Tree Populus Research Articles

Salinity-Induced Photorespiration in Populus Vascular Tissues Facilitate Nitrogen Reallocation.

Adaptation to abiotic stress is critical for the survival of perennial tree species. Salinity affects plant growth and productivity by interfering with major biosynthetic processes. Detrimental effects of salinity may vary between different plant tissues and cell types. However, spatial molecular mechanisms controlling plant responses to salinity stress are not yet thoroughly understood in perennial trees. We used laser capture microdissection in clones of Populus tremula x alba to isolate palisade and vascular cells of intermediary leaf from plants exposed to 150 mM NaCl for 10 days, followed by a recovery period. Cell-specific changes in proteins and metabolites were determined. Salinity induced a vascular-specific accumulation of proteins associated with photorespiration, and the accumulation of serine, 3-phosphoglycerate and NH4 + suggesting changes in N metabolism. Accumulation of the GLUTAMINE SYNTHETASE 2 protein, and increased GS1.1 gene expression, indicated that NH4 + produced in photorespiration was assimilated to glutamine, the main amino acid translocated in Populus trees. Further analysis of total soluble proteins in stems and roots showed the accumulation of bark storage proteins induced by the salinity treatments. Collectively, our results suggest that the salt-induced photorespiration in vascular cells mediates N-reallocation in Populus, an essential process for the adaptation of trees to adverse conditions.

Functional diversification within the heme-binding split-barrel family

Due to neofunctionalization, a single fold can be identified in multiple proteins that have distinct molecular functions. Depending on the time that has passed since gene duplication and the number of mutations, the sequence similarity between functionally divergent proteins can be relatively high, eroding the value of sequence similarity as the sole tool for accurately annotating the function of uncharacterized homologs. Here, we combine bioinformatic approaches with targeted experimentation to reveal a large multi-functional family of putative enzymatic and non-enzymatic proteins involved in heme metabolism. This family (homolog of HugZ (HOZ)) is embedded in the “FMN-binding split barrel” superfamily and contains separate groups of proteins from prokaryotes, plants, and algae, which bind heme and either catalyze its degradation or function as non-enzymatic heme sensors. In prokaryotes these proteins are often involved in iron assimilation, whereas several plant and algal homologs are predicted to degrade heme in the plastid or regulate heme biosynthesis. In the plant Arabidopsis thaliana, which contains two HOZ subfamilies that can degrade heme in vitro (HOZ1 and HOZ2), disruption of AtHOZ1 (AT3G03890) or AtHOZ2A (AT1G51560) causes developmental delays, pointing to important biological roles in the plastid. In the tree Populus trichocarpa, a recent duplication event of a HOZ1 ancestor has resulted in localization of a paralog to the cytosol. Structural characterization of this cytosolic paralog and comparison to published homologous structures suggests conservation of heme-binding sites. This study unifies our understanding of the sequence-structure-function relationships within this multi-lineage family of heme-binding proteins and presents new molecular players in plant and bacterial heme metabolism.

Potential of slurry from intensive dairy cattle farms for paulownia and populus trees, as organic fertilizer: i. effect on production

Intensive dairy cattle breeding have a relevant social and economic impact in Portugal, particularly in the northern region. This activity generates a high flow of livestock effluents (slurry), rich in important nutrients for plant growth, which can be introduced into forest production systems. These effluents can provide a good alternative to mineral fertilizers, not only from an economic perspective, but also from the point of view of soil fertility resilience. In the present study, the effect of increasing doses of slurry on tree growth was evaluated in clones of Paulownia and Populus, as they are genotypes that have a high efficiency in the mobilization of soil nutrients and in the capture of CO2 from the atmosphere, as well as high biomass calorific value. To this end, a demonstration field was installed, where the trees were planted with the compasses of: 2.5 x 1.5 m and 2.5 x 0.75 m, respectively for Paulownia and for Populus. In the field, the following treatments were performed: T0 - no fertilization, either mineral or organic; T1 - amount of slurry equivalent to 85 kg of N ha-1; T2 - amount of slurry equivalent to 170 kg of N ha-1; T3 - amount of slurry equivalent to 340 kg of N ha-1, either with or without inoculation prior to transplantation, with mycorrhizal arbuscular fungi and plant growth-promoting bacteria. Results evidenced a positive effect of the slurry application, both in the diameter at breast height and in total stand height, showing its fertilizing potential which should later be assessed on the ability to constitute an alternative or, simply, a complement to mineral fertilization.

