Poplar Genotypes Research Articles

Combined effects of elevated CO2 and Cd-contaminated soil on the growth, gas exchange, antioxidant defense, and Cd accumulation of poplars and willows

Few studies have explored the combined effects of elevated CO2 (EC) and Cd treatments on poplars and willows. The objective of this study is to determine how growth and phytoremediation efficiency are enhanced by EC. For this purpose, this study investigated the combined effects of EC and Cd treatments on the growth, gas exchange, antioxidant defense, and Cd accumulation in one poplar genotype (Populus×euramericana (Dode) cv. ‘Nanlin-95’ (NL95)) and one willow genotype (Salix jiangsuensis CL. ‘172’ (J172)), which were grown on three Cd-contaminated soil in six open-top chambers. Under Cd treatment, plant growth was decreased, Cd accumulation was increased, and the photosynthesis and malondialdehyde concentration were unchanged in leaves of two tree species. At EC levels for both species, plant growth, total Cd uptake, CO2 assimilation rate, and intrinsic water use efficiency were increased; stomatal conductance and transpiration rate were decreased; and Cd concentrations were unchanged. EC also decreased malondialdehyde content in two species grown in Cd-contaminated soil and increased antioxidant enzymatic activities in J172 grown in high Cd-contaminated soil. At EC, plant growth and total Cd uptake exhibited greater increase in high Cd-contaminated soil than in low Cd-contaminated soil. These findings suggest that EC stimulated plant growth by increasing leaf photosynthesis and enhanced phytoremediation efficiency, particularly at high levels of Cd exposure. EC decreased oxidative damage by stimulating photosynthesis and increasing antioxidant enzyme activities. Cd treatment inhibited the growth of two tree species, and this suppression was unrelated to photosynthesis. Under Cd treatment, the well-maintained photosynthesis is assumed responsible for decreasing reactive oxygen species accumulation and avoiding membrane lipid peroxidation.

Greater efficiency of water use in poplar clones having a delayed response of mesophyll conductance to drought.

Improvement of water use efficiency is a key objective to improve the sustainability of cultivated plants, especially fast growing species with high water consumption like poplar. It is well known that water use efficiency (WUE) varies considerably among poplar genotypes, and it was recently suggested that the use of the mesophyll-to-stomatal conductance ratio (gm/gs) would be an appropriate trait to improve WUE. The responses of 7-week-old cuttings of four hybrid poplar clones and one native Balsam poplar (Populus balsamifera L.) to a water stress-recovery cycle were examined to evaluate the relation between the gm/gs ratio and transpiration efficiency (TE), a leaf-level component of WUE. A contrasting gs response to water stress was observed among the five clones, from stomatal closure early on during soil drying up to limited closure in Balsam poplar. However in the hybrids, the decline in gm was consistently delayed by a few days compared with gs. Moreover, in the most water use-efficient hybrids, the recovery following rehydration occurred faster for gm than for gs. Thus, the delay in the response of gm to drought and its faster recovery upon rewatering increased the gm/gs of the hybrids and this ratio scaled positively with TE. Our results support the use of the gm/gs ratio to select genotypes with improved WUE, and the notion that breeding strategies focusing mainly on stomatal responses to soil drying should also look for a strong curvilinearity between net carbon assimilation rate and gs, the indication of a significant increase in gm/gs in the earlier stages of stomatal closure.

Genotypic variations in the dynamics of metal concentrations in poplar leaves: A field study with a perspective on phytoremediation

Poplar is commonly used for phytoremediation of metal polluted soils. However, the high concentrations of trace elements present in leaves may return to soil upon leaf abscission.To investigate the mechanisms controlling leaf metal content, metal concentrations and expression levels of genes involved in metal transport were monitored at different developmental stages on leaves from different poplar genotypes growing on a contaminated field.Large differences in leaf metal concentrations were observed among genotypes. Whereas Mg was remobilized during senescence, Zn and Cd accumulation continued until leaf abscission in all genotypes. A positive correlation between Natural Resistance Associated Macrophage Protein 1 (NRAMP1) expression levels and Zn bio-concentration factors was observed. Principal component analyses of metal concentrations and gene expression levels clearly discriminated poplar genotypes.This study highlights a general absence of trace element remobilization from poplar leaves despite genotype specificities in the control of leaf metal homeostasis.

