Biotic and abiotic factors causing the collapse of Robinia pseudoacacia L. veteran trees in urban environments.

Robinia pseudoacacia L. is an interesting example of how one plant species can be considered invasive or useful depending on its environment. In the past this tree species was planted for decorative purposes and for wood in Poland. For many years it was recommended in poor and degraded habitats because it facilitated late-successional plant species. The aim of this study was to verify if black locust can still be regarded as a resistant tree species in urban greenery. The health condition of old tree specimens growing along streets and in parks was compared. The occurrence of pests and pathogens on R. pseudoacacia trees was assessed and the content of mineral elements in leaves was examined. The research results showed that the health of black locust trees growing in the urban environment in Polish cities, especially along streets (in comparison to park sites), deteriorated significantly due to the interaction of harmful biotic and abiotic factors. Increased level of toxic metals (Fe, Zn, Pb, Mn and Cd) in plant tissues and the accumulation of pests and pathogens negatively influenced the health of R. pseudoacacia.

Effects of sodium sulfide application on the growth of Robinia pseudoacacia, heavy metal immobilization, and soil microbial activity in Pb-Zn polluted soil.

Sodium sulfide (Na2S) is usually used as an amendment in industrial sewage treatment. To evaluate the effects of Na2S on the growth of Robinia pseudoacacia (black locust), heavy metal immobilization, and soil microbial activity, the R. pseudoacacia biomass and nutrient content and the soil heavy metal bioavailability, enzyme activity, and arbuscular mycorrhizal (AM) fungal community were measured by a single-factor pot experiment. The Pb-Zn-contaminated soil was collected from a Pb-Zn mine that had been remediated by R. pseudoacacia for five years. Three pollution levels (unpolluted, mildly polluted, and severely polluted) were evaluated by the pollution load index. Na2S application increased the shoot biomass under severe and mild contamination. In soil, Na2S application decreased the bioavailable Pb and Zn contents under severe and mild contamination, which resulted in a decrease in the Pb and Zn content in R. pseudoacacia. However, Na2S application did not affect the total Pb content per plant and enhanced the total Zn content per plant because of the higher biomass of the plants under Na2S application. Increased phosphatase activity and increased available phosphorous content may promote the uptake of phosphorus in R. pseudoacacia. Moreover, Na2S application is beneficial to the diversity of AM fungi under mild and severe pollution. Overall, Na2S application has great potential for enhancing soil heavy metal immobilization, enhancing soil microbial activity, and improving the growth of R. pseudoacacia in polluted soils. Therefore, Na2S is suitable for use in Pb-Zn remediation to ameliorate environmental heavy metal pollution.

The Rpf84 gene, encoding a ribosomal large subunit protein, RPL22, regulates symbiotic nodulation in Robinia pseudoacacia.

MAIN CONCLUSION: A homologue of the ribosomal protein L22e, Rpf84, regulates root nodule symbiosis by mediating the infection process of rhizobia and preventing bacteroids from degradation in Robinia pseudoacacia. Ribosomal proteins (RPs) are known to have extraribosomal functions, including developmental regulation and stress responses; however, the effects of RPs on symbiotic nodulation of legumes are still unclear. Ribosomal protein 22 of the large 60S subunit (RPL22), a non-typical RP that is only found in eukaryotes, has been shown to function as a tumour suppressor in animals. Here, a homologue of RPL22, Rpf84, was identified from the leguminous tree R. pseudoacacia. Subcellular localization assays showed that Rpf84 was expressed in the cytoplasm and nucleus. Knockdown of Rpf84 by RNA interference (RNAi) technology impaired the infection process and nodule development. Compared with the control, root and stem length, dry weight and nodule number per plant were drastically decreased in Rpf84-RNAi plants. The numbers of root hair curlings, infection threads and nodule primordia were also significantly reduced. Ultrastructure analyses showed that Rpf84-RNAi nodules contained fewer infected cells with fewer bacteria. In particular, remarkable deformation of bacteroids and fusion of multiple symbiosomes occurred in infected cells. By contrast, overexpression of Rpf84 promoted nodulation, and the overexpression nodules maintained a larger infection/differentiation region and had more infected cells filled with bacteroids than the control at 45 days post inoculation, suggesting a retarded ageing process in nodules. These results indicate for the first time that RP regulates the symbiotic nodulation of legumes and that RPL22 may function in initiating the invasion of rhizobia and preventing bacteroids from degradation in R. pseudoacacia.

