf shape in response to [CO 2 ]. Leaves exhibited a basipetal gradient of leaf development, investigated by defining seven interveinal areas, with growth ceasing first at the leaf tip. Interestingly, and in contrast to other reports, no spatial differences in epidermal cell size were apparent across the lamina, whereas a clear basipetal gradient in cell production rate was found. These data suggest that the rate and timing of cell production was more important in determining leaf shape, given the constant cell size across the leaf lamina. The effect of elevated [CO 2 ] imposed on this developmental gradient suggested that leaf cell production continued longer in elevated [CO 2 ] and that basal increases in cell production rate were also more important than altered cell expansion for increased final leaf size and altered leaf shape in elevated [CO 2 ].Given the importance of forests for global bioproductivity, the consequences of increased atmospheric [CO 2 ] for the global carbon cycle are potentially extremely large (Malhi et al., 1999). Despite this, there are still relatively few large-scale, long-term experiments from which predictions about likely forest responses can be made. Few studies have been completed where trees are allowed to develop to canopy closure and where a "stable" response to [CO 2 ] is likely. Determining the response of leaf area development to elevated [CO 2 ] is important. It is still unknown whether forests of the future will maintain a higher leaf area index (LAI), as implied from smalltree studies (Ceulemans et al., 1997) or whether the long-term (decades) responses will be reduced allocation to foliage and lower LAI, as suggested by some modeling approaches (Medlyn and Dewar, 1996) or involve acclimation to limited nitrogen (Oren et al., 2001).Leaf growth is often stimulated in short-term response to elevated [CO 2 ] (Taylor et al., 1994;Pritchard et al., 1999), and both leaf cell expansion and cell production are sensitive to [CO 2 ] (Taylor et al., 1994). It is likely that these processes respond to additional carbohydrate from photosynthesis and, as such, altered atmospheric [CO 2 ] provides a critical insight into how carbon regulates plant development and growth (Masle, 2000). The importance of leaf develo...

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Summary• The effects of elevated CO 2 on leaf development in three genotypes of Populus were investigated during canopy closure, following exposure to elevated CO 2 over 3 yr using free-air enrichment.• Leaf quality was altered such that nitrogen concentration per unit d. wt ( N mass ) declined on average by 22 and 13% for sun and shade leaves, respectively, in elevated CO 2 . There was little evidence that this was the result of 'dilution' following accumulation of nonstructural carbohydrates. Most likely, this was the result of increased leaf thickness. Specific leaf area declined in elevated CO 2 on average by 29 and 5% for sun and shade leaves, respectively.• Autumnal senescence was delayed in elevated CO 2 with a 10% increase in the number of days at which 50% leaf loss occurred in elevated as compared with ambient CO 2 .• These data suggest that changes in leaf quality may be predicted following longterm acclimation of fast-growing forest trees to elevated CO 2 , and that canopy longevity may increase, with important implications for forest productivity.

The functional response to salt stress was studied in three Populus alba L. genotypes, from three localities of Italy: genotype 6K3 from northern Italy, genotype 2AS11 from middle-southern Italy, and genotype 14P11 from a coastal zone in southern Italy. Plants of the three genotypes were submitted to a progressive salt stress by NaCl irrigation during 4 weeks. The severity of symptoms was linked to the sodium accumulated in leaves of salt-stressed plants. Genotype 2AS11 accumulated significantly less sodium in leaves than the other genotypes, arising the hypothesis that mechanisms for sodium exclusion at root level could exist in this genotype. Its height growth was significantly reduced by salt stress but diameter growth and leaf abscission were not affected. 14P11 and 6K3 accumulated significantly more sodium in leaves and they were significantly affected by leaf abscission and reduction of height and diameter growth. Genotype 6K3 was the most salt-sensitive and its leaves had more extensive necrosis than 14P11 and 2AS11. The transcription profile of plasma membrane H + -ATPase and tonoplast H + -ATPase genes revealed a different regulation in response to salt stress in 2AS11 and 14P11, but it was unchanged in 6K3. The different response to salinity could be related to different ability to exclude sodium in roots and to different regulation of ion transport across membranes of leaf cells. The differences observed could be the expression of evolutive adaptation to the ecological conditions of original provenances. Likely a different genetic basis underlies the different degree of salt tolerance observed.