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Abstract: Short rotation coppice (SRC) willow plantations may play an important role in the future for biomass production purposes. However, the high planting density schemes combined with the frequent harvests occurring in such plantations could rapidly deplete soil resources. The use of genotypes able to produce greater amounts of biomass by using the least water and nutrients may help mitigating this risk. This study aimed at assessing among six willow genotypes (1) the variability of traits related to productivity (e.g. aboveground dry biomass or stem height), leaf and wood nitrogen (N) contents, N exportation rate and water-use efficiency (WUE) as estimated through bulk leaf carbon isotope discrimination (Δ13C), (2) the relationships among traits, and (3) the plasticity of these traits and of the relationships among them across different sites. The six genotypes were grown under SRC at three sites in northern France differing primarily in pedoclimatic characteristics for two years. A significant genotypic variability was found for all traits, except for the N exportation rate. The pedoclimatic context impacted the values of all traits, and the genotypic ranking for traits related to productivity and for Δ13C. Δ13C was negatively correlated with total shoot dry biomass and/or height irrespective of the site, meaning that the most productive genotypes were also the most efficient to use water. In conclusion, no antagonism was detected between biomass production and WUE. The most productive and most water-use efficient genotypes were the ones responsible for the highest nitrogen removal from the plantation during harvest. [Copyright &y& Elsevier]

We examined the relationships between xylem resistance to cavitation and 16 structural and functional traits across eight unrelated Populus deltoides× Populus nigra genotypes grown under two contrasting water regimes. The xylem water potential inducing 50% loss of hydraulic conductance ( Ψ50) 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 Ψ50 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 Ψ50 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. [ABSTRACT FROM AUTHOR]

Climate change will increase the risk of flooding in several areas of the world where Populus deltoides Marshall (eastern cottonwood) is planted, so it would be desirable for this species to select for flooding tolerance. The aims of this work were to explore the variability in growth, leaf traits and flooding tolerance in an F1 full-sib intraspecific progeny of P. deltoides , to analyze the correlations of leaf and growth traits with flooding tolerance and to assess their suitability for use in breeding programs. Two-month-old parental clones and their progeny of 30 full-sib F1 genotypes were grown in pots and subjected to two treatments: (i) plants watered to field capacity (control) and (ii) plants flooded up to 10 cm above soil level for 35 days. Growth (height, diameter and biomass partition) and leaf traits (leaf size and number, specific leaf area, leaf senescence, abscission, stomatal conductance, carbon isotope discrimination, stomatal index) were measured. Flooding tolerance for each genotype was estimated as the ratio of the biomass of stressed plants to the biomass of control plants. Results showed segregation in terms of flooding tolerance in the F1 progeny. A significant genotype effect was found for leaf size and number, carbon isotopic discrimination and stomatal conductance, but it did not correlate with flooding tolerance. Height, diameter and root-to-shoot ratio had a positive phenotypic correlation with flooding tolerance, and there was a positive genetic correlation of height and diameter with biomass on both treatments. The narrow sense heritability values for the traits analyzed ranged from 0 to 0.56. We conclude that growth traits are more adequate than leaf traits for selection to increase flooding tolerance. A vigorous initial growth would increase flooding tolerance in young poplar plants. [ABSTRACT FROM AUTHOR]

Climate change is expected to increase drought frequency and intensity which will threaten plant growth and survival. In such fluctuating environments, perennial plants respond with hydraulic and biomass adjustments, resulting in either tolerant or avoidant strategies. Plants' response to stress relies on their phenotypic plasticity. The goal of this study was to explore physiology of young Populus nigra in the context of a time‐limited and progressive water deficit in regard to their growth and stress response strategies. Fourteen French 1‐year‐old black poplar genotypes, geographically contrasted, were subjected to withholding water during 8 days until severe water stress. Water fluxes (i.e. leaf water potentials and stomatal conductance) were analyzed together with growth (i.e. radial and longitudinal branch growth, leaf senescence and leaf production). Phenotypic plasticity was calculated for each trait and response strategies to drought were deciphered for each genotype. Black poplar genotypes permanently were dealing with a continuum of adjusted water fluxes and growth between two extreme strategies, tolerance and avoidance. Branch growth, leaf number and leaf hydraulic potential traits had contrasted plasticities, allowing genotype characterization. The most tolerant genotype to water deficit, which maintained growth, had the lowest global phenotypic plasticity. Conversely, the most sensitive and avoidant genotype ceased growth until the season's end, had the highest plasticity level. All the remaining black poplar genotypes were close to avoidance with average levels of traits plasticity. These results underpinned the role of plasticity in black poplar response to drought and calls for its wider use into research on plants' responses to stress. [ABSTRACT FROM AUTHOR]

