Exogenous Organic Acids Research Articles

Modifying soil bacterial communities in saline mudflats with organic acids and substrates.

The high salinity of soil, nutrient scarcity, and poor aggregate structure limit the exploitation and utilization of coastal mudflat resources and the sustainable development of saline soil agriculture. In this paper, the effects of applying exogenous organic acids combined with biological substrate on the composition and diversity of soil bacterial community were studied in moderately saline mudflats in Jiangsu Province. A combination of three exogenous organic acids (humic acid, fulvic acid, and citric acid) and four biological substrates (cottonseed hull, cow manure, grass charcoal, and pine needle) was set up set up on a coastal saline mudflat planted with a salt-tolerant forage grass, sweet sorghum. A total of 120 kg ha-1 of organic acids and 5,000 kg ha-1 of substrates were used, plus two treatments, CK without application of organic acids and substrates and CK0 in bare ground, for a total of 14 treatments. No significant difference was found in the alpha diversity of soil bacterial community among all treatments (p ≥ 0.05), with the fulvic acid composite pine needle (FPN) treatment showing the largest increase in each index. The beta diversity differed significantly (p < 0.05) among all treatments, and the difference between citric acid-grass charcoal (CGC) and CK treatments was greater than that of other treatments. All treatments were effective in increasing the number of bacterial ASVs and affecting the structural composition of the community. Citric acid-cow manure (CCM), FPN, and CGC treatments were found to be beneficial for increasing the relative abundance of Proteobacteria, Chloroflexi, and Actinobacteria, respectively. By contrast, all treatments triggered a decrease in the relative abundance of Acidobacteria. Among the 12 different combinations of exogenous organic acid composite biomass substrates applied to the coastal beach, the CGC treatment was more conducive to increasing the relative abundance of the salt-tolerant bacteria Proteobacteria, Chloroflexi and Actinobacteria, and improving the community structure of soil bacteria. The FPN treatment was more conducive to increase the species diversity of the soil bacterial community and adjust the species composition of the bacterial community.

Effects of Exogenous Organic Acids on the Soil Metabolites and Microbial Communities of Panax notoginseng from the Forest Understory

Panax notoginseng (Sanqi) is a precious traditional Chinese medicine that is commonly cultivated using conventional management methods in agricultural systems in Yunnan, China, where it faces the challenge of continuous cropping obstacles (CCOs). However, the existence of Sanqi CCOs in Sanqi–pine agroforestry systems remains unclear. Here, we applied three types of exogenous organic acids (phthalic acid, palmitic acid, and phthalic + palmitic mixed organic acids) mainly derived from the root exudates of Sanqi to simulate the CCOs; then, we compared the effects on plant growth, soil physicochemical properties, soil microbes, and soil metabolites. We found that organic acid concentrations of &gt;250 mg/kg reduced the fresh weight of Sanqi and the levels of total nitrogen, ammonium nitrogen, soil water content, total phosphorus, and pH value; these concentrations also increased the soil bacterial and fungal α-diversity. The type of organic acid, as opposed to the concentration and duration of treatment, had a vital impact on the structure of the bacterial and fungal community in Sanqi soils. Moreover, the organic acid concentrations of &gt;250 mg/kg also decreased the complexity and stability of the bacterial and fungal network. In addition, the metabolic pathways in the soils under different organic acids included 17 differential metabolites (DMs), which were involved in steroid hormone biosynthesis. The structural equation models (SEMs) revealed that plant growth, soil edaphic factors, and soil metabolites had direct or indirect influences on soil microbial communities under different organic acid conditions. Our results suggest that any phthalic or palmitic acid concentrations at concentrations &gt;250 mg/kg are detrimental to multiple aspects of Sanqi cultivation, confirming the presence of Sanqi CCOs in Sanqi–pine agroforestry systems.

Effect of exogenous organic acids on chemical compositions and sensory attributes of fortified sweet wines from dehydrated grapes

In this study, acidity was regulated with the addition of exogenous tartaric acid and citric acid before bottling. The effect of exogenous organic acids on chemical compositions and sensory attributes of fortified sweet wines from dehydrated grapes were investigated. The results indicated that exogenous organic acids promoted the conversion of monomeric anthocyanins to copigmented anthocyanins in wines. Specifically, the combination of malvidin-3-O-glucoside and flavanols (catechin and epicatechin) was facilitated to form copigmented anthocyanins. Sensory analysis suggested that exogenous organic acids improved the balance of sugar and acidity and benefited the harmony in wines on the taste. Wines with a residual sugar and titratable acidity ratio of about 11:1 exhibited the more harmonious taste. In addition, it was also observed changes in the aroma profile related to volatile compounds, namely, more intense fruity aroma in wines with the addition of organic acids.

