Protective role of selenium against chromium stress involving metabolites and essential elements in Brassica juncea L. seedlings

Response strategies of slash pine (Pinus elliottii) to cadmium stress and the gain effects of inoculation with Herbaspirillum sp. YTG72 in alleviating phytotoxicity and enhancing accumulation of cadmium

Phytoremediation using fast-growing woody plants assisted by plant growth-promoting bacteria (PGPB) on cadmium (Cd)-contaminated sites is considered a promising technique; however, its remediation efficiency is still affected by multiple factors. In this study, the mining areas' soil conditions were simulated with different Cd addition levels (0, 3, 6, 9 mg kg-1) in order to investigate the response strategy to Cd stress of fast-growing economic tree species, slash pine (Pinus elliottii), and the effects of inoculation with the PGPB strain Herbaspirillum sp. YTG72 on the physiological activity and Cd accumulation of plants. The main results showed that there were significant (p < 0.05) increases in contents of chlorophyll and nutrient elements (P, K, Ca, and Mg) at low Cd addition level (3 mg kg-1) compared to non-Cd addition treatment. When the additive amount of Cd increased, the growth of plants was severely inhibited and the content of proline was increased, as well as Cd in plants. Besides, the ratios of K:P, Ca:P, and Mg:P in plants were negatively correlated with the contents of Cd in plants and soils. Inoculation of P. elliottii with the PGPB strain Herbaspirillum sp. YTG72 improved the physiological functions of the plants under Cd stress and activated the antioxidant system, reduced the accumulation of proline, and decreased the ratios of K:P, Ca:P, and Mg:P in plant. More importantly, planting P. elliottii in Cd-contaminated soil could significantly (p < 0.05) reduce the Cd content in the rhizosphere soil, and furthermore, inoculation treatment could promote the reduction of soil Cd content and increased the accumulation of Cd by root. The results of the present study emphasized the Cd response mechanism of P. elliottii based on multifaceted regulation, as well as the feasibility of strain Herbaspirillum sp. YTG72 assisted P. elliottii for the remediation on Cd-contaminated sites.

Effect of riboflavin on redox balance, osmolyte accumulation, methylglyoxal generation and nutrient acquisition in indian squash (Praecitrullus fistulosus L.) under chromium toxicity

The presence of high chromium (Cr) levels induces the buildup of reactive oxygen species (ROS), resulting in hindered plant development. Riboflavin (vitamin B2) is produced by plants, fungi, and microbes. It serves as a precursor to the coenzymes flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN), which play a crucial role in cellular metabolism. The objective of this work was to clarify the underlying mechanisms by which riboflavin alleviates Cr stress in Praecitrullus fistulosus L. Further, the role of riboflavin in growth, ions homeostasis, methylglyoxal detoxification, and antioxidant defense mechanism are not well documented in plants under Cr toxicity. We found greater biomass and minimal production of ROS in plants pretreated with riboflavin under Cr stress. Results manifested a clear abridge in growth, chlorophyll content, and nutrient uptake in Indian squash plants exposed to Cr stress. Findings displayed that Cr stress visibly enhanced oxidative injury reflected as higher malondialdehyde (MDA), hydrogen peroxide (H2O2), superoxide radical (O2•‒), methylglyoxal (MG) levels alongside vivid lipoxygenase activity. Riboflavin strengthened antioxidant system, enhanced osmolyte production and improved membrane integrity. Riboflavin diminished Cr accumulation in aerial parts that led to improved nutrient acquisition. Taken together, riboflavin abridged Cr phytotoxic effects by improving redox balance because plants treated with riboflavin had strong antioxidant system that carried out effective ROS detoxification. Riboflavin protected membrane integrity that, in turn, improved nutrient uptake in plants.

Interactive effects of selenium and arsenic on phenolic constituents and antioxidant activity in rice (Oryza sativa L.)

Arsenic (As) is a heavy toxic metalloid found in air, water and soil that adversely affects the plant growth by inducing oxidative stress in plants. Its contamination of rice is a serious problem throughout the world. Selenium (Se) is a beneficial micronutrient for plants that acts as an antioxidant at low doses and protect the plants against number of environmental stresses either by modulating the primary metabolic pathways or regulating the production of phenolic compounds. In the present investigation, effect of Se on different phenolics, enzymes related to their metabolism and antioxidative potential were studied in As stressed rice leaves. Rice plants were grown in pots containing sodium arsenate (2-10 mg As(V) kg-1 soil) and sodium selenate (0.5-1 mg Se kg-1 soil), both alone and in combination and leaf samples were analyzed for various biochemical parameters. Phenolic constituents increased in rice leaves with As(V) treatment from 2 to 5 mg kg-1 soil and leaves exposed to As(V) @ 5 mg kg-1 soil exhibited 1.7, 1.9 and 2.5 fold increase in total phenolics, o-dihydroxyphenols and flavonols, respectively at grain filling stage. Binary application of Se + As improved various phenolic constituents, FRAP, reducing power and antioxidant activities as compared to control. PAL, TAL and PPO activities increased from 1.3 to 4.6 fold in combined As + Se treatment at both the stages. Anthocyanin contents showed a decline (10.8 fold) with increasing As doses and its content improved at both the stages with maximum increase of 3.76 fold with As5+Se1 combination. Binary application of As + Se improved gallic acid, chlorogenic acid, 3-hydroxy benzoic acid and kaempferol contents than control whereas catechin and coumaric acid showed the reverse trend. Application of Se can modulate phenolic constituents in leaf and grains of rice Cv PR126 due to As stress that helped plants to adapt to excess As and resulted in improved plant growth.

In vitro chronic phytotoxicity of heavy metals and metalloids to Lepidium sativum (garden cress)

The paper presents the results of studies on the influence of selected concentrations (10-100 mg L-1) of heavy metals (Cd, Co, Cr, Cu, Fe, Hg, Mn, Mo, Ni, Pb, Zn) and metalloids (As, Sb, Se) on the germination and root elongation of garden cress (Lepidium sativum L). There are not many studies on phytotoxicity of heavy metals and metalloids with the complex use of single plant species so far. On the basis of the germination index (GI) and inhibition concentration IC50, the following order of phytotoxicity of the tested elements was determined: Se> As> Hg> Sb > Mo > Cd> Co > Zn > Ni. The other metals showed no phytotoxicity or even stimulating effect. In our study the stimulating effect of the majority of Pb concentrations and the lowest concentrations of Cd and Hg has been revealed. These metals do not play any role in living organisms, however some authors confirm their stimulating effect on plants at low concentrations. Toxic concentration of metals and metalloids calculated as IC50 are lower than the concentration calculated as GI (not phytotoxic). It is well known that seeds are more independent and tolerant to toxicants when they contain reserve substances which are used during the germination period. On the basis of conducted research, high tolerance of L. sativum to heavy metals and metalloids was found, which may indicate its usefulness for phytotoxicity assessment of leachate from contaminated soil or waste (e.g. foundry waste) and its application for bioremediation to manage heavy metal pollution of soils or foundry wastes containing heavy metals and metalloids. The understanding of heavy metal and metalloids toxicity will facilitate bioremediation.

