Lead tolerance and phytoremediation potential of Brazilian leguminous tree species at the seedling stage

Chinese sapindaceous tree species (Sapindus mukorosii) exhibits lead tolerance and long-term phytoremediation potential for moderately contaminated soils

Heavy metal pollution in metropolitan soils poses significant risks to human health and the entire ecosystem. Effective mitigation strategies and technologies are crucial for addressing these environmental issues. Fast-growing trees are an essential part of phytoremediation projects all over the world and provide long-term ecological benefits to mankind. This study assessed the lead tolerance and phytoremediation potential of a fast-growing soapberry tree species (Sapindus mukorossi) in moderately contaminated soil. Two independent experiments were conducted to assess its tolerance at (i) germination level and (ii) prolonged growth stage. In the germination experiments, seeds were exposed to lead (II) nitrate Pb (NO₃)₂ at various concentrations (0, 5, 10, 20, 50, 100, 200, 300, 400 and 500 μM) for 120 days. Results showed significant differences in germination time, germination index, seedling vigor index, energy of germination, final germination, germination inhibition, seedling height and root/shoot weight compared to the control experiments. In the prolonged growth experiments, seedlings were grown for six months in soils amended/spiked with different Pb concentrations (T0 = 0, T1 = 20, T2 = 50, T3 = 100, T4 = 150 and T5 = 200 mg kg-1 soil) and their biomass was determined. The highest biomass achieved in six months (T0: 12.62 g plant-1), followed by (T1: 12.33 g plant-1), (T2: 12.42 g plant-1), (T3: 11.86 g plant-1), (T4: 10.86 g plant-1) and (T5: 10.06 g plant-1) respectively. S. mukorossi showed no visible signs of Pb toxicity over a six-month period. During six months of exposure, the total Pb content in S. mucrossi tissues were classified as roots > leaves > stems. The highest cumulative absorption of Pb occurred between the fourth and fifth months of exposure. Maximum transfer factor (TF) was detected during the fourth month ranging from 0.888 to 1.012 for the different Pb concentrations. Furthermore, the growth behavior, lead accumulation, bioconcentration factors (BCF) and tolerance index (TI) indicated that S. mucrossi may tolerate moderate Pb concentrations for longer periods. These findings suggest that S. mukorossi may be deployed for long-term phytoremediation coupled with urban forest applications in the future.

Evaluation of heavy metal phytoremediation potential of six tree species of Faisalabad city of Pakistan during summer and winter seasons

Environmental pollution induced by heavy metals has been identified as a leading threat in the modern era. Woody tree species may play a crucial role in the removal of heavy metals from soil and air, thus minimizing pollution potential. The present study was designed to evaluate the phytoremediation potential of six tree species; Azadirachta indica, Cassia fistula, Conocarpus erectus, Eucalyptus camaldulensis, Morus alba, and Populus deltoids, respectively, in the industrial and residential areas of Faisalabad based on the concentrations of lead (Pb), zinc (Zn), cadmium (Cd), and copper (Cu) in their leaves and barks in winter (2018) and summer (2019) seasons. The seasonal contents of heavy metals in both the leaves and barks of these trees decreased in the order of: Zn > Pb > Cu > Cd at both study sites. The highest heavy metal contents were recorded in the leaves and barks of trees grown in the industrial areas as compared to residential areas, with leaves and barks having higher contents of heavy metals in the summer than winter. The tree species exhibited significantly different capacity for heavy metal accumulation, with the accumulation of Cd decreased in the order of: E. camaldulensis > M. alba > C. erectus > A. indica > P. deltoids > C. fistula, and while the order varied for different heavy metals. Overall, M. alba, E. camaldulensis and A. indica performed well in accumulating the targeted heavy metals from the ambient environment. Among the six tree species grown commonly in Faisalabad city, M. alba, E. camaldulensis, and A. indica are recommended for the industrial and residential areas due to their phytoremediation capacity for heavy metals.

Biomonitoring and phytoremediation potential of the leaves, bark, and branch bark of street trees for heavy metal pollution in urban areas

Tree tissues can accumulate heavy metals from the environment. We therefore aimed to evaluate the presence of the metals Pb, Cr, Mn, Cu, and Zn in four street tree species, namely Ailanthus altissima, Broussonetia papyrifera, Pinus tabuliformis, and Rhus typhina, along the highway side of Beijing, China. Sampling from the leaves, trunk bark, and branch annual segment bark of trees was conducted in the summer of 2021, and the concentration of heavy metals was determined. The results revealed the highest average concentration of total heavy metals in the R. typhina leaves (23.724 mg/kg) and barks (14.454 mg/kg). The maximum bio-concentration factor was noted for Zn in the B. papyrifera leaves (0.36) and P. tabuliformis barks (0.21). The maximum comprehensive bio-concentration index was observed for the B. papyrifera leaves (0.225) and P. tabuliformis bark (0.108). The maximum metal accumulation index was measured in the R. typhina leaves (29.682) and bark (12.407). Based on the air-originated metals, P. tabuliformis showed the highest dust collection capacity. In general, B. papyrifera and P. tabuliformis exhibited the highest absorption rate from the soil relative to the other studied species. R. typhina demonstrated the strongest phytoremediation ability for heavy metal pollution in air. In addition, our results proved that the branch annual segment bark of P. tabuliformis is an excellent record carrier that can be used to monitor heavy metal pollution in a specific time duration in an urban area.

Inga uruguensis response to lead: effects on growth and nitrogenous compounds

Abstract Lead (Pb) is a heavy metal considered one of the major soil pollutants. Phytoremediation is a sustainable and economically viable biological method for reducing Pb content in the environment. Inga uruguensis is a tree legume species that has characteristics favorable to phytoremediation, such as rapid growth and high biomass production. The objective of this work was an initial evaluation of tolerance and phytoremediation potential of I. uruguensis to Pb. The experiment was carried out in a greenhouse. In addition to the control, soil contamination was carried out with the following Pb doses: 100, 200, 300, 400, and 500 mg.dm³, with 5 repetitions in each treatment, totaling 30 vases. We assessed growth, number and mass of nodules, chlorophyll content, ureids, amino acid, protein and soluble carbohydrates in leaves, roots and nodules, tolerance index, dry matter, and tissues Pb content of I. uruguensis. The data were analyzed by the Tukey test using R and SISVAR software. There was no negative effect of Pb in soil on I. uruguensis growth, the symbiotic relationship with rhizobia was kept, even at high Pb content and the tolerance index was not lower than 0.69. Inga uruguensis has initial tolerance and potential to be used as phytoremediation in soils contaminated by Pb.

