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Trees are long-lived organisms that continuously adapt to their environments, a process in which epigenetic mechanisms are likely to play a key role. Via down regulation of the chromatin remodeler DECREASED IN DNA METHYLATION 1 (DDM1) in poplar (Populus tremula 9 Populus alba) RNAi lines, we examined how DNA methylation coordinates genomic and physiological responses to moderate water deficit.We compared the growth and drought response of two RNAi-ddm1 lines to wild-type (WT) trees under well-watered and water deficit/rewatering conditions, and analyzed their methylomes, transcriptomes, mobilomes and phytohormone contents in the shoot apical meristem. The RNAi-ddm1 lines were more tolerant to drought-induced cavitation but did not differ in height or stem diameter growth. About 5000 differentially methylated regions were consistently detected in both RNAi-ddm1 lines, colocalizing with 910 genes and 89 active transposable elements. Under water deficit conditions, 136 differentially expressed genes were found, including many involved in phytohormone pathways ; changes in phytohormone concentrations were also detected. Finally, the combination of hypomethylation and drought led to the mobility of two transposable elements. Our findings suggest major roles for DNA methylation in regulation of genes involved in hormone-related stress responses, and the maintenance of genome integrity through repression of transposable elements. more...

International audience; To buffer against the high spatial and temporal heterogeneity of the riparian habitat, riparian tree species, such as black poplar (Populus nigra L.), may display a high level of genetic variation and phenotypic plasticity for functional traits. Using a multisite common garden experiment, we estimated the relative contribution of genetic and environmental effects on the phenotypic variation expressed for individual leaf area, leaf shape, leaf structure and leaf carbon isotope discrimination (Delta C-13) in natural populations of black poplar. Twenty-four to 62 genotypes were sampled in nine metapopulations covering a latitudinal range from 48A degrees N to 42A degrees N in France and in Italy and grown in two common gardens at Orl,ans (ORL) and at Savigliano (SAV). In the two common gardens, substantial genetic variation was expressed for leaf traits within all metapopulations, but its expression was modulated by the environment, as attested by the genotype x environment (G x E) interaction variance being comparable to or even greater than genetic effects. For LA, G x E interactions were explained by both changes in genotype ranking between common gardens and increased variation in SAV, while these interactions were mainly attributed to changes in genotype ranking for Delta C-13. The nine P. nigra metapopulations were highly differentiated for LA, as attested by the high coefficient of genetic differentiation (Q(ST) = 0.50 at ORL and 0.51 at SAV), and the pattern of metapopulation differentiation was highly conserved between the two common gardens. In contrast, they were moderately differentiated for Delta C-13 (Q(ST) = 0.24 at ORL and 0.25 at SAV) and the metapopulation clustering changed significantly between common gardens. Our results evidenced that the nine P. nigra metapopulations present substantial genetic variation and phenotypic plasticity for leaf traits, which both represent potentially significant determinants of populations' capacities to respond, on a short-term basis and over generations, to environmental variations. more...

International audience; Trees have a long lifespan and must continually adapt to environmental pressures, notably in the context of climate change. Epigenetic mechanisms are doubtless involved in phenotypic plasticity and in stress memory; however, little evidence of the role of epigenetic processes is available for trees growing in fields. Here, we analyzed the possible involvement of epigenetic mechanisms in the winter-dormant shoot apical meristem of Populus × euramericana clones in memory of the growing conditions faced during the vegetative period. We aimed to estimate the range of genetic and environmentally induced variations in global DNA methylation and to evaluate their correlation with changes in biomass production, identify differentially methylated regions (DMRs), and characterize common DMRs between experiments. We showed that the variations in global DNA methylation between conditions were genotype dependent and correlated with biomass production capacity. Microarray chip analysis allowed detection of DMRs 6 months after the stressful summer period. The 161 DMRs identified as common to three independent experiments most notably targeted abiotic stress and developmental response genes. Results are consistent with a winter-dormant shoot apical meristem epigenetic memory of stressful environmental conditions that occurred during the preceding summer period. This memory may facilitate tree acclimation. more...