First vs. second rotation of a poplar short rotation coppice: Above-ground biomass productivity and shoot dynamics

Within the global search for renewable energy sources, woody biomass from short rotation coppice (SRC) cultures is a valuable option. So far there is a shortage of large-scale field yield data to support stakeholders. We investigated an operational-scale SRC plantation (POPFULL) with 12 poplar genotypes in Flanders during its first two biennial rotations. By inventorying shoot numbers and diameters, combined with allometric relationships, productivity related data were derived after each growing season. We observed significant variation in biomass yield and productivity-related characteristics among the 12 poplar genotypes, of which two recently selected. Genotype Hees (Populus deltoides × Populus nigra) and Skado (Populus trichocarpa × Populus maximowiczii, selected in 2005) reached the highest productivity among genotypes, i.e. 16 Mg ha−1 y−1 of dry matter (DM) yield in the second rotation, which was more than double than the poorest performing genotype Brandaris (a pure P. nigra). However, with many small shoots genotype Hees had a different growth strategy than Skado that resprouted with few, thicker and higher shoots. Biomass production increased from a plantation average of 4.04 Mg ha−1 y−1 of DM in the first (establishment) rotation to 12.24 Mg ha−1 y−1 in the second rotation. Mean height growth raised from 2.08 m y−1 during the first rotation to 2.99 m y−1 during the second rotation. The influence of the first coppicing on tree mortality was negligible. Monitoring of subsequent rotations over the plantations' lifetime – which counts for SRC bioenergy cultures in general – is essential to evaluate productivity in the long term.

Down-regulation of gibberellic acid in poplar has negligible effects on host-plant suitability and insect pest response

Endogenous levels and signaling of gibberellin plant hormones such as gibberellic acid (GA) have been genetically down-regulated to create semi-dwarf varieties of poplar. The potential benefits of semi-dwarf stature include reduced risk of wind damage, improved stress tolerance, and improved wood quality. Despite these benefits, modification of growth traits may have consequences for nontarget traits that confer defense against insect herbivores. According to the growth-differentiation balance hypothesis, reductions in growth may shift allocation of carbon from growth to chemical resistance traits, thereby altering plant defense. To date, host-plant suitability and pest response have not been comprehensively evaluated in GA down-regulated plants. We quantified chemical resistance and nitrogen (an index of protein) in GA down-regulated and wild-type poplar (Populus alba × P. tremula) genotypes. We also evaluated the performance of both generalist (Lymantria dispar) and specialist (Chrysomela scripta) insect pests reared on these genotypes. Our evaluation of resistance traits in four GA down-regulated genotypes revealed increased phenolic glycosides in one modified genotype and reduced lignin in two modified genotypes relative to the non-transgenic wild type. Nitrogen levels did not vary significantly among the experimental genotypes. Generalists reared on the four GA down-regulated genotypes exhibited reduced performance on only one modified genotype relative to the wild type. Specialists, however, performed similarly across all genotypes. Results from this study indicate that although some nontarget traits varied among GA down-regulated genotypes, the differences in poplar pest susceptibility were modest and highly genotype-specific.