Genome Wide Identification, Characterization and Evolutionary Analysis of T6SS in Burkholderia cenocepacia Strains

Pathogens of the Burkholderia genus are causing diseases in a diverse variety of hosts. After the discovery of T6SS, it was found to play a pivotal role in virulence and other pathogenicity factors in different pathogenic Burkholderia species. For this study, three strains of Burkholderia cenocepacia were selected from different ecological niches; J2315 from humans, MC0-3 from the rhizosphere of maize, and YG-3 from the Populus tree. The sequenced genomes were retrieved from PATRIC. It was found that B. cenocepacia J2315 and MC0-3 strains had only 1 cluster of T6SS in their genomes while the YG-3 strain had 3 clusters. The circular genomic map and phylogenetic tree suggested major differences in T6SS clusters 2 and 3 of the YG-3 strain from other clusters. From the results obtained in the study and reviewing the literature, it was concluded that all 3 strains harbor T6SS-1 type cluster that is involved in causing virulence in eukaryotic organisms and several bacterial species. This factor of causing virulence in the bacteria species might be helpful for B. cenocepacia strains J2315, MC0-3 and YG-3 in survival and niche adaptation.

Phytochrome-interacting factors play shared and distinct roles in regulating shade avoidance responses in Populus trees.

Plants adjust their growth and development in response to changing light caused by canopy shade. The molecular mechanisms underlying shade avoidance responses have been widely studied in Arabidopsis and annual crop species, yet the shade avoidance signalling in woody perennial trees remains poorly understood. Here, we first showed that PtophyB1/2 photoreceptors serve conserved roles in attenuating the shade avoidance syndrome (SAS) in poplars. Next, we conducted a systematic identification and characterization of eight PtoPIF genes in Populus tomentosa. Knocking out different PtoPIFs led to attenuated shade responses to varying extents, whereas overexpression of PtoPIFs, particularly PtoPIF3.1 and PtoPIF3.2, led to constitutive SAS phenotypes under normal light and enhanced SAS responses under simulated shade. Notably, our results revealed that distinct from Arabidopsis PIF4 and PIF5, which are major regulators of SAS, the Populus homologues PtoPIF4.1 and PtoPIF4.2 seem to play a minor role in controlling shade responses. Moreover, we showed that PtoPIF3.1/3.2 could directly activate the expression of the auxin biosynthetic gene PtoYUC8 in response to shade, suggesting a conserved PIF-YUC-auxin pathway in modulating SAS in tree. Overall, our study provides insights into shared and divergent functions of PtoPIF members in regulating various aspects of the SAS in Populus.

GWAS supported by computer vision identifies large numbers of candidate regulators of in planta regeneration in Populus trichocarpa.

Plant regeneration is an important dimension of plant propagation and a key step in the production of transgenic plants. However, regeneration capacity varies widely among genotypes and species, the molecular basis of which is largely unknown. Association mapping methods such as genome-wide association studies (GWAS) have long demonstrated abilities to help uncover the genetic basis of trait variation in plants; however, the performance of these methods depends on the accuracy and scale of phenotyping. To enable a large-scale GWAS of in planta callus and shoot regeneration in the model tree Populus, we developed a phenomics workflow involving semantic segmentation to quantify regenerating plant tissues over time. We found that the resulting statistics were of highly non-normal distributions, and thus employed transformations or permutations to avoid violating assumptions of linear models used in GWAS. We report over 200 statistically supported quantitative trait loci (QTLs), with genes encompassing or near to top QTLs including regulators of cell adhesion, stress signaling, and hormone signaling pathways, as well as other diverse functions. Our results encourage models of hormonal signaling during plant regeneration to consider keystone roles of stress-related signaling (e.g. involving jasmonates and salicylic acid), in addition to the auxin and cytokinin pathways commonly considered. The putative regulatory genes and biological processes we identified provide new insights into the biological complexity of plant regeneration, and may serve as new reagents for improving regeneration and transformation of recalcitrant genotypes and species.

