Summary• We examined the relationships among productivity, water use efficiency ( WUE) and drought tolerance in 29 genotypes of Populus × euramericana ( Populus deltoides × Populus nigra ), and investigated whether some leaf traits could be used as predictors for productivity, WUE and drought tolerance.• At Orléans, France, drought was induced on one field plot by withholding water, while a second plot remained irrigated and was used as a control. Recorded variables included stem traits (e.g. biomass) and leaf structural (e.g. leaf area) and functional traits [e.g. intrinsic water use efficiency ( W i ) and carbon isotope discrimination ( ∆ )].• Productivity and ∆ displayed large genotypic variability and were not correlated. ∆ scaled negatively with W i and positively with stomatal conductance under moderate drought, suggesting that the diversity for ∆ was mainly driven by stomatal conductance.• Most of the productive genotypes displayed a low level of drought tolerance (i.e. a large reduction of biomass), while the less productive genotypes presented a large range of drought tolerance. The ability to increase WUE in response to water deficit was necessary but not sufficient to explain the genotypic diversity of drought tolerance.

Summary• Here we tested whether some leaf traits could be used as predictors for productivity in a range of Populus deltoides × P. nigra clones. These traits were assessed in 3-yr-old rooted cuttings from 29 clones growing in an open field trial, in a five randomized complete block design, under optimal irrigation.• Variables were assigned to four groups describing productivity (above-ground biomass, total leaf area), leaf growth (total number of leaves increment rate), leaf structure (area of the largest leaf, specific leaf area, carbon and nitrogen contents), and carbon isotope discrimination in the leaves ( ∆ ).• High-yielding clones displayed larger total leaf area and individual leaf area, while no correlation could be detected between productivity and either leaf structure or ∆ . By contrast, ∆ was negatively correlated with number of leaves increment rate and leaf N content.• Our study shows that there is a potential to improve water-use efficiency in poplar without necessarily reducing the overall productivity. IntroductionPoplars are among the fastest growing trees under temperate latitudes. Their large productivity is associated with large water requirements and, as a consequence, a tight dependency of productivity upon water availability (Ceulemans et al ., 1988;Tschaplinski & Blake, 1989;Barigah et al ., 1994;Tschaplinski et al ., 1994;Zsuffa et al ., 1996). To sustain the extension of poplar cultivation from flood plains and bottomlands towards uplands where soil water availability is subjected to seasonal changes, more water-use efficient hybrids are required. One functional trait that could be of interest as an index for improved or maintained productivity under reduced water availability is the intrinsic water-use efficiency (Wi), i.e. the ratio between net CO 2 assimilation and stomatal conductance. Wi can be indirectly estimated at leaf level, via the carbon isotope discrimination ( ∆ ), assuming the occurrence of a linear and negative correlation between ∆ and intrinsic water-use efficiency shown both for cereals (Farquhar & Richards, 1984;Farquhar et al ., 1989) and for trees (Ponton et al ., 2001). This trait is easily measurable, highly heritable and has been used as a selection criterion in crop breeding Rebetzke et al ., 2002). Among tree species, it also displays a high heritability (Brendel et al ., 2002) and a large variability at interspecific level (Guehl et al ., 1998;Bonal et al ., 2000;Ponton et al ., 2001) and provenance level (Lauteri et al ., 1997;Roupsard et al ., 1998) as well as at family level (Brendel et al ., 2002). Such a clonal diversity of ∆ has already been shown to occur among poplars (Zhang et al ., 2004;Marron et al ., 2005). The detection of clones combining satisfactory productivity and high

Responses to successive drought and re-watering cycles (1-3 cycles) were compared in greenhouse-grown cuttings of Populus x canadensis (Moench) clones, 'Luisa_ Avanzo' and 'Dorskamp.' Total leaf number increment rate, duration of leaf expansion, total and individual leaf area expansion rates and stomatal conductance were recorded periodically during the experiment. Soil water content (SWC) and predawn leaf water potential (Psi(WP)) were measured four times during each drought cycle. In parallel, relative leaf water content (RWC) and specific leaf area (SLA) were estimated on leaves collected from the top to bottom of each cutting. Under well-watered conditions, 'Luisa_Avanzo' and 'Dorskamp' differed in their patterns of leaf area expansion. Although duration of leaf expansion was similar between clones, 'Luisa_ Avanzo' exhibited higher total leaf number increment rates and individual leaf area increases than 'Dorskamp.' As a result, 'Luisa_Avanzo' cuttings reached larger individual and total leaf areas than 'Dorskamp.' 'Dorskamp' leaves had lower SLA than 'Luisa_Avanzo' leaves. In response to successive drought cycles, both clones underwent decreases in total leaf number increment rates and in total leaf area expansion rates, but both whole-plant and individual leaf areas were drastically reduced only in 'Luisa_Avanzo.' 'Dorskamp' maintained a constant leaf area as a result of an increase in the duration of leaf expansion during drought and a significant stimulation of individual leaf area expansion rate and total leaf number increment rate in response to re-watering. Drought caused a greater decrease in SLA in 'Luisa_Avanzo' than in 'Dorskamp.' Expanded leaves of 'Dorskamp' were constitutively dense or thick, or both, whereas leaves of 'Luisa_Avanzo' became dense or thick, or both, only in response to drought. In both clones, re-watering caused partial recovery of SLA to control values. Our data confirm previous field and greenhouse observations that 'Dorskamp' is more tolerant to successive drought and re-watering cycles than 'Luisa_Avanzo.' We conclude that this difference is mainly attributable to the plasticity of 'Dorskamp' after re-watering, rather than to clonal differences in drought responses.