Physiological mechanisms of exogenous organic acids to alleviate aluminum toxicity in seedlings of mungbean, buckwheat, and rice

One of the major constraints for crop yield in acidic soils is the phytotoxicity of aluminum ions (Al3+), which primarily affects the roots. To mitigate the harmful effects of Al toxicity, plants use organic acids to chelate Al internally and externally. In this study, the effects of exogenous organic acids on Al toxicity in rice, mung bean, and buckwheat were investigated. Specifically, the study examined the ameliorative effect of three organic acids (oxalic acid, malic acid, and citric acid, each at a concentration of (100 μmol/L) on root elongation, fresh weight, Al content, organic acid key enzymes, and rhizosphere pH in hydroponic media containing (100 μmol/L) Al. The experimental results revealed species-specific responses to aluminum tolerance and the alleviating effects of different organic acids. Buckwheat was found to be the most aluminum-tolerant, followed by mung bean, while rice was the least tolerant. Exogenous application of oxalic acid promoted root elongation, increased root fresh weight, and enhanced the activity of the PEPC enzyme in mung bean. Malic acid, on the other hand, alleviated Al toxicity in rice by promoting root elongation, increasing root fresh weight, enhancing the activity of the PEPC enzyme, and decreasing the activity of the MDH enzyme. In buckwheat, citric acid application reduced Al toxicity by promoting root elongation, increasing root weight, and decreasing the activities of CS and GO enzymes. These findings indicate that different organic acids can reduce Al toxicity in different plant species by employing different physiological mechanisms.

Effects of exogenous organic acids on the growth, edaphic factors, soil extracellular enzymes, and microbiomes predict continuous cropping obstacles of Panax notoginseng from the forest understorey

Effects of exogenous organic acids on the growth, edaphic factors, soil extracellular enzymes, and microbiomes predict continuous cropping obstacles of Panax notoginseng from the forest understorey

Biomass composite with exogenous organic acid addition supports the growth of sweet sorghum (Sorghum bicolor 'Dochna') by reducing salinity and increasing nutrient levels in coastal saline-alkaline soil.

In coastal saline lands, organic matter is scarce and saline stress is high. Exploring the promotion effect of intervention with organic acid from biological materials on soil improvement and thus forage output and determining the related mechanism are beneficial to the potential cultivation and resourceful, high-value utilization of coastal mudflats as back-up arable land. Three exogenous organic acids [humic acid (H), fulvic acid (F), and citric acid (C)] were combined with four kinds of biomass materials [cottonseed hull (CH), cow manure (CM), grass charcoal (GC), and pine needle (PN)] and applied to about 0.3% of medium-salt mudflat soil. The salinity and nutrient dynamics of the soil and the growth and physiological differences of sweet sorghum at the seedling, elongation, and heading stages were observed under different treatments to screen for efficient combinations and analyze the intrinsic causes and influencing mechanisms. The soil salinity, nutrient dynamics, and forage grass biological yield during sweet sorghum cultivation in saline soils differed significantly (p < 0.05) depending on the type of organic acid-biomass composite applied. Citric acid-pine needle composite substantially reduced the soil salinity and increased the soil nutrient content at the seedling stage and improved the root vigor and photosynthesis of sweet sorghum by increasing its stress tolerance, allowing plant morphological restructuring for a high biological yield. The improvement effect of fulvic acid-pine needle or fulvic acid-cow manure composite was manifested at the elongation and heading stages. Citric acid-pine needle composite promoted the growth of saline sweet sorghum seedlings, and the effect of fulvic acid-pine needle composite lasted until the middle and late stages.