Итоги развития насаждений на территории сквера у культурно-развлекательного комплекса арена «Уралец» в г. Екатеринбурге

The article presents the results of a study of the species composition, structure, health condition, and stages of development of the plantings in the garden square near CEC «Uralets», Yekaterinburg – alley of the Palace of Sports in Yekaterinburg. This study shows that more than 56% of woody plants were Acer negundo L. and Populus balsamifera L. There were plantings in lines (20% of the area) consisting of trees of Populus and Malus genus, plantings in arrays (about 25% of the area) of Populus balsamifera L. and Fraxinus pennsylvanica Marsh. Other species were planted in groups (55% of the area). The health condition of the plantings was assessed as 2.9 (very poor). The most durable species Larix sibirica Ledeb., Quercus robur L., and Tilia cordata Mill. had the best average scores. Less durable species Prunus maackii Rupr., Pyrus ussuriensis Maxim., Malus baccata L., and Populus balsamifera L. were dying off. Planting density in mowed areas corresponded to the recommended norms, while this indicator was exceeded by an average of 25% in areas without care due to the invasion of Acer negundo L. Three stages were distinguished in the formation process of the garden square near CEC Uralets: 1) individual plots were created and natural plantations were integrated; 2) performed complete transformation of the landscape in the central part, planted ornamental plants and created a single space of the garden square; 3) decorative plantings were added to the garden square. During the reconstruction of the garden square that combines plantings of various origins and functions, step-by-step reconstruction is the most favorable – starting with less durable dying species and preserving Larix sibirica Ledeb., Quercus robur L., Tilia cordata Mill., and Betula pendula Roth.

SPL16 and SPL23 mediate photoperiodic control of seasonal growth in Populus trees.

Perennial trees in boreal and temperate regions undergo growth cessation and bud set under short photoperiods, which are regulated by phytochrome B (phyB) photoreceptors and PHYTOCHROME INTERACTING FACTOR 8 (PIF8) proteins. However, the direct signaling components downstream of the phyB-PIF8 module remain unclear. We found that short photoperiods suppressed the expression of miR156, while upregulated the expression of miR156-targeted SQUAMOSA-PROMOTER BINDING PROTEIN-LIKE 16 (SPL16) and SPL23 in leaves and shoot apices of Populus trees. Accordingly, either overexpression of MIR156a/c or mutagenesis of SPL16/23 resulted in the attenuation of growth cessation and bud set under short days (SD), whereas overexpression of SPL16 and SPL23 conferred early growth cessation. We further showed that SPL16 and SPL23 directly suppressed FLOWERING LOCUS T2 (FT2) expression while promoted BRANCHED1 (BRC1.1 and BRC1.2) expression. Moreover, we revealed that PIF8.1/8.2, positive regulators of growth cessation, directly bound to promoters of MIR156a and MIR156c and inhibited their expression to modulate downstream pathways. Our results reveal a connection between the phyB-PIF8 module-mediated photoperiod perception and the miR156-SPL16/23-FT2/BRC1 regulatory cascades in SD-induced growth cessation. Our study provides insights into the rewiring of a conserved miR156-SPL module in the regulation of seasonal growth in Populus trees.

Age-dependent seasonal growth cessation in Populus.