Combined interactions of amino acids and organic acids in heavy metal binding in plants

This research focused on the different approaches to the transport and internal chelation of metals with amino acids and organic acids in plants. Therefore, in the first phase, the plants studied were identified the characteristics of the bioaccumulation factors. Steria pumila, Echium angustifolium, Typha angustifolia, Sisymbrium austriacum were identified as hyperaccumulators (Cd, Ni), accumulators (Pb, Sn, and Se), excluders (Cr, Hg). On the other hand, the Sisymbrium austriacum only showed the characteristic of the accumulator for Cr. In the second phase, the combined effects of amino acids and organic acids on the chelation of heavy metals in plants were tested by a multi-linear regression model. Related to our hypothesis, Amino acids; Gly and Leu (Cd), Trp and Ile (Pb), Asp, Ser, and Leu (Cr), Ser (Hg), Trp and Glu (Ni), Asp, Thr, and Gly (Sn), Asn and Leu (Se), Organic acids; Malonic and Malic acid (Cd), Malonic acid (Pb), Oxalic and Malic acid (Cr), Oxalic, Succinic, Citric and Butyric acid (Hg), Malonic and Malic acid (Ni), Malonic, Maleic, and Malic acid (Sn), Malonic and Citric acid (Se) were concluded that had combined effect for heavy metal's phytochelation ability into plants.

Progress in phytoremediation of chromium from the environment

As chromium (Cr) in ecosystems affects human health through food chain exposure, phytoremediation is an environmentally friendly and efficient way to reduce chromium pollution in the environment. Here, we review the mechanism of absorption, translocation, storage, detoxification, and regulation of Cr in plants. The Cr(VI) form is more soluble, mobile, and toxic than Cr(III), reflecting how various valence states of Cr affect environmental risk characteristics, physicochemical properties, toxicity, and plant uptake. Plant root's response to Cr exposure leads to reactive oxygen species (ROS) generation and apoptosis. Cell wall immobilization, vacuole compartmentation, interaction of defense proteins and organic ligand with Cr, and removal of reactive oxygen species by antioxidants continue plant life. In addition, the combined application of microorganisms, genetic engineering, and the addition of organic acids, nanoparticles, fertilization, soil amendments, and other metals could accelerate the phytoremediation process. This review provides efficient methods to investigate and understand the complex changes of Cr metabolism in plants. Preferably, fast-growing, abundantly available biomass species should be modified to mitigate Cr pollution in the environment as these green and efficient remediation technologies are necessary for the protection of soil and water ecology.

Comparative physiological and metabolomics analyses using Ag⎯NPs and HAS31 (PGPR) to alleviate Cr stress in barley (Hordeum vulgare L.)

In the current industrial scenario, chromium (Cr) as a metal is of great importance but poses a major threat to the ecosystem because of its toxicity, but fewer studies have been conducted on its effects and alleviation strategies by using nanoparticles (NPs) and plant growth promoting rhizobacteria (PGPR). Taking into consideration the positive effects of silver⎯nanoparticles (Ag⎯NPs) and (HAS31) rhizobacteria in reducing Cr toxicity in plants, the present study was conducted. A pot experiment was conducted to determine the effects of single and/or combined application of different levels [0 (no Ag⎯NPS), 15 and 30 mM] of Ag⎯NPs and HAS31 [0 (no HAS31), 50 g and 100 g] on Cr accumulation, morpho-physiological and antioxidative defense attributes of barley (Hordeum vulgare L.) exposed to severe Cr stress [0 (without Cr stress), 50 and 100 μM)]. Results from the present study showed that the increasing levels of Cr in the soil significantly (P < 0.05) decreased plant growth and biomass, photosynthetic pigments, gas exchange attributes, sugars, and nutritional contents from the roots and shoots of the plants. In contrast, increasing levels of Cr in the soil significantly (P < 0.05) increased oxidative stress indicators in term of malondialdehyde, hydrogen peroxide, and electrolyte leakage, and also increased organic acid exudation patter in the roots of H. vulgare. Although, the activities of enzymatic antioxidants and the response of their gene expressions in the roots and shoots of the plants and non-enzymatic such as phenolic, flavonoid, ascorbic acid, and anthocyanin contents were increased by increasing the Cr concentration in the soil. The negative impacts of Cr injury were reduced by the application of PGPR (HAS31) and Ag⎯NPs, which increased plant growth and biomass, improved photosynthetic apparatus, antioxidant enzymes, and mineral uptake, as well as diminished the exudation of organic acids and oxidative stress indicators in roots of H. vulgare by decreasing Cr toxicity. Research findings, therefore, suggest that the application of PGPR (HAS31) and Ag⎯NPs can ameliorate Cr toxicity in H. vulgare, resulting in improved plant growth and composition under metal stress, as depicted by balanced exudation of organic acids.

Allantoin alleviates chromium phytotoxic effects on wheat by regulating osmolyte accumulation, secondary metabolism, ROS homeostasis and nutrient acquisition

Allantoin is a nitrogen metabolite with significant potential to mediate plant defense responses under salinity. However, the impact of allantoin on ions homeostasis and ROS metabolism has yet to be established in plants under Cr toxicity. In the current study, chromium (Cr) notably diminished growth, photosynthetic pigments, and nutrient acquisition in two wheat cultivars (Galaxy-2013 and Anaj-2017). Plants subjected to Cr toxicity displayed excessive Cr accumulation. Chromium produced substantial oxidative stress reflected as higher levels of O₂^•, H₂O₂, MDA, methylglyoxal (MG) and lipoxygenase activity. Plants manifested marginally raised antioxidant enzyme activities due to Cr stress. Further, reduced glutathione (GSH) levels diminished with a concurrent rise in oxidized glutathione levels (GSSG). Plants exhibited a considerable abridge in GSH:GSSG due to Cr toxicity. Allantoin (200 and 300 mg L¹) subsided metal phytotoxic effects by strengthening the activities of antioxidant enzymes and levels of antioxidant compounds. Plants administered allantoin displayed a considerable rise in endogenous H₂S and nitric oxide (NO) levels that, in turn, lessened oxidative injury in Cr-stressed plants. Allantoin diminished membrane damage and improved nutrient acquisition under Cr stress. Allantoin markedly regulated the uptake and distribution of Cr in wheat plants, abridging the degree of metal phytotoxic effect.