International audience; The adaptive capacity of long-lived organisms such as trees to the predicted climate changes, including severe and successive drought episodes, will depend on the presence of genetic diversity and phenotypic plasticity. Here, the involvement of epigenetic mechanisms in phenotypic plasticity toward soil water availability was examined in Populus×euramericana. This work aimed at characterizing (i) the transcriptome plasticity, (ii) the genome-wide plasticity of DNA methylation, and (iii) the function of genes affected by a drought-rewatering cycle in the shoot apical meristem. Using microarray chips, differentially expressed genes (DEGs) and differentially methylated regions (DMRs) were identified for each water regime. The rewatering condition was associated with the highest variations of both gene expression and DNA methylation. Changes in methylation were observed particularly in the body of expressed genes and to a lesser extent in transposable elements. Together, DEGs and DMRs were significantly enriched in genes related to phytohormone metabolism or signaling pathways. Altogether, shoot apical meristem responses to changes in water availability involved coordinated variations in DNA methylation, as well as in gene expression, with a specific targeting of genes involved in hormone pathways, a factor that may enable phenotypic plasticity. more...

Vulnerability to drought-induced cavitation is a key trait of plant water relations. Here, we summarize the available literature on vulnerability to drought-induced cavitation in poplars (Populus spp.), a genus of agronomic, ecological and scientific importance. Vulnerability curves and vulnerability parameters (including the water potential inducing 50% loss in hydraulic conductivity, P50) were collected from 37 studies published between 1991 and 2014, covering a range of 10 species and 12 interspecific hybrid crosses. Results of our meta-analysis confirm that poplars are among the most vulnerable woody species to drought-induced cavitation (mean P50 = -1.44 and -1.55 MPa across pure species and hybrids, respectively). Yet, significant variation occurs among species (P50 range: 1.43 MPa) and among hybrid crosses (P50 range: 1.12 MPa), within species and hybrid crosses (max. P50 range reported: 0.8 MPa) as well as in response to environmental factors including nitrogen fertilization, irradiance, temperature and drought (max. P50 range reported: 0.75 MPa). Potential implications and gaps in knowledge are discussed in the context of poplar cultivation, species adaptation and climate modifications. We suggest that poplars represent a valuable model for studies on drought-induced cavitation, especially to elucidate the genetic and molecular basis of cavitation resistance in Angiosperms. more...

DNA methylation is involved in the control of plant development and adaptation to the environment through modifications of chromatin compaction and gene expression. In poplar (Populus trichocarpa), a perennial plant, variations in DNA methylation have been reported between genotypes and tissues or in response to drought. Nevertheless, the relationships between gene-body DNA methylation, gene expression and chromatin compaction still need clarification. Here, DNA methylation was mapped in the noncondensed chromatin fraction from P. trichocarpa shoot apical meristematic cells, the center of plant morphogenesis, where DNA methylation variations could influence the developmental trajectory. DNase I was used to isolate the noncondensed chromatin fraction. Methylated sequences were immunoprecipitated, sequenced using Illumina/Solexa technology and mapped on the v2.0 poplar genome. Bisulfite sequencing of candidate sequences was used to confirm mapping data and to assess cytosine contexts and methylation levels. While the methylated DNase I hypersensitive site fraction covered 1.9% of the poplar genome, it contained sequences corresponding to 74% of poplar gene models, mostly exons. The level and cytosine context of gene-body DNA methylation varied with the structural characteristics of the genes. Taken together, our data show that DNA methylation is widespread and variable among genes in open chromatin of meristematic cells, in agreement with a role in their developmental trajectory. more...