In temperate and boreal regions, perennial plants adapt their annual growth cycle to the change of seasons. In natural forests, juvenile seedlings usually display longer growth seasons compared to adult trees to ensure their establishment and survival under canopy shade. However, how trees adjust their annual growth according to their age is not known. In this study, we show that age-dependent seasonal growth cessation is genetically controlled and found that the miR156-SPL3/5 module, a key regulon of vegetative phase change (VPC), also triggers age-dependent growth cessation in Populus trees. We show that miR156 promotes shoot elongation during vegetative growth, and its targets SPL3/5s function in the same pathway but as repressors. We find that the miR156-SPL3/5s regulon controls growth cessation in both leaves and shoot apices and through multiple pathways, but with a different mechanism compared to how the miR156-SPL regulon controls VPC in annual plants. Taken together, our results reveal an age-dependent genetic network in mediating seasonal growth cessation, a key phenological process in the climate adaptation of perennial trees.

Above- and belowground fungal biodiversity of Populus trees on a continental scale.

Understanding drivers of terrestrial fungal communities over large scales is an important challenge for predicting the fate of ecosystems under climate change and providing critical ecological context for bioengineering plant-microbe interactions in model systems. We conducted an extensive molecular and microscopy field study across the contiguous United States measuring natural variation in the Populus fungal microbiome among tree species, plant niche compartments and key symbionts. Our results show clear biodiversity hotspots and regional endemism of Populus-associated fungal communities explained by a combination of climate, soil and geographic factors. Modelling climate change impacts showed a deterioration of Populus mycorrhizal associations and an increase in potentially pathogenic foliar endophyte diversity and prevalence. Geographic differences among these symbiont groups in their sensitivity to environmental change are likely to influence broader forest health and ecosystem function. This dataset provides an above- and belowground atlas of Populus fungal biodiversity at a continental scale.

Microparticle-mediated CRISPR DNA delivery for genome editing in poplar.

The use of CRISPR/Cas9 is currently the method of choice for precise genome engineering in plants, including in the biomass crop poplar. The most commonly used method for delivering CRISPR/Cas9 and its components in poplar is via Agrobacterium-mediated transformation, that besides the desired gene-editing event also results in stable T-DNA integration. Here we explore the delivery of the gene-editing reagents via DNA-coated microparticle bombardment into the model tree Populus tremula x P. alba to evaluate its potential for developing transgene-free, gene-edited trees, as well as its potential for integrating donor DNA at specific target sites. Using an optimized transformation method, which favors the regeneration of plants that transiently express the genes on the delivered donor DNA, we regenerated gene-edited plants that are free of the Cas9 and the antibiotic resistance-encoding transgenes. In addition, we report the frequent integration of donor DNA fragments at the Cas9-induced double-strand break, opening opportunities toward targeted gene insertions.

Carbohydrate dynamics in Populus trees under drought: An expression atlas of genes related to sensing, translocation, and metabolism across organs.

In trees, nonstructural carbohydrates (NSCs) serve as long-term carbon storage and long-distance carbon transport from source to sink. NSC management in response to drought stress is key to our understanding of drought acclimation. However, the molecular mechanisms underlying these processes remain unclear. By combining a transcriptomic approach with NSC quantification in the leaves, stems, and roots of Populus alba under drought stress, we analyzed genes from 29 gene families related to NSC signaling, translocation, and metabolism. We found starch depletion across organs and accumulation of soluble sugars (SS) in the leaves. Activation of the trehalose-6-phosphate/SNF1-related protein kinase (SnRK1) signaling pathway across organs via the suppression of class I TREHALOSE-PHOSPHATE SYNTHASE (TPS) and the expression of class II TPS genes suggested an active response to drought. The expression of SnRK1α and β subunits, and SUCROSE SYNTHASE6 supported SS accumulation in leaves. The upregulation of active transporters and the downregulation of most passive transporters implied a shift toward active sugar transport and enhanced regulation over partitioning. SS accumulation in vacuoles supports osmoregulation in leaves. The increased expression of sucrose synthesis genes and reduced expression of sucrose degradation genes in the roots did not coincide with sucrose levels, implying local sucrose production for energy. Moreover, the downregulation of invertases in the roots suggests limited sucrose allocation from the aboveground organs. This study provides an expression atlas of NSC-related genes that respond to drought in poplar trees, and can be tested in tree improvement programs for adaptation to drought conditions.

The attractive host volatiles can enhance oviposition of Anoplophora glabripennis on a non-host tree.

