Unlocking the phytoremediation potential of organic acids: A study on alleviating lead toxicity in canola (Brassica napus L.)

Mitigating effect of γ-aminobutyric acid and gibberellic acid on tomato plant cultivated in Pb-polluted soil

Soil heavy metal pollution poses a significant environmental risk to human health and plant growth. Gibberellic acid (GA) and γ-aminobutyric acid (GABA) are effective methods for resolving this issue. GA regulates growth mechanisms such as seed germination, flowering, and stem elongation. Plants use GABA, a signaling molecule, to control physiological processes, growth, and responses to stress. This substance plays a crucial role in the interactions between hormones and plant defense, as evidenced by its effects on photosynthesis, food absorption, and stomatal behavior. The study aimed to determine how GABA and GA amendments affected tomato plants under no toxicity and Pb toxicity. The study included four treatments (0, GA, GABA, and GA + GABA) in four replications following a completely randomized design. Notably, the GA + GABA treatment led to considerable enhancements in fresh weight (88.98%), dry weight (68.28%), shoot length (39.98%), and root length (115.43%) compared to the control under Pb toxicity. Moreover, GA + GABA treatment significantly increased tomato chlorophyll a (161.72%), chlorophyll b (93.33%), and total chlorophyll content (112.45%) under Pb stress toxicity, confirming the effectiveness of GA + GABA treatment. In conclusion, GA + GABA is recommended as the best amendment to mitigate Pb stress in tomato plants. Our findings have broader implications for GA + GABA application, offering a potential technology to enhance sustainable crop production by improving plant growth and yield in Pb-contaminated soils. More investigations are suggested at field levels under different agroclimates on different crops for the declaration of GA + GABA as the best amendment for alleviating different heavy metal pollutions and sustainable agriculture productions.

Mitigating cadmium stress in rice (Oryza sativa L.) using succinic and oxalic acids with focus on cellular integrity and antioxidant responses

Soil contamination with toxic heavy metals [such as cadmium (Cd)] is becoming a serious global problem due to the rapid development of the social economy. Organic chelating agents such as succinic acid (SA) and oxalic acid (OA) are more efficient, environmentally friendly, and biodegradable compared to inorganic chelating agents and they enhance the solubility, absorption, and stability of metals. To investigate this, we conducted a pot experiment to assess the impact of SA (0.25 and 0.5 mM) and OA (0.25 and 0.5 mM) on enhancing the phytoremediation of Cd under its toxic concentration of 0.1 mM, using rice (Oryza sativa L.) plants. The research outcomes indicated that elevated levels of Cd stress in the soil significantly (P < 0.05) decreased plant growth and biomass, photosynthetic pigments, and gas exchange attributes. However, Cd stress also induced oxidative stress in the plants by increasing malondialdehyde (MDA) and hydrogen peroxide (H2O2), which also induced increased compounds of various enzymatic and non-enzymatic antioxidants and also the gene expression and sugar content. Furthermore, a significant (P < 0.05) increase in proline metabolism, the AsA-GSH cycle, and the pigmentation of cellular components was observed. In addition, scanning electron microscopy (SEM) revealed that Cd toxicity significantly affected double membranous organelles. Although, the application of SA and OA showed a significant (P < 0.05) increase in plant growth and biomass, gas exchange characteristics, enzymatic and non-enzymatic compounds, and their gene expression and also decreased oxidative stress. In addition, the application of SA and OA enhanced cellular fractionation and decreased the proline metabolism and AsA-GSH cycle in O. sativa plants. These results open new insights for sustainable agriculture practices and hold immense promise in addressing the pressing challenges of heavy metal contamination in agricultural soils.

Efficacy of malic and tartaric acid in mitigation of cadmium stress in Spinacia oleracea L. via modulations in physiological and biochemical attributes