Dynamic study of the lead (Pb) tolerance and accumulation characteristics of new dwarf bamboo in Pb-contaminated soil

Dwarf bamboo is a woody plant with potential for use in the remediation of Pb-contaminated soil. Due to its clonal growth habit, there are two keys to its application for continuous soil Pb remediation: 1) its ability to form shoots and grow into new bamboo normally under Pb stress and 2) the Pb tolerance and accumulation characteristics of this new bamboo. Here, 5 species of dwarf bamboo were treated with 2 levels of soil Pb stress (0 and 1500 mg kg^-1). In the roots of 3 of the species (Sasa argenteostriata, Sasaella glabra, and Indocalamus decorus), Pb tended to be distributed along the cell wall and transported to vacuoles. In the other 2 species (Sasa auricoma and Sasa fortunei), Pb was arranged linearly along the cell wall. Under Pb treatment, the new bamboo of all species showed gradual physiological adaptation to Pb stress. Correlations of the net photosynthetic rate, superoxide dismutase activity, and free proline levels with Pb content in new leaves in November were all higher than those in July, though that of malondialdehyde content decreased, suggesting that new dwarf bamboo exhibits good soil Pb stress tolerance. Sasa argenteostriata and Indocalamus decorus consistently maintained higher antioxidant enzyme activities and free proline levels than the other species under Pb treatment, and the total biomass per pot of the new bamboo decreased the least compared to that in the Pb-free treatment for these two species. Therefore, these bamboo species may be used in the long-term continuous remediation of Pb-contaminated soil.

Lonchocarpus cultratus, a Brazilian savanna tree, endures high soil Pb levels

Industrial revolution markedly increased the environmental contamination by different pollutants, which include the metal lead (Pb). The phytoremediation potential of native species from tropical regions is little known, especially for woody plants. The present study aimed to evaluate the performance of Lonchocarpus cultratus (Fabaceae), a tree species from the Brazilian savanna, grown in soil that was artificially contaminated with increasing Pb concentrations (control and 4 Pb treatments, 56, 120, 180, and 292 mg kg^-1) for 6 months. The biomass of L. cultratus was not depressed by exposure to Pb, despite the high accumulation of this metal (up to 7421.23 μg plant^-1), indicating a high plant tolerance to this trace metal. Lead was mainly accumulated in roots (from 67 to 99%), suggesting that the low root-to-shoot Pb translocation is a plant strategy to avoid Pb-induced damages in photosynthetic tissues. Accordingly, the content of chlorophylls a and b was maintained at similar levels between Pb-treated and control plants. Moreover, increments in leaf area were noticed in Pb-treated plants in comparison to the control plants (on average, 24.7%). In addition, root length was boosted in plants under Pb exposure (22.6-66.7%). In conclusion, L. cultratus is able to endure the exposure to high Pb concentrations in soil, being a potential plant species to be used for Pb phytostabilization in metal-contaminated soils in tropical regions.

Impacts of root pruning and magnetized water irrigation on the phytoremediation efficiency of Celosia argentea

Pot experiments were conducted to evaluate the effects of magnetized water irrigation (100 mT) and root cutting (three pruning intensities) on phytoremediation efficiency of Celosia argentea. In the absence of magnetic field treatment, low root cutting intensity increased the dry weight of the below-ground and aerial parts of C. argentea. Moderate and severe cutting intensities decreased the biomass yield of the plant roots by 11.3% and 31.0%, and increased the dry weight of aerial parts by 75.9% and 27.6%, respectively, alleviating the detrimental effects of these pruning treatments on the plant roots. In the presence of magnetic field treatment, 10% and 25% of pruning treatments increased the dry weight of plant roots by 52.1% and 33.8%, and 33% pruning treatment decreased it by 14.1%. Under both irrigation treatments, low and moderate root cutting strategies did not affect the take up of Cd by the plant roots, while severe cutting decreased it significantly. Enzyme activities decreased with the increment of pruning intensity, and magnetic field can alleviate the negative impact, increasing the capacity of the root pruned species to scavenge the excessive ROS induced by the accumulated Cd. The results showed that root pruning enhanced the phytoremediation efficiency of C. argentea, and this effect was enhanced when combined with magnetized water irrigation.

Mitigation of Pb toxicity by Mn in seedling of the cacao clonal CCN 51 genotype grown in soil: physiological, biochemical, nutritional and molecular responses

Lead (Pb) is a highly toxic metal for humans, animals and plants even at low concentrations in the soil. The ingestion of chocolate produced from contaminated beans can contribute to consumer exposure to Pb. While, Mn is an element essential for plants and participates as enzymatic cofactors in several metabolic pathways. The objective of this study was to evaluate the influence of Mn on mitigation of Pb toxicity in seedling of the cacao clonal CCN 51 genotype grown in soils with different doses of Pb, Mn and Mn+Pb, through physiological, biochemical, molecular and nutritional responses. It was found that the seedling of the cacao clonal CCN 51 genotype grown in soils with high Pb, Mn and Mn+Pb contents accumulated these heavy metals in the roots and leaves. Mn doses reduced the Pb uptake by root system and prevented that the Pb accumulated at toxic levels in the roots and leaves of the plants. High doses of Pb applied in soil were highly toxic to the plants, leading, in some cases, them to death. However, no Mn toxicity was observed in cocoa plants, even at high doses in the soil. Uptake of Pb and Mn by the roots and its transport into the aerial part of the plant promoted changes in photosynthesis, leaf gas exchange, respiration, carboxylation and in the instantaneous efficiency of carboxylation, reducing in the treatments with the highest concentrations of Pb, and the emission of chlorophyll fluorescence, affecting the efficiency of photosystem 2 and the production of photoassimilates. Besides that, Pb, Mn and Mn+Pb toxicities activated defense mechanisms in plants that alter the gene expression of met, psbA and psbO, increasing in plants subjected to high concentrations of Pb and the activity of the enzymes involved in the cellular detoxification of excess ROS at the leaf level. In addition, high uptake of Mn by root system was found to reduced Pb uptake in plants grown with Mn+Pb in the soil. Therefore, application of Mn in the soil can be used to mitigate the Pb toxicity in seedling of the cacao clonal CCN 51 genotype grown in contaminated soils.