Brassica juncea and the Se-hyperaccumulator Stanleya pinnata exhibit a different pattern of chromium and selenium accumulation and distribution while activating distinct oxidative stress-response signatures

Soils high in chromium and selenium exist in some countries, like China, India and the US. In the forms of chromate and selenate, these elements can compete during uptake by plants and lead to secondary effects on the absorption of the essential nutrient sulfur. In this study, we evaluated the potential of Brassica juncea and the Se-hyperaccumulator Stanleya pinnata to take-up and store chromium and selenium when applied individually or jointly, the effect on sulfur content, and the plant antioxidant responses. The aim is to advise the best use of these species in phytotechnologies. Plants were grown hydroponically with 50 μM chromate, 50 μM selenate and equimolar concentrations of both elements (50 μM chromate + 50 μM selenate). Our results suggest that B. juncea and S. pinnata possess transport systems with different affinity for chromate and selenate. The joint application of chromate and selenate restricted the accumulation of both elements, but the reduction of selenate uptake by chromate was more evident in B. juncea. On the other hand, selenate decreased chromium accumulation in B. juncea, whereas in S. pinnata such effect was evident only in roots. B. juncea plants stored more chromium and selenium than S. pinnata due to the higher biomass produced, but less selenium when treated with both elements. Chromate and selenate decreased sulfur accumulation in both species, but B. juncea was more sensitive to their toxicity when applied individually, as revealed by increased lipid peroxidation, hydrogen peroxide content in roots and antioxidant enzyme activity. This species can still be efficient for chromium and selenium phytoextraction as these elements in soil are less available than in hydroponics. In soils high in both elements, or low in selenium, S. pinnata is preferred for selenium phytoextraction and the biomass could be used for crop biofortification due its negligible chromium content.

Biochemical and proteomic insights revealed selenium priming induced phosphorus stress tolerance in common bean (Phaseolus vulgaris L.)

BACKGROUND

Mineral stress is one of the dominating abiotic stresses, which leads to decrease in crop production. Selenium (Se) seed priming is a recent approach to mitigate the plant's mineral deficiency stress. Although not an essential element, Se has beneficial effects on the plants in terms of growth, quality, yield and plant defense system thus, enhancing plant tolerance to mineral deficiency.

METHODS AND RESULTS

The present research was accomplished to find out the effect of Se priming on common bean plant (SFB-1 variety) under phosphorus (P) stress. The seeds were grown invitro on four different MGRL media which are normal MGRL media as control with non-Se primed seeds (Se^- P⁺), non -Se primed seeds grown on P deficient MGRL media (Se^- P^-), Se primed seeds grown on normal MGRL media (Se⁺P⁺) and Se primed seeds grown on P deficient MGRL media (Se⁺P ^-). The various morphological and biochemical parameters such as proline content, total sugar content, polyphenols and expression of proteins were analyzed under P stress. The results showed that Se priming has significantly (p ≤ 0.05) affected the morphological as well as biochemical parameters under normal and P stress conditions. The morphological parameters-length, weight, number of nodes and leaves of Se⁺P⁺, Se⁺P^- root and shoot tissue showed significant increase as compared to Se^-P⁺, Se^-P^-. Similarly various biochemical parameters such as total chlorophyll content, proline, total sugar content and polyphenols of Se⁺P⁺, Se⁺P^- increased significantly as compared to Se^-P⁺, Se^-P^-. The differential protein expression in both Se⁺P⁺, Se⁺P^- and Se^-P⁺, Se^-P^- plants were determined using MALDI-MS/MS. The differentially expressed proteins in Se⁺P⁺, Se⁺P^- plants were identified as caffeic acid-3-O-methyltransferase (COMT) and SecA protein (a subunit of Protein Translocan transporter), and are found responsible for lignin synthesis in root cell walls and ATP dependent movement of thylakoid proteins across the membranes in shoot respectively. The differential expression of proteins in plant tissues, validated morphological and biochemical responses such as maintaining membrane integrity, enhanced modifications in cellular metabolism, improved polyphenol activities and expression of defensive proteins against mineral deficiency.

CONCLUSIONS

The study provided an understanding of Se application as a potential approach increasing tolerance and yield in crop plants against mineral deficiency.

Chromium toxicity, speciation, and remediation strategies in soil-plant interface: A critical review

In recent decades, environmental pollution with chromium (Cr) has gained significant attention. Although chromium (Cr) can exist in a variety of different oxidation states and is a polyvalent element, only trivalent chromium [Cr(III)] and hexavalent chromium [Cr(VI)] are found frequently in the natural environment. In the current review, we summarize the biogeochemical procedures that regulate Cr(VI) mobilization, accumulation, bioavailability, toxicity in soils, and probable risks to ecosystem are also highlighted. Plants growing in Cr(VI)-contaminated soils show reduced growth and development with lower agricultural production and quality. Furthermore, Cr(VI) exposure causes oxidative stress due to the production of free radicals which modifies plant morpho-physiological and biochemical processes at tissue and cellular levels. However, plants may develop extensive cellular and physiological defensive mechanisms in response to Cr(VI) toxicity to ensure their survival. To cope with Cr(VI) toxicity, plants either avoid absorbing Cr(VI) from the soil or turn on the detoxifying mechanism, which involves producing antioxidants (both enzymatic and non-enzymatic) for scavenging of reactive oxygen species (ROS). Moreover, this review also highlights recent knowledge of remediation approaches i.e., bioremediation/phytoremediation, or remediation by using microbes exogenous use of organic amendments (biochar, manure, and compost), and nano-remediation supplements, which significantly remediate Cr(VI)-contaminated soil/water and lessen possible health and environmental challenges. Future research needs and knowledge gaps are also covered. The review's observations should aid in the development of creative and useful methods for limiting Cr(VI) bioavailability, toxicity and sustainably managing Cr(VI)-polluted soils/water, by clear understanding of mechanistic basis of Cr(VI) toxicity, signaling pathways, and tolerance mechanisms; hence reducing its hazards to the environment.

The impact of chromium ion stress on plant growth, developmental physiology, and molecular regulation

In recent years, heavy metals-induced soil pollution has increased due to the widespread usage of chromium (Cr) in chemical industries. The release of Cr into the environment has reached its peak causing hazardous environmental pollution. Heavy metal-induced soil pollution is one of the most important abiotic stress affecting the dynamic stages of plant growth and development. In severe cases, it can kill the plants and their derivatives and thereby pose a potential threat to human food safety. The chromium ion effect on plants varies and depends upon its severity range. It mainly impacts the numerous regular activities of the plant's life cycle, by hindering the germination of plant seeds, inhibiting the growth of hypocotyl and epicotyl parts of the plants, as well as damaging the chloroplast cell structures. In this review article, we tried to summarize the possible effects of chromium-induced stress on plant growth, developmental physiology, biochemistry, and molecular regulation and provided the important theoretical basis for selecting remedial plants in chromium-induced contaminated soils, breeding of low toxicity tolerant varieties, and analyzing the mechanism of plant resistance mechanisms in response to heavy metal stress.

