The short-term effect of cadmium on low molecular weight organic acid and amino acid exudation from mangrove (Kandelia obovata (S., L.) Yong) roots

In situ phytoextraction of Mn and NH4+-N from aqueous electrolytic manganese residue solution by Pistia stratiotes: Effects of Fe/Co presence and rhizospheric microbe synergistic involvement

The electrolytic manganese industry produces a large amount of electrolytic manganese residue (EMR). Soluble Mn, NH4+-N, and other pollutants may be released from the open-air stacked EMR and transported to the environment along with rainfall or surface runoff. Aqueous EMR solution (AES) generally contains various elements required for plant growth, and phytoremediation can be applied to remove these pollutants from AES. Since the contents of Fe and Co vary greatly in AES depending on the ore sources as well as the pre-treatment processes, the presence of bioavailable Fe and Co at different levels may affect plant growth, the rhizosphere microbes, and pollutant removal. The present study investigated the in-situ removal of Mn(II) and NH4+-N from AES solution using free floating aquatic plant Pistia stratiotes, focusing especially on the effects of Fe/Co presence and rhizospheric microbe synergistic involvement on contaminant removal. The results showed that 69.08% of Mn and 94.99% of NH4+-N were removed by P. stratiotes in 24 d. Both the presence of Fe(II) and Co(II) facilitated the Mn(II) immobilization and increased Mn(II) removal by 19-31% due to the enhanced peroxidase activity and the increased Mn accumulating in roots The complete removal of Mn from AES was found in the presence of Fe(II) at 2 mg L-1 or Co(II) at 0.5 mg L-1 and more than 51% accumulated Mn in the roots was stored in the vacuole and cytoplasm. BioMnOx was found on the surface of the roots, revealing that rhizofiltration, rhizospheric plaque/biofilm formation, and Mn biogeochemical cycle exert synergic effects on Mn(II) immobilization. The findings of the present study demonstrate the feasibility of using P. stratiotes in the treatment of aqueous EMR solutions and the presence of an appropriate amount of bio-available Fe and Co can promote the removal of Mn(II) and NH4+-N.

Salinity alleviates arsenic stress-induced oxidative damage via antioxidative defense and metabolic adjustment in the root of the halophyte Salvadora persica

MAIN CONCLUSION

Arsenic tolerance in the halophyte Salvadora persica is achieved by enhancing antioxidative defense and modulations of various groups of metabolites like amino acids, organic acids, sugars, sugar alcohols, and phytohormones. Salvadora persica is a facultative halophyte that thrives under high saline and arid regions of the world. In present study, we examine root metabolic responses of S. persica exposed to individual effects of high salinity (750 mM NaCl), arsenic (600 µM As), and combined treatment of salinity and arsenic (250 mM NaCl + 600 µM As) to decipher its As and salinity resistance mechanism. Our results demonstrated that NaCl supplementation reduced the levels of reactive oxygen species (ROS) under As stress. The increased activities of antioxidant enzymes like superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPX), and glutathione reductase (GR) maintained appropriate levels of ROS [superoxide (O2•-) and hydrogen peroxide (H2O2)] under salinity and/or As stress. The metabolites like sugars, amino acids, polyphenols, and organic acids exhibited higher accumulations when salt was supplied with As. Furthermore, comparatively higher accumulations of glycine, glutamate, and cystine under combined stress of salt and As may indicate its role in glutathione and phytochelatins (PCs) synthesis in root. The levels of phytohormones such as salicylate, jasmonate, abscisic acid, and auxins were significantly increased under high As with and without salinity stress. The amino acid metabolism, glutathione metabolism, carbohydrate metabolism, tricarboxylic acid cycle (TCA cycle), phenylpropanoid biosynthesis, and phenylalanine metabolism are the most significantly altered metabolic pathways in response to NaCl and/or As stress. Our study decoded the important metabolites and metabolic pathways involved in As and/or salinity tolerance in root of the halophyte S. persica providing clues for development of salinity and As resistance crops.

Insights into heavy metal tolerance mechanisms of Brassica species: physiological, biochemical, and molecular interventions

Heavy metal (HM) contamination of soil due to anthropogenic activities has led to bioaccumulation and biomagnification, posing toxic effects on plants by interacting with vital cellular biomolecules such as DNA and proteins. Brassica species have developed complex physiological, biochemical, and molecular mechanisms for adaptability, tolerance, and survival under these conditions. This review summarizes the HM tolerance strategies of Brassica species, covering the role of root exudates, microorganisms, cell walls, cell membranes, and organelle-specific proteins. The first line of defence against HM stress in Brassica species is the avoidance strategy, which involves metal ion precipitation, root sorption, and metal exclusion. The use of plant growth-promoting microbes, Pseudomonas, Psychrobacter, and Rhizobium species effectively immobilizes HMs and reduces their uptake by Brassica roots. The roots of Brassica species efficiently detoxify metals, particularly by flavonoid glycoside exudation. The composition of the cell wall and callose deposition also plays a crucial role in enhancing HMs resistance in Brassica species. Furthermore, plasma membrane-associated transporters, BjCET, BjPCR, BjYSL, and BnMTP, reduce HM concentration by stimulating the efflux mechanism. Brassica species also respond to stress by up-regulating existing protein pools or synthesizing novel proteins associated with HM stress tolerance. This review provides new insights into the HM tolerance mechanisms of Brassica species, which are necessary for future development of HM-resistant crops.