The increasing level of cadmium (Cd) contamination in soil due to anthropogenic actions is a significant problem. This problem not only harms the natural environment, but it also causes major harm to human health via the food chain. The use of chelating agent is a useful strategy to avoid heavy metal uptake and accumulation in plants. In this study, randomized design pot experiment was conducted to evaluate potential role of malic acid (MA) and tartaric acid (TA) foliar spray to mitigate Cd stress in Spinacia oleracea L plants. For Cd stress, S. oleracea plants were treated with CdCl2 solution (100 µM). For control, plants were given distilled water. One week after Cd stress, MA and TA foliar spray was employed at concentration of 100 and 150 µM for both. The results of this study revealed that Cd stress (100 µM) significantly reduced growth attributes, photosynthetic pigments and related parameters and gas exchange attributes. Cadmium stress also stimulated antioxidant defense mechanism in S. oleracea. Cd stressed plants had elevated levels of Cd metal ions in root and consumable parts (i.e. leaves) and caused severe oxidative damages in the form of increased lipid peroxidation and electrolytic leakage. MA and TA supplements at both low and high levels (100 and 150 µM) effectively reversed the devastating effects of Cd stress and improved growth, photosynthesis and defense related attributes of S. oleracea plants. These supplements also prevented excessive accumulation of Cd metal ions as indicated by lowered Cd metal contents in MA and TA treated plants. These findings demonstrated that MA and TA treatments can potentially reduce Cdl induced phytotoxicity in plants by reducing its uptake and enhancing photosynthesis and defense related parameters.

Fullerenol nanoparticles and AMF application for optimization of Brassica napus L. resilience to lead toxicity through physio-biochemical and antioxidative modulations

Crop plants are severely affected by heavy metals (HMs), leading to food scarcity and economical loss. Lead (Pb) is outsourced by use of lead-based fertilizers, batteries, mining, smelting and metal processing. It significantly reduces growth, development and yield of crops cultivated on contaminated sites. In this study, the ameliorative role of carbon based fullerenol nanoparticles (FNPs) in combination with Arbuscular Mycorrhizal Fungi (AMF) inoculation was examined in Brassica napus L. grown in Pb contaminated soil. A pot experiment in three-ways completely randomize fashion with three replicates was conducted. For Pb stress, a 200 µM PbCl2 solution was applied at a rate of 1 L per pot. FNPs were applied via foliar spray at a concentration of 3 mM. For AMF inoculation rhizospheric soil was collected from Sorghum bicolor fields and used in this experiment. Results of the study showed that Pb toxicity greatly reduced growth (shoot length; 15%, root length; 25%) of B. napus plants. It lowered photosynthesis (38%) and gas exchange related attributes. Pb contamination caused oxidative stress, evident from elevated level of malondialdehyde (62%), and reactive oxygen species (H2O2; 60%, OH-; 103% and O2•-; 23%). It also triggered the antioxidant defense system of B. napus. These plants also had high Pb metal ions in their root and shoot compared with control. Foliar application of FNPs along with AMF inoculation effectively mitigated oxidative stress caused by Pb via increasing antioxidant enzymes activities. Catalase, peroxidase, superoxide dismutase, ascorbate peroxidase, glutathione reductase, phenylalanine ammonia-lyase and polyphenol peroxidase activities were increased by 37, 19, 96, 200, 47, 117 and 47%, respectively. In conclusion, these treatments modulated photosynthetic machinery, antioxidant defense mechanism and nutrients uptake in B. napus plants to alleviate Pb stress. It is presumed that use of carbon-based nano particles in combination with AMF inoculation could effectively mitigate HMs stress in crop plants grown in contaminated soil.

Development of polyfunctionalized biochar modified with manganese oxide and sulfur for immobilizing Hg(II) and Pb(II) in water and soil and improving soil health

Mercury (Hg) and lead (Pb) pose significant risks to human health due to their high toxicity and bioaccumulative properties. This study aimed to develop a novel biochar composite (HMB-S), polyfunctionalized with manganese dioxide (α-MnO2) and sulfur functional groups, for the effective immobilization of Hg(II) and Pb(II) from contaminated environments. HMB-S demonstrated superior adsorption capacities of 190.1 mg/g for Hg(II) and 259.9 mg/g for Pb(II), which significantly surpasses the capacities of unmodified biochar (HB) and biochar functionalized solely with Mn (HMB). Mechanistic studies revealed that the immobilization of these metals by HMB-S involved ion exchange, mineral precipitation, surface complexation, and electrostatic interactions. In soil incubation experiments, HMB-S significantly decreased the levels of extractable Hg(II) and Pb(II) compared to the control, reducing the mobility of these metals and converting 17 % of Hg(II) and 26 % of Pb(II) into less bioavailable residual forms. Pot experiments confirmed that all tested biochar materials (HB, HMB, and HMB-S) promoted spinach growth in contaminated soils, with HMB-S being the most effective at lowering Hg(II) and Pb(II) uptake by plants. Additionally, analysis of soil microbial communities indicated that HMB-S altered community composition and increased the relative abundance of metal-resistant bacteria. These findings highlight the potential of polyfunctionalized biochar HMB-S as an effective remediation strategy for Hg and Pb contamination in soil and aqueous environments.