Optimizing Acetic Acid Application Strategy Can Effectively Promote the Remediation Performance of Oilseed Sunflower on Cd-Contaminated Soils

Applying exogenous organic acids is an effective method to improve the remediation efficiency of Cd-contaminated soils. To investigate the effects of exogenous acetic acid on Cd forms in rhizosphere soils and phytoremediation performance for Cd-contaminated soils, a potted experiment was performed with oilseed sunflower as the extractive plant. Acetic acid was applied at 1, 2, 3, 4, 5, and 6 mmol/kg at 20, 30, 40, and 50 days after seedling emergence. Soil without acetic acid was used as a control (CK). Emblematic chemical properties and different Cd forms in rhizosphere soils were inspected. Results showed that adding acetic acids improved the biomass of shoot and root; it increased firstly and then decreased with the increase of acetic acid concentrations. For all treatments, acetic acids increased sucrase activity and catalase activity but decreased amylase activity in rhizosphere soils. At 30 or 40 days after seedling emergence, the exchangeable Cd content, Fe-Mn oxide Cd content, and organic Cd content were lower, while the carbonate Cd content was greater. Adding acetic acids improved the removal rate of Cd, and when 1 mmol/kg acetic acid was applied at 40 days after seedling emergence, it was increased by 60%, which was the highest compared to CK. RDA showed that catalase activity, sucrase activity, carbonate Cd, and pH could promote the growth of oilseed sunflower, while organic Cd, Fe-Mn oxide Cd, total Cd, exchangeable Cd, and amylase activity inhibited the growth of oilseed sunflower. These findings suggest that acetic acid can improve the efficiency of phytoremediation in Cd-contaminated soils. In particular, the treatment with 1 mmol/kg acetic acid at 40 days after seedling emergence had the most obvious effect.

Organic acids in conjunction with various oilseed sunflower cultivars promote Cd phytoextraction through regulating micro-environment in root zone

Exogenous chelates in conjunction with phytoextraction have been demonstrated as a prominent potential for remediating Cd-contaminated soils. Among bath chelates, low molecular-weight organic acids have the advantages of smaller impact on plant growth and lower negative effects on soil environment. In this research, oilseed sunflowers were selected as the main extraction plants, and the Cd phytoextraction effects of different cultivars, varies organic acid types and concentrations combinations were systematically examined. Results showed that adding exogenous organic acids improved biomass of all issues, but the increase varied with the cultivar. For both cultivars, their biomass increase with acid application concavely, with the acid application rate at which the biomass peaked differed between the two cultivars. Adding exogenous acids boosted Cd concentration in roots and shoots in general, but it reduced the Cd content in seeds as well as the rhizosphere and bulk soil. In addition to its impact on Cd, application of organic acids also reduced nutrient and salinity in the rhizosphere and promoted root uptake of nutrients as a result. Cd content plant an soil was closely related to nutrients in soil and plant. The organic acids reduced soil Cd and nutrients in soil proximal to the roots. It was also found that adding acids reduced Cd content in the top 0–10 cm soil but increased the Cd content in the 10–30 cm soil, alluding a potential leaching. Therefore, adding organic acids to improve phytoremediation of Cd needs to proceed with care.

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.

Effect of Two Exogenous Organic Acids on the Excitation Effect of Soil Organic Carbon in Beijing, China

Significance: The study of the effects and pathways of catechol and pyrogallic acid on soil organic carbon mineralization has a positive effect on mastering soil carbon transformation. Methods and objectives: In this study, we took 0–20 cm soil from Pinus tabulaeformis forest as an object to investigate the effects of catechol and pyrogallic acid with different concentrations on soil organic carbon mineralization through a 60-day mineralization incubation test. The soil active carbon content and changes in soil microbial diversity and community composition were analyzed by using single exponential fitting, quantitative PCR, and high-throughput sequencing to explore the influencing mechanisms of catechol and pyrogallic acid on soil organic carbon excitation. Results: Catechol and pyrogallic acid had the effect of enhancing the soil organic carbon mineralization and soil active carbon content, and the higher the concentration, the stronger the enhancement effect. Catechol reduced the Ace index, Chao1 index, and Shannon index of bacteria and fungi, and further changed the relative abundance of two dominant groups (Proteobacteria and Acidobacteriota) in bacteria and Basidiomycota in fungi at high concentrations. There was no obvious regularity in the effects of pyrogallic acid on bacteria and fungi, but the Ace index and Chao1 index of bacteria underwent drastic and disordered changes. Conclusions: Catechol and pyrogallic acid can trigger positive excitation of the soil organic carbon through two pathways: increasing the soil active carbon content and modulating soil microorganisms, but the way they modulate soil microorganisms are different. Catechol regulates soil microorganisms by reducing the number, richness, and evenness of the bacteria and fungi species, as well as the community composition, while the way pyrogallic acid regulates only closely relates to the changes in the number, richness, and evenness of bacteria species.