Former Land Use and Host Genotype Influence the Mycorrhizal Colonization of Poplar Roots

The present paper analyses the community structure of ectomycorrhiza (ECM) and arbuscular mycorrhiza (AM) fungi associated with seven different poplar clone types growing in a patch system on soil from four different former land use types, originating from spruce forest, poplar stand, grassland and cornfield. We determined the extent to which ECM and AM play a role on the studied factors (genotype, former land use type and host growth). The diversity of ECM and AM fungal communities was estimated by morphological and molecular analyses of the 18S and ITS of the rDNA genes. Fifteen ECM fungal taxa and four AM groups were distinguished in the roots of the poplars grown for 18 months on soil originating from the respective land use types. The poplar clones showed significantly different rates of shoot length and AM colonization, especially concerning the occurrence of Glomus intraradices and Scutellospora sp. Populus deltoides had significantly higher Scutellospora sp. abundance. Although ECM abundance and diversity was high, no significant differences between the different land use types was found. However, some ECM fungi like Paxillus involutus, Laccaria proxima and Laccaria tortilis showed significant preferences for specific land use types. Our findings suggest that both factors, former land use type and poplar genotype, are important determinants of mycorrhizal colonization of the host plants.

Changes in belowground biomass after coppice in two Populus genotypes

Despite the potential of high-density, short-rotation woody biomass plantations to sequester carbon (C) in the soil, few studies have examined their belowground components. We were particularly interested in the biomass allocation patterns after the change from a single-stem (pre-coppice) to a multi-shoot (post-coppice) system, as well as in the fine root (Fr; ∅<2mm) mortality after coppice for their implications in the belowground C cycle. The root system of selected trees from two poplar (Populus spp.) genotypes – Skado and Koster – were excavated for the determination of coarse (Cr, ∅>5mm) and medium-sized (Mr, ∅2–5mm) roots. After two 2-year rotations, the Cr biomass (dry mass, DM) was higher in the P. trichocarpa × P. maximowiczii genotype Skado (187.0gDMm−2) than in the P. deltoides × P. nigra genotype Koster (155.4gDMm−2). Both genotypes showed a relatively shallow, but extensive root system. Allometric equations were fitted between DM of Cr and Mr, and basal area. The root:shoot ratio decreased exponentially with basal area, showing the same trend for single-stem and multi-shoot trees. The soil coring technique was used to determine Fr mass at different dates and at different soil depths. The highest Fr biomass was detected in the upper 15cm of the soil; no genotypic differences in Fr mass were detected at any soil depth. After coppice Fr mortality was significantly increased and weed root biomass significantly reduced. The coppice of the aboveground stems and shoots resulted in a high input of C into the soil and a large amount of C was stored in belowground tree biomass.

Phenotypic plasticity toward water regime: response of leaf growth and underlying candidate genes in Populus.

Phenotypic plasticity is considered as an important mechanism for plants to cope with environmental challenges. Leaf growth is one of the first macroscopic processes to be impacted by modification of soil water availability. In this study, we intended to analyze and compare plasticity at different scales. We examined the differential effect of water regime (optimal, moderate water deprivation and recovery) on growth and on the expression of candidate genes in leaves of different growth stages. Candidates were selected to assess components of growth response: abscisic acid signaling, water transport, cell wall modification and stomatal development signaling network. At the tree scale, the four studied poplar hybrids responded similarly to water regime. Meanwhile, leaf growth response was under genotype × environment interaction. Patterns of candidate gene expression enriched our knowledge about their functionality in poplars. For most candidates, transcript levels were strongly structured according to leaf growth performance while response to water regime was clearly dependent on genotype. The use of an index of plasticity revealed that the magnitude of the response was higher for gene expression than for macroscopic traits. In addition, the ranking of poplar genotypes for macroscopic traits well paralleled the one for gene expression.

Vapour pressure deficit during growth has little impact on genotypic differences of transpiration efficiency at leaf and whole-plant level: an example from Populus nigra L.

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 Populus 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.