Biomass allocation strategies and Pb-enrichment characteristics of six dwarf bamboos under soil Pb stress

Dwarf bamboos are clonal plants with potential applications in the remediation of heavy metal-polluted soils, although their pollution adaptation strategies are unknown. This study examined the biomass allocation strategies and lead (Pb) enrichment characteristics of various dwarf bamboo tissues by the end of the growing season and explored their potential for phytoremediation of Pb stress in the soils. Six dwarf bamboo genotypes were treated with three levels (0, 300, and 1500 mg kg^-1) of soil Pb stress. The majority of the bamboos adopted two biomass allocation strategies to adapt to Pb stress, namely, "reducing biomass allocation into new bamboo growth" and "increasing/stabilizing biomass allocation into rhizomes". Pb accumulation was highest in the roots, rhizomes, and old stems and showed the following trend: rhizomes/old stems> new roots/old roots> old leaves> new leaves> new stems among various tissues. Moreover, the six bamboos used three different Pb-enrichment strategies, as follows: (i) "rhizome domination and old stem synergy" (Sasaella glabra (Nakai) f. albo-striata Muroi, Sasa auricoma (Mitford) E.G. Camus, Sasa fortunei (Van Houtte) Fiori, and Shibataea lanceifolia C.H. Hu); (ii) "old stem domination and rhizome synergy" (Indocalamus decorus Q.H. Dai); and (iii) "old stem domination and new root synergy" (Sasa argenteostriata (Regel) E.G. Camus). In Pb-contaminated soils, genotypes with TFs greater than 1 were Sasa fortunei (Van Houtte) Fiori, Sasa argenteostriata (Regel) E.G. Camus, and Indocalamus decorus Q.H. Dai; in addition, only S. argenteostriata had BCF values greater than 1. Furthermore, this study provides the first evidence that S. argenteostriata can extract 0.22 and 0.58 mgplant^-1 of Pb ions in soil polluted with 300 and 1500 mg kg^-1 Pb, respectively. S. argenteostriata showed the greatest potential for phytoremediation among the bamboo genotypes in both Pb-contaminated urban and mining sites.

Growth and lead uptake by Parkinsonia aculeata L. inoculated with Rhizophagus intraradices

The increased lead (Pb) pollution in the biosphere has resulted in serious environmental problems, so it is essential to evaluate phytoremediation strategies for contaminated soils. This study evaluated the growth and Pd absorption capacity of Pakinsonia aculeata, inoculated with an arbuscular mycorrhizal fungus (Rhizophagus intraradices) over 18 weeks under greenhouse conditions. Treatments included inoculated and non-inoculated plants combined with six Pb concentrations (0, 40, 80, 160, 320, 640 mg·L^-1) in the form of Pb(NO₃)₂. Results showed that mycorrhizal colonization in inoculated plants ranged from 5.0 to 6.7% and favored plant growth. Pb levels and AMF-inoculation had no effects on chlorophyll fluorescence values. AMF-plants absorbed significantly more Pb in roots (237.97 mg·kg^-1) than control plants (202.85 mg·kg^-1), as well as high translocation to shoots (27.02 mg·kg^-1) under the high Pb dose. The increase in Pb concentration reduced the P concentration in roots, and the P and N concentrations in shoots; however, the absorption and translocation of Ca and Mg was increased in shoots. Inoculation of R. intraradices improved both growth and Pb uptake of P. aculeata, under greenhouse conditions suggesting that this tree species may be potentially studied for detoxifying Pb-polluted soils.

Phytoremediation potential of Khaya ivorensis and Cedrela fissilis in copper contaminated soil

Mineral exploration of copper (Cu) in the Amazon has significantly impacted the environment, leading to contamination of large areas that require remediation. Tropical tree species that can immobilize metals and restore plant cover should be selected for phytoremediation programs. The phytoremediation behavior of Khaya ivorensis and Cedrela fissilis was studied in Cu contaminated soil (60, 200, 400, and 600 mg kg^-1). K. ivorensis absorbed extremely high amounts of Cu in the roots (329 mg kg^-1) and excessive amounts in the shoot (52 mg kg^-1), while maintaining similar growth to control plants. C. fissilis seedlings presented a higher Dickson quality index. Bioaccumulation (BCF) and translocation (TF) factors were low in both species, indicating that even with the high amounts of copper absorbed, these contents were lower than the soil concentration (BCF < 1) and that most of Cu was compartmentalized in the roots (TF < 1). The tolerance index of K. ivorensis (>1) and C. fissilis (~1) indicate their ability to grow in Cu contaminated soil. These results suggest that these species could potentially be used as phytoremediators.

Hemp-Based Phytoaccumulation of Heavy Metals from Municipal Sewage Sludge and Phosphogypsum Under Field Conditions

The aim of this study was to evaluate the ability of three hemp cultivars to accumulate heavy metals under sewage sludge (SS) and phosphogypsum (PG) application. The field study was carried out from 2014 to 2016 on Luvisol (loamy sand) in Poland. The experiment scheme included five treatments—T0: the control without fertilization, T1: 170 kg N (nitrogen) ha−1 from sewage sludge, T2: 170 kg N ha−1 from sewage sludge and 100 kg ha−1 of phosphogypsum, T3: 170 kg N ha−1 from sewage sludge and 500 kg ha−1 of phosphogypsum, and T4: 170 kg N ha−1 from sewage sludge and 1000 kg ha−1 of phosphogypsum. It was found that the application of municipal sewage sludge enriched the soil with the bioavailable forms of heavy metals to the greatest extent and contributed to the highest increase in their contents in vegetative and generative organs of hemp plants. These parameters showed a phosphogypsum dose-dependent decline, which could hinder the phytoextraction process. The greatest extractions of heavy metal(loid)s (HMs) from the soil treated with SS and PG were achieved by the Tygra variety, which had the highest bioconcentration factor (BCF) and biomass yield.