Role of Root Exudates in Cadmium Accumulation of a Low-Cadmium-Accumulating Tobacco Line (Nicotiana tabacum L.)

Root exudates are tightly linked with cadmium (Cd) uptake by the root and thus affect plant Cd accumulation. A hydroponic experiment was carried out to explore the role of root exudates in Cd accumulation of a low-Cd-accumulating tobacco line (RG11) compared with a high-Cd- accumulating tobacco line (Yuyan5). Greater secretion of organic acids and amino acids by the roots was induced by an exogenous Cd addition in the two tobacco lines. The concentration of organic acid secreted by RG11 was only 51.1~61.0% of that secreted by Yuyan5. RG11 roots secreted more oxalic acid and acetic acid and less tartaric acid, formic acid, malic acid, lactic acid, and succinic acid than Yuyan5 under Cd stress. Oxalic acid accounted for 26.8~28.8% of the total organic acids, being the most common component among the detected organic acids, and was significantly negatively correlated with Cd accumulation in RG11. Propionic acid was only detected in the root exudates of RG11 under Cd stress. Lactic acid was positively linked with Cd accumulation in Yuyan5, being less accumulated in RG11. Similarly, RG11 secreted more amino acids than Yuyan5 under Cd stress. Aspartic acid, serine, and cysteine appeared in RG11 when it was exposed to Cd. Lysine was the most secreted amino acid in RG11 under Cd stress. RG11 roots secreted less lysine, histidine, and valine, but more phenylalanine and methionine than Yuyan5 under Cd stress. The results show that organic acids and amino acids in root exudates play a key role in Cd uptake by the root, and this contribution varied with cultivar/genotype. However, further research is still needed to explore the mechanisms underlying low Cd translocation to the leaf, which may be the key contribution of low Cd accumulation in RG11 to the security of tobacco leaf.

Effects of cadmium and flooding on the formation of iron plaques, the rhizosphere bacterial community structure, and root exudates in Kandelia obovata seedlings

In the rhizosphere, plant root exudates (REs) serve as a bridge between plant and soil functional microorganisms, which play a key role in the redox cycle of iron (Fe). This study examined the effects of periodic flooding and cadmium (Cd) on plant REs, the rhizosphere bacterial community structure, and the formation of root Fe plaques in the typical mangrove plant Kandelia obovata, as well as the relationship between REs and Fe redox cycling bacteria. Based on two-way analysis of variance, flooding and Cd had a considerable effect on the REs of K. obovata. DOC, NH₄⁺-N, NO₃^--N, dissolved inorganic phosphorus, acetic acid, and malonic acid concentrations in REs of K. obovata increased considerably with the increase of Cd concentration under 5 and 10 h flooding conditions. Fe plaque development in the plant root was stimulated by flooding and Cd, although flooding was more effective. After Cd treatment, the ways in which Fe-oxidizing bacteria (FeOB) and Fe-reducing bacteria (FeRB) were enriched in the rhizosphere and rhizoplane of plants were different. Thiobacillus and Sideroxydans (dominant FeOB) were more abundant in the plant rhizosphere, whereas Acinetobacter (dominant FeRB) was more abundant in the rhizoplane. Cd considerably decreased the relative abundance of unclassified_f_Gallionellaceae in the rhizosphere and rhizoplane but dramatically enhanced the relative abundance of Thiobacillus, Shewanella, and unclassified_f_Geobacteraceae. Unclassified_f_Geobacteraceae and Thiobacillus exhibited substantial positive correlations with citric acid and DOC in REs in the rhizosphere and rhizoplane but strong negative correlations with Sideroxydans. The findings indicate that Cd and flooding treatments may play a role in the production and breakdown of Fe plaque in K. obovata roots by affecting the relative abundance of Fe redox cycling bacteria in the rhizosphere and rhizoplane.

Bioaccumulation and physiological changes in the fruiting body of Agaricus bisporus (Large) sing in response to cadmium

The bioremediation of heavy metals contaminated soils with macrofungi is a new and promising approach; hence Agaricus bisporus (Large) sing has potentially shown accumulating ability to Cd contamination. This study focused on the tolerance response by A. bisporus to different contents of Cd in the closed cup and the flat stage of fruiting body development. The contents of Cd, soluble protein, sugar, low molecular weight organic acids (LMWOAs), and antioxidant activity were investigated. The bioaccumulation factor and transfer factor results revealed that Cd accumulated in the cap of A. bisporus more than that in the stipe with the highest content being 18.38 mg kg^-1 dry weight at the closed cup stage under 414.28 mg kg^-1 Cd stress. High Cd content stress increased soluble protein, proline, and malonaldehyde contents at both stages; while higher peroxidase, catalase, ascorbic acid peroxidase activities, and LMWOAs contents were only recorded at the closed cup stage. On the other hand, Superoxide dismutase activities and soluble sugar content showed a complex trend. Overall, these results have successfully established that A. bisporus could resort to modulating its metabolism to avoid the destructive effects of Cd stress and could successfully accumulate Cd in the soil, which is a promising prospect for the remediation of Cd-contaminated soils.

