Silicon deposition in roots minimizes the cadmium accumulation and oxidative stress in leaves of cowpea plants

Health risk assessment of bread wheat grown under cadmium and nickel stress and impact of silicic acid application on its growth, physiology, and metal uptake

Heavy metal stress poses a significant threat to the productivity of agricultural systems and human health. Silicon (Si) is widely reported to be very effective against the different heavy metal stresses in crops. According to reports, it can help plants that are under cadmium (Cd) and nickel (Ni) stress. The presented work investigated how silicon interacted in Cd- and Ni-stressed wheat and mitigated metal toxicity. A pot experiment was carried out in which wheat crop was irrigated with Cd- and Ni-contaminated water. Application of Cd and Ni-contaminated water to wheat significantly reduced the root and shoot growth parameters and physiological and biochemical factors while increasing the antioxidant enzymatic activity and bioaccumulation of Cd and Ni metal in shoot and root as compared to the control. Application of Si led to an improvement in physiological parameters, i.e., greenness of leaves, i.e., SPAD values (17% and 26%), membrane stability (26% and 25%), and growth parameters i.e., root surface area (42% and 23%), root length (81% and 79%), root dry weight (456% and 190%), root volume (64% and 32%), shoot length (41% and 35%), shoot dry weight of shoot (111% and 117%), and overall grain weight (62% and 72%) under Cd and Ni stress, respectively. It increased the activity of antioxidant activity (max. up to 20%) whereas decreased the metal bioaccumulation of Cd and Ni in the roots and shoot (max. up to 62%) of wheat. It was concluded that the application of Si potentially increases antioxidant activity and metal chelation resulting in decreased oxidative damage and reducing the effect of Cd and Ni stress on wheat which improves growth and physiological parameters as well as inhibits Cd and Ni inclusion in food chain under Cd and Ni toxicity reducing health risks associated with these metals.

Influence of Blanching Time on the Phytochemical and Nutritive Value of Cowpea (Vigna unguiculata L. Walp) Leafy Vegetable

Seasonal production limits the effective utilization of cowpea, which is regarded as food and a cash income crop in most African rural communities. To reduce the bacterial content and inactivate the naturally occurring enzymes that could induce undesirable changes during storage of the vegetable, blanching is applied. However, loss in flavor and nutritional value is experienced. Therefore, a study was conducted to investigate the effects of different blanching times on cowpea at a constant temperature, on its nutritive value. A 2 kg sample of fresh leaves was divided into four portions and blanched at 95°C for 0 (control), 2.5, 5, and 10 min. The study was arranged in a completely randomized design in triplicate. Collected data included moisture, total phenolic, antioxidant activity, and minerals [calcium (Ca), potassium (K), cobalt (Co), chromium (Cr), and silicon (Si)]. Compared to the control, total phenolic increased at 2.5 min to 9.83 mg GAE/g but then decreased by 4.99 and 4.60 mg GAE/g at 5 and 10 min of blanching time, respectively. Similarly, antioxidant activity increased at 2.5 min to 1025 μg AAE/g of WM, but reduced by 751.71 and 641.80 μg AAE/g of WM at 5 and 10 min, respectively. Ca increased at 2.5, 5, and 10 min by 69.10, 62.47, and 74.53 mg/L, respectively. Similarly, K increased at 2.5, 5, and 10 min by 31.57, 49.13, and 46.03 mg/L, respectively. Contrarily, Co decreased by 7.65, 7.37, and 9.29 mg/L at 2.5, 5, and 10 min of blanching, respectively. Similarly, Cr also decreased at 2.5, 5, and 10 min, by 0.23, 0.35, and 0.56 mg/L, respectively. Si increased at 2.5 and 10 min by 4.15 and 3.31 mg/L and reduced at 5 min by 1.61 mg/L. Therefore, blanching time of 2.5 min at a constant temperature of 95°C increased the tested nutritive elements, except for moisture, Co, and Cr.