(13) C discrimination between atmosphere and bulk leaf matter (Δ(13) C(lb) ) is frequently used as a proxy for transpiration efficiency (TE). Nevertheless, its relevance is challenged due to: (1) potential deviations from the theoretical discrimination model, and (2) complex time integration and upscaling from leaf to whole plant. Six hybrid genotypes of Populus deltoides×nigra genotypes were grown in climate chambers and tested for whole-plant TE (i.e. accumulated biomass/water transpired). Net CO(2) assimilation rates (A) and stomatal conductance (g(s) ) were recorded in parallel to: (1) (13) C in leaf bulk material (δ(13) C(lb) ) and in soluble sugars (δ(13) C(ss) ) and (2) (18) O in leaf water and bulk leaf material. Genotypic means of δ(13) C(lb) and δ(13) C(ss) were tightly correlated. Discrimination between atmosphere and soluble sugars was correlated with daily intrinsic TE at leaf level (daily mean A/g(s) ), and with whole-plant TE. Finally, g(s) was positively correlated to (18) O enrichment of bulk matter or water of leaves at individual level, but not at genotype level. We conclude that Δ(13) C(lb) captures efficiently the genetic variability of whole-plant TE in poplar. Nevertheless, scaling from leaf level to whole-plant TE requires to take into account water losses and respiration independent of photosynthesis, which remain poorly documented. more...

International audience; Six Populus deltoides Bartr. ex Marsh. × P. nigra L. genotypes were selected to investigate whether stem xylem anatomy correlated with gas exchange rates, water-use efficiency (WUE) and growth performance. Clonal copies of the genotypes were grown in a two-plot common garden test under contrasting water regimes, with one plot maintained irrigated and the other one subjected to moderate summer water deficit. The six genotypes displayed a large range of xylem anatomy, mean vessel and fibre diameter varying from about 40 to 60 μm and from 7.5 to 10.5 μm, respectively. Decreased water availability resulted in a reduced cell size and an important rise in vessel density, but the extent of xylem plasticity was both genotype and trait dependent. Vessel diameter and theoretical xylem-specific hydraulic conductivity correlated positively with stomatal conductance, carbon isotope discrimination and growth performance-related traits and negatively with intrinsic WUE, especially under water deficit conditions. Vessel diameter and vessel density measured under water deficit conditions correlated with the relative losses in biomass production in response to water deprivation; this resulted from the fact that a more plastic xylem structure was generally accompanied by a larger loss in biomass production. more...

Article en open access; International audience; Xylem resistance to drought-induced cavitation is a key trait of plant water relations. This study assesses the genetic variation expressed for stem cavitation resistance within a population of a riparian species, the European black poplar (Populus nigra L.), and explores its relationships with xylem anatomy, water-use efficiency (WUE), and growth. Sixteen structural and physiological traits related to cavitation resistance, xylem anatomy, growth, bud phenology, and WUE were measured on 33 P. nigra genotypes grown under optimal irrigation in a 2-year-old clonal experiment in a nursery. Significant genetic variation was expressed for the xylem tension inducing 50% loss of hydraulic conductivity (Ψ50) within the studied population, as attested by the high value of broad-sense heritability estimated for this trait (H 2 ind = 0.72). Stem cavitation resistance was associated with xylem structure: the more cavitation-resistant genotypes exhibited lower hydraulic efficiency and higher mechanical reinforcement as assessed from stem xylem cross sections. By contrast, Ψ50 was not significantly related to shoot height increment, total above-ground dry mass, or bulk leaf carbon isotope discrimination, a proxy for intrinsic WUE. These findings indicate that the trade-offs between xylem resistance to cavitation, hydraulic efficiency, and mechanical reinforcement can occur at the within-population level. Given that the studied genotypes were exposed to the same environmental conditions and evolutionary drivers in situ, the trade-offs detected at this scale are expected to reflect true functional relationships. more...