Effects of exogenous organic acids and flooding on root exudates, rhizosphere bacterial community structure, and iron plaque formation in Kandelia obovata seedlings

The rhizosphere of coastal wetland plants is the active interface of iron (Fe) redox transformation. However, coupling mechanism between organic acids (OAs) exuded by plant roots and Fe speciation transformation participated by Fe redox cycling bacteria in the rhizosphere is still unclear. Effects of four common OAs (citric acid, malic acid, tartaric acid, and oxalic acid) on root exudation, rhizosphere bacterial community structure, root Fe plaque, and Fe redox cycling bacterial communities of Kandelia obovata were investigated in this study. Long-term flooding (10 h) was conducive to K. obovata seedlings exuding additional dissolved organic carbon (DOC) and nitrogen and phosphorus organic matter (NH₄⁺-N, NO₃^--N, and dissolved inorganic phosphorus [DIP]) under each OA level. DOC, NH₄⁺-N, NO₃^--N, and DIP in root exudates increased significantly with the increase of exogenous OA level. Notably, long flooding time corresponds to an evidently increasing trend. Exogenous OAs also significantly increased contents of formic and oxalic acids in root exudates. Exogenous OAs and flooding enhanced the rhizosphere effect of K. obovata and significantly enhanced bacterial diversity of the rhizosphere and relative abundance of dominant bacteria in rhizoplane. Bacterial diversity in the rhizosphere of K. obovata seedlings was significantly higher than that in the rhizoplane under the same level of OAs and flooding. Fe plaque content of K. obovata root decreased significantly and the relative abundance of typical Fe-oxidizing bacteria, such as Gallionella, unclassified_f__Gallionellaceae, and Sideroxydans, decreased significantly in the rhizosphere but increased significantly in the rhizoplane with the increase of the treatment level of exogenous OAs. This finding is likely due to the Fe³⁺ reduction caused by acidification of rhizosphere environment after exogenous OA treatment rather than the result of chemotactic colonization of Fe redox cycling bacteria in the rhizoplane.

Low-Altitude Boundary of Abies faxoniana Is More Susceptible to Long-Term Open-Top Chamber Warming in the Eastern Tibetan Plateau

With global climate change, for evaluating warming effect on subalpine forest distribution, the substantial effects of long-term warming on tree growth and soil nutrients need to be explored. In this study, we focused on different responses in the boundaries of trees and soils to warming. Using the open-top chamber (OTC), a 10-year artificial warming experiment was conducted to evaluate the impacts of warming on Abies faxoniana at three different altitudes. We determined metabolites and nutrient concentrations in needles of A. faxoniana and characterized the soil chemistries. Many kinds of sugars, amino acids, and organic acids showed higher contents at high altitude (3,500 m) compared with low altitude (2,600 m), which could have been due to the temperature differences. Warming significantly decreased needle sugar and amino acid concentrations at high altitude but increased them at low altitude. These results indicated contrasting physiological and metabolic responses of A. faxoniana to long-term warming at different altitudes. Furthermore, we found that OTC warming significantly increased the concentrations of soil extractable sodium, aluminum (Al), and manganese (Mn), while decreased potassium (K) and phosphorus (P) concentrations and pH values at low altitude rather than at middle (3,000 m) or high altitude. The soil carbon and nitrogen contents were increased only at the middle altitude. In A. faxoniana at low altitudes, more mineral nutrients iron, K, and P were demand, and a mass of Al, Mn, and zinc was accumulated under warming. Soil P limitation and heavy metals accumulation are disadvantageous for trees at low altitudes with warming. Therefore, compared with high altitudes, A. faxoniana growing at low boundary in alpine regions is expected to be more susceptible to warming.

Silicon Nanoparticles Improve the Shelf Life and Antioxidant Status of Lilium

The production of ornamentals is an economic activity of great interest, particularly the production of Lilium. This plant is very attractive for its color and shapes; however, the quality of its flower and its shelf life can decrease very fast. Therefore, it is of the utmost importance to develop techniques that allow for increasing both flower quality and shelf life. Nanotechnology has allowed for the use of various materials with unique characteristics. These materials can induce a series of positive responses in plants, among which the production of antioxidant compounds stands out. The objective of this study was to determine the impact of the application of silicone nanoparticles (SiO2 NPs) on the quality, shelf life, and antioxidant status of Lilium. For this, different concentrations of SiO2 NPs (0, 200, 400, 600, 800, and 1000 mg L-1) were applied in two ways, foliar and soil, as two independent experiments. The contents of enzymatic (superoxide dismutase, glutathione peroxidase, catalase, ascorbate peroxidase, and phenylalanine ammonia lyase) and non-enzymatic (phenols, flavonoids, and glutathione) antioxidant compounds, the mineral content, flower quality, and shelf life were analyzed. The results showed that the application of SiO2 NPs through the foliar method induced a greater flowers' shelf life (up to 21.62% more than the control); greater contents of Mg, P, and S (up to 25.6%, 69.1%, and 113.9%, respectively, compared to the control); more photosynthetic pigment (up to 65.17% of total chlorophyll); more glutathione peroxidase activity (up to 69.9%); more phenols (up to 25.93%); and greater antioxidant capacity as evaluated by the DPPH method (up to 5.18%). The use of SiO2 NPs in the production of Lilium is a good alternative method to increase flower quality and shelf life.