Metal tolerance and Cd phytoremoval ability in Pisum sativum grown in spiked nutrient solution

In the presented study, the effects of cadmium (Cd) stress and silicon (Si) supplementation on the pea plant (Pisum sativum L.) were investigated. The tendency to accumulate cadmium in the relevant morphological parts of the plant (roots and shoots respectively)-bioaccumulation, the transfer of this element in the plant (translocation) and the physiological parameters of the plant through indicators of oxidative stress were determined. Model studies were carried out at pH values 6.0 and 5.0 plant growth conditions in the hydroponic cultivation. It was shown that Cd accumulates mostly in plant roots at both pH levels. However, the Cd content is higher in the plants grown at lower pH. The Cd translocation factor was below 1.0, which indicates that the pea is an excluder plant. The contamination of the plant growth environment with Cd causes the increased antioxidant stress by the growing parameters of the total phenolic content (TPC), polyphenol oxidase activity (PPO), the malondialdehyde (MDA) and lipid peroxidation (LP). The results obtained showed that the supplementation with Si reduces these parameters, thus lowering the oxidative stress of the plant. Moreover, supplementation with Si leads to a lower content of Cd in the roots and reduces bioaccumulation of Cd in shoots and roots of pea plants.

Nano silicon dioxide reduces cadmium uptake, regulates nutritional homeostasis and antioxidative enzyme system in barley seedlings (Hordeum vulgare L.) under cadmium stress

Cadmium (Cd) toxicity is one of the most severe environmental threats inhibiting crop growth and productivity. Strategies to mitigate the adverse effects of Cd stress on plants are under scrutiny. Nano silicon dioxide (nSiO₂) is an emerging material and could protect plants against abiotic stress. Can nSiO₂ alleviate Cd toxicity in barley, and the possible mechanisms are poorly understood. A hydroponic experiment was conducted to study the mitigation effects of nSiO₂ on Cd toxicity in barley seedlings. The results showed that the application of nSiO₂ (5, 10, 20, and 40 mg/L) increased barley plant growth and chlorophyll and protein content, improving photosynthesis, compared with Cd-treated alone. Specifically, 5-40 mg/L nSiO₂ addition increased net photosynthetic rate (Pn) by 17.1, 38.0, 30.3, and - 9.7%, respectively, relative to the Cd treatment alone. Furthermore, exogenous nSiO₂ reduced Cd concentration and balanced mineral nutrient uptake. The application of 5-40 mg/L nSiO₂ decreased Cd concentration in barley leaves by 17.5, 25.4, 16.7, and 5.8%, respectively, relative to the Cd treatment alone. Moreover, exogenous nSiO₂ lowered malondialdehyde (MDA) content by 13.6-35.0% in roots, and by 13.5-27.2% in leaves, respectively, compared with Cd-treated alone. Besides, nSiO₂ altered antioxidant enzyme activities and alleviated detrimental effects on Cd-treated plants, attaining maximal values at 10 mg/L nSiO₂. These findings revealed that exogenous nSiO₂ application may be a viable option for addressing Cd toxicity of barley plants.

Interactive effect of silicon and nitric oxide effectively contracts copper toxicity in Salvia officinalis L

Excess copper (Cu) causes the toxic effects in plants and health hazards to humans. Therefore, in this study, the effect of sodium silicate (1 mM Si) and sodium nitroprusside (200 µM SNP as a releasing NO), was assessed on Cu tolerance in Salvia officinalis L. plants exposed to 400 µM CuSO4. Results revealed that the combined supplementation with Si and SNP rather than the single application of these chemicals lowered Cu concentrations and translocation factor and increased Mg, Zn, and Fe concentrations in roots and shoots. Furthermore, combined treatment more efficiently decreased electrolyte leakage enhanced the activities of POD and APX in the leaves and roots, and improved relative water content and the content of Chl. a and Chl. b in leaves and consequently further increased tolerance index. Silicon supply enhanced NO content and applying Si + SNP more than the treatment of Si alone increased Si concentrations in the roots and shoots under Cu stress. Therefore, the reciprocal interaction of Si and NO might enhance Cu tolerance in plants, and the combined application of Si and SNP might be a promising strategy to decrease heavy metal accumulation in medicinal plants grown in polluted lands.