Effects of Exogenous Organic Acids and Flooding on Root Exudates, Rhizosphere Bacterial Community Structure, and Iron Plaque Formation in Kandelia Obovata Seedlings

Effects of Exogenous Organic Acids and Flooding on Root Exudates, Rhizosphere Bacterial Community Structure, and Iron Plaque Formation in Kandelia Obovata Seedlings

Effects of Exogenous Organic Acids on Cd Tolerance Mechanism of Salix variegata Franch. Under Cd Stress.

Chelate induction of organic acids has been recognized to enhance metal uptake and translocation in plants, but the underlying mechanism remains unclear. In this study, seedlings of Salix variegata were hydroponically exposed to the combinations of Cd (0 and 50 μM) and three exogenous organic acids (100 μM of citric, tartaric, or malic acid). Plant biomass, antioxidant enzymes, non-protein thiol compounds (NPT) content, and the expression of candidate genes associated with Cd accumulation and tolerance were determined. Results showed that Cd significantly inhibited plant biomass but stimulated the activity of antioxidant enzymes in the roots and leaves, while the lipid peroxidation increased as well. Respective addition of three organic acids greatly enhanced plant resistance to oxidative stress and reduced the lipid peroxidation induced by Cd, with the effect of malic acid showing greatest. The addition of organic acids also significantly increased the content of glutathione in the root, further improving the antioxidant capacity and potential of phytochelatin biosynthesis. Moreover, Cd induced the expression level of candidate genes in roots of S. variegata. The addition of three organic acids not only promoted the expression of candidate genes but also drastically increased Cd accumulation in S. variegata. In summary, application of citric, tartaric, or malic acid alleviated Cd-imposed toxicity through the boost of enzymatic and non-enzymatic antioxidants and candidate gene expression, while their effects on Cd tolerance and accumulation of S. variegata differed.

Change in phytoextraction of Cd by rapeseed (Brassica napus L.) with application rate of organic acids and the impact of Cd migration from bulk soil to the rhizosphere

Adding exogenous low-molecular weight organic acids is an effective technique to improve phytoremediation of Cd-contaminated soil and has been well documented, but how acid application rate affects remediation efficiency and its underlying limiting factors remains elusive. We investigated this using pot experiments with rapeseed (Brassica napus L.) as the model plant. Plastic pots packed with a sandy loam contaminated by Cd at 4.838 mg/kg were amended with acetic acid, oxalic acid, citric acid, malic acid and tartaric acid, respectively, at an application rate gradient varying from 0.0 to 12.0 mmol/kg. Plants in each pot were harvested after growing for five months, and we then measured the exchangeable, carbonate, Fe–Mn oxide, organic and residual Cd in the rhizosphere, as well as Cd in both roots and shoots. The results showed that all organic acids improved plant uptake of Cd and, compared with the control without acid addition, they could improve Cd uptake by more than 100%. The enhanced Cd extraction was due to the increase in exchangeable Cd in the rhizosphere. Plant Cd was weakly correlated to the amount of Cd lost from a unit volume of the rhizosphere due to root extraction (R2 = 0.06), but a good negative correlation was found between them after normalizing the lost Cd by root biomass (R2 = 0.36). Mass balance analysis revealed that the average Cd content in soil (rhizosphere and bulk soils combined) was much higher than the Cd content in the rhizosphere, and the improved Cd mobility after acid addition was thus due to the increased chelation. As diffusion of ligands in water is one order in magnitude smaller than diffusion of Cd ions, our results suggested that Cd migration from the bulk soil into the rhizosphere was a major factor limiting Cd phytoextraction by rapeseed after adding the exogenous organic acids.

Ralstonia solanacearum promotes pathogenicity by utilizing l-glutamic acid from host plants.

Ralstonia solanacearum is an important bacterial pathogen that can infect a broad range of plants worldwide. A previous study showed that R. solanacearum could respond to exogenous organic acids or amino acids to modulate cell motility. However, it was unclear whether R. solanacearum uses these compounds to control infection. In this study, we found that R. solanacearum GMI1000 uses host plant metabolites to enhance the biosynthesis of virulence factors. We demonstrated that l‐glutamic acid from host plants is the key active component associated with increased extracellular polysaccharide production, cellulase activity, swimming motility, and biofilm formation in R. solanacearum GMI1000. In addition, l‐glutamic acid also promoted colonization of R. solanacearum cells in the roots and stems of tomato plants and accelerated disease incidence. Furthermore, genetic screening and biochemical analysis suggested that RS01577, a hybrid sensor histidine kinase/response regulator, is involved in l‐glutamic acid signalling in R. solanacearum. Mutations in RS01577 and exogenous addition of l‐glutamic acid to the GMI1000 wild‐type strain had overlapping effects on both the transcriptome and biological functions of R. solanacearum, including on motility, biofilm formation, and virulence. Thus, our results have established a new interaction mechanism between R. solanacearum and host plants that highlights the complexity of the virulence regulation mechanism and may provide new insight into disease control.