Shade tolerance plays an important role in biomass production of different poplar genotypes in a high-density plantation

Poplar (Populus sp.) canopy provides considerable shading in high-density plantations, imposing adverse impacts on crown characteristics relating to biomass production, such as leaf area index (LAI) and photosynthesis. However, it is yet unclear how the LAI, photosynthesis, and biomass production of poplars would respond to shading. In this study, the shading responses were investigated in three poplar genotypes with contrasting biomass performance in a high-density plantation (1667ha−1) established six years prior to the study. A three dimensional laser scanner and a portable photosynthesis system were used to measure the canopy structure and photosynthetic parameters. Principal component analysis (PCA) and the partial least squares method (PLS) were used to determine the importance of measured variables to LAI and biomass. Among all the factors studied, LAI was one of the most related factors to biomass. The genotype with greater shade tolerance ability had higher LAI. Moreover, the high biomass genotype showed a low light compensation point (LCP) and respiration rate (Rd) accompanied with a high ratio of photosynthesis to respiration (Amax/Rd) and a high maximum photosynthetic rate (Amax). This characteristic was more distinct in the middle (M) and lower (L) crown layers than in the upper (U) crown layer. Therefore, high LAI, Amax and Amax/Rd accompanied by low LSP and Rd are the key traits in obtain a high biomass genotype. Thus, improving shade tolerance in leaves of the middle and lower canopy layer to obtain a high LAI and biomass production can be an effective breeding method.

Evaluation of biomass production potential and heating value of hybrid poplar genotypes in a short-rotation culture in Italy

Ensuring increased and sustainable biomass production is critical for European countries. Short-rotation coppice (SRC) plantations on agricultural lands have a great potential to increase biomass supply for biofuels, bioenergy, and bioproducts. In Italy, SRC is based on the use of fast growing species, high planting density, and short harvesting cycles. In this study, the performance of new genotypes to be used in SRC plantations for biomass production was evaluated after three biennial rotations. At the trial plantation in Mira (Northern Italy), six different poplar (Populus) genotypes, belonging to different interspecific hybrids were studied. This plantation provided the opportunity to study the relationships between survival, biomass production and other growth parameters over multiple rotations and for a wide genotypic range. Biomass production differed significantly among rotations starting from 16Mgha−1year−1 in the first, peaking at 20Mgha−1year−1 in the second, and decreasing to 17Mgha−1year−1 in the third rotation. At the end of each rotation, significant differences among genotypes were observed in number of shoots per stool and per ha, stem diameter, tree height, and biomass production. Mean survival rates became significantly different from the other rotations only in the third rotation during which survival rate ranged from 95% for (P.×generosa)×P. nigra ‘Monviso’ to 75% for (P.×generosa)×P. nigra ‘AF6’, but non-significant difference was observed among genotypes. Skewness and inequality of shoot size distributions were genotype-dependent and increased with rotations. Highest biomass production was found for genotypes P.×canadensis ‘83.148.041’, ‘Monviso’ and (P.×generosa)×P. trichocarpa ‘AF8’ with mean annual dry mass production of 21.7, 19.5 and 19.3Mgha−1year−1, respectively. Genotype×rotation interactions were significant on shoot size, diameter and number, but not on survival and biomass production highlighting that genotypes behaved differently over rotations. Moisture and ash content, wood specific gravity, and higher heating value were determined at the end of the third biennial rotation, and significant differences among genotypes were found. This study is critical for investigating the behavior of novel poplar genotypes with potential for commercial biomass production over multiple coppice rotations.

Carbon isotope compositions (δ13C) of leaf, wood and holocellulose differ among genotypes of poplar and between previous land uses in a short‐rotation biomass plantation

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 (δ(13) 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 δ(13) Cwood and δ(13) Cholocellulose were tightly and positively correlated, but the offset varied significantly among genotypes (0.79-1.01‰). Leaf phenology was strongly correlated with δ(13) 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 δ(13) C. Trees growing on former pasture with higher N-availability displayed higher δ(13) C as compared with trees growing on former cropland. The positive relationships between δ(13) Cleaf 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 δ(13) C 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.

Seasonal variations in photosynthesis, intrinsic water-use efficiency and stable isotope composition of poplar leaves in a short-rotation plantation.

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.