Synergistic effects of tree-herb intercropping on the phytoremediation efficiency of cadmium and lead contaminated soil

Tree-herb intercropping has emerged as an effective strategy for the remediation of soil contamination. In this study, the effects of intercropping willow with herbaceous plants Lolium perenne L., Iris lactea Pall. and Bidens pilosa L. were investigated on the phytoremediation of Cd- and Pb-contaminated soil. After a 90-day of cultivation, the results showed that intercropping stimulated the phytoremediation efficiency through increased metal accumulation in plants. Intercropping caused a significant (p < 0.05) increase in willow biomass ranging from 48.07 % to 95.58 % by promoting photosynthesis activities and antioxidant responses. Metal contents in willow leaves and roots were also observably (p < 0.05) enhanced, indicating a beneficial effect in tree-herb intercropping systems. The biomass and metal accumulation of I. lactea Pall. and B. pilosa L. decreased due to competitive interactions with willow in the intercropping treatments. However, intercropping with willow (p < 0.05) significantly increased the Pb contents of L. perenne L. Intercropping improved the absorption of bioavailable fractions of Cd and Pb by willow and herbs in comparison to the monoculture. The decrease in soil Cd contents was partly due to the chemical changes induced by root exudates, which enhanced the transfer of Cd from the soil to the plants. Willow showed a tendency for Cd accumulation, whereas herbs exhibited Pb accumulation, reflecting the complementarity of metal accumulation in tree-herb intercropping patterns. Intercropping willow with B. pilosa L. was found to be an effective method for the remediation of Cd-contaminated soil, whereas the combination of willow with L. perenne L. proved suitable for the Pb-contaminated soil. These findings might support the potential of tree-herb intercropping as an effective strategy for enhancing the phytoremediation of contaminated soils.

Citric acid-driven cadmium uptake and growth promotion mechanisms in Brassica napus

Citric acid (CA) is well-known for mitigating cadmium (Cd) toxicity in plants. Yet, the underlying mechanisms driving growth promotion, Cd detoxification/tolerance, and enhanced phytoremediation processes remain incompletely understood. This study investigated the effects of CA application (2.5 mM) on Brassica napus grown in Cd-contaminated (30 mg kg-1) growth medium through a controlled pot experiment. Cd exposure alone significantly impaired various plant physiological parameters in B. napus. Whereas CA application significantly (p < 0.05) enhanced physiological attributes, Cd detoxification and tolerance by modulating key genes involved in photosynthesis and Cd transport, including the metal-transporting P1B-type ATPases (Cd/zinc heavy metal-transporting ATPase 1; HMA1) and light-harvesting chlorophyll a/b-binding 3 (LHCB3). Notably, CA application increased Cd accumulation in stems and leaves by 4% and 35%, respectively, enhancing bioconcentration factors (BCF) by 12% in stems and 40% in leaves while reducing root BCF by 10%. This translocation was facilitated by the upregulation of HMA4, HMA2, and plant Cd resistance (PCR2) genes in plant leaves, improving Cd mobility within the plant. Furthermore, CA induced a 34% increase in phytochelatins and a 32% upregulation in metallothioneins, accompanied by a significant reduction in oxidative stress markers, including a 40% decrease in hydrogen peroxide and a 44% decline in malondialdehyde levels in leaves. Enhanced antioxidant enzyme activity and osmolyte accumulation further contributed to improved Cd detoxification/sequestration in leaves, reduced oxidative stress, and improved photosynthetic efficiency, resulting in enhanced plant biomass production and Cd tolerance. Transcriptomic analysis showed that CA treatment substantially influenced the expression of 12,291 differentially expressed genes (DEGs), with 750 common genes consistently downregulated in CK vs Cd treatment group but upregulated in Cd vs Cd-CA treatment group. Additionally, CA modulated 11 DEGs associated with 32 gene ontologies in the citrate pathway under Cd stress, highlighting its targeted regulatory effect on metabolic pathways involved in Cd stress response. This study offers novel insights into the synergistic role of CA in promoting plant growth and regulating Cd uptake in B. napus, highlighting its potential to enhance phytoremediation strategies.