The mechanism of silicon on alleviating cadmium toxicity in plants: A review

Cadmium is one of the most toxic heavy metal elements that seriously threaten food safety and agricultural production worldwide. Because of its high solubility, cadmium can easily enter plants, inhibiting plant growth and reducing crop yield. Therefore, finding a way to alleviate the inhibitory effects of cadmium on plant growth is critical. Silicon, the second most abundant element in the Earth's crust, has been widely reported to promote plant growth and alleviate cadmium toxicity. This review summarizes the recent progress made to elucidate how silicon mitigates cadmium toxicity in plants. We describe the role of silicon in reducing cadmium uptake and transport, improving plant mineral nutrient supply, regulating antioxidant systems and optimizing plant architecture. We also summarize in detail the regulation of plant water balance by silicon, and the role of this phenomenon in enhancing plant resistance to cadmium toxicity. An in-depth analysis of literature has been conducted to identify the current problems related to cadmium toxicity and to propose future research directions.

Abatement of Cd in rice grain and toxic risks to human health by the split application of silicon at transplanting and jointing period

Silicon (Si) has the potential to ameliorate the toxic effects of cadmium (Cd) on rice growth and mitigate Cd-uptake by rice under Cd-contaminated soil. However, it is not completely clear whether there are differences in the impacts of different Si management on the chemical behavior of Cd in soil-rice system under Cd-contaminated paddy field. Here, pot trials were conducted to explore the effects of three modes of Si application (T-applying Si at transplanting stage, J-applying Si at jointing stage, TJ-applying Si at transplanting stage and jointing stage with a ratio of 50% to 50%) on the accumulation of Cd in rice grain and the toxic risk of Cd on human health in rice consumption under Cd-polluted soil (4.21 mg·kg^-1), and that without Si application was used as control (CK). Results showed that rice growth and Cd-retention in root were enhanced by Si application, and the retention of Cd in TJ root was the highest, reaching 82.36%∼84.06% of total Cd absorbed by rice plant. TJ also elevated soil pH and CEC value significantly during the whole growth period, diminished Cd availability and converted exchangeable-Cd into residual-Cd in soil. Moreover, Si application reduced Cd concentration in iron plaque, while TJ had the lowest concentration of DCB-Cd and the highest molar ratios of Fe/Cd and Mn/Cd. The bioaccessibility of Cd from grains and cooked rice were decreased by Si application. Compared with T and J, the hazard quotient of digestion from cooked white rice of TJ in gastric phase was reduced by 19.61% and 21.94%, respectively. In brief, TJ had more efficiency on reducing the Cd availability in soil during the rice growing period, promoting the retention of Cd in root, decreasing Cd uptake by rice plant and distribution to grains, as well as the bioaccessibility of Cd from cooked rice. These results also provide a novel strategy of Si application to decrease the risk of Cd migration in the soil-rice-humans system and simultaneously promote rice yields.

Silicon- induced nitric oxide burst modulates systemic defensive responses of Salvia officinalis under copper toxicity

In this study, the role of nitric oxide (NO) burst in modulating Si-induced defensive responses in leaves and roots of Salvia officinalis under copper (Cu) stress were investigated. The result showed that 400 μM Cu markedly reduced shoot dry weight, but increased electrolyte leakage (EL) in leaves and both Si and sodium nitroprusside (SNP as the NO donor) improved these attributes in a dose-dependent manner. Interestingly, Cu toxicity systemically boosted a NO burst in both roots and shoots and applying Si and SNP markedly intensified it. The application of Si and SNP alone as well as their combination improved growth parameters and systemically alleviated Cu-induced lipid peroxidation and H₂O₂ accumulation through lowering Cu accumulation, increasing proline content, enhancing the activities of catalase (CAT) and superoxide dismutase (SOD) in both roots and leaves and up-regulating expression of SOD gene in leaves of S. officinalis. NO generation was substantially arrested and the responses induced by Si were significantly suppressed by pretreatment with 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxy l-3-oxide (cPTIO) as a NO scavenger, Nx-Nitro- L-arginine methyl ester hydrochloride (L-NAME) as a nitric oxide synthase inhibitor, and tungstate as a nitrate reductase inhibitor. These novel results indicate that Si can induce Cu tolerance through triggering NO generation which systemically modulates defensive reactions in both roots and leaves of Salvia officinalis.