Comparative resource-use efficiencies and growth ofPopulus trichocarpaandPopulus balsamiferaunder glasshouse conditions

Amongst other traits, ideal poplar genotypes for afforestation programs would be fast growing and have high resource-use efficiencies. Black cottonwood (Populus trichocarpa Torr. &amp; 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 (gs), intrinsic water-use efficiency (A/gs), photosynthetic nitrogen-use efficiency (PNUE), leaf and stemwood13C/12C isotope ratios (δ13C), 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 gsand Ds, but higher A/gsand δ13C. Variation within provenances, in most traits, exceeded variation between species or provenances. δ13C and A/gswere highly correlated across all genotypes. Variation in A/gsseemed primarily related to gs, although positive correlations were found between δ13C 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.

Physiological adaptations of five poplar genotypes grown under SRC in the semi-arid Mediterranean environment

Key message The genotype ‘Neva’ under high plant density showed the highest biomass yield and optimal physiological strategies and could be the most suitable choice under semi-arid environment

Resistant poplar genotypes inhibit pseudogall formation by the wooly poplar aphid, Phloeomyzus passerinii Sign

Phloeomyzus passerinii can induce a pseudogall within the bark of susceptible poplar genotypes, while in resistant genotypes the induction seems to be impeded by lignification processes. Phloeomyzus passerinii is a major pest of poplar stands in Europe, North Africa and the Near East. This aphid feeds in the cortical parenchyma of different poplar species and hybrids, and can affect their growth and survival through unknown mechanisms. In some genotypes, however, resistance prevents either the settlement or the development of aphid colonies. For a better understanding of tree reactions to aphid probing, we compared the anatomical and biochemical modifications undergone within the bark of stem cuttings, after different delays of either aphid colonization or mechanical wounding. To assess how resistance may modulate tree reactions, the comparison was performed using three poplar genotypes exhibiting different resistance levels. In these three genotypes, mechanical wounding induced a similar, localized, wound periderm. In contrast, aphid colonization triggered more extended reactions, which differed among genotypes. In the susceptible genotype, aphids induced a reaction tissue, characterized after a month by thin-walled hypertrophied cells and a depletion of soluble phenolic compounds and starch. Anatomical features of this reaction tissue suggest that the aphid initiates a pseudogall in the cortical tissues of its susceptible host. In the resistant genotypes, however, the differentiation of the reaction tissue was totally or partially inhibited, probably because of extended lignification processes. The implications of a pseudogall induction on susceptible hosts’ physiology, and the impact of lignification on aphid development and behavior, are discussed.

Microsatellite analysis of Icelandic populations of the poplar fungal pathogen Melampsora larici-populina shows evidence of repeated colonization events

The basidiomycete Melampsora larici-populina causes foliar rust on Populus species from the sections Aigeiros and Tacamahaca, causing reduction in biomass production and economic losses. In the present study, samples of Icelandic M. larici-populina were collected for analysis of genetic diversity and population structure. A total of 439 isolates, collected at 15 locations, and analysed using 22 microsatellite markers were compared to data from French M. larici-populina populations. Twenty-one of the loci analysed were polymorphic, with an average of 3.4 alleles per locus. The mean observed and expected heterozygosities for all populations were 0.35 and 0.38. Evidence was found for a substructure within the Icelandic population with three subpopulations being the most likely scenario with low levels of gene flow. The population structure seen here is most likely shaped by both isolation and genetic drift as well as repeated events of colonization. In the future it can therefore be expected that regional poplar rust genotypes in Iceland change by two different modes; on one hand by transport of spores within the country and on the other hand by repeated colonization events. The results reported here underline the importance of closely monitoring the development of fungal diseases in Iceland, and to carefully select for resistance in Icelandic plant breeding programs.