Foliar application of biosynthetic nano-selenium alleviates the toxicity of Cd, Pb, and Hg in Brassica chinensis by inhibiting heavy metal adsorption and improving antioxidant system in plant

Biosynthetic nano-selenium (bio-SeNP), as a plant growth regulator, has better bioavailability and lower toxicity than selenite and selenate. This study investigated the beneficial role of bio-SeNP in mitigating the adverse effects of multiple heavy metals (HMs, e.g., Cd, Pb, and Hg) on growth and yield of pak choi (Brassica chinensis) grown in slightly or heavily polluted (SP or HP) soil by regulating metabolic and antioxidant systems. The results revealed that foliar application of bio-SeNP (5, 10, 20 mg L^-1 Se) at the 6-leaf stage greatly reduced the levels of Cd, Pb, and Hg in shoots and roots of pak choi. Application of 5 mg L^-1 bio-SeNP significantly (p < 0.05) decreased the translocation factor (TF) of Cd, Pb, and Hg from root to shoot by 9.83%, 44.21%, and 46.99% for SP soil, 24.17%, 56.00%, and 39.36% for HP soil, respectively. Meanwhile, all bio-SeNP treatments led to a significant improvement in plants growth by enhancing the antioxidant defense system (e.g., AsA-GSH) and promoting chlorophyll synthesis as well as suppressed the lipid peroxidation products contents (MDA) in shoots. Moreover, the enhanced levels of mineral nutrient elements (e.g., Ca, Mg, Fe, or Zn) and organic selenium (e.g., selenocystine, Se-methylselenocysteine, and selenomethionine) in the edible shoots of bio-SeNP-treated pak choi plant under multiple HMs stress indicated the positive impacts of bio-SeNP on the improvement of shoot quality and nutritional values. Collectively, our results indicated that bio-SeNP play an important role in the management of multiple HMs-induced adverse effects on pak choi. Foliar application of bio-SeNP at appropriate concentration (≤ 5 mg L^-1 Se) can be considered as a promising agronomic measure for safety leafy vegetable production in multiple HMs polluted soils when bio-SeNP application.

Harzianopyridone Supplementation Reduced Chromium Uptake and Enhanced Activity of Antioxidant Enzymes in Vigna radiata Seedlings Exposed to Chromium Toxicity

This study explains the scarce information on the role of harzianopyridone (HZRP) in the alleviation of chromium (Cr) stress alleviation in Vigna radiata (L.). To this end, V. radiata seedlings primed with HZRP at 1 and 2 ppm were exposed to 50 mg kg^-1 Cr for 30 days. Cr stress reduced growth, chlorophyll (Chl) content, net photosynthetic rate, gas-exchange attributes along with enhanced oxidative damages, i.e., electrolyte leakage (EL), hydrogen peroxide (H₂O₂), and malondialdehyde (MDA). Application of HZRP enhanced intercellular carbon dioxide (CO₂) concentration, stomatal conductance, and net photosynthetic rate with decreased activity of the chlorophyllase (Chlase) enzyme in V. radiata seedlings exposed to Cr stressed conditions. To maintain Cr-induced oxidative damages, HZRP treatment increased the levels of antioxidant metabolites (phenolic and flavonoids) and the activity of antioxidative enzymes [superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD)] in V. radiata seedlings grown in normal and Cr-polluted potted soil. In addition to this, glycine betaine content was also increased in plants grown in Cr-contaminated soil. It is proposed the potential role of supplementation of HZRP in mitigating Cr stress. Further research should be conducted to evaluate the potential of HZRP in the mitigation of abiotic stresses in plants.

Effects of Chromium Toxicity on Physiological Performance and Nutrient Uptake in Two Grapevine Cultivars (Vitis vinifera L.) Growing on Own Roots or Grafted onto Different Rootstocks

Chromium toxicity is considered within the most severe and dangerous nutritional disorders, and it can often be observed in crops grown in industrial areas. The present study aims to determine the effects of Cr(VI) toxicity on the growth, nutrition, and physiological performance of grapevines. In a pot hydroponic experiment, own-rooted Merlot and Cabernet Franc grapevine cultivars or cultivars grafted onto 1103P and 101-14 Mgt rootstocks were exposed to 120 μM Cr(VI). Leaf interveinal chlorosis appeared after forty-five days of treatment. Overall leaf chlorosis and brown root coloration after sixty days was reported. A significant effect on the majority of the measured parameters due to the Cr(VI) treatment was observed. Chromium stress increased the total Cr concentrations in all parts of the vines, i.e., leaves, shoots, roots, and trunks. When comparing between the studied plant sections, the roots presented the highest Cr concentrations, ranging from 396 to 868 mg kg−1 d. w., and then, in descending order, the Cr concentrations ranged from 41 to 102 mg kg−1 d. w. in the trunks, from 2.0 to 3.3 mg kg−1 d. w. in the leaves, and from 1.9 to 3.0 mg kg−1 d. w. in the shoots. Between the assessed rootstocks, 1103P was identified to be a better excluder of Cr concentration in the roots and other aerial parts of the vines. Additionally, chromium toxicity negatively affected the concentrations and compartmentalization of the most important nutrients. Leaf chlorophyll (Chl) concentration decreased down to approximately 53% after sixty days of Cr stress. Chromium toxicity significantly reduced the stem water potential (SWP), net CO2 assimilation rate (A), stomatal conductance (gs), and PSII maximum quantum yield in all the cases of grafted or own-rooted vines. At this stage, chromium stress increased the leaf total phenolic content from 46.14% in Merlot vines to 75.91% in Cabernet Franc vines.

Taurine modulates dynamics of oxidative defense, secondary metabolism, and nutrient relation to mitigate boron and chromium toxicity in Triticum aestivum L. plants

The present study was undertaken to appraise the efficacy of exogenous taurine in alleviating boron (B) and chromium (Cr) toxicity. Taurine protects cell membranes from lipid peroxidation due to its function as a ROS scavenger. However, there exists no report in the literature on the role of taurine in plants under abiotic stresses. The present investigation indicated the involvement of exogenous taurine in mediating plant defense responses under B and Cr toxicity. Wheat plants manifested a significant drop in growth, chlorophyll molecules, SPAD values, relative water content, nitrate reductase activity, and uptake of essential nutrients under B, Cr, and combined B-Cr toxicity. Plants showed significant oxidative damage due to enhanced cellular levels of superoxide radicals (O₂^•-), hydrogen peroxide (H₂O₂), malondialdehyde (MDA), relative membrane permeability, and activity of lipoxygenase (LOX). Additionally, a significant negative correlation existed in B and Cr levels with the uptake of essential nutrients. Taurine substantially improved growth, photosynthetic pigments, and nutrient uptake by regulating ROS scavenging, secondary metabolism, and ions homeostasis under stress. Taurine protected plants from the detrimental effects of B and Cr by upregulating the production of nitric oxide, hydrogen sulfide, glutathione, and phenolic compounds.