Zinc toxicity in seedlings of three trees from the Fabaceae associated with arbuscular mycorrhizal fungi

Due to diverse human activities zinc (Zn) may reach phytotoxic levels in the soil. Here, we evaluated the differential sensibility of three Brazilian tree species from the Fabaceae to increasing soil Zn concentrations and its physiological response to cope with excess Zn. A greenhouse experiment was conducted with the species: Mimosa caesalpiniaefolia, Erythrina speciosa and Schizolobium parahyba, and the addition of 0, 200, 400 and 600 mg Zn kg^-1 to the soil. Plants were harvested after three months of cultivation, and growth, root symbiosis, biochemical markers and elemental composition were analyzed. Soil Zn addition reduced seedling growth, irrespective of the species, with a strong reduction in M. caesalpiniaefolia. Regarding root symbiosis, in N₂-fixing species, nitrogenase activity was reduced by the highest Zn concentrations. Zn addition caused plants nutritional imbalances, mainly in roots. The content of photosynthetic pigments in leaves decreased up to 40%, suggesting that high Zn contents interfered with its biosynthesis, and altered the content of foliar polyamines and free amino acids, depending on the species and the soil Zn concentration. Zn toxicity in M. caesalpiniaefolia plants was observed at available soil Zn concentrations greater than 100 mg kg^-1 (DTPA-extractable), being the most sensitive species and E. speciosa was moderately sensitive. S. parahyba was a moderately tolerant species, which seems to be related to polyamines accumulation and to mycorrhizal association. This last species has the potential for revegetation of areas with moderately high soil Zn concentration and for phytostabilization purposes. Future research evaluating the tolerance to multiple metal stress under field conditions should confirm S. parayba suitability in Zn contaminated areas of tropical regions.

Bioremediation Methods for the Recovery of Lead-Contaminated Soils: A Review

Currently, the pollution of soils by heavy metals is a problem of paramount relevance and requires the development of proper remediation techniques. In particular, lead is a frequently detected soil contaminant that poses adverse effects to the environment and human health. In this review, we provide an overview of the bioremediation treatments promoted by plants (phytoremediation), fungi, or bacteria that could be applied to areas polluted by lead. These restoration processes have the advantage of being environmentally friendly and cost-effective solutions that exploit plants to immobilize and extract contaminants from soil and water, and fungi and bacteria to degrade them. Phytoremediation is an extensively studied and mature practice, with many in-the-field applications where numerous plant species have been employed. In contrast, bioremediation processes promoted by fungi and bacteria are very promising but, up to now, studies have been mostly performed at a laboratory scale with only a few implementations in real-world situations; therefore, further research is needed.

Genotypic variation in growth and lead accumulation among Brassica juncea accessions

Selecting (inter-varietal) Brassica juncea for tolerance to metal-contamination has been proposed as a strategy to develop superior genotypes for phytoextraction of lead (Pb) through selection and breeding techniques. To understand the differences among accessions of a single species to Pb accumulation, a pot experiment was conducted with three B. juncea accessions under levels of Pb added to the soil (0, 90, 180, and 540 mg kg^-1). The duration of the growth period was 100 d. Pb concentration levels did not affect the flowering of B. juncea accessions. Plant height, shoot dry matter, and root dry matter were reduced linearly when soil Pb concentration increased to 540 mg kg^-1. A significant interaction between Pb concentration levels and accessions was observed for Pb concentration in shoots and roots, indicating genotypic variation in Pb absorption. The concentration of Pb in shoots in accession PI 180266 was 51% higher compared to accessions PI 649105 and PI 432379 when soil Pb concentration increased to 540 mg kg^-1. It can be concluded that the B. juncea accessions differed significantly in Pb uptake, and the selection of tolerant cultivars might be helpful for Pb phytoremediation of contaminated soils.

Bioaccumulation of heavy metals air pollutants by urban trees

Leaf and bark of trees are tools for assessing the effects of the heavy metals pollution and monitoring the environmental air quality. In this study, the possibility of using leaves and bark of two urban trees, namely, Ficus nitida and Eucalyptus globulus as a bioindicator of atmospheric pollution was evaluated by determining the composition of heavy elements in the tree leaves, bark, soil, and the atmospheric dust. Two common tree species, namely, F. nitida and E. globulus were selected in the heavily industrial zone of surrounding Minya governorate, Upper Egypt. Two urban areas with heavy traffic load (sites 1 and 2), three industrial zones (sites 3, 4, and 5) and an uncontaminated area as a control were selected (site 6). Sampling from leaf, bark, soil, deposited dust of trees was carried out in winter and summer seasons (from November 2016 to March 2017). The concentrations of heavy metals in dust, soil, leaves, and bark possess the same trend: Pb>Cu>Cd. The highest concentration of cadmium, lead, and copper was found in the leaf of F. nitida and E. globulus higher than bark samples of the studied species, supporting the idea suggesting that tree leaves can be used as a good indicator of heavy metals accumulation. A high and statistically significant correlation (p < .05) was found between Pb concentrations in the atmospheric dust and those in the leaves of both species throughout the two growing seasons, confirming that the main source of incorporated Pb is the atmospheric dust. Otherwise, the obtained results showed that F. nitida tree does not seem to be a good accumulator of Cu. According to the obtained results, F. nitida and E. globulus trees are more likely to capture cadmium and lead from air, so planting these trees in industrial areas with such atmospheric pollutants would be beneficial.

Assisting Phytoremediation of Heavy Metals Using Chemical Amendments

Phytoremediation is one of the safer, economical, and environment-friendly techniques in which plants are used to recover polluted soils, particularly those containing toxic organic substances and heavy metals. However, it is considered as a slow form of remediation, as plants take time to grow and flourish. Various amendments, including the augmentation of certain chemical substances i.e., ethylenediamine tetraacetic acid (EDTA), ethylene glycol tetra acetic acid (EGTA), and sodium dodecyl sulfate (SDS) have been used to induce and enhance the phytoextraction capacity in plants. Several reports show that chemical amendments can improve the metal accumulation in different plant parts without actually affecting the growth of the plant. This raises a question about the amount and mechanisms of chemical amendments that may be needed for potentially good plant growth and metal phytoremediation. This review provides a detailed discussion on the mechanisms undertaken by three important chemical amendments that are widely used in enhancing phytoremediation (i.e., EDTA, EGTA, and SDS) to support plant growth as well as soil phytoremediation. A core part of this review focuses on the recent advances that have been made using chemical amendments in assisting metal phytoremediation.