Poplars are fast-growing, high-yielding forest tree species, whose cultivation as second-generation biofuel crops is of increasing interest and can efficiently meet emission reduction goals. Yet, breeding elite poplar trees for drought resistance remains a major challenge. Worldwide breeding programs are largely focused on intra/interspecific hybridization, whereby Populus nigra L. is a fundamental parental pool. While high-throughput genotyping has resulted in unprecedented capabilities to rapidly decode complex genetic architecture of plant stress resistance, linking genomics to phenomics is hindered by technically challenging phenotyping. Relying on unmanned aerial vehicle (UAV)-based remote sensing and imaging techniques, high-throughput field phenotyping (HTFP) aims at enabling highly precise and efficient, non-destructive screening of genotype performance in large populations. To efficiently support foresttree breeding programs, ground-truthing observations should be complemented with standardized HTFP. In this study, we develop a high-resolution (leaf level) HTFP approach to investigate the response to drought of a full-sib F2 partially inbred population (termed here 'POP6'), whose F1 was obtained from an intraspecific P. nigra controlled cross between genotypes with highly divergent phenotypes. We assessed the effects of two water treatments (well-watered and moderate drought) on a population of 4603 trees (503 genotypes) hosted in two adjacent experimental plots (1.67 ha) by conducting low-elevation (25 m) flights with an aerial drone and capturing 7836 thermal infrared (TIR) images. TIR images were undistorted, georeferenced, and orthorectified to obtain radiometric mosaics. Canopy temperature (Tc) was extracted using two independent semi-automated segmentation techniques, eCognition- and Matlab-based, to avoid the mixed-pixel problem. Overall, results showed that the UAV platform-based thermal imaging enables to effectively assess genotype variability under drought stress conditions. Tc derived from aerial thermal imagery presented a good correlation with ground-truth stomatal conductance (gs) in both segmentation techniques. Interestingly, the HTFP approach was instrumental to detect droughttolerant response in 25% of the population. This study shows the potential of UAV-based thermal imaging for field phenomics of poplar and other tree species. This is anticipated to have tremendous implications for accelerating forest tree genetic improvement against abiotic stress. [ABSTRACT FROM AUTHOR]

Whole genome resequencing of 51 Populus nigra (L.) individuals from across Western Europe was performed using Illumina platforms. A total number of 1 878 727 SNPs distributed along the P. nigra reference sequence were identified. The SNP calling accuracy was validated with Sanger sequencing. SNPs were selected within 14 previously identified QTL regions, 2916 expressional candidate genes related to rust resistance, wood properties, water-use efficiency and bud phenology and 1732 genes randomly spread across the genome. Over 10 000 SNPs were selected for the construction of a 12k Infinium Bead-Chip array dedicated to association mapping. The SNP genotyping assay was performed with 888 P. nigra individuals. The genotyping success rate was 91%. Our high success rate was due to the discovery panel design and the stringent parameters applied for SNP calling and selection. In the same set of P. nigra genotypes, linkage disequilibrium throughout the genome decayed on average within 5-7 kb to half of its maximum value. As an application test, ADMIXTURE analysis was performed with a selection of 600 SNPs spread throughout the genome and 706 individuals collected along 12 river basins. The admixture pattern was consistent with genetic diversity revealed by neutral markers and the geographical distribution of the populations. These newly developed SNP resources and genotyping array provide a valuable tool for population genetic studies and identification of QTLs through natural-population based genetic association studies in P. nigra. [ABSTRACT FROM AUTHOR]

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 (Δ13C) in natural populations of black poplar. Twenty-four to 62 genotypes were sampled in nine metapopulations covering a latitudinal range from 48°N to 42°N in France and in Italy and grown in two common gardens at Orléans (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 × environment (G × E) interaction variance being comparable to or even greater than genetic effects. For LA, G × 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 Δ13C. The nine P. nigra metapopulations were highly differentiated for LA, as attested by the high coefficient of genetic differentiation (QST = 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 Δ13C (QST = 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. [ABSTRACT FROM AUTHOR]