Melatonin Promotes SGT1-Involved Signals to Ameliorate Drought Stress Adaption in Rice

Drought has become one of the environmental threats to agriculture and food security. Applications of melatonin (MT) serve as an effective way to alleviate drought stress, but the underlying mechanism remains poorly understood. Here, we found that foliar spray of 100-µM MT greatly mitigated the severe drought stress-induced damages in rice seedlings, including improved survival rates, enhanced antioxidant system, and adjusted osmotic balance. However, mutation of the suppressor of the G2 allele of skp1 (OsSGT1) and ABSCISIC ACID INSENSITIVE 5 (OsABI5) abolished the effects of MT. Furthermore, the upregulated expression of OsABI5 was detected in wild type (WT) under drought stress, irrespective of MT treatment, whereas OsABI5 was significantly downregulated in sgt1 and sgt1abi5 mutants. In contrast, no change of the OsSGT1 expression level was detected in abi5. Moreover, mutation of OsSGT1 and OsABI5 significantly suppressed the expression of genes associated with the antioxidant system. These results suggested that the functions of OsSGT1 in the MT-mediated alleviation of drought stress were associated with the ABI5-mediated signals. Collectively, we demonstrated that OsSGT1 was involved in the drought response of rice and that melatonin promoted SGT1-involved signals to ameliorate drought stress adaption.

Selenium nanoparticles reduced cadmium uptake, regulated nutritional homeostasis and antioxidative system in Coriandrum sativum grown in cadmium toxic conditions

Nanotechnology has become a valuable novel approach to manage several environmental challenges through providing innovative and effective solutions. Heavy metal stress is an important abiotic limiting factor. Seed priming with selenium (Se) alleviates various kinds of environmental stresses; yet, the potential of seed priming with selenium nanoparticles (SeNPs) under cadmium (Cd) stress for coriander crop has never been evaluated. This research work was designed to explore the effects of seed priming with three levels (0, 5, 10 and 15 mg L^-1) of SeNPs solution on the physio-biochemical characteristics, nutrition, antioxidative defense system and growth of coriander under Cd stress. Cadmium toxicity reduced chlorophyll content, photosynthetic activity and growth of treated plants. Moreover, Cd stressed plants exhibited modulations in proline level, together with decreased water potential, and leaf osmotic potential. However, SeNPs increased growth attributes, chlorophyll content, total soluble sugars, leaf relative water content, and gas exchange parameters in treated plants which were conversely decreased by Cd toxicity. The seeds priming with SeNPs promoted antioxidant response by increasing catalase (CAT), ascorbate peroxidase (APX) and peroxidase (POX) activity and safeguarding cellular structures through scavenging free radicals and reactive oxygen species. Furthermore, Cd stressed plants displayed an upper level of MDA (1.91 fold) while SeNPs improved membranous integrity through detoxification of hydrogen peroxide. Additionally, SeNPs enhanced nutrients contents (P, K, Ca, Mg, Zn), metal tolerance index and diminished Cd content in plants resulting in the improved growth and development of Cd affected coriander plants.

Se changed the component of organic chemicals and Cr bioavailability in pak choi rhizosphere soil

Rhizosphere organic chemicals response and its role on Cr/Se adsorption are of great importance to understand Cr/Se bioavailability in Cr-contaminated soil with the application of Se. In the current work, the processes were carried out using rhizobox experiment (Brassica campestris L. ssp. chinensis Makino). The results showed that in soil contaminated by 200 mg kg^-1 Cr(III), Se(IV) complexed with Cr(III) and carboxylic acid (cis-9,10-Epoxystearic acid, hexadecanedioic acid) reduced Cr(VI) to Cr(III), thus increasing of Cr adsorption, furtherly decreasing Cr bioavailability. While in soil contaminated by 120 mg kg^-1 Cr(VI), Se(VI) competed for adsorption sites with Cr(VI) and salicylic acid activated insoluble Cr(III), thus decreasing Cr adsorption, finally increasing Cr bioavailability. Moreover, with Cr contamination, Se bioavailability in soil was enhanced by the secretion of carboxylic acid, which can reduce Se to lower valent state and compete the adsorption sites and complex with Se oxyanion. These results yielded a better understanding of rhizosphere dynamics regulating by Se application in Cr-contaminated soil. Moreover, the current study supplemented the theoretical basis for beneficial elements application as an environment-friendly resource to facilitate cleaner production in heavy metal contaminated soil.

Selenium restores mitochondrial dysfunction to reduce Cr-induced cell apoptosis in Chinese cabbage (Brassica campestris L. ssp. Pekinensis) root tips

Chromium (Cr) disrupts the growth and physiology of plants. Selenium (Se) is considered as a promising option to help plants ameliorate Cr toxicity. To investigate the effects of exogenous Se on reactive oxygen species (ROS) burst and programmed cell death (PCD) in root tip cells under Cr stress, hydroponic experiments were carried out with Chinese cabbage seedlings grown in Hoagland solution containing 1 mg L^-1 Cr and 0.1 mg L^-1 Se. Results showed that Se scavenged the overproduction of H₂O₂ and O₂^－·, and alleviated the level of lipid peroxidation in root tips stressed by Cr. Moreover, Se effectively prevented DNA degradation and reduced the number of apoptotic cells in root tips. Compared with Cr treatment, Se supplementation reduced the content of ROS and malondialdehyde in mitochondria by 38.23% and 17.52%, respectively. Se application decreased the opening degree of mitochondrial permeability transition pores by 32.30%, increased mitochondrial membrane potential by 40.91%, alleviated the release of cyt c from mitochondria into cytosol by 18.42% and caused 57.40% decrease of caspase 3-like protease activity, and thus restored mitochondrial dysfunction caused by Cr stress. In addition, the alteration of Se on mitochondrial physiological properties maintained calcium homeostasis between mitochondria and cytosol, which further contributed to reducing the appearance of Cr-induced PCD. Findings suggested that Se restored mitochondrial dysfunction, which further rescued root tip cells from PCD, consequently activating defense strategies to protect plants from Cr toxicity and maintaining plant growth.

Seed soaking with melatonin promotes seed germination under chromium stress via enhancing reserve mobilization and antioxidant metabolism in wheat

Chromium (Cr) pollution has serious harm to crop growth, while little is known on the role of melatonin (MT) on seed germination and physiology in Cr-stressed wheat. The effects of seed soaking with MT on growth, reserve mobilization, osmotic regulation and antioxidant capacity of wheat seeds during germination under hexavalent chromium (100 μM) stress were investigated. The results indicated that Cr toxicity decreased the seed germination rate by 16% and suppressed the growth of germinated seeds compared to unstressed seeds. MT in the concentration-dependent manner increased germination rate and promoted subsequent growth when seeds were exposed to Cr stress, but the effect could be counteracted at high concentration. Seed soaking with MT (100 μM) markedly decreased Cr accumulation in seeds, radicals and coleoptiles by 15%, 6% and 15%, respectively, and enhanced α-amylase activity and soluble sugar and free amino acids content in seeds to improve reserve mobilization under Cr stress, compared with Cr treatment. Furthermore, decreasing the level of osmotic regulators (soluble sugar and soluble protein) in radicles under MT combined with Cr treatment confirmed the reduction of osmotic stress caused by Cr stress. Importantly, MT pretreatment reduced H₂O₂ content by 19% and O₂·^- release rate by 45% in radicles under Cr toxicity compared with Cr-stressed wheat, in terms of promoting scavenging ability and decreasing production ability, which was to upregulate the activities and encoding genes expression levels of superoxide dismutase (SOD), catalase (CAT), ascorbic acid peroxidase (APX) and peroxidase (POD) and to downregulate plasma membrane-bound NADPH oxidase (NOX) encoding genes (TaRbohD, TaRbohF) expression, respectively. In all, these results provided evidence that seed soaking with MT could be a potentially method to protect wheat seeds from Cr toxicity, which effectively ameliorated germination under Cr stress by enhancing reserve mobilization and antioxidant metabolism in wheat.