In plants organogenesis, cell differentiation and dedifferentiation are fundamental processes allowing high developmental plasticity. Such plasticity involved epigenetic mechanisms but limited knowledge is available concerning quantitative aspects. Three sugarbeet (Beta vulgaris L. altissima) cell lines originating from the same mother plant and exhibiting graduate states of morphogenesis were used to assess whether these differences could be related or not to changes in DNA methylation levels. Methylcytosine percentages from 18.3 to 28.8% and distinct levels of DNA methyltransferase (EC 2.1.1.37) activities were shown in the three cell lines. The lowest methylcytosine percentage was associated to organogenesis. In order to test the plasticity of these cell lines, various treatments causing DNA hypo or hypermethylation were performed at different times and concentrations. In this collection of treated lines with+/-10% of methylcytosine percentages, loss of organogenic properties and cell dedifferentiation were observed. As cell wall formation fits well with cell differentiation state, the lignification process was further investigated in treated and untreated lines as a biochemical marker of the phenotypic changes. For example, peroxidase specific activities (EC 1.11.1.7) varied from 0.7 to 0.02 pkat mg(-1) of protein in organogenic and dedifferentiated lines, respectively. A negative relationship between peroxidase activities, incorporation of cell wall-bound phenolic compounds as ferulate and sinapate derivatives and methylcytosine percentages was obtained. This is the first biochemical evidence that phenotypic changes in plant cells induced by DNA hypo- or hypermethylating treatments are correlated in a linear relationship to modifications of the cell wall differentiation state. more...

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. more...

One hundred Norway spruce (Picea abies (L.) Karst.) clones (three ramets per clone) were analyzed for phloem phenol composition and concentration before and 10 days after wound inoculation with sterile malt agar. Fifty clones (Experiment 1) belonged to the same provenance, whereas the remaining clones (Experiment 2) belonged to five provenances from three geographic areas. In Experiment 2, two additional ramets from the same clones were mass inoculated with Ceratocystis polonica (Siem.) C. Moreau to quantify the resistance of each clone. Tree response to wound inoculations was characterized by increased catechin concentration in both experiments, accompanied by increases in astringin and decreases in piceid in Experiment 1. In both experiments, we observed a diverse group of phenolic compounds whose concentrations increased (catechin, astringin) or did not vary (taxifolin glucoside) in response to wound inoculations, whereas concentrations of a homogeneous group of stilbene compounds decreased (piceid) or did not vary (isorhapontin, unidentified stilbene). In Experiment 2, provenances from the alpine and Hercynian-Carpatic areas differed from provenances from the Baltic area with respect to the relative importance of these two groups of compounds, further indicating that the two groupings of phenolic compounds structure the Norway spruce populations. Eighty days after mass inoculation, the percentage of healthy sapwood, which was taken as a measure of tree resistance, indicated that clones from the Baltic area were less resistant to mass inoculations than clones from the alpine and Hercynian-Carpatic areas. We conclude that the degree of resistance of Norway spruce trees to mass inoculations with a bark beetle-associated fungus can be predicted based on the diversity of constitutive phloem phenols and the ability to induce phenol synthesis in response to wounding. more...

International audience; The B-type response regulators (B-type RRs), final elements of a signaling pathway called "histidine/aspartate phosphorelay system" in plants, are devoted to the regulation of response genes through a transcription factor activity. Signal transduction consists in the transfer of a phosphoryl group from a transmembrane histidine kinase (HK) which recognizes a given stimulus to nuclear RRs via cytosolic shuttle phosphotransfer proteins (HPts). In Arabidopsis, the receptors HK are to date the major characterized candidates to be responsible for initiation of osmotic stress responses. However, little information is available concerning the signaling partners acting downstream of HKs. In Populus, three HPts and five B-type RRs were previously identified as interacting partners of HK1, the Arabidopsis AHK1 homolog. Here, we report the isolation of RR18, a member of the B-type RR family, which shares high sequence similarities with ARR18 characterized to act in the osmosensing signaling pathway in Arabidopsis, from poplar cuttings subjected to osmotic stress conditions. By using yeast and in planta interaction assays, RR18 was further identified as acting downstream of HK1 and its three preferential HPt partners. Besides, our results are in favor of a possible involvement of both RR18 and RR13, the main expressed poplar B-type RR, in the osmotic signaling pathway. Nonetheless, different behaviors of these two B-type RRs in this pathway need to be noted, with one RR, RR13, acting in an early phase, mainly in roots of poplar cuttings, and the other one, RR18, acting in a late phase, mainly in leaves to supply an adequate response. (C) 2015 Elsevier Masson SAS. All rights reserved. more...