Populus species with wide geographic ranges display strong adaptation to local environments. We studied the clinal patterns in phenology and ecophysiology in allopatric Populus species adapted to similar environments on different continents under common garden settings. As a result of climatic adaptation, both Populus tremula L. and Populus balsamifera L. display latitudinal clines in photosynthetic rates (A), whereby high-latitude trees of P. tremula had higher A compared to low-latitude trees and nearly so in P. balsamifera (p = 0.06). Stomatal conductance (gs) and chlorophyll content index (CCI) follow similar latitudinal trends. However, foliar nitrogen was positively correlated with latitude in P. balsamifera and negatively correlated in P. tremula. No significant trends in carbon isotope composition of the leaf tissue (δ13C) were observed for both species; but, intrinsic water-use efficiency (WUEi) was negatively correlated with the latitude of origin in P. balsamifera. In spite of intrinsically higher A, high-latitude trees in both common gardens accomplished less height gain as a result of early bud set. Thus, shoot biomass was determined by height elongation duration (HED), which was well approximated by the number of days available for free growth between bud flush and bud set. We highlight the shortcoming of unreplicated outdoor common gardens for tree improvement and the crucial role of photoperiod in limiting height growth, further complicating interpretation of other secondary effects. [ABSTRACT FROM AUTHOR]

Poplar genotypes differ in transpiration efficiency ( TE) at leaf and whole-plant level under similar conditions. We tested whether atmospheric vapour pressure deficit ( VPD) affected TE to the same extent across genotypes. Six P opulus nigra genotypes were grown under two VPD. We recorded (1) 13 C content in soluble sugars; (2) 18 O enrichment in leaf water; (3) leaf-level gas exchange; and (4) whole-plant biomass accumulation and water use. Whole-plant and intrinsic leaf TE and 13 C content in soluble sugars differed significantly among genotypes. Stomatal conductance contributed more to these differences than net CO2 assimilation rate. VPD increased water use and reduced whole-plant TE. It increased intrinsic leaf-level TE due to a decline in stomatal conductance. It also promoted higher 18 O enrichment in leaf water. VPD had no genotype-specific effect. We detected a deviation in the relationship between 13 C in leaf sugars and 13 C predicted from gas exchange and the standard discrimination model. This may be partly due to genotypic differences in mesophyll conductance, and to its lack of sensitivity to VPD. Leaf-level 13 C discrimination was a powerful predictor of the genetic variability of whole-plant TE irrespective of VPD during growth. [ABSTRACT FROM AUTHOR]

The efficiency of water use to produce biomass is a key trait in designing sustainable bioenergy-devoted systems. We characterized variations in the carbon isotope composition (δ13 C) of leaves, current year wood and holocellulose (as proxies for water use efficiency, WUE) among six poplar genotypes in a short-rotation plantation. Values of δ13 Cwood and δ13 Cholocellulose were tightly and positively correlated, but the offset varied significantly among genotypes (0.79-1.01‰). Leaf phenology was strongly correlated with δ13 C, and genotypes with a longer growing season showed a higher WUE. In contrast, traits related to growth and carbon uptake were poorly linked to δ13 C. Trees growing on former pasture with higher N-availability displayed higher δ13 C as compared with trees growing on former cropland. The positive relationships between δ13Cleaf and leaf N suggested that spatial variations in WUE over the plantation were mainly driven by an N-related effect on photosynthetic capacities. The very coherent genotype ranking obtained with δ13C in the different tree compartments has some practical outreach. Because WUE remains largely uncoupled from growth in poplar plantations, there is potential to identify genotypes with satisfactory growth and higher WUE. [ABSTRACT FROM AUTHOR]

In order to efficiently grow Salix for biomass production in warmer climates, varieties with a desirable response to drought and nutrient-limiting conditions are needed. The main objective of this study was to investigate the genetic background of biomass production and nitrogen (N) economy in contrasting conditions of water and nutrient availability and to identify candidate genes with a putative function in the expression of the different traits. Quantitative trait loci (QTLs) analysis was conducted using data from 198 individuals of a back-cross population between S. viminalis and S. schwerinii grown in a greenhouse under three combinations of N and water supply. In total, 60 QTLs were identified for biomass and N economy traits in the different treatments. Most of the QTLs mapped to linkage groups II, III, VI, X, XIII and B. At linkage groups III, VI and X, QTLs for both N economy and biomass traits co-located. The phenotypic variation explained by each QTL varied from 7.7 to 41.9 % of the total variance. QTLs for N economy traits explained most of the variation. Gene ontology (GO) analyses, performed on QTL intervals for each trait and projected to the Populus trichocarpa genome, revealed that genomic intervals connected to 19 traits were enriched for at least one GO term. Candidate genes were selected among genes linked to the enriched GO terms. These results represent a first necessary step for additional mapping and functional studies and encourage the development of marker-assisted breeding of Salix varieties adapted to drier climates. [ABSTRACT FROM AUTHOR]