Menadione sodium bisulfite alleviated chromium effects on wheat by regulating oxidative defense, chromium speciation, and ion homeostasis

Menadione sodium bisulfite (MSB) is a crucial growth regulator mediating plant defense response. MSB-mediated regulation of defense mechanisms in wheat under chromium (Cr) toxicity has not been reported in the literature. Therefore, the present study was undertaken to appraise the efficacy of exogenous MSB on circumventing Cr phytotoxic effects on wheat. We also compared the effects of water-soluble MSB with that of water-insoluble menadiol diacetate (MD). The levels used in the present investigation for MSB and MD were 100 and 200 mg L-1. Wheat plants grown in soil contaminated with 25 mg kg-1 Cr in the form of K2Cr2O7 showed a notable reduction in growth, chlorophyll molecules, relative water contents, grain yield, total soluble sugars, phenolics, flavonoids, ascorbic acid, activities of antioxidant enzymes (SOD, POD, CAT), and uptake of essential nutrients (K, P, and Ca). Cr toxicity caused a noticeable accretion in total free amino acids, proline, malondialdehyde, H2O2, O2•-, relative membrane permeability, methylglyoxal contents, activities of enzymes (lipoxygenase, glutathione-S-transferase, and ascorbate peroxidase), nitric oxide and H2S contents, glutathione and oxidized glutathione contents, total Cr contents, and Cr6+ and Cr3+ accumulation. MSB application significantly reduced lipid peroxidation, ROS overproduction, methylglyoxal levels, total Cr contents, and maintained higher Cr3+:Cr6+ ratio in aerial parts. Besides, Cr-mediated inhibition in essential nutrient uptake was significantly circumvented by exogenous MSB. Consequently, MSB enhanced wheat growth by lessening oxidative damage, total Cr contents in aerial parts, and strengthening antioxidant enzyme activities. MD was not effective in mediating defense responses in wheat under Cr toxicity.

Uptake, transport, and metabolism of selenium and its protective effects against toxic metals in plants: a review

Selenium (Se) is an essential trace element of fundamental importance to humans, animals, and plants. However, the uptake, transport, and metabolic processes of Se and its underlying mechanisms in plants have not been well characterized. Here, we review our current understanding of the adsorption and assimilation of Se in plants. First, we discussed the conversion of Se from inorganic Se into organic forms, the mechanisms underlying the formation of seleno-amino acids, and the detoxification of Se. We then discussed the ways in which Se protects plants against toxic metal ions in the environment, such as by alleviating oxidative stress, regulating the activity of antioxidant enzymes, sequestering metal ions, and preventing metal ion uptake and accumulation. Generally, this review will aid future research examining the molecular mechanisms underlying the antagonistic relationships between Se and toxic metals in plants.

Selenium yeast promoted the Se accumulation, nutrient quality and antioxidant system of cabbage (Brassica oleracea var. capitata L.)

The application of Se yeast as a Se source to cultivate Se-rich cabbage has a significant effect on cabbage growth and quality indices. Results showed that total plant weight, head weight, and head size in cabbage were notably increased by 48.4%, 88.3%, and 25.4% under 16 mg/kg Se yeast treatment, respectively. Compare with the control, a high proportion of 3874% of Se accumulation in cabbage head was also detected in 16 mg/kg Se yeast treatment. Selenocystine (SeCys₂) and Methyl-selenocysteine (MeSeCys) were the main Se speciations in the cabbage head. Application of 8 mg/kg Se yeast improved cabbage quality and antioxidant system indices, including free amino acid, soluble sugar, ascorbic acid, phenolic acid, glucosinolates, and SOD activity, which had 81.6%, 46.5%, 34.9%, 12.3%, 44.8%, 25.2% higher than that of the control, respectively. In summary, considering 8 mg/kg Se yeast as the appropriate level of Se enrichment during cabbage cultivation. These findings enhanced our understanding of the effects of Se yeast on the growth and quality of cabbage and provided new insights into Se-enrichment vegetable cultivation.

Chromium (VI)-Induced Alterations in Physio-Chemical Parameters, Yield, and Yield Characteristics in Two Cultivars of Mungbean (Vigna radiata L.)

Chromium (Cr) presently used in various major industries and its residues possess a potent environmental threat. Contamination of soil and water resources due to Cr ions and its toxicity has adversely affected plant growth and crop productivity. Here, deleterious effects of different levels of Cr (VI) treatments i.e., 0, 30, 60, 90, and 120 μM on two mungbean cultivars, Pusa Vishal (PV) and Pusa Ratna (PR), in hydroponic and pot conditions were evaluated. Germination, seedling growth, biomass production, antioxidant enzyme, electrolytic leakage, oxidative stress (hydrogen peroxide and malondialdehyde), and proline content were determined to evaluate the performance of both cultivars under hydroponic conditions for 15 days. The hydroponic results were further compared with the growth and seed yield attributes of both the genotypes in pot experiments performed over 2 years. Seedling growth, biomass production, total chlorophyll (Chl), Chl-a, Chl-b, nitrogen content, plant height, seed protein, and seed yield decreased significantly under the 120 μM Cr stress level. Activities of antioxidant enzymes superoxide dismutase, catalase, ascorbate peroxidase and peroxidase increased in the leaves following Cr exposure at 60-90 μM but declined at 120 μM. Cr-induced reductions in growth and seed yield attributes were more in the sensitive than in the tolerant cultivar. Cr accumulation in the roots, stems, leaves, and seeds increased with an increase in Cr concentrations in the pot conditions. Furthermore, for both cultivars, there were significant negative correlations in morpho-physiological characteristics under high Cr concentrations. Overall results suggest that (PR) is more sensitive to Cr stress (PV) at the seedling stage and in pot conditions. Furthermore, (PV) can be utilized to study the mechanisms of Cr tolerance and in breeding programs to develop Cr-resistant varieties.