Growth and accumulation of Pb by roots and shoots of Brassica juncea L

In this study, different soil Pb concentrations [24 (control), 80, 136, 362, and 1150 mg kg^-1] were used to analyze the tolerance threshold and accumulation potential of Brassica juncea L. in a pot experiment under greenhouse conditions. In addition to growth and Pb accumulation, the following contamination indices were calculated: transfer coefficient (TC), translocation factor (TF), and tolerance index (TI). Growth and Pb accumulation were determined at 60 days after emergence. The Pb concentrations were determined using the flame atomic absorption spectrometry (FAAS). The plant height was affected by soil Pb contamination, and it decreased from 1.37 to 0.83 m when the soil Pb concentration increased from 24 (control) to 1150 mg kg^-1, respectively. The Pb concentration in the shoots and roots increased as the Pb concentration in the soil increased, reaching 94 mg kg^-1 in shoots and 783 mg kg^-1 in roots when was grown under 1150 mg kg^-1 of Pb. TF was <1 at all levels of contamination. The TI values suggested that B. juncea presented Pb tolerance in Pb contaminated soils. Our findings indicate that B. juncea has the potential to accumulate Pb in soil under tropical conditions.

Woody species Rhus chinensis Mill. seedlings tolerance to Pb: Physiological and biochemical response

Screening potential plant species is a crucial consideration in phytoremediation technology. Our previous study demonstrated that Rhus chinensis Mill. seedlings had potentials for phytoremediation of Pb contaminated soil. However, its bioaccumulation and tolerance characteristics remain unclear. Seedling growth, LMWOAs secreted by roots, Pb subcellular distribution and chemical forms, and mineral elements in R. chinensis tissues were evaluated under different Pb concentrations (0, 25, 50, 100, 200 and 400 mg/L) in culture solution at 14 days after planting. R. chinensis did not show visual symptoms of Pb toxicity under lower Pb treatments; however, Pb significantly declined the growth of seedlings under higher Pb treatments. Higher Pb stress also decreased the concentrations of nitrogen in leaves, but increased the concentrations of P and K in roots. Pb stress also decreased Mn concentrations in leaves. A great quantity of Pb was uptake and mostly retained in R. chinensis roots. Nonetheless, R. chinensis can still concentrate 459.3 and 1102.7 mg/kg Pb in leaves and stems, respectively. Most of Pb in R. chinensis tissues was stored in the cell wall with HAc-, HCl-, and NaCl-extractable form. LMWOAs secreted by R. chinensis roots showed a strong positive correlation with Pb concentrations in all plant tissues and with P in roots. Our results suggested that Pb deposited in the cell wall and integration with phosphate or oxalate might be responsible for the tolerance of R. chinensis under Pb stress in short period.

Long-term responses of Scots pine (Pinus sylvestris L.) and European beech (Fagus sylvatica L.) to the contamination of light soils with diesel oil

Research into trees plays a very important role in evaluations of soil contamination with diesel oil. Trees are ideal for reclaiming contaminated soils because their large biomass renders them more resistant to higher concentrations of pollutants. In the literature, there is a general scarcity of long-term studies performed on trees, in particular European beeches. The aim of this study was to evaluate the responses of Scots pines and European beeches grown for 8 years on soil contaminated with diesel oil. Selected morphological and physiological parameters of trees were analyzed. The biomass yield of Scots pines was not significantly correlated with increasing concentrations of diesel oil, but it was more than 700% higher than in European beeches. Scots pines were taller and had a larger stem diameter than European beeches during the 8-year study. The diameter of trees grown on the most contaminated soil was reduced 1.5-fold in Scots pines and more than twofold in European beeches. The length of Scots pine needles from the most contaminated treatment decreased by 50% relative to control needles. The shortest needles were heaviest. The fluctuating asymmetry (FA) of needle length was highest in Scots pines grown on the most contaminated soil, whereas the reverse was noted in the FA of needle weight. Diesel oil decreased the concentrations of chlorophylls a and b, total chlorophyll, and carotenoids. The Fv/Fm ratio of needles and leaves was influenced by the tested concentrations of diesel oil. The results of the study indicate that the Scots pine better adapts (grows more rapidly and produces higher biomass) to long-term soil contamination with diesel oil than the European beech. In European beeches, growth inhibition and leaf discoloration (a decrease in chlorophyll content) were observed already after the first year of the experiment, which indicates that 1-year-old seedlings of European beech are robust bioindicators of soil contamination with diesel oil.

Growth, biochemical response and nutritional status of Angico-Vermelho (Parapiptadenia rigida (Bentham) Brenan) under the application of soil amendment in Cu-contaminated soil

Forest species Angico-Vermelho (Parapiptadenia rigida (Bentham) Brenan) is an alternative for the revegetation of areas contaminated with high levels of heavy metals such as copper (Cu). However, excess Cu may cause toxicity to plants, which is why the use of soil amendments can facilitate cultivation by reducing the availability of Cu in the soil. The aim of this study was to assess how the use of amendment can contribute to growth and nutritional status as well as reduce oxidative stress in Angico-Vermelho grown in Cu-contaminated soil. Samples of a Typic Hapludalf soil containing high Cu content were used for the application of four amendments (limestone, organic compost, Ca silicate and zeolite), in addition to a control treatment. The treatments were arranged in a completely randomized design, with four replicates. The use of amendments decreased Cu content available in soil and contributed to improve both plant nutritional status and its antioxidant response expressed by enzymatic activity. The application of the amendments, especially zeolite and Ca silicate, increased dry matter yield of Angico-Vermelho. Thus, the results presented here suggest that the use of amendments contributes to improving Cu-contaminated soils and favors revegetation with Angico-Vermelho.

Cd and Pb accumulation characteristics of phytostabilizer Athyrium wardii (Hook.) grown in soils contaminated with Cd and Pb

Interactions between heavy metals in soil could affect soil heavy metal availability and plant uptake. Thus, in this study, Cd and Pb accumulation as well as plant growth of the mining ecotype (ME) and non-mining ecotype (NME) of Athyrium wardii (Hook.) in response to the exposure of Cd and Pb was investigated by a pot experiment. Although the exposure of Cd in combination with Pb further inhibited the growth of the two ecotypes in comparison with the exposure of single Cd or Pb, the ME presented lower biomass decline for the whole plant (22.0%-70.0%) than the NME among most treatments. The presence of Pb promoted Cd accumulation both in above-ground and under-ground parts of the ME. Cd concentrations in under-ground parts of the ME decreased when exposed to higher concentrations of Pb (> 600 mg kg^-1). Meanwhile, the presence of Cd inhibited Pb accumulation in above-ground parts of the ME and promoted Pb accumulation in under-ground parts of the ME. Pb concentrations in under-ground parts of the ME decreased when soil Cd concentrations were more than 25 mg kg^-1. The partial correlation analysis further demonstrated that the interactions between Cd and Pb stimulated Cd accumulation both in above-ground and under-ground parts of the ME and Pb accumulation in under-ground parts of the ME, while inhibited Pb accumulation in above-ground parts of the ME, showing great benefit for Pb phytostabilization by the ME. Among treatments, the bioaccumulation coefficients for Cd and Pb of the ME, varying from 2.71-31.05 and 20.09-78.06, were much higher than those of the NME. The translocation factors for Cd and Pb of the ME, varying from 0.26-0.52 and 0.01-0.10, were lower than those of the NME. These results indicate that the ME presented greater potential for the phytostabilization of soil contamination with Cd and Pb, especially for Pb.