Influence of Potassium Nutrition and Exogenous Organic Acids on Iron Uptake by Monocot and Dicot Plants

Iron (Fe) is a vital element for the survival and proliferation of all plants; therefore, Fe-biofortification by the application of chemical and organic fertilizers is being as an effective approach to fight hidden hunger retards the growth and development of crop plants. Two experiments were carried out to investigate the effect of potassium and exogenous organic acids on iron uptake by two different plants: one is monocotyledon, maize (Zea mays L.) and the second is dicotolydon pea (Pisum sativum L.) grown under controlled conditions. The seedlings were grown in sand culture in a greenhouse experiment and irrigated with one-tenth strength modified nutrient solution of Hoagland and Arnon as a base solution (pH 7.5), containing different iron treatments (0, 1, and 5 ppm as FeSO4·7H2O) combined with potassium nutrition (0, 5, 10, and 50 ppm as K2SO4). After 30 days, the best interaction treatment was selected for further experiment including 5.0 ppm Fe as FeSO4.7H2O and 50 ppm K as K2SO4 in combination with 1 × 10-5 mole/liter of one of the following organic acids: Citric acid, Oxalic acid, Formic acid, Acetic acid, Propionic acid, Tartaric acid, Succinic acid, Fumaric acid, Malic acid, Glutamic acid, besides the free organic acid nutrient solution as a control. Results revealed that the interaction between 5.0 ppm Fe and 50 ppm K was the best interaction treatment for increasing biomass production and iron uptake of maize and pea seedlings under applied condition. Furthermore, exogenous application of organic acids improves uptake and translocation of nutrient such as iron, potassium and phosphorus by the maize and pea plants. In conclusion, potassium nutrition and exogenous organic acids have the potential to stimulate Fe-uptake of monocot and dicot plants and mediate iron-biofortified crops.

The effects of exogenous organic acids on the growth, photosynthesis and cellular ultrastructure of Salix variegata Franch. Under Cd stress

We studied the effects of three organic acids (citric acid, tartaric acid and malic acid) on the biomass, photosynthetic pigment content and photosynthetic parameters of Salix variegata under Cd stress and observed the ultrastructure of mesophyll cells in each treatment. Cd stress significantly reduced photosynthesis by reducing the content of pigments and disrupting chloroplast structure, which consequently decreased the biomass. However, respective addition of three organic acids greatly increased the biomass of S. variegata under Cd stress. Among them, the effect of malic acid or tartaric acid on shoot and total biomass accumulation was greater than that of citric acid. The alleviation of biomass probably related with the photosynthetic process. Results revealed that treatment with each organic acid enhanced the net photosynthesis rate under Cd stress. Malic acid promoted plant growth and biomass by increasing the chlorophyll content and mitigating damage to the photosynthetic apparatus resulting from Cd stress. Tartaric acid had little impact on the photosynthetic pigment content, but it was important in mitigating the ultrastructural damage of plants caused by Cd. Addition of citric acid significantly increased the carotenoid as well as the number and volume of chloroplasts in mesophyll cells, while the mitigation of structural damage in the photosynthetic apparatus was weaker than that in tartaric acid or malic acid treatment. It is concluded that application of tartaric acid or malic acid is effective in increasing the growth potential of S. variegata under Cd stress and thus can be a promising approach for the phytoremediation of Cd-contaminated soil.

The activation and extraction systems using organic acids and Lentinus edodes to remediate cadmium contaminated soil

To develop a high efficient and eco-friendly approach to remediate cadmium (Cd) contaminated soil, we designed the activation and extraction systems, on the basis of combined effects between the ability of organic acids to activate Cd and the ability of mushroom accumulator (Lentinus edodes) to extract Cd. The results showed that the proportion of acetic acid-extractable Cd significant increased with the application of exogenous organic acids. Additionally, soil microecology analysis indicated that exogenous organic acids evidently enhanced the numbers of microbial cells and the activities of soil enzymes. Besides, high throughput sequencing analysis revealed exogenous organic acids improved the diversity and structure of soil bacterial community after remediation. Particularly, the combination application of mushroom and exogenous citric acid had highest accumulation efficiency of Cd, and its efficiency was 59.19% higher than single mushroom treatment. Hence, exogenous organic acids could alleviate soil microecology and increase mycoextraction efficiency, which suggested it was a feasible route to remediate Cd contaminated soil by the activation and extraction systems.