Alternative Pathway Is Involved in Hydrogen Peroxide-Enhanced Cadmium Tolerance in Hulless Barley Roots

Hulless barley, grown in the Qinghai Tibet Plateau, has a wide range of environmental stress tolerance. Alternative pathway (AP) and hydrogen peroxide (H₂O₂) are involved in enhancing plant tolerance to environmental stresses. However, the relationship between H₂O₂ and AP in hulless barley tolerance to cadmium (Cd) stress remains unclear. In the study, the role and relationship of AP and H₂O₂ under Cd stress were investigated in hulless barley (Kunlun14) and common barley (Ganpi6). Results showed that the expression level of alternative oxidase (AOX) genes (mainly AOX1a), AP capacity (V_alt), and AOX protein were clearly induced more in Kunlun14 than in Ganpi 6 under Cd stress; moreover, these parameters were further enhanced by applying H₂O₂. Malondialdehyde (MDA) content, electrolyte leakage (EL) and NAD(P)H to NAD(P) ratio also increased in Cd-treated roots, especially in Kunlun 14, which can be markedly alleviated by exogenous H₂O₂. However, this mitigating effect was aggravated by salicylhydroxamic acid (SHAM, an AOX inhibitor), suggesting AP contributes to the H₂O₂-enhanced Cd tolerance. Further study demonstrated that the effect of SHAM on the antioxidant enzymes and antioxidants was minimal. Taken together, hulless barley has higher tolerance to Cd than common barley; and in the process, AP exerts an indispensable function in the H₂O₂-enhanced Cd tolerance. AP is mainly responsible for the decrease of ROS levels by dissipating excess reducing equivalents.

Fluorescent dissolved organic matter facilitates the phytoavailability of copper in the coastal wetlands influenced by artificial topography

Dissolved organic matter (DOM) is a crucial driver in ecosystem services and a central part of the carbon transport and biological cycle in land-sea interaction. DOM exhibits characteristic environmental behavior in the coastal zone, but its sustainability is affected by expanding artificial topography (AT) construction. It requires combining analyses on AT-induced response of field fluorescent DOM (fDOM) and its quenching pattern under metal-complexation. Herein, we conducted systemic investigations into the spatiotemporal dynamics of fDOM compositions with further in-lab verification to study its Cu-binding capacity. We detected three humid-like fDOM components sensitive to AT. The total fDOM intensity was positively correlated with low molecular weight organic acid (LMWOA) extractable Cu and the Cu pools in above-ground biomass. The enriched fDOM serves as an ecological engineer by increasing the Cu mobility, confirmed by an in-lab fluorescence titration. The application of LMWOA greatly enhanced the intensity of one fDOM component, elevated its conditional stability constant, and decreased its quenched proportion, implying that LMWOA might extract part of Cu from fDOM complexation. The present work provides an "fDOM-LMWOA pump" explanation to suggest that fDOM is a novel ecological regulator on vegetation growth under the AT-induced matter accumulation.

Low-level arsenite boosts rhizospheric exudation of low-molecular-weight organic acids from mangrove seedlings (Avicennia marina): Arsenic phytoextraction, removal, and detoxification

Arsenic (As) contamination in mangrove wetlands has become a major concern. However, the impact of As on mangroves and the rhizospheric mechanism remains unclarified. In this study, various properties and responses of mangrove seedlings were investigated after exposure to arsenite (As³⁺). The results indicate that low-level As promoted the secretion of Low-molecular-weight organic acids (LMWOA, 4.5-6.59 mg/kg root in dry weight) and Fe plaque formation in their rhizospheres. Citric, oxalic, and malic acid were the three main components (84.3%-86.8%). Low-level As (5 and 10 μmol/L) also inhibited the rate of radial oxygen loss (ROL) but increased the accumulation of plant As (stem > leaf > root) and plaque As (0.23-1.13 mg/kg root in dry weight). We selected model LMWOAs to further examine As migration and speciation over time in As-enriched sediments (0, 20 and 40 mg/kg). The results reveal that LMWOAs promoted sediment As mobilisation and followed the order of citric acid > malic acid > oxalic acid. The hydrolysis and precipitation of Fe³⁺ and the complexation with organic ligand led to aqueous As and Fe sedimentation and, conversely, increased solution pH and re-translocated free As. The tolerance mechanisms include lowering ROL, translocating As and releasing LMWOAs to reduce its toxicity, and facilitating the fixation in sediment of oxidised As. The present study highlights the fact that mangroves are potentially favourable for As phytoextraction, removal and detoxification.

Non-targeted metabolomics reveal the impact of phenanthrene stress on root exudates of ten urban greening tree species

Different root exudations can modify the bioavailability of persistent organic pollutants (POPs). Among these exudations, the low molecular weight organic acids play an imperative role in this process. The study was conducted to analyze the effect of phenanthrene (PHE) stress on root exudation variations and changes in its chemical composition in ten urban greening tree species, namely Loropetalum chinense, Gardenia ellis, Photinia fraseri, Ligustrum japonicum, Rhododendron simsii, Osmanthus fragrans, Gardenia jasminoides, Buxus sinica, Camellia sasanqua, and Euonymus japonicas. The experiment was carried out in three PHE concentration treatments (0 mg kg^-1 (CK), 200 mg kg^-1 (PHE_L), 2000 mg kg^-1 (PHE_H)). The root exudates were collected and analyzed by GC-MS method. In total, 673 compounds were identified either with high or low abundance among all species and treatments. Compounds identified in CK, PHE_L, and PHE_H were 240, 180, and 256, respectively. The results illustrated that carbohydrates, phenols, and esters were the dominant compounds, accounted for more than 92%. Principal component analysis depicted that tree species grown in PHE_H showed obvious alteration in compounds of root exudation, whereas little difference was noticed between PHE_L and CK. Phenols (80%) were the most abundant, while nitriles contributed a small portion. Moreover, among all species, R. simsii released the maximum number of compounds, and L. japonicum released the least number of compounds accounting for 89 and 46, respectively. The results achieved here to illustrate that plant type, and PHE stress can significantly change the concentrations and species of root exudates. This study provides the scientific reference for understanding the phenanthrene responsive changes in root exudates and phytoremediation of polycyclic aromatic hydrocarbons (PAHs), as well as a screening of urban greening tree species.