Transcriptomic Characterization of the Effects of Selenium on Maize Seedling Growth

Selenium (Se) is a trace mineral element in soils that can be beneficial to plants in small amounts. Although maize is among the most economically important crops, there are few reports on the effects of Se on maize seedling growth at the molecular level. In this study, the growth of maize seedlings treated with different concentrations of Na₂SeO₃ was investigated, and the physiological characteristics were measured. Compared with the control, a low Se concentration promoted seedling growth, whereas a high Se concentration inhibited it. To illustrate the transcriptional effects of Se on maize seedling growth, samples from control plants and those treated with low or high concentrations of Se were subjected to RNA sequencing. The differentially expressed gene (DEG) analysis revealed that there were 239 upregulated and 106 downregulated genes in the low Se treatment groups, while there were 845 upregulated and 1,686 downregulated DEGs in the high Se treatment groups. Both the Gene Ontology (GO) and the Kyoto Encyclopedia of Genes and Genomes (KEGG) annotation analyses showed a low concentration of the Se-stimulated expression of "DNA replication" and "glutathione (GSH) metabolism"-related genes. A high concentration of Se repressed the expression of auxin signal transduction and lignin biosynthesis-related genes. The real-time quantitative reverse transcription PCR (qRT-PCR) results showed that in the low Se treatment, "auxin signal transduction," "DNA replication," and lignin biosynthesis-related genes were upregulated 1.4- to 57.68-fold compared to the control, while, in the high Se concentration treatment, auxin signal transduction and lignin biosynthesis-related genes were downregulated 1.6- to 16.23-fold compared to the control. Based on these transcriptional differences and qRT-PCR validation, it was found that a low dosage of Se may promote maize seedling growth but becomes inhibitory to growth at higher concentrations. This study lays a foundation for the mechanisms underlying the effects of Se on maize seedling growth.

Selenium mitigates salt-induced oxidative stress in durum wheat (Triticum durum Desf.) seedlings by modulating chlorophyll fluorescence, osmolyte accumulation, and antioxidant system

Hydroponic experiments were conducted to investigate the effects of different concentrations of sodium selenate (Na₂SeO₄) and sodium selenite (Na₂SeO₃) on durum wheat seed germination and seedling growth under salt stress. The treatments used were 0 and 50 mM NaCl solutions, each supplemented with Na₂SeO₄ or Na₂SeO₃ at 0, 0.1, 1, 2, 4, 8, or 10 μM. Salt alone significantly inhibited seed germination and reduced seedling growth. Addition of low concentrations (0.1-4 μM) of Na₂SeO₄ or Na₂SeO₃ mitigated the adverse effects of salt stress on seed germination, biomass accumulation, and other physiological attributes. Among them, 1 μM Na₂SeO₄ was most effective at restoring seed germination rate, germination energy, and germination index, significantly increasing these parameters by about 12.35, 24.17, and 11.42%, respectively, compared to salt-stress conditions. Adding low concentrations of Na₂SeO₄ or Na₂SeO₃ to the salt solution also had positive effects on chlorophyll fluorescence indices, decreased the concentrations of free proline and malondialdehyde, as well as electrolyte leakage, and increased catalase, superoxide dismutase, and peroxidase activities in roots and shoots. However, high concentrations (8-10 μM) of Na₂SeO₄ or Na₂SeO₃ disrupted seed germination and seedling growth, with damage caused by Na₂SeO₃ being more severe than that by Na₂SeO₄. It is thus clear that exogenous selenium can improve the adaptability of processing wheat to salt stress and maintain higher photosynthetic rate by decreasing the accumulation of reactive oxygen species and alleviating the degree of membrane lipid peroxidation. Na₂SeO₄ was more effective than Na₂SeO₃ at all given concentrations.

Quality enhancement of Dendrobium officinale and banana juice through probiotic fermentation using beneficial lactic acid-producing bacteria

Consumption of functional juice is becoming increasingly popular. This study aimed to evaluate the effect of probiotic fermentation with Bacillus sp. DU-106; Lactobacillus planturum Lp-43 and Lactobacillus rhamnosus Lr-156 on the biochemical and functional characteristics, antioxidant activities and storage stability of Dendrobium officinale and banana (DOB) juice. The cell levels of the lactic acid bacteria reached near 9 log copies/mL after fermentation at 32 °C for 36 h and could keep in this level after storage at 4 °C for 28 days. After fermentation, total acidity, flavonoids and polysaccharide contents of DOB juice increased, while pH, dissolved oxygen, nitrite, nitrate, vitamin C, β-carotene, sugar contents and antioxidant capacity slightly decreased. Compared with nonfermented DOB juice, polyphenol oxidase (PPO) and peroxidase (POD) activities of fermented DOB decreased significantly, but the color value and the content of total phenols and vitamin C decreased slightly in fermented DOB juice during storage. Overall, the probiotic fermentation with selected strains could improve the quality the DOB juice, enhance antioxidant capacity and storage stability, and destroy accumulation of nitrite and nitrate during storage. Findings of this study would help in the development of beneficial beverages in industrial production.

Ameliorative effect of co-application of Bradyrhizobium japonicum EI09 and Se to mitigate chromium stress in Capsicum annum L

The present study was conducted to explore the potential of Bradyrhizobium japonicum EI09 (EI09) and selenium (Se) alone or in combination to mitigate hexavalent chromium (Cr⁶⁺) stress in Capsicum annum L. Chromium stressed plants exhibited significant reduction in biomass, chlorophyll content and gas exchange characteristics. The inoculated seedlings subjected to Cr⁶⁺stress showed improvement in growth, proline content, gas exchange attributes and total soluble proteins. Likewise, inoculated C. annum seedlings exhibited augmented activity of ascorbate peroxidase (APX), catalase (CAT), peroxidase (POD) and superoxide dismutase (SOD) under Cr⁶⁺ stress. The Cr⁶⁺ stress mitigation in inoculated seedlings was ascribed to reduction in malondialdehyde (MDA) content, hydrogen peroxide (H₂O₂) besides increase activity of flavonoids, proline, phenolic content along with modulation of antioxidative enzymes. The growth-enhancing attributes of bacteria such as indole acetic acid (IAA) content and 1-aminocyclopropane-1-carboxylate deaminase (ACCD) activity enhanced growth in Cr⁶⁺-stressed plants. Moreover, co-treatment of EI09 and 5 µM Se effectively mitigated Cr (VI) stress in C. annum plants. Current studies provide a novel insight into potential of B. japonicum EI09 and Se in reduction of Cr⁶⁺ toxicity in C. annum plants.

Organic chelates decrease phytotoxic effects and enhance chromium uptake by regulating chromium-speciation in castor bean (Ricinus communis L.)