In this study the seasonal variation in carbon, water and energy fluxes as well as in net primary productivity ( NPP) of different tree components is presented for a 2-year-old poplar ( Populus spp.) plantation. A thorough ecophysiological study was performed at ecosystem scale, at tree and at leaf level, in this high-density bioenergy plantation. Seasonal variation in NPP and fluxes was analysed in relation to meteorological parameters at the field site. The growing season length in terms of carbon uptake was controlled by leaf area development until the maximum leaf area index ( LAImax) was reached. Afterwards, a shift to belowground carbon allocation was observed. A dry period in spring caused a reduced leaf area production as well as a decrease in net ecosystem exchange and gross primary production ( GPP) due to stomatal closure. Water use efficiency and fine root growth increased in response to limiting soil water availability in the root zone. When soil water availability was not limiting, GPP was controlled by a decrease in solar radiation and air temperature. The results of this study indicate that the productivity of recently established bioenergy plantations with fast-growing trees is very sensitive to drought. The interaction between soil water availability and factors controlling ecosystem GPP is crucial in assessing the CO2 mitigation potential under future climate conditions. [ABSTRACT FROM AUTHOR]

In order to uncover the genetic basis of phenotypic trait variation, we used 448 unrelated wild accessions of black cottonwood ( Populus trichocarpa) from much of its range in western North America. Extensive data from large-scale trait phenotyping (with spatial and temporal replications within a common garden) and genotyping (with a 34 K Populus single nucleotide polymorphism ( SNP) array) of all accessions were used for gene discovery in a genome-wide association study ( GWAS)., We performed GWAS with 40 biomass, ecophysiology and phenology traits and 29 355 filtered SNPs representing 3518 genes. The association analyses were carried out using a Unified Mixed Model accounting for population structure effects among accessions., We uncovered 410 significant SNPs using a Bonferroni-corrected threshold ( P < 1.7 × 10−6). Markers were found across 19 chromosomes, explained 1-13% of trait variation, and implicated 275 unique genes in trait associations. Phenology had the largest number of associated genes (240 genes), followed by biomass (53 genes) and ecophysiology traits (25 genes)., The GWAS results propose numerous loci for further investigation. Many traits had significant associations with multiple genes, underscoring their genetic complexity. Genes were also identified with multiple trait associations within and/or across trait categories. In some cases, traits were genetically correlated while in others they were not. [ABSTRACT FROM AUTHOR]

Photosynthetic carbon assimilation and transpirational water loss play an important role in the yield and the carbon sequestration potential of bioenergy-devoted cultures of fast-growing trees. For six poplar (Populus) genotypes in a short-rotation plantation, we observed significant seasonal and genotypic variation in photosynthetic parameters, intrinsic water-use efficiency (WUEi) and leaf stable isotope composition (δ13C and δ18O). The poplars maintained high photosynthetic rates (between 17.8 and 26.9 μmol m−2 s−1 depending on genotypes) until late in the season, in line with their fast-growth habit. Seasonal fluctuations were mainly explained by variations in soil water availability and by stomatal limitation upon photosynthesis. Stomatal rather than biochemical limitation was confirmed by the constant intrinsic photosynthetic capacity (Vcmax) during the growing season, closely related to leaf nitrogen (N) content. Intrinsic water-use efficiency scaled negatively with carbon isotope discrimination (Δ13Cbl) and positively with the ratio between mesophyll diffusion conductance (gm) and stomatal conductance. The WUEi – Δ13Cbl relationship was partly influenced by gm. There was a trade-off between WUEi and photosynthetic N-use efficiency, but only when soil water availability was limiting. Our results suggest that seasonal fluctuations in relation to soil water availability should be accounted for in future modelling studies assessing the carbon sequestration potential and the water-use efficiency of woody energy crops. [ABSTRACT FROM AUTHOR]