Effects of phenolic acids on free radical scavenging and heavy metal bioavailability in kandelia obovata under cadmium and zinc stress

Current mechanism studies in plant heavy metal tolerance do not consider the effects of different phenolic acids on the bioavailability of heavy metals and the comparison with antioxidant enzyme system in the hydroxyl radical scavenging capacity. In present study, by a set of pot culture experiments with adding cadmium (Cd) and zinc (Zn) to the sediments, the effects of different phenolic acids on the toxicity of Cd and Zn in Kandelia obovata and the dominant role in scavenging hydroxyl radicals were evaluated. The results showed that 100 mg kg^-1 Zn treatment promoted the growth of plant under high concentrations of Cd and Zn stress. Under the stress of Cd and Zn, the phenolic acids were mainly metabolized by phenylpropanoid and flavonoid pathways, supplemented by shikimate and monolignol pathways in K. obovata. Eleven phenolic acids with different abilities of scavenging free radicals were detected in the plant, including pyrogallic acid (Gal), coumaric acid (Cou), protocatechuic acid (Pro), chlorogenic acid (Chl), 4-hydroxy benzoic acid (Hyd), caffeic acid (Caf), vanillic acid (Van), ferulic acid (Fer), benzoic acid (Ben), and salicylic acid (Sal). By adding phenolic acids to the sediments, chlorogenic acid (Chl), pyrogallic acid (Gal), cinnamic acid (Cin), and coumaric acid (Cou) behave as more reactive in changing Cd or Zn into residual fractions than the others, and chlorogenic acid (Chl), pyrogallic acid (Gal), ferulic acid (Fer) and caffeic acid (Caf) have higher ability of scavenging hydroxyl radicals than the others. In summary, K. obovata tends to synthesize phenolic acids with strong scavenging ability of free radicals and changing the bioavailability of Cd and Zn under high concentration of Cd and Zn stress. Phenolic acids played a crucial role in the mitigative effect of heavy metal stress via scavenging free radicals and involving in the process of Cd and Zn uptake and tolerance. The results will provide important theoretical basis and method guidance for mangrove wetland conservation.

Remediation of cadmium-contaminated soils using Brassica napus: Effect of nitrogen fertilizers

The physico-chemical characteristics of N fertilizers remain poorly understood with respect to their use with rape (Brassica napus L.) to remediate Cd-contaminated soil. In this work, eight types of fertilizer (comprising physico-chemical alkaline, neutral, and acidic N fertilizers) were employed to assess the effect of soil remediation via rape at different levels of Cd contamination (0, 5, and 10 mg kg^-1 Cd). The results show that the pH of rhizosphere soils was significantly higher under physico-chemical alkaline N fertilizer treatments than under physico-chemical acidic and neutral N fertilizer treatments. The physico-chemical characteristics of N fertilizers affected the rhizosphere soil pH and promoted Cd phytoextraction and accumulation by rape. In the 5 mg kg^-1 Cd-contaminated soil, the Cd accumulation and bioconcentration factor value in the shoots and the Cd translocation factor value were highest with the addition of NH₄Cl, a physico-chemical acidic N fertilizer. Among the physico-chemical alkaline N fertilizers, Ca(NO₃)₂ enabled the highest accumulation of Cd in rape shoots when soil was contaminated with 10 mg kg^-1 Cd. Thus, administering physico-chemical acidic N fertilizer to soils with lower Cd concentrations provides better remediation effects by rape, whereas physico-chemical alkaline N fertilizers are more effective in soils with higher Cd concentrations. These results show that physico-chemical N fertilizers can be employed to enhance the remediation of Cd-contaminated soil by rape and simultaneously improve the yield of this crop, with implications for environmental health and sustainable agricultural development.

The alleviation effect of iron on cadmium phytotoxicity in mangrove A. marina. Alleviation effect of iron on cadmium phytotoxicity in mangrove Avicennia marina (Forsk.) Vierh

Cd has high activity and bioavailability and is a poisonous element to plants. As a critical ecosysterm, mangroves are subjected to serious Cd pollution. In this research, the hypothesis was presented that improving Fe bioavailability would alleviate Cd phytotoxicity to Avicennia marina (Forsk.) Vierh. To test this, we examined the effect of four exogenous Fe and three Cd concentrations on A. marina. The results showed that a significant positive correlation excited between moderate exogenous Fe concentration and Cd tolerance of A. marina. Moderate exogenous Fe concentration directly or indirectly promoted the formation of Fe plaque, which immobilised more Cd on the root surface and decreased Cd absorption in roots. Furthermore, an exogenous Fe application increased plant biomass and Fe accumulation in A. marina tissues. This improved the competition between Fe and Cd within the plants. Therefore, an Fe application facilitated a decrease in Cd toxicity within A. marina. Simultaneously, a moderate Fe concentration caused an increase in low-molecular-weight organic acid (LMWOA) secretion from the roots. Meanwhile, Cd can be chelated/complexed by LMWOAs. It also played a crucial role in Cd detoxification in A. marina. In conclusion, Fe application accelerated the growth and enhanced Cd tolerance of A. marina. Therefore, improving Fe bioavailability will protect mangroves from Cd contamination.