Effect of planting alien Robinia pseudoacacia trees on homogenization of Central European forest vegetation.

Biological homogenization is a process of biodiversity loss driven by the introduction and invasion of widespread species and the extinction of specialized, endemic species. This process has accelerated in recent years due to intensive human activities. We focused our study on large areas of forest vegetation that have not yet been intensively studied. Forest management, especially the planting of alien trees, could play a key role in the homogenization process because alien trees can act as habitat 'transformers' influencing vegetation through creating different environmental conditions. Several types of native forests (hardwood floodplain forests, oak forests, and oak-hornbeam forests) have in many regions been replaced by Robinia pseudoacacia plantations. The huge diversity of native broadleaved deciduous forests in the Pannonian and Carpathian regions, with many local differences and considerable geographical variability, could be exposed to the homogenization process due to the planting of Robinia pseudoacacia. We used 282 paired plots of Robinia pseudoacacia-dominated forests and native forests with a distance of 50-250 m among them under the same environmental conditions to avoid the influence of the variability of local environmental conditions on the forest undergrowth. We found out that the replacement of native forests by plantations of Robinia pseudoacacia plays a crucial role in the homogenization process in forest vegetation by unifying microenvironmental conditions of stands and removing the geographically specified variability of plant communities from previous four classes to single one. The replacement reduced total species pool from 422 to 372 species and supported the occurrence of widespread, generalist plant species in the undergrowth.

Increased nitrogen supply promoted the growth of non-N-fixing woody legume species but not the growth of N-fixing Robinia pseudoacacia.

Nitrogen (N) is an essential macronutrient for plant development and growth, and the deposition of N has increased in recent decades. Legumes that fix N can also provide N for nearby species. However, N in soil inhibits N fixation. We tested the effects of N fertilisation on one N-fixing (Robinia pseudoacacia) and two non-N-fixing (Sophora japonica and Senna surattensis) woody legume species, which were subjected to five different N levels (0, 1.5, 2.9, 5.9 and 11.4 mg N per plant day-1) under greenhouse conditions. The growth of the two non-N-fixing species was promoted by N supply, while that of R. pseudoacacia was unaffected. Among the three species, R. pseudoacacia had the largest specific leaf area and chlorophyll concentration, S. japonica had the largest root-to-shoot ratio and main root-to-lateral root ratio, and S. surattensis had the largest leaf N and phosphorus concentrations. The N-fixing species was mostly unaffected by N supply. The growth, leaf chlorophyll concentration, and leaf number in the non-N-fixing species were promoted by N supply. The N-fixing species showed better growth in low-N environments, while under increased N deposition, its growth was similar to that of the non-N-fixing species.

An introduced plant affects aquatic-derived carbon in the diets of riparian birds.

Non-native plants can impact riparian ecosystem function through diverse terrestrial and aquatic pathways, with cascading effects on food webs. Invasion-mediated vegetation changes can depress terrestrial arthropod communities and alter arthropod flux across the aquatic-terrestrial interface. We investigated the effects of a non-native woody plant, Robinia neomexicana, on insect contributions to riparian songbird diets. This plant was introduced over 100 years ago to the Clear Creek drainage in northwestern Colorado (USA) from its native range, which extends into southern Colorado. We used stable isotope analysis of insects and avian feces to 1) assess whether the relative contributions of aquatic- and terrestrial-derived arthropod prey differed between reference sites and sites invaded by R. neomexicana, and 2) quantify the amount of aquatic- and terrestrial-derived resources consumed by an insectivorous songbird assemblage. Two species of insectivorous songbirds consumed more aquatic insects in invaded sites compared to reference sites. This change in terrestrial- and aquatic-derived prey in bird diets in response to a near-range plant invasion suggests that the introduction of novel species from more distant native ranges could produce similar or stronger effects. Overall, the songbird community consumed approximately 34% aquatic resources, which highlights the importance of these subsidies to riparian consumers. Our investigation of insect subsidies demonstrates how introduced species can indirectly affect food webs and provides insight into the plasticity of riparian consumer responses.