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The variability of ecophysiological traits associated with productivity (e.g., water relations, leaf structure, photosynthesis and nitrogen (N) content) and susceptibility to fungal and insect infection were investigated in five poplar clones (Populus deltoides Batr.—Lux clone; Populus nigra L.—58-861 clone and Populus × canadensis Mönch.—Luisa Avanzo, I-214 and Adige clones) during their growing season. The objective of the study was to determine their physiological responses under summer constraints (characteristic of the Mediterranean climate) and to propose clone candidates for environmental restoration activities such as phytoremediation. Relative water content, the radiometric water index and 13C isotope discrimination (Δ13C) results reflected improved water relations in Adige and Lux during summer drought. Leaf structural parameters such as leaf area, leaf mass per area, density (D) and thickness (T) indicated poorer structural adaptations to summer drought in clone 58-861. Nitrogen content and Δ13C results pointed to a stomatal component as the main limitant of photosynthesis in all clones. Adige and Lux showed enhanced photoprotection as indicated by the size and the de-epoxidation index of the xanthophyll-cycle pool, and also improved antioxidant defence displayed by higher ascorbate, reduced glutathione, total phenolics and α-tocopherol levels. Photoprotective and antioxidative responses allowed all clones to maintain a high maximum quantum yield of PSII (Fv/Fm) with the exception of Luisa Avanzo and 58-861 which experienced slight photoinhibition in late spring. The study of susceptibility to rust (Melampsora sp.) and lace bug (Monosteira unicostata Muls. and Rey) infections showed Adige and Lux to be the most tolerant. Overall, these two clones presented high adaptability to summer conditions and improved resistance to abiotic and biotic stress, thereby making them highly commendable clones for use in environmental remediation programmes. [ABSTRACT FROM PUBLISHER]

It is hoped that lignocellulosic sources will provide energy security, offset carbon dioxide enrichment of the atmosphere, and stimulate the development of new economic sectors. However, little is known about the productivity and sustainability of plant cell-wall energy industries. In this study, we used 16 global circulation models to project the global distribution of relative water availability in the coming decades and summarized the available data on the water-use efficiency of tree-and grass-based bioenergy systems. The data on bioenergy water use were extremely limited. Productivity was strongly correlated with water-use efficiency, with C[sub 4] grasses having a distinct advantage in this regard. Our analysis of agro-climatic drivers of bioenergy productivity suggests that relative water availability will be one of the most important climatic changes to consider in the design of bioenergy systems. [ABSTRACT FROM AUTHOR]

Context: The reforestation of degraded tropical pastures requires innovative tree planting systems that consider land user needs. Aim: We established a silvopastoral reforestation system and assessed the effects of companion trees on the native timber tree Tabebuia rosea in Panama. Timber tree individuals were established in (1) solitary plantings (TSol) or in companion plantings with (2) Guazuma ulmifolia (TGua) or (3) the nitrogen-fixing Gliricidia sepium (TGli). Methods: We quantified growth characteristics and herbivory of T. rosea, and analyzed leaf chemistry (including the stable isotopes δN and δC) and structure (leaf mass per area). Results: Companion trees significantly affected stem diameter growth of T. rosea. Stem diameter growth was as high in TGli trees as in TSol trees but was reduced in TGua trees. Furthermore, TGua trees had higher leaf water content, and lower δC and lower leaf mass-to-area ratio than TGli trees, suggesting there were effects of shading by G. ulmifolia on T. rosea. Herbivory was high but not affected by planting regimes. Leafing phenology did not differ between planting regimes and G. sepium did not increase nitrogen content in T. rosea leaves. Conclusion: Companion tree planting can support timber tree growth in silvopastoral reforestations, but adequate species selection is crucial for successful implementation of this planting system. Tree-tree interactions seem to be more relevant for timber tree growth than herbivory in the studied system. [ABSTRACT FROM AUTHOR]

Background: Hybrid poplars species are candidates for biomass production but breeding efforts are needed to combine productivity and water use efficiency in improved cultivars. The understanding of the genetic architecture of growth in poplar by a Quantitative Trait Loci (QTL) approach can help us to elucidate the molecular basis of such integrative traits but identifying candidate genes underlying these QTLs remains difficult. Nevertheless, the increase of genomic information together with the accessibility to a reference genome sequence (Populus trichocarpa Nisqually-1) allow to bridge QTL information on genetic maps and physical location of candidate genes on the genome. The objective of the study is to identify QTLs controlling productivity, architecture and leaf traits in a P. deltoides x P. trichocarpa F1 progeny and to identify candidate genes underlying QTLs based on the anchoring of genetic maps on the genome and the gene ontology information linked to genome annotation. The strategy to explore genome annotation was to use Gene Ontology enrichment tools to test if some functional categories are statistically over-represented in QTL regions. Results: Four leaf traits and 7 growth traits were measured on 330 F1 P. deltoides x P. trichocarpa progeny. A total of 77 QTLs controlling 11 traits were identified explaining from 1.8 to 17.2% of the variation of traits. For 58 QTLs, confidence intervals could be projected on the genome. An extended functional annotation was built based on data retrieved from the plant genome database Phytozome and from an inference of function using homology between Populus and the model plant Arabidopsis. Genes located within QTL confidence intervals were retrieved and enrichments in gene ontology (GO) terms were determined using different methods. Significant enrichments were found for all traits. Particularly relevant biological processes GO terms were identified for QTLs controlling number of sylleptic branches: intervals were enriched in GO terms of biological process like 'ripening' and 'adventitious roots development'. Conclusion: Beyond the simple identification of QTLs, this study is the first to use a global approach of GO terms enrichment analysis to fully explore gene function under QTLs confidence intervals in plants. This global approach may lead to identification of new candidate genes for traits of interest. [ABSTRACT FROM AUTHOR]