Seasonal and clonal variations of microbial biomass and processes in the rhizosphere of poplar plantations

Plant nutrient acquisition and dynamics are closely coupled with microbial activities and rhizosphere processes. This study was conducted using an in situ rhizobox approach and incubation experiments, with the primary goal to elucidate rhizosphere effects of different poplar clones at different sampling seasons on soil microbial processes and biomass content. Evaluations included pH, inorganic N (NO3−-N and NH4+-N), microbial biomass carbon (MBC) and nitrogen (MBN), enzyme activities related to N cycling, as well as net N mineralization and nitrification rates in rhizosphere soils of three poplar clones (Nanlin-895, NL-80351 and 75) at three periods of the year. The magnitude of rhizosphere effects varied with time and poplar genotypes. Significant variations in microbial biomass content and enzyme activities were observed among the tested three poplar clones; and microbial biomass content was positively correlated with soil NO3−-N concentration. The obtained results suggested that genotypes (clones or cultivars) had profound impacts on the rhizosphere microbial communities; and NO3−-N concentration rather than total inorganic N was the primary determinant of seasonal dynamics of microbial biomass and processes in the rhizosphere soils of poplar trees in a seasonally flooded soil.

Distinct responses to ozone of abaxial and adaxial stomata in three Euramerican poplar genotypes

Ozone induces stomatal sluggishness, which impacts photosynthesis and transpiration. Stomatal responses to variation of environmental parameters are slowed and reduced by ozone and may be linked to difference of ozone sensitivity. Here we determine the ozone effects on stomatal conductance of each leaf surface. Potential causes of this sluggish movement, such as ultrastructural or ionic fluxes modification, were studied independently on both leaf surfaces of three Euramerican poplar genotypes differing in ozone sensitivity and in stomatal behaviour. The element contents in guard cells were linked to the gene expression of ion channels and transporters involved in stomatal movements, directly in microdissected stomata. In response to ozone, we found a decrease in the stomatal conductance of the leaf adaxial surface correlated with high calcium content in guard cells compared with a slight decrease on the abaxial surface. No ultrastructural modifications of stomata were shown except an increase in the number of mitochondria. The expression of vacuolar H(+) /Ca(2+) -antiports (CAX1 and CAX3 homologs), β-carbonic anhydrases (βCA1 and βCA4) and proton H(+) -ATPase (AHA11) genes was strongly decreased under ozone treatment. The sensitive genotype characterized by constitutive slow stomatal response was also characterized by constitutive low expression of genes encoding vacuolar H(+) /Ca(2+) -antiports.

Ozone affects ascorbate and glutathione biosynthesis as well as amino acid contents in three Euramerican poplar genotypes

Ozone is an air pollutant that causes oxidative stress by generation of reactive oxygen species (ROS) within the leaf. The capacity to detoxify ROS and repair ROS-induced damage may contribute to ozone tolerance. Ascorbate and glutathione are known to be key players in detoxification. Ozone effects on their biosynthesis and on amino acid metabolism were investigated in three Euramerican poplar genotypes (Populus deltoides Bartr. × Populus nigra L.) differing in ozone sensitivity. Total ascorbate and glutathione contents were increased in response to ozone in all genotypes, with the most resistant genotype (Carpaccio) showing an increase of up to 70%. Reduced ascorbate (ASA) concentration at least doubled in the two most resistant genotypes (Carpaccio and Cima), whereas the most sensitive genotype (Robusta) seemed unable to regenerate ASA from oxidized ascorbate (DHA), leading to an increase of 80% of the oxidized form. Increased ascorbate (ASA + DHA) content correlated with the increase in gene expression in its biosynthetic pathway, especially the putative gene of GDP-l-galactose phosphorylase VTC2. Increased cysteine availability combined with increased expression of γ-glutamylcysteine synthetase (GSH1) and glutathione synthetase (GSH2) genes allows higher glutathione biosynthesis in response to ozone, particularly in Carpaccio. In addition, ozone caused a remobilization of amino acids with a decreased pool of total amino acids and an increase of Cys and putrescine, especially in Carpaccio. In addition, the expression of genes encoding threonine aldolase was strongly induced only in the most tolerant genotype, Carpaccio. Reduced ascorbate levels could partly explain the sensitivity to ozone for Robusta but not for Cima. Reduced ascorbate level alone is not sufficient to account for ozone tolerance in poplar, and it is necessary to consider several other factors including glutathione content.