Phenolic metabolism and related heavy metal tolerance mechanism in Kandelia Obovata under Cd and Zn stress

In the present study, a set of pot culture experiments was conducted to reveal how the metabolism process of phenolic compounds was affected by cadmium (Cd) and zinc (Zn) and to further uncover heavy metal tolerance mechanisms in Kandelia obovata. After 60d of treatment, the biomass and chlorophyll a content in the leaves were suppressed, but total phenolic compounds in roots and leaves were improved by the increasing gradient of Cd or Zn concentrations; Total phenolic compounds significantly increased by 3.6-44.6% in the roots, and by 0.4-126.6% in the leaves. At the meantime, the activity of Shikimate dehydrogenase (SKDH), cinnamyl alcohol dehydrogenase (CAD), and polyphenol oxidase (PPO) in the roots increased by 11.2-307.6%, 12.4-175.4% and - 2.7-392.8%, and the results were 3.4-69.5%, 1.7-40.0%, 16.0-99.7% in the leaves. Higher toxicity of Cd than Zn, as well as slight alleviating effect of 100 mg kg^-1 Zn on 2.5 mg kg^-1 Cd were found. Additionally, a significantly positive correlation coefficients for relationship between phenolic metabolism related enzyme activity and Cd/Zn contamination levels was found, and leaf SKDH, leaf CAD, and leaf PPO activities were moderately correlated with leaf Cd (r = 0.39, r = 0.43, and r = 0.57, respectively) and leaf Zn (r = 0.44, r = 0.41, r = 0.19, respectively) content, which indicate that Cd and Zn play a previously unrecognized but major role in phenolic compounds synthesis, transport, and metabolism in K. obovata. The results also provided evidence that the application of high levels of Cd and Zn was accompanied by three phenolic metabolism pathways participating in heavy metal tolerance process.

Short-term cadmium exposure induces gas exchanges, morphological and ultrastructural disturbances in mangrove Avicennia schaueriana young plants

Mangroves have been subject to more metal contamination, including cadmium (Cd). This study evaluated if a relatively short Cd exposure may induce metabolic, morphological and ultrastructural cell disturbance in Avicennia schaueriana. Cd induced evident constraints to seedlings since there was reduction in leaf gas exchanges and the plants did not survive for more than 10 days at a higher Cd exposure in controlled conditions. The highest Cd accumulation was observed in roots and gradually less in stem and leaves. Cadmium induced lignin deposition was observed in xylem cells of all vegetative organs. Intense sclerification in xylem cells, endoderm and change in the hypoderm organization were also detected. Cadmium clearly induced chloroplast deformities with ruptures of its membranes, thylakoids and core and provoked cytoplasm disorganization. These metal constraints under natural conditions for long term can lead to the accumulation of cellular and metabolic damages and jeopardize seedlings establishment and local biodiversity.

Accumulation and speciation of Cd in Avicennia marina tissues

Avicennia marina is a high-Cd-tolerant species in the mangrove wetlands. A hydroponic experiment was carried out to research the accumulation and chemical form distribution of Cd in the tissues of A. marina under different concentrations and durations of Cd stress. It was found that the concentrations of Cd in plant tissues followed the order of root > stem > leaf. The data suggested that root activity decreased, Cd accumulation ability weakened in roots, and the translocation factor increased in stems and leaves with the increase of stress duration. With a proactive defense mechanism, most Cd was bound to pectates, organic acids, and protein, especially in roots and stems with the most proportion of 88.51 and 78.91%, respectively, having lower biological activities. The Cd bounded to water-soluble organic acid and free inorganic aminophenol-Cd showed the lowest concentration. The pectates, organic acids, and protein-integrated Cd seem the most important in affecting Cd detoxification for A. marina; this mechanism of change in Cd biological activities decreases the toxicity of this aggressive pollutant and presents new knowledge about the tolerance of mangrove plants.

Physiological mechanisms of a wetland plant (Echinodorus osiris Rataj) to cadmium detoxification

Physiological responses of Echinodorus osiris Rataj plant under cadmium (Cd) stress (5 and 15 mg L^-1) were studied by researching the change of non-enzymatic antioxidants and the exudation of root organic acids. There was a significant increase of ascorbic acid, glutathione, and non-protein thiols in the plant, and the increment was much obvious in roots than that in leaves with increased Cd stress. The accumulation of Cd was associated with mitochondrial structural damages in roots, while the organelle structure, such as chloroplast, in leaves remains intact. In exudates collected from the plants in the treatment with 15 mg L^-1 Cd, oxalate, citric, and succinic acids responded intensively than other organic acids.