Isolation, characterization, and selection of heavy metal-resistant and plant growth-promoting endophytic bacteria from root nodules of Robinia pseudoacacia in a Pb/Zn mining area.

Multiple heavy metals (HMs) commonly coexist in mining areas, which highlights the necessity to select multiple HM-resistant plant growth-promoting bacteria for improving phytoremediation efficiency. In this study, we isolated and characterized 82 endophytic bacteria from the root nodules of black locust (Robinia pseudoacacia) grown in a Pb-Zn mining area. There were 80 isolates showing resistance to four HMs, 0.01-18.0 mM/L for Cd, 0.2-40.0 mM/L for Zn, 0.3-2.2 mM/L for Pb, and 0.2-1.4 mM/L for Cu. Indole-3-acetic acid production, siderophore production, and 1-aminocyclopropane-1-carboxylate deaminase activity were detected in 43, 50, and 17 isolates, respectively. Two symbiotic isolates selected with the highest potential for HM resistance and PGP traits, designated Mesorhizobium loti HZ76 and Agrobacterium radiobacter HZ6, were evaluated for promotion of plant growth and metal uptake by R. pseudoacacia seedlings grown in pots containing different levels of Cd, Zn, Pb, or Cu. HZ76 significantly increased plant shoot biomass, while HZ6 did not, compared with non-inoculated controls. The results indicate that inoculation with HZ76 or HZ6 relieved HM stress in the plants, depending on the type and concentration of HM in the treatment. Mesorhizobium loti HZ76 may be a better candidate for application in phytoremediation than A. radiobacter HZ6. The microsymbiosis between HM-resistant rhizobia and R. pseudoacacia is an interesting mutualistic system for phytoremediation in mining areas contaminated with multiple HMs.

[Ecological and physiological mechanisms of growth decline of Robinia pseudoacacia plantations in the Loess Plateau of China: A review.] / 黄土高原人工刺槐林生长衰退的生态生理机制.

Robinia pseudoacacia has been widely planted in the Loess Plateau of China for soil and water conservation. The growth decline of R. pseudoacacia plantations has become a recently emerging challenge for the revegetation program and sustainable forest management in this region. As to the scientific definition, identified criteria and quantitative indices have not yet been comprehensively quantified, our current understanding of the ecological and physiological mechanisms for growth decline of R. pseudoacacia plantations is limited. The knowledge could enrich the basic theories of vegetation restoration and benefit the sustainable development of the afforestation project in the Loess Plateau. Through the comprehensive compilation of literatures on forest decline and tree mortality in the Loess Plateau and other regions across the world, this review summarized the mechanisms and recent research progress on growth decline for R. pseudoacacia plantations in the Loess Plateau, primarily demonstrated from ecological (e.g., climatic change, soil desiccation, the imbalance of community structure and the misconduct of forest management) and physiological (e.g., hydraulic failure, carbon starvation, genetic and molecular regulation) perspectives. Finally, we highlighted the research gap with regard to growth decline of R. pseudoacacia plantations in the Loess Plateau.

Experimental arid land afforestation in Central Anatolia, Turkey.

The afforestation of arid lands faces many challenges, and perhaps the most important key for success is choosing one or more species that are adapted well for local environmental conditions. We explored species that would be suitable for the steppe region of Central Anatolia. Intensive site preparation included ripping the subsoil (to 80 cm) and plowing the upper soil before planting seedlings of Elaeagnus angustifolia, Robinia pseudoacacia, Fraxinus angustifolia, and Pinus nigra were used as tree species. We also tested the success of several shrub species: Amygdalus orientalis, Calligonum polygonoides, and Spartium junceum. After five growing seasons, E. angustifolia showed the highest survival, with 80% of planted seedlings remaining. For the shrubs, A. orientalis was the most successful species with a 95% survival rate. Broad-leaved trees grew a cumulative average of 34 cm in height in 5 years, whereas P. nigra seedings grew only 9 cm. The greatest height growth occurred in the shrubs, with A. orientalis gaining 40 cm in height in 5 years. Overall, E. angustifolia and A. orientalis appeared best suited for afforestation in these areas. R. pseodoacacia and F. angustifolia may also be used as alternative species.

