Background and AimsExtreme water stress episodes induce tree mortality, but the physiological mechanisms causing tree death are still poorly understood. This study tests the hypothesis that a potted ...tree's ability to survive extreme monotonic water stress is determined by the cavitation resistance of its xylem tissue.MethodsTwo species were selected with contrasting cavitation resistance (beech and poplar), and potted juvenile trees were exposed to a range of water stresses, causing up to 100 % plant death.Key ResultsThe lethal dose of water stress, defined as the xylem pressure inducing 50 % mortality, differed sharply across species (1·75 and 4·5 MPa in poplar and beech, respectively). However, the relationships between tree mortality and the degree of cavitation in the stems were similar, with mortality occurring suddenly when >90 % cavitation had occurred.ConclusionsOverall, the results suggest that cavitation resistance is a causal factor of tree mortality under extreme drought conditions.
The winter-dormant shoot apical meristem of the tree species Populus keeps an epigenetic memory of environmental variations that arose during the preceding vegetative period.
Abstract
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.
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 (H²ind = 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.
The importance of tree genetic variability in the ability of forests to respond and adapt to environmental changes is crucial in forest management and conservation. Along with genetics, recent ...advances have highlighted “epigenetics” as an emerging and promising field of research for the understanding of tree phenotypic plasticity and adaptive responses. In this paper, we review recent advances in this emerging field and their potential applications for tree researchers and breeders, as well as for forest managers. First, we present the basics of epigenetics in plants before discussing its potential for trees. We then propose a bibliometric and overview of the literature on epigenetics in trees, including recent advances on tree priming. Lastly, we outline the promises of epigenetics for forest research and management, along with current gaps and future challenges. Research in epigenetics could use highly diverse paths to help forests adapt to global change by eliciting different innovative silvicultural approaches for natural- and artificial-based forest management.
Riparian forests remain largely understudied in the context of climate change in comparison to other forest ecosystems although they serve multiple socio-ecological functions. We evaluated local ...adaptation and adaptive potential in Populus nigra L., an emblematic tree species of European riparian forests. We set up a reciprocal transplant experiment and measured 17 structural and functional traits among 10 progenies of two genetically differentiated populations. The populations originated from two separate watersheds differing in climate conditions and a total of 1200 seedlings were grown in 1-m3 mesocosms at both sites for one growing season. Traits measured were related to growth, leaf physiology and xylem water transport. The populations showed similar biomass suggesting no local adaptation for overall performance but displayed distinct trait syndromes and plastic abilities. The southern population primarily adjusted through changes in allocation and leaf water-use efficiency while the northern population primarily adjusted through changes in specific leaf area. Genetic variation within populations was most of the times equal or larger than between populations. This combined with the generally moderate to high heritability values and the observed plasticity suggested significant adaptive potential. Altogether, our findings reveal that although evolution may not lead to obvious differentiation between populations in global performance, integrated multi-trait approaches are highly valuable to shed light on how evolution may shape distinct underlying functional strategies among populations resulting in a similar outcome.
Summary
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 downregulation of the chromatin ...remodeler DECREASED IN DNA METHYLATION 1 (DDM1) in poplar (Populus tremula × 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.
Summary
Ectomycorrhizas are an intrinsic component of tree nutrition and responses to environmental variations. How epigenetic mechanisms might regulate these mutualistic interactions is unknown.
By ...manipulating the level of expression of the chromatin remodeler DECREASE IN DNA METHYLATION 1 (DDM1) and two demethylases DEMETER‐LIKE (DML) in Populus tremula × Populus alba lines, we examined how host DNA methylation modulates multiple parameters of the responses to root colonization with the mutualistic fungus Laccaria bicolor. We compared the ectomycorrhizas formed between transgenic and wild‐type (WT) trees and analyzed their methylomes and transcriptomes.
The poplar lines displaying lower mycorrhiza formation rate corresponded to hypomethylated overexpressing DML or RNAi‐ddm1 lines. We found 86 genes and 288 transposable elements (TEs) differentially methylated between WT and hypomethylated lines (common to both OX‐dml and RNAi‐ddm1) and 120 genes/1441 TEs in the fungal genome suggesting a host‐induced remodeling of the fungal methylome. Hypomethylated poplar lines displayed 205 differentially expressed genes (cis and trans effects) in common with 17 being differentially methylated (cis).