Stable isotope analysis is a powerful tool for tracing biogeochemical processes in the carbon and water cycles. One particularly powerful approach is to employ multiple isotopes where the simultaneous assessment of the D/H,18O/16O and/or 13C/12C in different compounds provide a unique means to investigate the coupling of water and 5 carbon fluxes at various temporal and spatial scales. Here, we present a research update on recent advances in our process-based understanding of the utilization of carbon, oxygen and hydrogen isotopes to lend insight into carbon and water cycling. We highlight recent technological developments and approaches, their strengths and methodological precautions with examples covering scales from minutes to centuries and from the leaf to the globe. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM PUBLISHER]

The stem lenticel is a highly specialized tissue of woody plants that has evolved to balance stem water retention and gas exchange as an adaptation to local environments. In this study, we applied genome-wide association studies and selective sweeping analysis to characterize the genetic architecture and genome-wide adaptive signatures underlying stem lenticel traits among 303 unrelated accessions of P. tomentosa, which has significant phenotypic and genetic variations according to climate region across its natural distribution. In total, we detected 108 significant single-nucleotide polymorphisms, annotated to 88 candidate genes for lenticel, of which 9 causative genes showed significantly different selection signatures among climate regions. Furthermore, PtoNAC083 and PtoMYB46 showed significant association signals and abiotic stress response, so we overexpressed these two genes in Arabidopsis thaliana and found that the number of stem cells in all three overexpression lines was significantly reduced by PtoNAC083 overexpression but slightly increased by PtoMYB46 overexpression, suggesting that both genes are involved in cell division and expansion during lenticel formation. The findings of this study demonstrate the successful application of an integrated strategy for dissecting the genetic basis and landscape genetics of complex adaptive traits, which will facilitate the molecular design of tree ideotypes that may adapt to future climate and environmental changes. [ABSTRACT FROM AUTHOR]

We explored the relationship between tree growth, water use, and related hydraulic traits in Populus deltoides Bartr. ex Marsh.and hybrid clones, to examine potential trade-offs between growth and water use efficiency. Nine genotypes, six P. deltoides and three hybrid clones, that represented genotypes with high (Group H), intermediate (Group I), and low (Group L) growth performance were selected for study, based on year-two standing stem biomass in a replicated field trial. In year four, tree growth, transpiration (Et), canopy stomatal conductance (Gs), whole-tree hydraulic conductance (Gp), and carbon isotope discrimination (Δ13C) were measured. Tree sap flux was measured continuously using thermal dissipation probes. We hypothesized that Group H genotypes would have increased growth efficiency (GE), increased water use efficiency of production (WUEp, woody biomass growth/Et), lower Δ13C, and greater Gp than slower growing genotypes. Tree GE increased with relative growth rate (RGR), and mean GE in Group H was significantly greater than L, but not I. Tree WUEp ranged between 1.7 and 3.9 kg biomass m3 H2O−1, which increased with RGR. At similar levels of Et, WUEp was significantly greater in Group H (2.45 ± 0.20 kg m−3), compared to I (2.03 ± 0.18 kg m−3) or L (1.72 ± 0.23 kg m−3). Leaf and wood Δ13C scaled positively with stem biomass growth but was not correlated with WUEp. However, at a similar biomass increment, clones in Group H and I had significantly lower leaf Δ13C than Group L. Similarly, Group H clones had a significantly lower wood Δ13C than Group L, supporting our hypothesis of increased WUE in larger trees. Tree physiological and hydraulic traits partially explain differences in WUEp and Δ13C, and suggest that clone selection and management activities that increase tree biomass production will likely increase tree and stand WUE. However, more research is needed to discern the underlying hydraulic mechanisms responsible for the higher WUE exhibited by large trees and distinct clones. [ABSTRACT FROM AUTHOR]