Summary The mechanisms by which drought-induced oxidative stress is tolerated in poplar were appreciated by the ability of leaf extracts to scavenge reactive oxygen species (ROS) generated in vitro by a xanthine oxidase-hypoxanthine system. Their effects were detected by the protection towards the denaturation of malate dehydrogenase (MDH) of crude and dialysed extracts of a drought-sensitive ( Populus × euramericana , Luisa Avanzo) and a drought-tolerant ( P × euramericana , Dorskamp) poplar exposed to control and osmotic stress (-0.336 MPa). At the beginning of the incubation period in presence of ROS, MDH from crude extract of control Dorskamp was protected against the denaturation; boiled crude extracts of control Dorskamp also protected MDH from the dialysed extract of control Luisa Avanzo. In control Dorskamp, cysteine, proline, polyamines and ascorbate added in vitro were efficient ROS scavengers. Compared with the control condition, the rate of MDH denaturation in presence of ROS decreased in a dialysed extract of stressed Luisa Avanzo but increased in that of stressed Dorskamp. Under stress conditions, proline in vitro enhanced the rate of MDH denaturation by ROS in Luisa Avanzo, but competed as ascorbate with ROS in Dorskamp. In Dorskamp, the 12h-exposure to osmotic stress resulted in a decrease in catalase and gutathione reductase activities, in an enhancement of superoxide dismutase, ascorbate peroxidase and contents of some putative leaf antioxidants (putrescine and glutathione), but did not affect the contents of spermine, spermidine and ascorbate. The consequences of extract/solute effects are discussed for each clone as regards the drought response. more...

International audience; In poplar, we identified proteins homologous to yeast proteins involved in osmosensing multistep phosphorelay Sln1p‐Ypd1p‐Ssk1p. This finding led us to speculate that Populus cells could sense osmotic stress by a similar mechanism. This study focuses on first and second protagonists of this possible pathway: a histidine‐aspartate kinase (HK1), putative osmosensor and histidine phosphotransfer proteins (HPt1 to 10), potential partners of this HK. Characterization of HK1 showed its ability to homodimerize in two‐hybrid tests and to act as an osmosensor with a kinase activity in yeast, by functional complementation of sln1Δ sho1Δ strain. Moreover, in plant cells, plasma membrane localization of HK1 is shown. Further analysis on HPts allowed us to isolate seven new cDNAs, leading to a total of 10 different HPts identified in poplar. Interaction tests showed that almost all HPts can interact with HK1, but two of them exhibit stronger interactions, suggesting a preferential partnership in poplar. The importance of the phosphorylation status in these interactions has been investigated with two‐hybrid tests carried out with mutated HK1 forms. Finally, in planta co‐expression analysis of genes encoding these potential partners revealed that only three HPts are co‐expressed with HK1 in different poplar organs. This result reinforces the hypothesis of a partnership between HK1 and these three preferential HPts in planta. Taken together, these results shed some light on proteins partnerships that could be involved in the osmosensing pathway in Populus. more...