Silicon Application Modulates Growth, Physio-Chemicals, and Antioxidants in Wheat (Triticum aestivum L.) Exposed to Different Cadmium Regimes

Silicon (Si) application enhanced the tolerance of plants against different environmental stresses. Therefore, objective of the study revealed that foliar applied Si alleviates the adverse effect of Cd by enhancing the growth, metabolite accumulation, strengthening the antioxidant defense system, reducing oxidative injury, improving plant nutrient status, and decreasing the Cd uptake in wheat. The surface sterilized seeds of Sahar-2006 (tolerant) and Inqalab-91 (sensitive) having the differential metal tolerance capacity were sown in plastic pots containing normal and Cd spiked sandy loamy soil. The design of experiments was completely randomized with 3 replicates per treatment. Two weeks after germination, plants were sprayed with different concentrations of Si (1.5 and 3 mM) with 0.1% surfactant in the form of Tween-20. The plants were harvested after 2 weeks of Si application to determine various attributes. High concentration of Cd (25 mg kg-1) decreased growth-related-attributes, essential nutrient uptake and increase the levels of oxidative stress indicators. The application of Si increased the growth-related attributes, photosynthetic pigments, essential nutrient uptake and also enhanced the activities of various antioxidant compounds (superoxide dismutase (SOD), peroxidase (POD, ascorbate peroxidase (APX) and catalase (CAT) by decreasing the contents of oxidative stress indicators and Cd uptake in root and shoot of both wheat cultivars. Sahar-2006 cultivar showed more tolerance to Cd regimes than that of Inqalab-91 as clear from greater plant dry masses. Thus, our results showed that the applied Si level (3 mM) is an efficient strategy for field use in the areas, where slightly Cd polluted soils limit the agriculture production.

Silicon in mitigation of abiotic stress-induced oxidative damage in plants

Accumulation of reactive oxygen species (ROS), and their destructive effects on cellular organelles are the hallmark features of plants exposed to abiotic stresses. Plants are well-equipped with defensive mechanisms like antioxidant systems to deal with ROS-induced oxidative stress. Silicon has been emerged as an important regulator of plant protective mechanisms under environmental stresses, which can be up-taken from soil through a system of various silicon-transporters. In plants, silicon is deposited underneath of cuticles and in the cell wall, and help plant cells reduce deleterious effects of stresses. Furthermore, silicon can provide resistance to ROS-toxicity, which often accounts for silicon-mediated improvement of plant tolerance to different abiotic constraints, including salinity, drought, and metal toxicity. Silicon enhances the ROS-detoxification ability of treated plants by modulating the antioxidant defense systems, and the expression of key genes associated with oxidative stress mitigation and hormone metabolism. Silicon also displays additive roles in ROS-elimination when supplied with other external stimuli. Here, we discuss recent findings on how silicon is able to modulate antioxidant defense of plants in response to oxidative stress triggered by different abiotic constraints. We also review interactions of silicon with other signaling molecules, including nitric oxide, ROS, polyamines, and phytohormones in the mediation of plant protection against abiotic stress-induced oxidative damage.

Effects of rice straw and rice straw ash on rice growth and α-diversity of bacterial community in rare-earth mining soils

Pot experiments were carried out to study the effects of rice straw (RS) and rice straw ash (RSA) on the growth of early rice and α-diversity of bacterial community in soils around rare earth mining areas of Xunwu and Xinfeng counties in South Jiangxi of China. The results showed that the exploitation of rare earth resources leads to soil pollution around rare earth mining areas and affects the growth of rice, and the content of rare earth elements (REEs) in rice was positively correlated with that in soils and negative correlated with dry weight of rice; The addition of RS to soils around REE mining area can inhibit growth of early rice, and the dry weight of rice grains, shoots, roots is lower when compared with the controls, while the content of REEs is higher. The α-diversity of soil bacterial decreases, which promotes the growth of Pseudorhodoferax, Phenylobacterium and other bacteria of the same kind, and inhibits the growth of beneficial bacteria. The addition of RSA to soils had no significant effect on α-diversity of soil bacterial but promoted the growth of Azospira and other beneficial bacteria, inhibited the growth of Bryobacter and other bacteria of the same kind, significantly improved the dry weight of grains, shoots and roots of early rice, and reduced the content of REEs in these parts of rice. It can be concluded that RS is unsuitable to be added to the planting soil of early rice in REE mining area, while RSA is suitable.