Potential of trees leaf/ bark to control atmospheric metals in a gas and petrochemical zone

Leaf and bark of trees are tools for assessing the effects of the heavy metals pollution and monitoring the environmental air quality. The aim of this study was to evaluate the presence of Ni, Pb, V, and Co metals in four tree/shrub species (Conocarpus erectus, Nerium oleander, Bougainvillea spectabilis willd, and Hibiscus rosa-sinensis) in the heavily industrial zone of Asaloyeh, Iran. Two industrial zones (sites 1 and 2), two urban areas (sites 3 and 4), and two rural areas (sites 5 and 6) in the Asaloyeh industrial zone and an uncontaminated area as a control were selected. Sampling from leaf and bark of trees was carried out in spring 2016. The metals content in the washed and unwashed leaf and bark was investigated. The results showed that four studied metals in N. oleander, C. erectus, and B. spectabilis willd in all case sites were significantly higher than that of in the control site (p < 0.05). The highest concentration of metals was found in sites 3, 4, and 6; this was due to dispersion of the pollutants from industrial environments by dominant winds. The highest comprehensive bio-concentration index (CBCI) was found in leaf (0.37) and bark (0.12) of N. oleander. The maximum metal accumulation index (MAI) in the samples was found in leaf of N. oleander (1.58) and in bark of H. rosa-sinensis (1.95). The maximum bio-concentration factor (BCF) was seen for cobalt metal in the N. oleander leaf (0.89). The nickel concentration in washed-leaf samples of C. erectus was measured to be 49.64% of unwashed one. In general, the N. oleander and C. erectus species were found to have the highest absorption rate from the atmosphere and soil than other studied species, and are very suitable tools for managing air pollution in highly industrialized areas.

Effects of Oxalic Acid on Arsenic Uptake and the Physiological Responses of Hydrilla verticillata Exposed to Different Forms of Arsenic

A hydroponic experiment was conducted to investigate the effects of oxalic acid (OA) on arsenic (As) uptake and the physiological responses of Hydrilla verticillata exposed to 3 mg L^-1 of As in different forms. Plant As(III) uptake was significantly increased by 200-2000 µg L^-1 OA. However, an increase of As(V) uptake was only shown with 1000 µg L^-1 OA, and no significant difference was observed with dimethylarsinate treatment. Peroxidase and catalase activities, and the contents of photosynthetic pigments, soluble sugar and proline, were significantly increased by 1000 µg L^-1 OA during As(III) treatment. Superoxide dismutase and proline were also increased significantly by 1000 µg L^-1 OA when plants were exposed to As(V). In DMA treatment, proline was significantly increased by 500 µg L^-1 OA. Therefore, As-induced oxidative stress is relieved by OA, but it depends on OA concentration and the form of As. Our results may be useful for the phytoremediation of waste water containing As and OA.

Comparative of Quercus spp. and Salix spp. for phytoremediation of Pb/Zn mine tailings

A pot experiment was conducted to evaluate the feasibility of using tree seedlings for the phytoremediation of lead/zinc (Pb/Zn) mine tailings. Seedlings of three Quercus spp. (Q. shumardii, Q. phellos, and Q. virginiana) and rooted cuttings of two Salix spp. (S. matsudana and S. integra) were transplanted into pots containing 50 and 100 % Pb/Zn mine tailings to evaluate their tolerance of heavy metals. The five species showed different tolerance levels to the Pb/Zn tailings treatments. Q. virginiana was highly tolerant to heavy metals and grew normally in the Pb/Zn tailings. The root systems showed marked differences between the Quercus spp. and Salix spp., indicating that different mechanisms operated to confer tolerance of heavy metals. The maximum efficiency of photosystem II photochemistry value of the five species showed no differences among the treatments, except for Q. shumardii. All species showed low metal translocation factors (TFs). However, S. integra had significantly higher TF values for Zn (1.42-2.18) and cadmium (1.03-1.45) than did the other species. In this respect, Q. virginiana showed the highest tolerance and a low TF, implying that it is a candidate for phytostabilization of mine tailings in southern China. S. integra may be useful for phytoextraction of tailings in temperate regions.

Phytoremediation potential of transplanted bare-root seedlings of trees for lead/zinc and copper mine tailings

Selecting plant species that can overcome unfavorable conditions and increase the recovery of degraded mined lands remains a challenge. A pot experiment was conducted to evaluate the feasibility of using transplanted tree seedlings for the phytoremediation of lead/zinc and copper mine tailings. One-year-old bare-root of woody species (Rhus chinensis Mill, Quercus acutissima Carruth, Liquidambar formosana Hance, Vitex trifolia Linn. var. simplicifolia Cham, Lespedeza cuneata and Amorpha fruticosa Linn) were transplanted into pots with mine tailings and tested as potential metal-tolerant plants. Seedling survival, plant growth, root trait, nutrient uptake, and metal accumulation and translocation were assessed. The six species grew in both tailings and showed different tolerance level. A. fruticosa was highly tolerant of Zn, Pb and Cu, and grew normally in both tailings. Metal concentrations were higher in the roots than in the shoots of the six species. All of the species had low bioconcentration and translocation factor values. However, R. chinensis and L. formosana had significantly higher translocation factor values for Pb (0.88) and Zn (1.78) than the other species. The nitrogen-fixing species, A. fruticosa, had the highest tolerance and biomass production, implying that it has great potential in the phytoremediation of tailing areas in southern China.