Exogenous malic acid alleviates cadmium toxicity in Miscanthus sacchariflorus through enhancing photosynthetic capacity and restraining ROS accumulation

Malic acid (MA) plays an important role in the regulation of plant growth, stomatal aperture, nutrition elements homeostasis and toxic metals tolerance. However, little is known about the effects of exogenous MA on physiological and biochemical responses to toxic metals in plants. To measure the alleviation roles of exogenous MA against cadmium (Cd), we determined the effects of MA on plant growth, net photosynthetic rate (Pn), reactive oxygen species (ROS) accumulation and the activities of anti-oxidant enzymes in the leaves of Miscanthus sacchariflorus (M. sacchariflorus) under Cd stress. The Cd exposure alone significantly inhibited plant growth and Pn, but increased the accumulation of ROS even though the anti-oxidant enzymes were markedly activated in the leaves of M. sacchariflorus. Treatment with MA significantly enhanced plant growth and decreased Cd accumulation accompanied by increasing Pn under Cd stress as compared to Cd stress alone, especially when treatment with high concentration of MA (200μM) was used. In addition, Cd and MA indicated synergistic effects by further increasing the activities and genes expression of partial anti-oxidant enzymes, thus resulting in higher glutathione accumulation and reduction of ROS production. The results showed that application of MA alleviated Cd-induced phytotoxicity and oxidant damage through the regulation of both enzymatic and non-enzymatic anti-oxidants under Cd stress in M. sacchariflorus.

Response of phenolic metabolism to cadmium and phenanthrene and its influence on pollutant translocations in the mangrove plant Aegiceras corniculatum (L.) Blanco (Ac)

Polyphenolic compounds are abundant in mangrove plants, playing a pivotal role in the detoxification of pollutants extruded from surrounding environments into plant tissues. The present study aimed to examine the variations of phenolic compounds, namely total polyphenolics, soluble tannins, condensed tannins and lignin, in the mangrove plant Aegiceras corniculatum (L.) due to the presence of exogenous cadmium and phenanthrene and to explore the influence of phenolic metabolism on biological translocation of these pollutants from roots to leaves. After a 6-week exposure to cadmium and phenanthrene, significant accumulations of both pollutants were observed. All determined phenolic compounds in both leaves and roots at high dosage levels were enhanced compared to the uncontaminated plant. Elevations of polyphenols in both treatments are possibly a result of stimulation in the activity of phenylalanine ammonia-lyase (PAL) and the enrichment of soluble sugar. Additionally, a significantly positive dosage relationship between polyphenolic metabolism intensity and phenanthrene contamination levels was found, while the trend observed in cadmium treatment was weak since cadmium at high levels inhibited phenolic production. The enrichment of polyphenols led to a decline in the biological translocation of these pollutants from roots to leaves. The immobilization of pollutants in the plant roots is possibly linked to the adsorption potential of polyphenols. These results will improve the understanding of the tolerance of mangrove plants to exogenous pollutants and will guide the selection of plants in phytoremediation because of the variability of polyphenol concentrations among species.

Response of low-molecular-weight organic acids in mangrove root exudates to exposure of polycyclic aromatic hydrocarbons

Low-molecular-weight organic acids (LMWOAs) represent an important component of root exudates. They play a pivotal role in the degradation of polycyclic aromatic hydrocarbons (PAHs) in sediments as they influence PAH bioavailability and degrader colonization. This study examined variations of LMWOAs in mangrove root exudates (Aegiceras corniculatum and Kandelia obovata) after exposure in phenanthrene and pyrene solution for 7 and 40 days, respectively. After 7 days of treatment, total root exudates and six types of LMWOA in root exudates from both mangrove species were enhanced. The largest increase was found in oxalic acid, i.e., the dominant component in determined LMWOAs. Coupled with the enhancement in LMWOA exudation rates, root metabolism intensities, measured as the dehydrogenase activity, increased. In contrast, after 40 days of exposure, the exudation rate of total LMWOAs had dropped markedly in PAH-contaminated groups compared to the control, indicating that PAHs negatively impacted root metabolism and activities due to their toxicity. The largest decrease was also found in oxalic acid, suggesting that the biological reactions related with oxalic acid are vulnerable under PAH stresses.

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 β-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.

Organic acid compounds in root exudation of Moso Bamboo (Phyllostachys pubescens) and its bioactivity as affected by heavy metals

Moso bamboo (Phyllostachys pubescens) has great potential as phytoremediation material in soil contaminated by heavy metals. A hydroponics experiment was conducted to determine organic acid compounds of root exudates of lead- (Pb), zinc- (Zn), copper- (Cu), and cadmium (Cd)-tolerant of Moso bamboo. Plants were grown in nutrients solution which included Pb, Zn, Cu, and Cd applied as Pb(NO₃)₂ (200 μM), ZnSO₄·7H₂O (100 μM), CuSO₄·5H₂O (25 μM), and CdCl₂ (10 μM), respectively. Oxalic acid and malic acid were detected in all treatments. Lactic acid was observed in Cu, Cd, and control treatments. The oxalic was the main organic acid exudated by Moso bamboo. In the sand culture experiment, the Moso bamboo significantly activated carbonate heavy metals under activation of roots. The concentration of water-soluble metals (except Pb) in sand were significantly increased as compared with control. Organic acids (1 mM mixed) were used due to its effect on the soil adsorption of heavy metals. After adding mixed organic acids, the Cu and Zn sorption capacity in soils was decreased markedly compared with enhanced Pb and Cd sorption capacity in soils. The sorption was analyzed using Langmuir and Freundlich equations with R ² values that ranged from 0.956 to 0.999 and 0.919 to 0.997, respectively.