Our findings suggest a central role of host and fungal DNA methylation in the ability to form ectomycorrhizas including not only poplar genes involved in root initiation, ethylene and jasmonate‐mediated pathways, and immune response but also terpenoid metabolism.
See also the Commentary on this article by Zimmermann & Gaillard, 238: 2259–2260.
•Planting density affects growth and WUE depending on pedoclimatic conditions.•Under favorable conditions, increasing planting density accentuates plant competition for light.•Under limiting water ...availability increasing planting density accentuates plant competition for water acquisition.•No trade-off between growth potential and WUE.
Poplar coppice plantations for biomass production can be conducted under either short rotation coppice (SRC) or short rotation forestry (SRF) systems, depending on planting density and rotation length. It is likely that differences in planting density affect tree physiology through competition for resource acquisition, including light, water and nutrients. In this paper, we hypothesized that the effects of planting density on growth and water-use efficiency (assessed through bulk leaf carbon isotope discrimination, Δ13C) in poplar depend on site characteristics in terms of soil fertility and water availability. To test this hypothesis, 56 Populus deltoides×P. nigra genotypes were planted under both SRC and SRF and replicated at two sites differing for pedoclimatic conditions. At the most favorable site for growth, trees grown at the higher density (SRC) displayed higher stem height, lower stem circumference, higher specific leaf area, higher mass-based leaf nitrogen contents and higher Δ13C, indicating that increased tree density mainly accentuated competition for light. Under less favorable conditions, trees grown under SRC still displayed lower stem circumference, higher specific leaf area and higher mass-based leaf nitrogen contents. However, stem height remained unaffected by increasing planting density while Δ13C was lower, likely because of increased competition for water availability. Genotypic rankings across planting densities were overall conserved while they were significantly modified across sites, suggesting that rankings for genotypic performances were much less affected by planting density than by site. Realized growth measured after 2years (height and circumference) was weakly correlated with Δ13C, but a negative relationship between Δ13C and growing season leaf increment rate was observed in most cases. The absence of trade-off between growth and water-use efficiency combined with the large genotypic variations observed for these traits confirms the potential for selecting genotypes with high water-use efficiency without counter-selecting on biomass production in P. deltoides×P. nigra.
Tests were carried out to determine whether variations in the hydraulic architecture of eight Populus deltoidesxPopulus 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 kplant, indicating that highpressure 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 wateruse 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. deltoidesxP. nigra genotypes.
We compared the temporal dynamics of growth, leaf gas exchange, xylem hydraulic dysfunction and non-structural carbohydrates (NSC) of two poplar hybrid genotypes subjected to a differential nitrogen ...(N) supply under contrasting water deficits. Moderate water deficit generated intermediate embolism rates (45–70%) but had marginal effects on NSC. Severe water deficit generated progressive hydraulic failure and NSC reduction, leading to tree death within 90 days. At death, NSC in perennial tissues were not entirely depleted because of remaining soluble sugars. Higher N availability primarily affected growth and NSC dynamics (mainly starch), not embolism dynamics. The faster growing genotype benefiting most from N addition ceased growth and photosynthesis almost simultaneously, with no effect of N addition on process cessation, leading to progressive starch depletion during severe drought. In contrast, the slower growing genotype ceased growth before photosynthesis, leading to a transient increase in starch concentration during early drought stages, but this tended to be suppressed by higher N availability. Altogether, our findings indicate that carbon starvation alone is unlikely in hydraulically vulnerable species such as poplar even under prolonged moderate water deficit, but starch relative depletion covaries in time with progressive hydraulic failure under lethal water deficit. Nutritional status has the potential to shape drought responses, primarily through adjustments in carbon source-sink relations rather than in intrinsic xylem hydraulics, in a genotype-dependent manner. Exploring how hydraulic and carbon safety margins can be coordinated within species would be valuable in understanding how variation in drought response strategies can be exploited in breeding/selection.
•Carbon starvation alone is unlikely in hydraulically vulnerable species such as poplar even under prolonged moderate drought.•Starch relative depletion, not soluble sugars, covaries with hydraulic failure under severe drought progression.•Nitrogen shapes drought responses through changes in carbon source-sink relations rather than intrinsic xylem hydraulics.
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