An evaluation of EDTA additions for improving the phytoremediation efficiency of different plants under various cultivation systems

Previous studies have shown that phytoremediation usually requires soil amendments, such as chelates, to mobilize low bioavailability heavy metals for better plant absorption and, consequently, for remediation efficiency. A total dry biomass of 3.39 and 0.0138 kg per plant was produced by a phytoremediator, Eucalyptus globulus, and a nitrogen fixing crop, Cicer arietinum (chickpea), respectively. The accumulation of Pb in E. globulus and chickpea reached 1170.61 and 1.33 mg per plant (700 and 324 mg kg(-1)), respectively, under an ethylene diamine tetraacetic acid (EDTA) treatment, which was a five and sixfold increase over the value in untreated experiments, respectively. EDTA enhanced the phytoremediation efficiency and increased the heavy metal concentration in the soil solution. In pot experiments, approximately 27 % of the initial Pb leached from the spiked soil after EDTA and 25 mm artificial precipitation additions into soil without plants, which was considerably larger than the value under the same conditions without EDTA application (7 %). E. globulus planted in a mixed culture had higher water use efficiency than monocultures of either species in field experiments, and E. globulus intercepted almost all of the artificial precipitation in the pot experiments. This study demonstrates that E. globulus can maximize the potential of EDTA for improving the phytoremediation efficiency and minimizing its negative effects to the environment simultaneously by absorbing the metal-rich leachate, especially in a mixed culture of E. globulus and chickpeas.

Magnesium Alleviates Adverse Effects of Lead on Growth, Photosynthesis, and Ultrastructural Alterations of Torreya grandis Seedlings

Magnesium (Mg²⁺) has been shown to reduce the physiological and biochemical stress in plants caused by heavy metals. To date our understanding of how Mg²⁺ ameliorates the adverse effects of heavy metals in plants is scarce. The potential effect of Mg²⁺ on lead (Pb²⁺) toxicity in plants has not yet been studied. This study was designed to clarify the mechanism of Mg²⁺-induced alleviation of lead (Pb²⁺) toxicity. Torreya grandis (T. grandis) seedlings were grown in substrate contaminated with 0, 700 and 1400 mg Pb²⁺ per kg^-1 and with or without the addition of 1040 mg kg^-1 Mg²⁺. Growth parameters, concentrations of Pb²⁺ and Mg²⁺ in the plants' shoots and roots, photosynthetic pigment, gas exchange parameters, the maximum quantum efficiency (Fv/Fm), root oxidative activity, ultrastructure of chloroplasts and root growth were determined to analyze the effect of different Pb²⁺ concentrations on the seedlings as well as the potential ameliorating effect of Mg²⁺ on the Pb²⁺ induced toxicity. All measurements were tested by a one-way ANOVA for the effects of treatments. The growth of T. grandis seedlings cultivated in soils treated with 1400 mg kg^-1 Pb²⁺ was significantly reduced compared with that of plants cultivated in soils treated with 0 or 700 mg kg^-1 Pb²⁺. The addition of 1040 mg kg^-1 Mg²⁺ improved the growth of the Pb²⁺-stressed seedlings, which was accompanied by increased chlorophyll content, the net photosynthetic rate and Fv/Fm, and enhanced chloroplasts development. In addition, the application of Mg²⁺ induced plants to accumulate five times higher concentrations of Pb²⁺ in the roots and to absorb and translocate four times higher concentrations of Mg²⁺ to the shoots than those without Mg²⁺ application. Furthermore, Mg²⁺ addition increased root growth and oxidative activity, and protected the root ultrastructure. To the best of our knowledge, our study is the first report on the mechanism of Mg²⁺-induced alleviation of Pb²⁺ toxicity. The generated results may have important implications for understanding the physiological interactions between heavy metals and plants, and for successful management of T. grandis plantations grown on soils contaminated with Pb²⁺.

Physiological analyses indicate superoxide dismutase, catalase, and phytochelatins play important roles in Pb tolerance in Eremochloa ophiuroides

Phytoremediation is considered to be a promising approach to restore or stabilize soil contaminated by lead (Pb). Turfgrasses, due to their high biomass yields, are considered to be suitable for use in phytoextraction of soil contaminated with heavy metal. It has been demonstrated that centipedegrass (Eremochloa ophiuroides (Munro) Hack., Poaceae) is a good turfgrass for restore of soil contaminated by Pb. However, the enhanced tolerant mechanisms in metallicolous (M) centipedegrass accessions remain unknown. In this study, we made a comparative study of growth performance, Pb accumulation, antioxidant levels, and phytochelatin concentrations in roots and shoots from M and nonmetallicolous (NM) centipedegrass accessions. Results showed that turf quality and growth rate were less repressed in M accessions than in NM accession. Pb stress caused generation of reactive oxygen species in centipedegrass with relatively lower levels in M accessions. Antioxidant activity analysis indicated that superoxide dismutase and catalase played important roles in Pb tolerance in M accessions. M accessions accumulated more Pb in roots and shoots. Greatly increased phytochelatins and less repressed sulfur contents in roots and shoots of M accessions indicated that they correlated with Pb accumulation and tolerance in centipedegrass.

Changes in chemical forms, subcellular distribution, and thiol compounds involved in Pb accumulation and detoxification in Athyrium wardii (Hook.)

Athyrium wardii is one of the dominant plant species flourishing on the Pb-Zn mine tailings in Sichuan Province, China. A greenhouse pot experiment was conducted to evaluate the chemical forms, subcellular distribution, and thiol compounds in A. wardii under different Pb treatments. The results showed that plants of the mining ecotype (ME) of A. wardii were more tolerant to Pb than those of the non-mining ecotype (NME) in spite of accumulation of higher Pb concentrations. The Pb concentrations in shoots and roots of the ME were 3.2∼8.6 times and 3.0∼24.6 times higher than those of the NME, respectively. The ME was more efficient in Pb uptake than the NME. Moreover, 27.8∼39.0% of the total Pb in ME was sodium chloride (NaCl) extractable and 38.0∼48.5% was acetic acid (HAc) extractable, whereas only a minority of total Pb was in ethanol and H2O extractable. In subcellular level, 77.4∼88.8% of total Pb was stored in the cell walls of ME and 9.0∼18.9% in soluble fractions. Increasing Pb concentrations enhanced sequestration of Pb into the cell walls and soluble fractions of ME tissues to protect organelles against Pb. Synthesis of non-protein thiols (NP-SH) and phytochelatins (PCs) in roots of ME significantly enhanced in response to Pb stress, and significant increases in glutathione (GSH) were observed in shoots of ME. Higher levels of NP-SH, GSH, and PCs were observed in roots of the ME comparing with NME, especially under high Pb treatments. The results indicated that Pb was localized mainly in cell wall and soluble fraction of ME plants with low biological activity by cell wall deposition and vacuolar compartmentalization, which might be the important adapted Pb detoxification mechanisms of ME.