Assessing the effect of industrial wastewater on soil properties and physiological and nutritional responses of Robinia pseudoacacia, Cercis siliquastrum and Caesalpinia gilliesii seedlings.

The main aim of this study is to investigate the physical and chemical properties, including soil salinity, acidity, concentrations of macro-nutrients (phosphorus, potassium, and Calcium-Magnesium) and sodium adsorption ratio to the soil, physiological and nutritional traits of three plant species including Caesalpinia gilliesii, Robinia pseudoacacia, and Cercis siliquastrum. First, some sample were taken from the agricultural soils irrigated with wastewater. The results of initial soil test revealed that the irrigation with wastewater significantly increased sodium adsorption ratio (SAR), electrical conductivity (ECe), cation exchange capacity (CEC) of the soil (p < 0.05). Secondly, the effect of industrial wastewater on the responses of three plants were investigated. According to the results, the highest shoot fresh weight was observed in C. gilliesii seedlings treated with T100%, which is 35% higher than the control treatment. The highest concentration of shoot phosphorus in the three plants was respectively 0.54, 0.72, and 1% in those treated with T100% and 0.41, 0.48, and 0.83% in the control treatment. The amount of shoot potassium in the three plants treated with T100% was respectively 0.84, 0.48, and 1%, while it was 0.43, 0.4, and 0.1 in the control treatment, respectively (p < 0.05). According to the current concerns about increased EC, SAR, and Na in C. gilliesii treated with T100%, as compared to the control treatment (50, 386, and 412), and the positive effects of wastewater on soil properties (CEC, pH, and K) and morpho-physiological responses of the plant, it is recommended to use wastewater with continuous monitoring to prevent the pollution of water and soil resource.

How ecosystems change following invasion by Robinia pseudoacacia: Insights from soil chemical properties and soil microbial, nematode, microarthropod and plant communities.

Biological invasions are a global threat to biodiversity. Since the spread of invasive alien plants may have many impacts, an integrated approach, assessing effects across various ecosystem components, is needed for a correct understanding of the invasion process and its consequences. The nitrogen-fixing tree Robinia pseudoacacia (black locust) is a major invasive species worldwide and is used in forestry production. While its effects on plant communities and soils are well known, there have been few studies on soil fauna and microbes. We investigated the impacts of the tree on several ecosystem components, using a multi-trophic approach to combine evidence of soil chemical properties and soil microbial, nematode, microarthropod and plant communities. We sampled soil and vegetation in managed forests, comparing those dominated by black locust with native deciduous oak stands. We found qualitative and quantitative changes in all components analysed, such as the well-known soil nitrification and acidification in stands invaded by black locust. Bacterial richness was the only component favoured by the invasion. On the contrary, abundance and richness of microarthropods, richness of nematodes, and richness and diversity of plant communities decreased significantly in invaded stands. The invasion process caused a compositional shift in all studied biotic communities and in relationships between the different ecosystem components. We obtained clear insights into the effects of invasion of managed native forests by black locust. Our data confirms that the alien species transforms several ecosystem components, modifying the plant-soil community and affecting biodiversity at different levels. Correct management of this aggressive invader in temperate forests is urgently required.

Investigations of microbial degradation of polycyclic aromatic hydrocarbons based on 13C-labeled phenanthrene in a soil co-contaminated with trace elements using a plant assisted approach.