International audience; An epigenetic control of vernalization has been demonstrated in annual plants such as Arabidopsis and cereals, but the situation remains unclear in biennial plants such as sugar beet that has an absolute requirement for vernalization. The role of DNA methylation in flowering induction and the identification of corresponding target loci also need to be clarified. In this context, sugar beet (Beta vulgaris altissima) genotypes differing in bolting tolerance were submitted to various bolting conditions such as different temperatures and/or methylating drugs. DNA hypomethylating treatment was not sufficient to induce bolting while DNA hypermethylation treatment inhibits and delays bolting. Vernalizing and devernalizing temperatures were shown to affect bolting as well as DNA methylation levels in the shoot apical meristem. In addition, a negative correlation was established between bolting and DNA methylation. Genotypes considered as resistant or sensitive to bolting could also be distinguished by their DNA methylation levels. Finally, sugar beet homologues of the Arabidopsis vernalization genes FLC and VIN3 exhibited distinct DNA methylation marks during vernalization independently to the variations of global DNA methylation. These vernalization genes also displayed differences in mRNA accumulation and methylation profiles between genotypes resistant or sensitive to bolting. Taken together, the data suggest that the time course and amplitude of DNA methylation variations are critical points for the induction of sugar beet bolting and represent an epigenetic component of the genotypic bolting tolerance, opening up new perspectives for sugar beet breeding. more...

Tests were carried out to determine whether variations in the hydraulic architecture of eight Populus deltoides×Populus nigra genotypes could be related to variations in leaf function and growth performance. Measurements were performed in a coppice plantation on 1-year-old shoots under optimal irrigation. Hydraulic architecture was characterized through estimates of hydraulic efficiency (the ratio of conducting sapwood area to leaf area, A(X):A(L); leaf- and xylem-specific hydraulic conductance of defoliated shoots, k(SL) and k(SS), respectively; apparent whole-plant leaf-specific hydraulic conductance, k(plant)) and xylem safety (water potential inducing 50% loss in hydraulic conductance). The eight genotypes spanned a significant range of k(SL) from 2.63 kg s(-1) m(-2) MPa(-1) to 4.18 kg s(-1) m(-2) MPa(-1), variations being mostly driven by k(SS) rather than A(X):A(L). There was a strong trade-off between hydraulic efficiency and xylem safety. Values of k(SL) correlated positively with k(plant), indicating that high-pressure flowmeter (HPFM) measurements of stem hydraulic efficiency accurately reflected whole-plant water transport efficiency of field-grown plants at maximum transpiration rate. No clear relationship could be found between hydraulic efficiency and either net CO(2) assimilation rates, water-use efficiency estimates (intrinsic water-use efficiency and carbon isotope discrimination against (13)C), or stomatal characteristics (stomatal density and stomatal pore area index). Estimates of hydraulic efficiency were negatively associated with relative growth rate. This unusual pattern, combined with the trade-off observed between hydraulic efficiency and xylem safety, provides the rationale for the positive link already reported between relative growth rate and xylem safety among the same eight P. deltoides×P. nigra genotypes. more...

Tests were carried out to determine whether variations in the hydraulic architecture of eight Populus deltoides×Populus nigra genotypes could be related to variations in leaf function and growth performance. Measurements were performed in a coppice plantation on 1-year-old shoots under optimal irrigation. Hydraulic architecture was characterized through estimates of hydraulic efficiency (the ratio of conducting sapwood area to leaf area, AX:AL; leaf- and xylem-specific hydraulic conductance of defoliated shoots, kSL and kSS, respectively; apparent whole-plant leaf-specific hydraulic conductance, kplant) and xylem safety (water potential inducing 50% loss in hydraulic conductance). The eight genotypes spanned a significant range of kSL from 2.63 kg s−1 m−2 MPa−1 to 4.18 kg s−1 m−2 MPa−1, variations being mostly driven by kSS rather than AX:AL. There was a strong trade-off between hydraulic efficiency and xylem safety. Values of kSL correlated positively with kplant, indicating that high-pressure flowmeter (HPFM) measurements of stem hydraulic efficiency accurately reflected whole-plant water transport efficiency of field-grown plants at maximum transpiration rate. No clear relationship could be found between hydraulic efficiency and either net CO2 assimilation rates, water-use efficiency estimates (intrinsic water-use efficiency and carbon isotope discrimination against 13C), or stomatal characteristics (stomatal density and stomatal pore area index). Estimates of hydraulic efficiency were negatively associated with relative growth rate. This unusual pattern, combined with the trade-off observed between hydraulic efficiency and xylem safety, provides the rationale for the positive link already reported between relative growth rate and xylem safety among the same eight P. deltoides×P. nigra genotypes. more...