Alleviating effects of silicate, selenium, and microorganism fertilization on lead toxicity in ginger (Zingiber officinale Roscoe)

The aim of this work was exploring the effects of silicon, selenium, and a microorganism fertilizer on alleviating the effects of lead (Pb) toxicity in ginger. Ginger plants were grown in soil containing 500 mg/kg Pb(NO3)2 without (CK) or with Si, Se, or microorganism fertilizer (T1, T2, T3) as soil conditioners. Morphology indexes, Pb accumulation and distribution rates, and antioxidant enzyme activities were investigated. The Pb transfer and Pb absorption coefficients were calculated, and Pb accumulation in plant organs at various developmental stages were determined. All three soil conditioners alleviated Pb stress in ginger plants. The rhizome fresh weight in T1, T2, and T3 was increased by 96.06, 85.81, and 41.58%, respectively, compared with CK. The accumulation of Pb in organs was lower in all treatments than in CK. The chlorophyll and carotenoid contents in leaves, and root activity, root length, and the tolerance index, were higher in the treatments than in CK. The reactive oxygen species content in ginger leaves and roots was significantly lower in all treatments than in CK. Soil conditioners alleviated the negative effects of Pb stress on ginger plants: Si was the most effective, followed by Se, and then the microorganism fertilizer.

Proteomics analyses revealed the reduction of carbon- and nitrogen-metabolism and ginsenoside biosynthesis in the red-skin disorder of Panax ginseng

Red-skin disorder (RSD), a non-infectious disorder in Panax ginseng, impairs the quality and yield of ginseng and impedes continuous cropping. Since the mechanism of this disorder is unknown, there are no effective prevention measures for RSD. The proteomic changes in RSD ginseng were analysed in this study by two-dimensional electrophoresis (2-DE) and isobaric tags for relative and absolute quantification (iTRAQ). The differential expression of 137 proteins (60 from 2-DE and 77 from iTRAQ) was identified in RSD ginseng as compared with healthy ginseng. Most changes are related to carbon- and nitrogen- metabolism, redox homeostasis, and stress resistance. We also found that the concentration of metal elements, such as iron (Fe), aluminium (Al), and manganese (Mn), was significantly increased in RSD ginseng. These increased metals would be chelated with phenols to form red spots on the ginseng epidermis. Moreover, RSD disturbed the carbon and nitrogen metabolism and affected the biosynthesis of nutrients (sugar, proteins, amino acids) and active components (ginsenosides), which reduced the survival rate and medicinal value of ginseng. These differences between RSD and healthy ginseng will contribute to the understanding of RSD mechanism.

Silicon nanoparticles enhanced the growth and reduced the cadmium accumulation in grains of wheat (Triticum aestivum L.)

The application of silicon (Si) under heavy metal stress is well known, but the use of Si nanoparticles (NPs) under metal stress in not well documented. Thus, the experiments were performed to investigate the impacts of soil and foliar applied Si NPs on wheat (Triticum aestivum L.) growth and cadmium (Cd) accumulation in grains under Cd toxicity. The plants were grown under natural environmental conditions and were harvested after physiological maturity (124 days after sowing). The results demonstrated that Si NPs significantly improved, relative to the control, the dry biomass of shoots, roots, spikes and grains by 24-69%, 14-59%, 34-87%, and 31-96% in foliar spray and by 10-51%, 11-49%, 25-69%, and 27-74% in soil applied Si NPs, respectively. The Si NPs enhanced the leaf gas exchange attributes and chlorophyll a and b concentrations, whereas diminished the oxidative stress in leaves which was indicated by the reduced electrolyte leakage and enhancement in superoxide dismutase and peroxidase activities in leaf under Si NPs treatments over the control. When compared with the control, the foliar spray of Si NPs reduced the Cd contents in shoots, roots, and grains by 16-58%, 19-64%, and 20-82%, respectively, whereas soil applied Si NPs reduced the Cd concentrations in shoots, roots, and grains by 11-53%, 10-59%, and 22-83%, respectively. In comparison with the control, Si concentrations significantly (p ≤ 0.05) increased in the shoots and roots in both foliar and soil supplementation of Si NPs. Our results suggested that Si NPs could improve the yield of wheat and more importantly, reduce the Cd concentrations in the grains. Thus, the use of Si NPs might be a feasible approach in controlling Cd entry into the human body via crops.