Co-contaminations of soils with organic and inorganic pollutants are a frequent environmental problem. Due to their toxicity and recalcitrance, the heterogeneous pollutants may persist in soil. The hypothesis of this study was that degradation of polycyclic aromatic hydrocarbons (PAHs) is enhanced if heavy metals in soil are immobilized and their bioavailability reduced. For metal immobilization and enhanced biodegradation, distinct mineral and organic soil amendments (iron oxides, gravel sludge, biochar) were deployed in an incubation batch experiment. The second part of the experiment consisted of a greenhouse pot experiment applying fast-growing and pollution-tolerant woody plants (willow and black locust). Soil amendments initially immobilized NH4NO3-extractable zinc, cadmium, and lead; after 100 days of incubation, soil amendments showed reductions only for cadmium and a tendency to enhance arsenic mobility. In order to monitor the remediation success, a 13C-phenanthrene (PHE) label was applied. 13C-phospholipid fatty acid analysis (13C-PLFA) further enabled the identification of PHE-degrading soil microorganisms. Both experiments exhibited a similar PLFA profile. Gram-negative bacteria (esp. cy17:0, 16:1ω7 + 6, 18:1ω7c) were the most significant microbial group taking up 13C-PHE. Plants effectively increased the label uptake by gram-positive bacteria and increased the biomass of the fungal biomarker, although their contribution to the degradation process was minor. Plants tended to prolong PAH dissipation in soil; at the end of the experiment, however, all treatments showed equally low total PAH concentrations in soil. While black locust plants tended not to take up potentially toxic trace elements, willows accumulated them in their leaves. The results of this study show that the chosen treatments did not enhance the remediation of the experimental soil.

Quantitative response relationships between net nitrogen transformation rates and nitrogen functional genes during artificial vegetation restoration following agricultural abandonment.

A comprehensive understanding of how microbial associated with nitrogen (N) cycling respond to artificial vegetation restoration is still lacking, particularly in arid to semi-arid degraded ecosystems. We compared soil net N mineralization rates and the abundance of bacteria, archaea, and eleven N microbial genes on the northern Loess Plateau of China during the process of artificial vegetation restoration. The quantitative relationships between net N mineralization rates and N microbial genes were determined. We observed a significant difference of net transformation rates of NH4+-N (Ra), NO3--N (Rd), and total mineralization (Rm), which rapidly decreased in 10-year soils and steadily increased in the 10-30-year soils. Different N functional microbial groups responded to artificial vegetation restoration distinctly and differentially, especially for denitrifying bacteria. Stepwise regression analysis suggested that Ra was collectively controlled by AOA-amoA and Archaea; Rd was jointly governed by narG, napA, nxrA, and bacreria; and Rm was jointly controlled by napA, narG, nirK, nirS, norB, nosZ, and nxrA.

[Effect of several methods of scarification and osmotic stress on seed germination of Robinia pseudoacacia L.] / Effet de plusieurs méthodes de scarification et du stress osmotique sur la germination des graines de Robinia pseudoacacia L.

The planting of several species adapted to forests areas in the Saharan Atlas would allow one to better fight against the desertification of the vegetation cover of these fragile areas. Thanks to its rapid growth on degraded soils, Robinia pseudoacacia L. has an advantage in the repopulation of these areas undergoing desertification. Operation of this large-scale tree requires good control of germination conditions and growth of plants. The scarification of the seeds of Robinia pseudoacacia L. is necessary to allow the absorption of water by the seeds. Our results show that mechanical scarification with sulphuric acid (75minutes), boiling water (90minutes) and scarification by microwaves (700W) (105seconds) give the best germination rates. The presence of PEG6000 in the imbibition's solutions reduces considerably the germination rate of the seeds of R. pseudoacacia L. A 70 % decrease in the optimal rate of germination is observed when the osmotic pressure of the imbibition solution is at -4.65 bar.

Tolerance or avoidance: drought frequency determines the response of an N2 -fixing tree.

Climate change is increasing drought frequency, which may affect symbiotic N2 fixation (SNF), a process that facilitates ecosystem recovery from disturbance. Here, we assessed the effect of drought frequency on the ecophysiology and SNF rate of a common N2 -fixing tree in eastern US forests. We grew Robinia pseudoacacia seedlings under the same mean soil moisture, but with different drought frequency caused by wet-dry cycles of varying periodicity. We found no effect of drought frequency on final biomass or mean SNF rate. However, seedlings responded differently to wet and dry phases depending on drought frequency. Under low-frequency droughts, plants fixed carbon (C) and nitrogen (N) at similar rates during wet and dry phases. Conversely, under high-frequency droughts, plants fixed C and N at low rates during dry phases and at high rates during wet phases. Our findings suggest that R. pseudoacacia growth is resistant to increased drought frequency because it employs two strategies - drought tolerance or drought avoidance, followed by compensation. SNF may play a role in both by supplying N to leaf tissues for acclimation and by facilitating compensatory growth following drought. Our findings point to SNF as a mechanism for plants and ecosystems to cope with drought.