The Asian longhorned beetle (ALB), Anoplophora glabripennis, is a serious wood borer of hardwood trees. Populus deltoides 'Shalinyang' (PdS) is attractive to ALB adults for oviposition but highly resistant to their offspring. Investigation of the chemicals regulating ALB oviposition is scarce in previous studies until now. To determine which chemicals emitted by PdS were attractive and induced oviposition behavior by ALB on non-host poplar tree species, we first: collected and identified the bio-active volatiles produced by PdS using coupled gas chromatography-mass spectrometry (GC-MS) and coupled gas chromatography-electroantennographic detector (GC-EAD); then evaluated which chemicals were attractive in a Y-tube olfactometer bioassay; and finally screened key compounds affecting ALB oviposition using a 'chemical-stimulated oviposition on non-host tree' bioassay. (E)-2-Hexenal, hexyl acetate, (Z)-3-hexenol acetate, 1-hexanol, (Z)-3-hexenol, β-caryophyllene, and salicylaldehyde emitted from PdS were attractive to ALB. When (E)-2-hexenal, 1-hexanol, (Z)-3-hexenol acetate, and (Z)-3-hexenol were applied to the non-host tree Populus tomentosa, oviposition by ALB females was significantly increased. Furthermore, the mean number of oviposition pits increased as the (Z)-3-hexenol concentrations increased. Further tests on synergy between pairs of chemicals showed that (Z)-3-hexenol stimulated production of the most oviposition pits, but that the percentage of effective oviposition pits (those containing an egg and larva and not empty) decreased. (Z)-3-Hexenol is the main chemical component inducing ALB oviposition. These results increase understanding about the oviposition behavior of ALB and could help improve management strategies that regulate ALB behavior by planting mixed-species forests resistant to ALB. © 2023 Society of Chemical Industry.

SOIL PHYSICO CHEMICAL PROPERTIES, WHEAT GRAIN YIELD AND ECONOMICS OF POPULUS DELTOIDES BASED AGRO FORESTRY SYSTEM

The research was carried out to determine the effect of Populus deltoides trees on the soil physical and chemical properties and grain yield of wheat crop under the agro ecological conditions of Faisalabad (Punjab), Pakistan. Two fields were selected; one with wheat monoculture and the other had boundary trees of Populus deltoides. The average age of the s Populus trees was 5 years. Number of trees was 40 per acre. The average height and diameter at breast height of trees was 12 meters and 42.43 cm respectively. The wheat crop was sown in October, 2020 in both fields and harvested in April, 2021. A quadrate of one meter square was used to collect the data. The quadrate was put at various distances (2-24 meter) from tree stems and the plants growing there were harvested. The grain yield (gm-2) per quadrate was determined with the help of an electric weighing balance. The soil samples were selected from both fields and various chemical and physical properties of soil were determined. The soil pH was 7.32 and 7.95 in wheat monoculture and Populus+wheat field respectively. Total nitrogen was 0.05 and 0.08%in both fields respectively as mentioned above. Organic matter was 0.64 and 0.81 in wheat monoculture and Populus based field respectively. The effect of Populus trees on grain yield of wheat was negative form 2m to 10 meter distance from trees. While from 12m to 24 m distance, wheat grain yield was more or less equal in both fields i.e. with and without trees. Net income was Rs. 67437.69 and Rs. 104781.03 per acre without and with trees, respectively. On the basis of this research, it is concluded that wheat + Poplar is more suitable option for farmers. So farmers should adopt this system to generate more money while keeping the inputs same.