Effects of exogenous organic acids on the resistance of Changbai larch (Larix olgensis) seedlings to mixed Pb and Cd stress

ABSTRACTMixed Pb and Cd soil contamination is an issue in Northeast China. We examined the effects of exogenous organic acids on the resilience of Changbai larch (Larix olgensis) seedlings, a pioneering forestry species in afforestation and vegetation restoration in Northeast China, under such stress. Mixed Pb and Cd stress led to significantly higher Pb and Cd content in the leaves and fine roots, malondialdehyde content in the leaves, superoxide dismutase activity, and soluble protein content in the leaves. Lower biomass of the roots, stems, and leaves was observed, with the roots showing the sharpest reduction in biomass. However, the application of organic acids mitigated or reversed these effects. This was most pronounced following treatment with 0.2 mmol·L−1 or 1.0 mmol·L−1 organic acids for 20 days. Citric acid had the greatest positive effect compared with succinic acid and oxalic acid. We suggest that exogenous organic acids have the potential to alleviate Pb and Cd-induced oxidation injury symptoms in Changbai larch seedlings, and may enhance resilience to mixed Pb and Cd stress.

Exogenous Oxalic Acid and Citric Acid Improve Lead (Pb) Tolerance of Larix olgensis A. Henry Seedlings

We investigated the beneficial role of different concentrations of exogenous oxalic acid (OA) or citric acid (CA) for improving Pb tolerance and mitigating Pb-induced physiological toxicity in Changbai larch (Larix olgensis A. Henry) seedlings in northeast China. The seedlings were exposed to 100 mg·kg−1 Pb in soil alone or in combination with OA or CA irrigation for 10, 20, or 30 days. Pb-induced damage in L. olgensis was evident from elevated lipid peroxidation that significantly inhibited plant growth. Malondialdehyde (MDA) contents also increased in the presence of elevated Pb; however, superoxide dismutase (SOD) and peroxidase (POD) activities, as well as proline and pigment contents, all decreased. The damage increased in controls over the application periods. Pb contents in fine roots and leaves generally decreased with low-concentration organic acids (&lt;1.0 mmol·L−1), but often increased at 5.0 and 10.0 mmol·L−1. Alternatively, when Pb-stressed plants were exposed to an organic acid (especially 5.0 or 10.0 mmol·L−1 for 10 days), the damage, as indicated by the physiological parameters, was reversed, and plant growth was promoted; CA was more effective in inducing these changes than OA. Therefore, exogenous organic acids have the potential to alleviate Pb-induced oxidative injuries, and can improve the tolerance of L. olgensis seedlings to Pb stress. Under lower OA and CA concentrations, the detoxification mechanism appears to be an external resistance mechanism; however, under higher concentrations (5.0–10.0 mmol·L−1) internal resistance mechanisms appear dominant. It is also possible that the two mechanisms work in tandem.

An aerated axenic hydroponic system for the application of root treatments: exogenous pyruvate as a practical case

BackgroundHydroponic systems are a convenient platform for plant cultivation when treatments are applied to the roots because they provide precise control of the composition of the growth medium, ensuring the availability of different compounds. A problem arises when axenic conditions are needed but the treatment of choice (exogenous organic acids or sugars) promote the growth of unwanted microorganisms. Moreover, axenic conditions are usually applied in liquid and semi-liquid growing systems, where oxygen availability can be compromised, if no aeration is provided.ResultsThe driver for the development of this hydroponic system was the application of the organic acid pyruvate to the roots of plants grown under aerated axenic conditions. No contamination was detected in the nutrient solution, even after the addition of pyruvate. The system was validated in pea plants treated with either pyruvate or herbicides inhibiting amino acid biosynthesis. The effects on ethanol fermentation were compared by analysing the enzymatic activity, protein content and transcriptional levels in plants treated with either pyruvate or herbicides.ConclusionsThe developed system enables the study of the exogenous application of organic acids in the nutrient solution under axenic conditions and without oxygen limitation. This system allows the study of the effect of any type of treatments applied to roots under aerated axenic hydroponic systems at physiological and molecular levels. The role of pyruvate in the induction of fermentation by herbicides cannot be simply explained by an increase in substrate availability.