Plant growth in amended molybdenum mine waste rock.

This greenhouse study examined the use of organic and inorganic soil amendments in waste rock material from the former Questa Molybdenum Mine in northern New Mexico to promote beneficial soil properties. Waste rock material was amended with 11 soil amendment treatments that included municipal composted biosolids, Biosol®, inorganic fertilizer, and two controls (pure waste rock and sand). Elymus trachycaulus and Robinia neomexicana growth performance and plant chemistry were assessed across all treatments over a period of 99 and 141 days, respectively. Even though waste rock material had more than 200 times the molybdenum concentration of native soils, adverse effects were not observed for either species. The two main limiting factors in this study were soil nutritional status and soil water retention. The biosolid amendment was found to provide the greatest buffer against these limiting factors due to significant increases in both nutrition and soil water retention. As a result, both species responded with the highest levels of biomass production and the least amount of required water demands. Use of organic amendments such as biosolids, even though short lived in the soil, may provide plants the necessary growth stimulus to become more resilient to the harsh conditions found on many mine reclamation sites.

De novo sequencing and comparative transcriptome analysis of adventitious root development induced by exogenous indole-3-butyric acid in cuttings of tetraploid black locust.

BACKGROUND: Indole-3-butyric acid (IBA) is applied to the cuttings of various plant species to induce formation of adventitious roots (ARs) in commercial settings. Tetraploid black locust is an attractive ornamental tree that is drought resistant, sand tolerant, can prevent sand erosion and has various commercial uses. To further elucidate the mechanisms of AR formation, we used Illumina sequencing to analyze transcriptome dynamics and differential gene expression at four developmental stages in control (CK) and IBA-treated groups. RESULTS: The short reads were assembled into 127,038 unitranscripts and 101,209 unigenes, with average lengths of 986 and 852 bp. In total, 10,181 and 14,924 differentially expressed genes (DEGs) were detected in the CK and IBA-treated groups, respectively. Comparison of the four consecutive developmental stages showed that 282 and 260 DEGs were shared between IBA-treated and CK, suggesting that IBA treatment increased the number of DEGs. We observed 1,721 up-regulated and 849 down-regulated genes in CI vs. II, 849 up-regulated and 836 down-regulated genes in CC vs. IC, 881 up-regulated and 631 down-regulated genes in CRP vs. IRP, and 5,626 up-regulated and 4,932 down-regulated genes in CAR vs. IAR, of which 25 up-regulated DEGs were common to four pairs, and these DEGs were significantly up-regulated at AR. These results suggest that substantial changes in gene expression are associated with adventitious rooting. GO functional category analysis indicated that IBA significantly up- or down-regulated processes associated with regulation of transcription, transcription of DNA dependent, integral to membrane and ATP binding during the development process. KEGG pathway enrichment indicated that glycolysis/gluconeogenesis, cysteine and methionine metabolism, photosynthesis, nucleotide sugar metabolism, and lysosome were the pathways most highly regulated by IBA. We identified a number of differentially regulated unigenes, including 12 methionine-related genes and 12 ethylene-related genes, associated with the KEGG pathway cysteine and methionine metabolism. The GO enrichment, pathway mapping, and gene expression profile analyses revealed molecular traits for root induction and initiation. CONCLUSION: Our study presents a global view of the transcriptomic profiles of tetraploid black locust cuttings in response to IBA treatment and provides new insights into the fundamental mechanisms associated with auxin-induced adventitious rooting.

Elevated CO2 benefits the soil microenvironment in the rhizosphere of Robinia pseudoacacia L. seedlings in Cd- and Pb-contaminated soils.