• Populus trichocarpa is widespread across western North America spanning extensive variation in photoperiod, growing season and climate. We investigated trait variation in P. trichocarpa using over 2000 trees from a common garden at Vancouver, Canada, representing replicate plantings of 461 genotypes originating from 136 provenance localities. • We measured 40 traits encompassing phenological events, biomass accumulation, growth rates, and leaf, isotope and gas exchange-based ecophysiology traits. With replicated plantings and 29,354 single nucleotide polymorphisms (SNPs) from 3518 genes, we estimated both broad-sense trait heritability (H(2)) and overall population genetic structure from principal component analysis. • Populus trichocarpa had high phenotypic variation and moderate/high H(2) for many traits. H(2) ranged from 0.3 to 0.9 in phenology, 0.3 to 0.8 in biomass and 0.1 to 0.8 in ecophysiology traits. Most traits correlated strongly with latitude, maximum daylength and temperature of tree origin, but not necessarily with elevation, precipitation or heat : moisture indices. Trait H(2) values reflected trait correlation strength with geoclimate variables. The population genetic structure had one significant principal component (PC1) which correlated with daylength and showed enrichment for genes relating to circadian rhythm and photoperiod. • Robust relationships between traits, population structure and geoclimate in P. trichocarpa reflect patterns which suggest that range-wide geographical and environment gradients have shaped its genotypic and phenotypic variability., (© 2013 The Authors. New Phytologist © 2013 New Phytologist Trust.)

Balsam poplar (Populus balsamifera L.) is a transcontinental tree species in North America, making it an ideal species to study intra-specific hybrid vigour as a tool for increasing genetic gain in growth. We tested the hypothesis that intra-specific breeding of disparate populations of balsam poplar would lead to the expression of hybrid vigour and we determined the role of endogenous hormones linked to ecophysiological and growth performance. In September 2009, three field trials were established in Canada (two in Alberta (AB), i.e., Fields AB1 and AB2, and one in Quebec (QC), i.e., Field QC1) in conjunction with Alberta-Pacific Forest Industries Inc. and the Ministry of Forests, Wildlife and Parks, Quebec. Five male parents from each province as well as five female parents from QC and four female parents from AB were used for breeding intra-regional and inter-regional crosses. Based on a significant difference at year six for height and diameter, from the AB1 and AB2 field trials, the AB × QC cross-type was selected for further study. Cuttings from the AB × QC cross-type were grown in a randomized complete block design under near-optimal greenhouse conditions. Families were identified as slow- or fast-growing, and the relationship between hormone levels and growth performance of the genotypes within the families were examined. In late June, after 34 days of growth, internode tissue samples collected from each progeny were analyzed for gibberellic acids, indole-3-acetic acid, and abscisic acid content. Stem volume of two-month-old rooted cuttings, grown under optimal greenhouse conditions, was positively and significantly correlated with the photosynthetic rate, greenhouse growth, and stem volume of 8-year-old field-grown trees (Fields AB1 values: r = 0.629 and p = 0.012; AB2 values: r = 0.619 and p = 0.014, and QC1 values: r = 0.588 and p = 0.021, respectively). We determined that disparate and native populations of balsam poplar can be bred to produce superior progeny with enhanced stem growth traits. [ABSTRACT FROM AUTHOR]

Leaf stable carbon isotope (δ13C) composition provides comprehensive insight into plant carbon cycles and water use efficiency and has also been widely used to evaluate the response of plants to environmental change. In the present study, leaf δ13C was analyzed in samples of Caragana microphylla Lam., C. liouana Zhao, and C. korshinskii Kom. from 38 populations. These species provide great environmental benefits and economic value and are distributed east to west continuously across northern China. We studied the relationship of δ13C to altitude, mean annual precipitation (MAP), mean annual temperature (MAT), mean annual relative humidity (RH), leaf nitrogen (N), and phosphorus (P) concentrations to examine the patterns and controls of leaf δ13C variation in each species. Results indicated that, across the three species, leaf δ13C significantly decreased with MAP, RH, and leaf N and P concentrations, while it increased with altitude and MAT. However, patterns and environmental controls of leaf δ13C varied proportionally with species. C. korshinskii was mainly controlled by MAP and leaf N concentration, C. liouana was controlled by both MAT and MAP, and C. microphylla was mainly controlled by MAT. Further analysis indicated significant differences in leaf δ13C between species, which tended to increase from C. microphylla to C. korshinskii. Overall, these results suggest that the three Caragana species may respond differently to future climate change due to different controlling factors on leaf δ13C variation, as well as differentiation in water use efficiency, which likely contributes to the geographical distribution of these species. [ABSTRACT FROM AUTHOR]