International audience; Genetic differences in delta C-13 (isotopic composition of dry matter carbon) have been evidenced among poplar genotypes at juvenile stages. To check whether such differences were maintained with age in trees growing in plantations, we investigated the time course of delta C-13 as recorded in annual tree rings from different genotypes growing at three sites in southwestern France and felled at similar to 15-17 years. Wood cores were cut from tree discs to record the time course of annual basal area increment (BAI). The isotopic ratio delta C-13 was recorded in bulk wood and in extracted cellulose from the annual rings corresponding to the period 1996-2005. Discrimination against C-13 between atmosphere and tissues (delta C-13) was computed by taking into account the inter-annual time course of delta C-13 in the atmosphere. Annual BAI increased steadily and stabilized at about 8 years. An offset in delta C-13 of similar to 1 parts per thousand was recorded between extracted cellulose and bulk wood. It was relatively stable among genotypes within sites but varied among sites and increased slightly with age. Site effects as well as genotype differences were detected in delta C-13 recorded from the cellulose fraction. Absolute values as well as the genotype ranking of delta C-13 remained stable with age in the three sites. Genotype means of delta C-13 were not correlated to annual BAI. We conclude that genotypic differences of delta C-13 occur in older poplar trees in plantations, and that the differences as well as the genotype ranking remain stable while trees age until harvest. more...

We examined the relationships between xylem resistance to cavitation and 16 structural and functional traits across eight unrelated Populus deltoides x Populus nigra genotypes grown under two contrasting water regimes. The xylem water potential inducing 50% loss of hydraulic conductance (Psi(50)) varied from -1.60 to -2.40 MPa. Drought-acclimated trees displayed a safer xylem, although the extent of the response was largely genotype dependant, with Psi(50) being decreased by as far as 0.60 MPa. At the tissue level, there was no clear relationship between xylem safety and either xylem water transport efficiency or xylem biomechanics; the only structural trait to be strongly associated with Psi(50) was the double vessel wall thickness, genotypes exhibiting a thicker double wall being more resistant. At the leaf level, increased cavitation resistance was associated with decreased stomatal conductance, while no relationship could be identified with traits associated with carbon uptake or bulk leaf carbon isotope discrimination, a surrogate of intrinsic water-use efficiency. At the whole-plant level, increased safety was associated with higher shoot growth potential under well-irrigated regime only. We conclude that common trade-offs between xylem resistance to cavitation and other physiological traits that are observed across species may not necessarily hold true at narrower scales. more...

International audience; Genotypic variability for productivity, wateruse efficiency and leaf traits in 33 genotypes selected from an F1 progeny of Populus deltoides Bartr. ex Marsh • Populus trichocarpa L. was explored under optimal and moderate water-deficit conditions. Saplings of the 33 genotypes were grown in a two-plot open field at INRA Orle´ ans (France) and coppiced every year. A moderate water deficit was induced during two successive years on one plot by withholding irrigation, while the second one remained irrigated (control). Stem biomass and leaf structure (e.g., specific leaf area and leaf area) were measured in 2004 and 2005 and functional leaf traits (e.g., carbon isotope discrimination, D) were measured only in 2004. Tolerance to water deficit was estimated at genotype level as the ability to limit losses in biomass production in water deficit versus control trees. Stem biomass, leaf structure and D displayed a significant genotypic variability whatever the irrigation regime. For all traits, genotype ranks remained stable across years for similar irrigation conditions. Carbon isotope discrimination scaled negatively with productivity and leaf nitrogen content in controls. The most productive genotypes were the least tolerant to moderate water deficit. No relationship was evidenced between D and the level of tolerance to water deficit. The relationships between traits evidenced in this collection of P. deltoides • P. trichocarpa F1 genotypes contrast with the ones that were previously detected in a collection of P. deltoides • Populus nigra L. cultivars tested in the same field trial. more...