There is limited information available on changes in the uptake of essential nutrients and secondary metabolites accumulation in castor bean under Cr toxicity. Besides, the role of organic chelates (EDTA and citric acid) mediated improvement in Cr uptake by castor bean is mostly unknown. Three independent experiments (sand, hydroponics, and soil) were executed to determine the Cr phytoextraction potential of Ricinus communis L. In the sand experiment, optimum doses of organic chelates (EDTA and citric acid) were selected. These optimum doses of chelates were used in the hydroponics and soil experiments. The results of hydroponics and soil experiments manifested a significant decrease in growth characteristics and leaf pigments in response to Cr stress applied as K2Cr2O7 (a source of Cr6+). The application of organic chelates (2.5 and 5 mM) showed a noticeable improvement in oxidative defense and secondary metabolites accumulation that might have decreased oxidative injury reflected as lower hydrogen peroxide (H2O2) and malondialdehyde (MDA) contents. Moreover, chelates improved the uptake of essential nutrients (K+, Ca2+, Mg2+, Fe2+ and P) alongside significant enhancement in total Cr contents of plants. Our results advocated that chelates application resulted in greater endogenous levels of Cr3+ in plants compared with Cr6+ which is more toxic. In nutshell, organic chelates improved growth by regulating Cr species, ion homeostasis and secondary metabolites accumulation in Ricinus communis L.

Chromium Morpho-Phytotoxicity

Chromium (Cr) is considered as one of the chronic pollutants that cause damage to all living forms, including plants. Various industries release an excessive amount of Cr into the environment. The increasing accumulation of Cr in agricultural land causes a significant decrease in the yield and quality of economically important crops. The Cr-induced biochemical, molecule, cytotoxic, genotoxic, and hormonal impairments cause the inhibition of plant growth and development. In the current study, we reviewed Cr morpho-phytotoxicity related scientific reports published between 2009 to 2019. We mainly focused on the Cr-induced inhibition of seed germination and total biomass production. Furthermore, Cr-mediated reduction in the root, branches, and leave growth and development were separately discussed. The Cr uptake mechanism and interference with the macro and micro-nutrient uptake were also discussed and visualized via a functional model. Moreover, a comprehensive functional model has been presented for the Cr release from the industries, its accumulation in the agricultural land, and ultimate morpho-phytotoxicity. It is concluded that Cr-reduces plant growth and development via its excess accumulation in the plant different parts and/or disruption of nutrient uptake.

Effect of Molybdenum on Plant Physiology and Cadmium Uptake and Translocation in Rape (Brassica napus L.) under Different Levels of Cadmium Stress

This study investigated the beneficial effect of molybdenum (Mo) application on rape plants (Brassica napus L.) grown in a soil polluted by cadmium (Cd). A pot experiment was conducted to determine how different concentrations of exogenous Mo (0, 50, 100, and 200 mg/kg) affect plant physiology, biomass, photosynthesis, cation uptake, and Cd translocation and enrichment in rape plants under Cd stress (0.5 and 6.0 mg/kg). Under single Cd treatment, plant physiological and biochemical parameters, biomass parameters, leaf chlorophyll fluorescence parameters, and macroelement uptake of rape plants decreased, while their malonaldehyde content, proline content, non-photochemical quenching coefficient, and Cd uptake significantly increased, compared to those of the control group (p-values < 0.05). High-Cd treatment resulted in much larger changes in these parameters than low-Cd treatment. Following Mo application, the accumulation of malondialdehyde and proline decreased in the leaves of Cd-stressed plants; reversely, the contents of soluble protein, soluble sugar, and chlorophyll, and the activities of superoxide dismutase and glutathione peroxidase, all increased compared to those of single Cd treatment (p-values < 0.05). Exogenous Mo application promoted shoot and root growth of Cd-stressed plants in terms of their length, fresh weight, and dry weight. The negative effect of Cd stress on leaf chlorophyll fluorescence was substantially mitigated by applying Mo. Exogenous Mo also improved the uptake of inorganic cations, especially potassium (K⁺), in Cd-stressed plants. After Mo application, Cd uptake and accumulation were inhibited and Cd tolerance was enhanced, but Cd translocation was less affected in Cd-stressed plants. The mitigation effect of Mo on Cd stress in rape was achieved through the immobilization of soil Cd to reduce plant uptake, and improvement of plant physiological properties to enhance Cd tolerance. In conclusion, exogenous Mo can effectively reduce Cd toxicity to rape and the optimal Mo concentration was 100 mg/kg under the experimental conditions.

A multivariate analysis of comparative effects of heavy metals on cellular biomarkers of phytoremediation using Brassica oleracea

The biochemical/physiological variations in plant responses to heavy metals stress govern plant's ability to phytoremediate/tolerate metals. So, the comparative effects of different types of heavy metals on various plant responses can better elucidate the mechanisms of metal toxicity and detoxification. This study compared the physiological modifications, photosynthetic performance and detoxification potential of Brassica oleracea under different levels of chromium (Cr), nickel (Ni) and selenium (Se). All the heavy metals induced a severe phytotoxicity to B. oleracea in terms of chlorophyll contents, Ni being the most toxic (76% decrease). Brassica oleracea showed high lipid oxidation: 87% and 273%, respectively in leaves and roots. Furthermore, all the metals increased the activities of catalase and peroxidase, while decreased superoxide dismutase and ascorbate peroxidase. Interestingly, heavy metals decreased hydrogen peroxide contents perhaps due to their possible transformation to another form of reactive oxygen species such as hydroxyl radical. Among the three metals, Ni was more phytotoxic than Cr and Se. Moreover, the phytoremediation/tolerance potential of B. oleracea to Ni, Cr and Se stress varied with the type of metal, their applied levels, response variables and plant organ type (root/shoot). The multivariate analysis separated different plant response variables and heavy metal treatments into different groups based on their correlations.

Selenium induces changes of rhizosphere bacterial characteristics and enzyme activities affecting chromium/selenium uptake by pak choi (Brassica campestris L. ssp. Chinensis Makino) in chromium contaminated soil

Understanding the chemical response and characteristics of bacterial communities in soil is critical to evaluate the effects of selenium (Se) supplement on plant growth and chromium (Cr)/Se uptake in Cr contaminated soil. The rhizosphere soil characteristics of pak choi (Brassica campestris L. ssp. Chinensis Makino) were investigated in soil contaminated with different levels and forms of Cr when supplemented with Se. Although inhibition of plant growth caused by Cr stress was not completely alleviated by Se, Cr content in plant tissues decreased in Cr(VI)120Se5 treatment (Cr(VI): 120 mg kg^-1 soil; Se: 5 mg kg^-1 soil) and its bioavailability in soil decreased in Cr(III)200Se5 (Cr(III): 200 mg kg^-1 soil; Se: 5 mg kg^-1 soil) treatment. Moreover, antagonism of Cr and Se on soil enzyme activities and bacterial communities were revealed. Notably, results of Cr(VI) reduction and Se metabolism functional profiles confirmed that bacterial communities play a critical role in regulating Cr/Se bioavailability. Additionally, the increases of Se bioavailability in Cr contaminated soil were ascribed to oxidation of Cr(VI) and reduction of Se reductases proportions, as well as the enhancing of pH in soil. These findings reveal that Se has the potential capacity to sustain the stability of microdomain in Cr contaminated soil.