Comparative assessment of pantothenic, aspartic, ascorbic and tartaric acids assisted Pb-phytoextraction by sunflower (Helianthus annuus L.)

Phytoextraction of lead (Pb) is a challenging task due to its extremely low mobility within soil and plant systems. In this study, we tested the influence of some novel chelating agents for Pb-phytoextraction using sunflower. The Pb was applied at control (0.0278 mM) and 4.826 mM Pb as Pb(NO3)2 through soil-spiking. After 10 days of Pb addition, four different organic ligands (aspartic, ascorbic, tartaric, and pantothenic acids) were added to the soil at 1 mM concentration each. respectively. In the absence of any chelate, sunflower plants grown at 4.826 mM Pb level accumulated Pb concentrations up to 104 µg g-1 DW in roots, whereas 64 µg g-1 DW in shoot. By contrast, tartaric acid promoted significantly Pb accumulation in roots (191 µg g-1 DW; + 45.5%) and shoot (131.6 µg g-1 DW; + 51.3%). Pantothenic acid also resulted in a significant Pb-uptake in the sunflower shoots (123 µg g-1 DW; + 47.9%) and in roots (177.3 µg g-1 DW; + 41.3%). The least effective amongst the chelates tested was aspartic acid, but it still contributed to + 40.1% more Pb accumulation in the sunflower root and shoots. In addition, plant growth, biochemical, and ionomic parameters were positively regulated by the organic chelates used. Especially, an increase in leaf Ca, P, and S was evident in Pb-stressed plants in response to chelates. These results highlight that the use of biocompatible organic chelates positively alters plant physio-biochemical traits contributing to higher Pb-sequestration in sunflower plant parts.

Interaction of titanium dioxide nanoparticles with PVC-microplastics and chromium counteracts oxidative injuries in Trachyspermum ammi L. by modulating antioxidants and gene expression

The emergence of polyvinyl chloride (PVC) microplastics (MPs) as pollutants in agricultural soils is increasingly alarming, presenting significant toxic threats to soil ecosystems. Ajwain (Trachyspermum ammi L.), a plant of significant medicinal and culinary value, is increasingly subjected to environmental stressors that threaten its growth and productivity. This situation is particularly acute given the well-documented toxicity of chromium (Cr), which has been shown to adversely affect plant biomass and escalate risks to the productivity of such economically and therapeutically important species. The present study was conducted to investigate the individual effects of different levels of PVC-MPs (0, 2, and 4 mg L-1) and Cr (0, 150, and 300 mg kg-1) on various aspects of plant growth. Specifically, we examined growth and biomass, photosynthetic pigments, gas exchange attributes, oxidative stress responses, antioxidant compound activity (both enzymatic and nonenzymatic), gene expression, sugar content, nutritional status, organic acid exudation, and Cr accumulation in different parts of Ajwain (Trachyspermum ammi L.) seedlings, which were also exposed to varying levels of titanium dioxide (TiO2) nanoparticles (NPs) (0, 25, and 50 µg mL-1). Results from the present study showed that the increasing levels of Cr and PVC-MPs in soils significantly decreased plant growth and biomass, photosynthetic pigments, gas exchange attributes, sugars, and nutritional contents from the roots and shoots of the plants. Conversely, increasing levels of Cr and PVC-MPs in the soil increased oxidative stress indicators in term of malondialdehyde, hydrogen peroxide, and electrolyte leakage, and also increased organic acid exudation pattern in the roots of T. ammi seedlings. Interestingly, the application of TiO2-NPs counteracted the toxicity of Cr and PVC-MPs in T. ammi seedlings, leading to greater growth and biomass. This protective effect is facilitated by the NPs' ability to sequester reactive oxygen species, thereby reducing oxidative stress and lowering Cr concentrations in both the roots and shoots of the plants. Our research findings indicated that the application of TiO2-NPs has been shown to enhance the resilience of T. ammi seedlings to Cr and PVC-MPs toxicity, leading to not only improved biomass but also a healthier physiological state of the plants. This was demonstrated by a more balanced exudation of organic acids, which is a critical response mechanism to metal stress.