Factors affecting in vitro regeneration in the model tree Populus trichocarpa: II. Heritability estimates, correlations among explant types, and genetic interactions with treatments among wild genotypes

To enhance the sensitivity of an ongoing Genome Wide Association Study (GWAS) for in vitro shoot regeneration and genetic transformation, a wide range of factors that can affect regeneration rate and, therefore, expression of genetic diversity were studied. Included were explant types and sources; direct versus indirect regeneration; nitrogen salts and micronutrient levels in basal medium; sucrose, auxin, and cytokinin types and levels; light intensity and quality; melatonin and serotonin; antibiotics (to exogenously control contaminants); antioxidants; and the ethylene-mitigating agent silver nitrate. Genotype was a statistically significant source of variance in response to nearly every treatment and for every measure of regeneration noted in shoots and roots, and initial generation of callus. Nonetheless, broad-sense heritability estimates varied widely as a result of variable degrees of environmental modification and statistical interactions with genotype. Compared to leaves, stem and petiole explants were highly correlated in their expression of genetic variation in response to treatments, a likely result of their functional similarity. Based on literature review, this study appears to be the most intensive analysis of plant natural genetic variation in response to in vitro manipulations published. It should help to guide development of improved and highly heritable treatments in other plant species that employ organogenic regeneration systems.

Different responses to joint exposure to cadmium and zinc depends on the sex in Populus cathayana

The alarming increase in soil contamination by heavy metals, such as cadmium and zinc demands immediate attention. The dioecious tree Populus cathayana, a phytoremediation plant, plays an important role in rehabilitating heavy metal contaminated areas. In this study, male and female P. cathayana plants were treated with Cd (20 mg kg−1) and different levels of Zn (25, 50, or 100 mg kg−1) to study their physiological responses. The results showed that Cd exposure alone caused stress by inhibiting the growth of both male and female plants. In both males and females, photosynthesis and antioxidant enzymes activities decreased substantially under Cd stress alone. Cd was largely located in the roots, but Zn was present in the shoots of both sexes. Zn supplementation considerably increased the photosynthetic rate from 14.62 % to 60.45 % and also enhanced the antioxidant enzymes activities from 24.11 % to 86.21 %. Zn treatment decreased the translocation ability of Cd compared to the Cd-only treatment, alleviating Cd toxicity. In addition, when sufficient Zn was made available, males showed a high degree of Cd accumulation, low root-to-shoot translocation, elevated antioxidant defense abilities, and an increased photosynthetic rate, while females were less responsive to Cd stress than males. Thus, combined exposure to Cd and Zn caused differential responses in plant growth and physiological processes between males and females P. cathayana. Male plants exhibit better Cd tolerance and accumulation capacity under optimum Zn supplementation. This study increases the fundamental knowledge regarding P. cathayana plants, which can be applied to enhance their remediation capacity in Cd-contaminated soils.

Factors affecting in vitro regeneration in the model tree Populus trichocarpa I. Medium, environment, and hormone controls on organogenesis

In preparation for a major GWAS (Genome Wide Association Study) of plant regeneration and transformation, a large number of factors were examined for their effects on indirect regeneration rate in diverse wild genotypes—seeking a high rate of regeneration, but also highly genetically variable and heritable treatments. Many of the factors examined have never before been reported on for their effects on callus, shoot, or root organogenesis in poplar (Populus). Stems had the highest regeneration potential, followed by petioles and leaves, with greenhouse grown explant sources superior to in vitro growth explant sources. Changes of ± 50% to Murashige and Skoog (MS) basal medium salts and micronutrients had a minor effect on regeneration. Many popular treatments that were evaluated also had little to no useful effect at the levels studied, including activated charcoal, ascorbic acid, silver nitrate, melatonin, serotonin, sucrose concentration, and lipoic acid. As a result of this wide exploration, treatment combinations that substantially elevated regeneration in diverse genotypes were identified, enabling GWAS.