Tobacco plants (Nicotiana benthamiana) were influenced by silicon and were not infected by dodder (Cuscuta europaea)

The effect of silicon (Si) on tobacco (Nicotiana benthamiana) development and dodder (Cuscuta europaea) - tobacco interaction were studied. Three Si application approaches were tested: tobacco seed priming (2.5 mM Si and 5 mM Si; 2.5S, 5S), watering tobacco plants with Si solution (2.5 mM Si and 5 mM Si; 2.5W, 5W) and foliar application (1 mM Si and 2.5 mM Si; 1F, 2.5F). Dodder was not able to infect the host plant in almost all Si treatments. Only in the control and 2.5W treatments was dodder able to infect its host. A significant increase in all observed antioxidant enzymes activities (POX, CAT and SOD) occurred in the plants of 2.5W treatment after infection in comparison with the uninfected 2.5W treatment and control plants, which indicated the importance of antioxidant enzymes activities in the plant parasite - host interaction. Resistance of Si treated plants to dodder could have been due to the changes in the cell wall properties of the epidermis and cortex where activity of POX was confirmed histochemically. The growth and development of tobacco shoots were evaluated after four and eight weeks of cultivation in the individual Si treatments. The development of shoots was enhanced after eight weeks of cultivation in the 2.5S, 5S, 2.5W and 5W treatments in comparison with the control treatment. However, a negative effect of Si was observed in 1F and 2.5F treatments. In the majority of cases, the plants treated with Si had decreased chlorophyll content when compared to control, except for chl a in 5W plants after 8 weeks of cultivation. Contrary to this, carotenoids increased in all Si treated plants after eight weeks cultivation in comparison with the control. The secondary xylem formation in tobacco was enhanced after 4 and 8 weeks cultivation in shoots of plants receiving the 2.5S, 5S, 2.5W and 5W treartments. The cambium was the most active in producing secondary xylem in the 2.5S treatment. Protein profile and antioxidant enzymes activities (POX, CAT and SOD) were altered by Si treatment. After 8 weeks of cultivation, activities of POX were significantly decreased in 2.5S, 5S, 2.5W and 5W in comparison with control. Catalase was decreased in 2.5S, 5S and 5W in comparison with the control, however, 1F and 2.5F treatments had significantly increased CAT and SOD activities. The specific activity of POX was confirmed histochemically in Si treated plants in the cell walls of several stem tissues like the epidermis, cortex and pith. A small amount of H2O2 was detected in leaves in the control and Si treated plants. The amount of O2- decreased in all treatments with time. The highest Si concentration in the plants (almost 800 mg . kg-1 d. w.) was detected in the 2.5W, 5W treatments.

Menadione sodium bisulphite mediated growth, secondary metabolism, nutrient uptake and oxidative defense in okra (Abelmoschus esculentus Moench) under cadmium stress

Menadione sodium bisulphite (MSB) mediates plant defense responses under abiotic stresses. In present experiment, Cd stress (1 mM) resulted in significant reduction in growth, relative water contents, chlorophyll and uptake of essential nutrients in two okra cultivars (Shabnum and Arka Anamika). Cd-induced reduction in these variables was more in cv. Arka Anamika compared with cv. Shabnum 786. Cd caused oxidative damage in the form of higher cellular levels of MDA and H₂O₂. MSB applications (0, 50, 100, 150 and 200 μM) had differential effect on growth and key physio-biochemical attributes. Higher MSB dose (200 μM) was lethal as it further aggravated damages under Cd toxicity. However, plants treated with 100 μM MSB exhibited lesser oxidative damage due to better oxidative defense in the form of stimulated activities of antioxidant enzymes (SOD, POD, CAT and APX) and increased concentration of non-enzymatic antioxidants (phenolics, flavonoids and ascorbic acid). Moreover, 100 μM MSB mitigated Cd effect on the uptake of Ca, K, and Mg. MSB also reduced the uptake and transport of Cd to aerial parts of plants. The results of present study revealed MSB-induced slight oxidative burst that induced the accumulation of reactive oxygen species (ROS) scavenging defense proteins under Cd stress.