Amongst other traits, ideal poplar genotypes for afforestation programs would be fast growing and have high resource-use efficiencies. Black cottonwood (Populus trichocarpa Torr. & A. Gray) and balsam poplar (Populus balsamifera L.) are closely related species that together extend over much of the forested area of Canada. Within their respective ranges, however, black cottonwood attains much greater size than balsam poplar. Two populations of each species, each with three replicates of 9-10 genotypes, were grown from stem cuttings for 60 days in a greenhouse under long days to examine variation in biomass, height growth, net photosynthesis (A), stomatal conductance ([g.sub.s]), intrinsic water-use efficiency (A/[g.sub.s]), photosynthetic nitrogenuse efficiency (PNUE), leaf and stemwood [sup.13]C/[sup.12]C isotope ratios (δ[sup.13]C), stomatal density (Ds), and leaf amphistomaticity. There were no significant differences in A, PNUE, biomass, or height growth between species. On average, black cottonwood had lower [g.sub.s] and Ds, but higher A/[g.sub.s] and δ[sup.13]C. Variation within provenances, in most traits, exceeded variation between species or provenances. δ[sup.13]C and A/[g.sub.s] were highly correlated across all genotypes. Variation in A/[g.sub.s] seemed primarily related to [g.sub.s], although positive correlations were found between δ[sup.13]C and A in the P. balsamifera populations, which more generally met expectations for sink-driven differences in water-use efficiency. There is potential to identify fast-growing genotypes with relatively high use efficiencies for both water and nitrogen. Key words: balsam poplar, black cottonwood, water-use efficiency, [sup.13]C/[sup.12]C isotope ratio, nitrogen-use efficiency. Parmi certains traits, dans le cadre de programmes de boisement, les genotypes ideaux du peuplier comprendraient des proprietes de croissance rapide et d'efficacite elevee d'utilisation des ressources. Le peuplier occidental (Populus trichocarpa Torr. & A. Gray) et le peuplier baumier (Populus balsamifera L.) sont deux especes etroitement reliees qui s'etendent conjointement sur une bonne partie du territoire forestier du Canada. A l'interieur de leurs etendues respectives cependant, le peuplier occidental atteint une taille largement superieure a celle du peuplier baumier. Deux populations de chaque espece comprenant chacune trois replicats de 9-10 genotypes ont ete cultivees en serre a partir de segments de tige pendant 60 jours, sous des conditions d'eclairage de longue duree, afin d'examiner la variation de biomasse, de croissance en hauteur, de photosynthese nette (A), de conductance stomatique ([g.sub.s]), d'efficacite intrinseque d'utilisation de l'eau (A/[g.sub.s]), d'efficacite photosynthetique d'utilisation de l'azote (PNUE), des rapports isotopiques [sup.13]C/[sup.12]C des feuilles et du bois de fut (δ[sup.13]C), de la densite stomatique (Ds) et de l'amphistomaticite. Il n'y avait pas de differences significatives sur le plan de la A, de la PNUE, de la biomasse ou de la croissance en hauteur entre les especes. En moyenne, le peuplier occidental presentait une [g.sub.s] et une Ds plus faibles, mais une A/[g.sub.s] et un δ[sup.13]C plus eleves. La variation a l'interieur d'une provenance donnee excedait la variation observee entre les especes ou entre les provenances et ce, pour la plupart des traits. La correlation entre les rapports δ[sup.13]C et A/[g.sub.s] etait forte a travers tous les genotypes. La variation de A/[g.sub.s] semblait etre principalement reliee a [g.sub.s], meme si des correlations positives etaient trouvees entre le δ[sup.13]C et A chez les populations de peuplier baumier, ce qui, de maniere plus generale, repond aux attentes en ce qui concerne des differences d'efficacite d'utilisation de l'eau dictees par les puits. Il existe un potentiel d'identification des genotypes de croissance rapide avec des efficacites d'utilisation d'eau et d'azote relativement elevees. [Traduit par la Redaction] Mots-cles : peuplier baumier, peuplier occidental, efficacite d'utilisation de l'eau, rapport isotopique [sup.13]C/[sup.12]C, efficacite d'utilisation de l'azote., Introduction Populus L. tree species, including cottonwoods and aspens (henceforth referred to as poplars) (Taylor 2002), are among the favoured tree species for use in afforestation. These trees are fast [...]