Soil contamination by heavy metals in combination with elevated atmospheric CO2 has important effects on the rhizosphere microenvironment by influencing plant growth. Here, we investigated the response of the R. pseudoacacia rhizosphere microenvironment to elevated CO2 in combination with cadmium (Cd)- and lead (Pb)-contamination. Organic compounds (total soluble sugars, soluble phenolic acids, free amino acids, and organic acids), microbial abundance and activity, and enzyme activity (urease, dehydrogenase, invertase, and ß-glucosidase) in rhizosphere soils increased significantly (p < 0.05) under elevated CO2 relative to ambient CO2; however, l-asparaginase activity decreased. Addionally, elevated CO2 alone affected soil microbial community in the rhizosphere. Heavy metals alone resulted in an increase in total soluble sugars, free amino acids, and organic acids, a decrease in phenolic acids, microbial populations and biomass, and enzyme activity, and a change in microbial community in rhizosphere soils. Elevated CO2 led to an increase in organic compounds, microbial populations, biomass, and activity, and enzyme activity (except for l-asparaginase), and changes in microbial community under Cd, Pb, or Cd + Pb treatments relative to ambient CO2. In addition, elevated CO2 significantly (p < 0.05) enhanced the removal ratio of Cd and Pb in rhizosphere soils. Overall, elevated CO2 benefited the rhizosphere microenvironment of R. pseudoacacia seedlings under heavy metal stress, which suggests that increased atmospheric CO2 concentrations could have positive effects on soil fertility and rhizosphere microenvironment under heavy metals.

Effects of salt stress on eco-physiological characteristics in Robinia pseudoacacia based on salt-soil rhizosphere.

Robinia pseudoacacia is the main arbor species in the coastal saline-alkali area of the Yellow River Delta. Because most studies focus on the aboveground parts, detailed information regarding root functioning under salinity is scare. Root traits of seedlings of R. pseudoacacia including morphological, physiological and growth properties under four salinity levels (CK, 1‰, 3‰ and 5‰ NaCl) were studied by the pot experiments to better understand their functions and relationships with the shoots. The results showed that seedling biomass decreased by the reduction of root, stem and leaf biomass with the increase of salinity levels. With increasing salinity levels, total root length (TRL) and total root surface area (TRSA) decreased, whereas specific root length (SRL) and specific root area (SRA) increased. Salt stress decreased root activity (RA) and the maximum net photosynthetic rate (Amax) and increased the water saturation deficit (WSD) significantly in the body. Correlation analyses showed significantly correlations between root morphological and physiological parameters and seedling biomass and shoot physiological indexes. R. pseudoacacia seedlings could adapt to 1‰ salinity by regulating the root morphology and physiology, but failed in 5‰ salinity. How to adjust the water status in the body with decreasing water uptake by roots was an important way for R. pseudoacacia seedlings to adapt to the salt stress.

Growth under elevated air temperature alters secondary metabolites in Robinia pseudoacacia L. seedlings in Cd- and Pb-contaminated soils.

Plant secondary metabolites play a pivotal role in growth regulation, antioxidant activity, pigment development, and other processes. As the global climate changes, increasing atmospheric temperatures and contamination of soil by heavy metals co-occur in natural ecosystems, which alters the pH of rhizosphere soil and influences the bioavailability and mobility of metals. Elevated temperatures in combination with heavy metals are expected to affect plant secondary metabolites, but this issue has not been extensively examined. Here, we investigated secondary metabolites in Robiniapseudoacacia seedlings exposed to elevated temperatures using a passive warming device in combination with Cd- and Pb-contaminated soils. Heavy metals significantly stimulated the accumulation of saponins, phenolic compounds, and flavonoids in leaves and stems; alkaloid compounds increased in leaves and decreased in stems, and condensed tannins fluctuated. Elevated temperatures, alone and in combination with Cd and Pb, caused increases in secondary metabolites in the plant tissues. Phenolic compounds showed the greatest changes among the secondary metabolites and significant interactive effects of temperature and metals were observed. These results suggest that slightly elevated temperature could enhance protective and defense mechanisms of Robinia pseudoacacia seedlings exposed to heavy metals by stimulating the production of secondary metabolites.