Effects of root exudates on the leachability, distribution, and bioavailability of phenanthrene and pyrene from mangrove sediments

In this study, column leaching experiments were used to evaluate the leachability, distribution and bioavailability of phenanthrene and pyrene by root exudates from contaminated mangrove sediments. We observed that root exudates significantly promoted the release and enhanced the bioavailability of phenanthrene and pyrene from sediment columns. The concentration of phenanthrene and pyrene and cumulative content released from the analyzed sediment samples following root exudate rinsing decreased in the following order: citric acid > oxalic acid > malic acid. After elution, the total concentrations of phenanthrene and pyrene in sediment layers followed a descending order of bottom (9-12 cm) > middle (5-7 cm) > top (0-3 cm). Furthermore, a positive correlation between leachate pH values and PAH concentrations of the leachate was found. Consequently, the addition of root exudates can increase the leachability and bioavailability of phenanthrene and pyrene.

Effect of enhanced reactive nitrogen availability on plant-sediment mediated degradation of polycyclic aromatic hydrocarbons in contaminated mangrove sediment

As land-ocean interaction zones, mangrove systems receive substantial polycyclic aromatic hydrocarbons (PAHs) from sewage and combustion of fossil fuel. In this study, we investigated the relationship between dissolved inorganic nitrogen (DIN) availability and degradation rate of phenanthrene, a typical PAH compound, in mangrove plant-sediment systems, using Avicennia marina as a model plant. After 50 day incubation, phenanthrene removal ratios in sediments ranged from 53.8% to 97.2%. In non-rhizosphere sediment, increasing DIN accessibility increased microbial biomass and total microbial activity, while enhancements in population size of phenanthrene degradation bacteria (PDB) and phenanthrene degradation rates were insignificant. In contrast, the presence of excessive DIN in rhizosphere sediment resulted in a significantly large number of PDB, leading to a rapid dissipation rate of phenanthrene. The differences in degradation rates and abundances of degrader in sediment may be explained by the enhanced root activity due to the elevation in DIN accessibility.

Roles of rhizosphere and root-derived organic acids in Cd accumulation by two hot pepper cultivars

Cultivars of hot pepper (Capsicum annuum L.) have different abilities to accumulate Cd in their fruits. Previously, we suggested that low-Cd cultivars take up more Cd, but can better prevent the Cd translocation from roots to aerial parts. However, the mechanisms involved in those processes are still unclear. In this study, we explored the roles of rhizosphere soil Cd fractions and root secretions of low molecular weight organic acids in the uptake, translocation, and accumulation of Cd in a low-Cd and high-Cd cultivar. The results showed that there was no significant difference in exchangeable Cd between rhizosphere soils of the two cultivars, which might be related to their similar root's Cd uptake ability. The total content of low molecular weight organic acids released from roots of the low-Cd cultivar was almost equal to that released from roots of the high-Cd cultivar at the same Cd level; however, the composition of low molecular weight organic acids were determined by cultivars and Cd exposure levels. In the higher Cd (10 μM) treatment, the roots of the low-Cd cultivar excreted significantly less tartaric acid and more oxalic and acetic acids than those of the high-Cd cultivar. Additionally, there was no difference in the concentration of citric or succinic acid between the two cultivars. These results indicate that some kinds of low molecular weight organic acids efflux from hot pepper roots played an important role in the difference of Cd accumulation between low- and high-Cd cultivars.

Interactive effects of cadmium and pyrene on contaminant removal from co-contaminated sediment planted with mangrove Kandelia obovata (S., L.) Yong seedlings

The interactive effects of cadmium (Cd) and pyrene (Pyr) on contaminant removal from co-contaminated sediment planted with Kandelia obovata were investigated by a pot experiment. We found that dry weight of plant was significantly decreased under high level of Cd-Pyr combined stress. High Pyr caused the increase of Cd toxicity to K. obovata under high Cd stress because more Cd translocated to the plant tissues. Cd toxicity inhibited Pyr degradation in co-contaminated sediments and higher Pyr degradation was found in the rhizosphere than that in the non-rhizosphere sediment under high Cd treatment. The total number of microorganisms in sediments tended to decrease with increasing Cd under Cd-Pyr combined stress and more amount existed in the rhizosphere sediment. In conclusion, Cd and Pyr removal by K. obovata can influence interactions between these two pollutants in co-contaminated sediment. This suggests that this mangrove can effectively remedy sites co-contaminated with these two types of contamination.

Research on the nitrogen cycle in rhizosphere of Kandelia obovata under ammonium and nitrate addition

The present study investigated nitrogen process in rhizosphere of Kandelia obovata under nitrogen input. Results showed that nitrogen additions significantly increased 4 kinds of enzyme activities (Urease, Nitrate reductase, Nitrite reductase and hydroxylamine reductase). The pH value increased to 7.1 under ammonium addition, but decreased to 6.9 under nitrate addition. Potential Nitrification Intensity (PNI) increased 200-1500% compared with control under ammonium addition, and Potential Denitrification Intensity (PDI) increased more than 200% under nitrate addition. Ten types of organic acids were detected from root exudates, which mainly included oxalic acid, tartaric acid, formic acid, acetic acid, and citric acid. The abundance of 5 kinds of microbial functional groups (nifH, AOA, AOB, nirS, nirK) responded differently. Total nitrogen in organs of K. obovata increased more than 200%. This indicated that nitrogen additions accelerated the transformation of nitrogen directly and stimulated the exudation of root exudates and 5 kinds of genes indirectly.