Diazotrophs-assisted phytoremediation of heavy metals: a novel approach

Heavy metals, which have severe toxic effects on plants, animals, and human health, are serious pollutants of the modern world. Remediation of heavy metal pollution is utmost necessary. Among different approaches used for such remediation, phytoremediation is an emerging technology. Research is in progress to enhance the efficiency of this plant-based technology. In this regard, the role of rhizospheric and symbiotic microorganisms is important. It was assessed by enumeration of data from the current studies that efficiency of phytoremediation can be enhanced by assisting with diazotrophs. These bacteria are very beneficial because they bring metals to more bioavailable form by the processes of methylation, chelation, leaching, and redox reactions and the production of siderophores. Diazotrophs also posses growth-promoting traits including nitrogen fixation, phosphorous solubilization, phytohormones synthesis, siderophore production, and synthesis of ACC-deaminase which may facilitate plant growth and increase plant biomass, in turn facilitating phytoremediation technology. Thus, the aim of this review is to highlight the potential of diazotrophs in assisting phytoremediation of heavy metals in contaminated soils. The novel current assessment of literature suggests the winning combination of diazotroph with phytoremediation technology.

Enhanced Pb Absorption by Hordeum vulgare L. and Helianthus annuus L. Plants Inoculated with an Arbuscular Mycorrhizal Fungi Consortium

The effect of an arbuscular mycorrhizal fungi (AMF) consortium conformed by (Glomus intraradices, Glomus albidum, Glomus diaphanum, and Glomus claroideum) on plant growth and absorption of Pb, Fe, Na, Ca, and (32)P in barley (Hordeum vulgare L.) and sunflower (Helianthus annuus L.) plants was evaluated. AMF-plants and controls were grown in a substrate amended with powdered Pb slag at proportions of 0, 10, 20, and 30% v/v equivalent to total Pb contents of 117; 5,337; 13,659, and 19,913 mg Pb kg(-1) substrate, respectively. Mycorrhizal root colonization values were 70, 94, 98, and 90%, for barley and 91, 97, 95, and 97%, for sunflower. AMF inoculum had positive repercussions on plant development of both crops. Mycorrhizal barley absorbed more Pb (40.4 mg Pb kg(-1)) shoot dry weight than non-colonized controls (26.5 mg Pb kg(-1)) when treated with a high Pb slag dosage. This increase was higher in roots than shoots (650.0 and 511.5 mg Pb kg(-1) root dry weight, respectively). A similar pattern was found in sunflower. Plants with AMF absorbed equal or lower amounts of Fe, Na and Ca than controls. H. vulgare absorbed more total P (1.0%) than H. annuus (0.9%). The arbuscular mycorrizal consortium enhanced Pb extraction by plants.

Phytoremediation: role of terrestrial plants and aquatic macrophytes in the remediation of radionuclides and heavy metal contaminated soil and water

Nuclear power reactors are operating in 31 countries around the world. Along with reactor operations, activities like mining, fuel fabrication, fuel reprocessing and military operations are the major contributors to the nuclear waste. The presence of a large number of fission products along with multiple oxidation state long-lived radionuclides such as neptunium ((237)Np), plutonium ((239)Pu), americium ((241/243)Am) and curium ((245)Cm) make the waste streams a potential radiological threat to the environment. Commonly high concentrations of cesium ((137)Cs) and strontium ((90)Sr) are found in a nuclear waste. These radionuclides are capable enough to produce potential health threat due to their long half-lives and effortless translocation into the human body. Besides the radionuclides, heavy metal contamination is also a serious issue. Heavy metals occur naturally in the earth crust and in low concentration, are also essential for the metabolism of living beings. Bioaccumulation of these heavy metals causes hazardous effects. These pollutants enter the human body directly via contaminated drinking water or through the food chain. This issue has drawn the attention of scientists throughout the world to device eco-friendly treatments to remediate the soil and water resources. Various physical and chemical treatments are being applied to clean the waste, but these techniques are quite expensive, complicated and comprise various side effects. One of the promising techniques, which has been pursued vigorously to overcome these demerits, is phytoremediation. The process is very effective, eco-friendly, easy and affordable. This technique utilizes the plants and its associated microbes to decontaminate the low and moderately contaminated sites efficiently. Many plant species are successfully used for remediation of contaminated soil and water systems. Remediation of these systems turns into a serious problem due to various anthropogenic activities that have significantly raised the amount of heavy metals and radionuclides in it. Also, these activities are continuously increasing the area of the contaminated sites. In this context, an attempt has been made to review different modes of the phytoremediation and various terrestrial and aquatic plants which are being used to remediate the heavy metals and radionuclide-contaminated soil and aquatic systems. Natural and synthetic enhancers, those hasten the process of metal adsorption/absorption by plants, are also discussed. The article includes 216 references.

Combined effect and mechanism of acidity and lead ion on soybean biomass

Heavy metal pollution and soil acidification are serious global environmental issues. The combined pollution from acidification and heavy metal has become a new environmental issue in regions where the two issues simultaneously occur. However, studies on combined pollution are still limited. In the current study, we investigated the combined effect and mechanism of acidity and heavy metal [lead ion (Pb(2+))] on soybean biomass as well as on growth, nitrogen nutrition, and antioxidant system in soybean roots. Results showed that the combined treatment with acidity and Pb(2+) decreased the soybean biomass. At pH 4.5, the soybean biomass in the combined treatment with acidity and 0.9 mmol L(-1) Pb(2+) was lower than that in the combined treatment with acidity and Pb(2+) at 0.3 or 1.5 mmol L(-1). This result was also observed at pH 3.5 and 3.0. The combined treatment with acidity and Pb(2+) also resulted in the following consequences: root growth inhibition; decrease in nitrate, ammonium, and malondialdehyde contents; increase in nitrite reductase activity; and decrease in peroxidase activity. The extent at which the test indexes decreased/increased in the combined treatment was higher than that in the single acidity treatment. The correlation analysis results indicated that the decrease in the soybean biomass in the combined treatment with acidity and Pb(2+) resulted from the decrease in the root growth, nitrate-nitrogen assimilation, and peroxidase activity.