A facultative ectomycorrhizal association is triggered by organic nitrogen

Increasing nitrogen (N)deposition often tends to negatively impact the functions of belowground ectomycorrhizal networks, although the exact molecular mechanisms underlying this trait are still unclear. Here, we assess how the root-associated fungus Clitopilus hobsonii establishes an ectomycorrhiza-like association with its host tree Populus tomentosa and how this interaction is favored by organic N over mineral N. The establishment of a functional symbiosis in the presence of organic N promotes plant growth and the transfer of 15N from the fungus to above ground plant tissues. Genomic traits and in planta transcriptional signatures suggest that C.hobsonii may have a dual lifestyle with saprotrophic and mutualistic traits. For example, several genes involved in the digestion of cellulose and hemicellulose are highly expressed during the interaction, whereas the expression of multiple copies of pectin-digesting genes is tightly controlled. Conversely, the nutritional mutualism is dampened in the presence of ammonium (NH4+) or nitrate (NO3-). Increasing levels of NH4+ led to a higher expression of pectin-digesting genes and a continuous increase in hydrogen peroxide production in roots, whereas the presence of NO3- resulted in toxin production. In summary, our results suggest that C.hobsonii is a facultative ectomycorrhizal fungus. Access to various forms of N acts as an on/off switch for mutualism caused by large-scale fungal physiological remodeling. Furthermore, the abundance of pectin-degrading enzymes with distinct expression patterns during functional divergence after exposure to NH4+ or organic N is likely to be central to the transition from parasitism to mutualism.

Populus tremula × P. alba Microshoot Secondary Metabolism Response after Paenibacillus sp. Inoculation In Vitro

Bacterial biostimulants are an eco-friendly alternative to chemical fertilizers. However, before their introduction into open ecosystems, broad-scope studies need to be carried out. Paenibacillus sp. was shown to positively affect poplar root growth. It was hypothesized that alongside these improvements, the Paenibacillus sp. inoculant may affect its host’s secondary metabolism. Populus tremula × P. alba microshoots were inoculated in vitro. Microshoots were tested for chlorophyll, carotenoid, total flavonoid (TFC), total phenol content (TPC) and free radical scavenging capacity during primary growth after 4, 6 and 8 weeks. The results showed that the inoculation decreased shoot phenolics and free radical scavenging capacity after 6 and 8 weeks. Chlorophyll b amounts increased after 6 and 8 weeks. Carotenoid content decreased after 6 weeks, while chlorophyll a and carotenoid levels increased after 8 weeks. Correlation and principal component analyses showed that the inoculant changed the way in which the photosynthesis pigment content relates to TPC, TFC and radical scavenging activity. Overall, these data suggest that the inoculant does statistically significantly affect Populus tree secondary metabolism in the later stages of the initial growth period. This effect may potentially be compensatory in nature.

Limited mitigating effects of elevated CO2 in young aspen trees to face drought stress

Elevated atmospheric CO 2 concentration (eCO 2 ) is expected to mitigate the adverse effects of moderate drought on leaf and whole-tree functioning. However, tree responses to eCO 2 under severe drought and throughout the growing season remain largely unknown. One-year-old Populus tremula L. trees were grown in two controlled treatment chambers under ambient and elevated CO 2 conditions, while progressive drought was imposed early (spring/summer 2019) and late (summer/autumn 2018) during the growing season. Leaf level responses to eCO 2 (i.e., stomatal conductance, leaf carbon assimilation and leaf respiration) were monitored in concert with whole-tree level responses (i.e., canopy conductance, radial stem growth, stem CO 2 efflux, xylem water potential and non-structural carbohydrates (NSC)). At the leaf level, eCO 2 lowered the drought susceptibility of stomatal closure and delayed drought-induced reduction in leaf carbon assimilation during late season drought, but these responses were not observed during the early season drought. Drought effects on whole-tree functioning and NSC depletion remained unaltered by eCO 2 . Under moderate drought, stem volumetric growth ceased earlier than photosynthesis, while leaf and stem respiratory metabolism were maintained at 30 % of well-watered levels even under severe drought, independent of the CO 2 treatment and timing of drought. Therefore, the ability of eCO 2 to mitigate drought was mainly limited to leaf processes during the late season and under moderate drought (> − 2 MPa), while drought offset any beneficial effect of eCO 2 at the whole-tree level. These results urge us to revisit predictions of forests' potential to sequester carbon under climate change scenarios. • Mitigating effects of elevated CO 2 under drought are limited in young Populus trees. • Elevated CO 2 mitigates drought at the leaf level, while whole-tree processes remain unaffected. • Mitigating effects of elevated CO 2 were limited to a late-season drought event.