International audience; • We examined the relationships among roductivity, 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 (Wi) and carbon isotope discrimination (∆)]. • Productivity and ∆ displayed large genotypic variability and were not correlated. ∆ scaled negatively with Wi 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. more...

To test if some leaf parameters are predictors of productivity in a range of Populus deltoides (Bartr.) Marsh. x P. nigra L. clones, we assessed leaf traits and productivity in 2-month-old rooted cuttings from 31 clones growing in 4-l pots in a greenhouse, under conditions of controlled temperature and optimal irrigation. We evaluated four groups of variables describing (1) productivity (total biomass), (2) leaf growth (total leaf number increment and total leaf area increment rate), (3) leaf structure (specific leaf area and nitrogen and carbon contents) and (4) carbon isotope discrimination (delta), which is negatively correlated with time-integrated water-use efficiency. High-yielding clones did not necessarily display high leaf growth rates, but they displayed a larger total leaf area, lower specific leaf area and lower leaf nitrogen concentration than clones with low productivity. Total leaf area was mainly controlled by maximal individual leaf area and total leaf area increment rate (r = 0.51 and 0.56, respectively). Carbon isotope discrimination did not correlate with total biomass, but it was associated with total number of leaves and total leaf area increment rate (r = 0.39 and 0.45, respectively). Therefore, leaf area and specific leaf area were better indicators of productivity than leaf growth traits. The observed independence of delta from biomass production provides opportunities for selecting poplar clones combining high productivity and high water-use efficiency. more...

International audience; • 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. more...

International audience; We compared responses to drought and re-watering of greenhouse-grown cuttings of Populus × euramericana (Dode) Guinier clones, Luisa Avanzo and Dorskamp. Total leaf area, leaf number, leaf area increment and stomatal conductance were evaluated periodically during a 29-day drought period and for 16 days after re-watering. Soil water content and predawn leaf water potential (Øwp) were measured on Days 29 and 45. On the same days, relative water content (RWC), specific leaf area (SLA), nitrogen, chlorophyll, soluble sugars, total phenols, flavanols and antioxidant activity were determined for leaves taken from the bottom to the top of each cutting. Leaves of Luisa Avanzo cuttings grew more rapidly than leaves of Dorskamp and exhibited higher SLA, but lower concentrations of nitrogen, chlorophyll and soluble sugars and lower antioxidant activity per unit area. On Day 29, after withholding water, both clones had closed their stomata, reduced rates of leaf growth, and lower Øwp and RWC; however, the clones differed in their responses to soil water depletion. Compared to Dorskamp, Luisa Avanzo closed its stomata earlier and maintained higher Øwp, but lower RWC and leaf sugar concentrations. Antioxidant activity of leaf methanolic extracts decreased in response to water stress only in Luisa Avanzo. Leaf physiology and its modulation by water stress were age dependent in Luisa Avanzo. more...

Two Norway spruce (Picea abies Karst.) clones, one resistant and the other susceptible to mass inoculation with Ophiostoma polonicum Siem., were compared with regard to their phenolic compositions and chalcone synthase (CHS) and stilbene synthase activities of their phloem before and at 6 and 12 d after artificial inoculation with sterile malt agar or O. polonicum. In unwounded phloem, the resistant clone differed from the susceptible clone by the presence of taxifolin glycoside, lower concentrations of stilbene glycosides, and higher CHS activity. After inoculation, (+)-catechin concentration and CHS activity dramatically increased around the wound, particularly in the resistant clone. Stilbene synthase activity also increased, but more slowly and to a lower level, whereas the concentrations of stilbenes remained stable. Tanning ability decreased in the susceptible clone, whereas it remained stable in the resistant one. It is proposed that the induced phenolic response of Norway spruce phloem consists of an activation of the phenolic pathway, finally leading to tannins and insoluble polymers. It is suggested that resistance to O. polonicum depends on the ability of the tree to easily activate the flavonoid pathway. more...