Influence of magnesium on copper phytotoxicity to and accumulation and translocation in grapevines

Boron-mediated amelioration of copper toxicity in Citrus sinensis seedlings involved reduced concentrations of copper in leaves and roots and their cell walls rather than increased copper fractions in their cell walls

Little information is available on how boron (B) supplementation affects plant cell wall (CW) remodeling under copper (Cu) excess. 'Xuegan' (Citrus sinensis) seedlings were submitted to 0.5 or 350 µM Cu × 2.5 or 25 µM B for 24 weeks. Thereafter, we determined the concentrations of CW materials (CWMs) and CW components (CWCs), the degree of pectin methylation (DPM), and the pectin methylesterase (PME) activities and PME gene expression levels in leaves and roots, as well as the Cu concentrations in leaves and roots and their CWMs (CWCs). Additionally, we analyzed the Fourier transform infrared (FTIR) and X-ray diffraction (XRD) spectra of leaf and root CWMs. Our findings suggested that adding B reduced the impairment of Cu excess to CWs by reducing the Cu concentrations in leaves and roots and their CWMs and maintaining the stability of CWs, thereby improving leaf and root growth. Cu excess increased the Cu fractions in leaf and root pectin by decreasing DPM due to increased PME activities, thereby contributing to citrus Cu tolerance. FTIR and XRD indicated that the functional groups of the CW pectin, hemicellulose, cellulose, and lignin could bind and immobilize Cu, thereby reducing Cu cytotoxicity in leaves and roots.

Joint effects of CuO nanoparticles and perfluorooctanoic acid on cabbage (Brassica pekinensis L.)

Coexisting nanoparticles (NPs) may change plant accumulation and toxicity of perfluorooctanoic acid (PFOA) in soil, but research is very scarce. In this study, cabbage (Brassica pekinensis L.) was exposed to single or combined treatments of PFOA (2 mg/kg and 4 mg/kg) and copper oxide NPs (nCuO, 200 mg/kg and 400 mg/kg) for 40 days. At harvest, biomass, photosynthesis index, and nutrient composition of cabbage, as well as plant accumulation of PFOA and Cu, were measured. Results showed that nCuO and PFOA were adverse to cabbage growth by decreasing chlorophyll contents, inhibiting photosynthesis and transpiration, and interfering with the utilization of nutrient components. Besides, they also affected each other's plant utilization and transmission. Especially, nCuO at a high dose (400 mg/kg) significantly increased the transport of coexisting PFOA (4 mg/kg) content (by 124.9% and 118.2%) to cabbage shoots. The interaction mechanism between nCuO and PFOA is unknown, and more research is needed to evaluate their composite phytotoxicity.

Distinct responses of frond and root to increasing nutrient availability in a floating clonal plant

Current knowledge on responses of aquatic clonal plants to resource availability is largely based on studies manipulating limited resource levels, which may have failed to capture the “big picture” for aquatic clonal plants in response to resource availability. In a greenhouse experiment, we grew the floating clonal plant Spirodela polyrhiza under ten nutrient levels (i.e., 1/64×, 1/32×, 1/16×, 1/8×, 1/4×, 1/2×, 1×, 2×, 4× and 8×full-strength Hoagland solution) and examined their responses in terms of clonal growth, morphology and biomass allocations. The responses of total biomass and number of ramets to nutrient availability were unimodal. A similar pattern was found for frond mass, frond length and frond width, even though area per frond and specific frond area fluctuated greatly in response to nutrient availability. In contrast, the responses of root mass and root length to nutrient availability were U-shaped. Moreover, S. polyrhiza invested more to roots under lower nutrient concentrations. These results suggest that nutrient availability may have distinct influences on roots and fronds of the aquatic clonal plant S. polyrhiza, resulting in a great influence on the whole S. polyrhiza population.

Modeling Alleviative Effects of Ca, Mg, and K on Cu-Induced Oxidative Stress in Grapevine Roots Grown Hydroponically

The aim of this study was to determine the pattern of alleviation effects of calcium (Ca), magnesium (Mg), and potassium (K) on copper (Cu)-induced oxidative toxicity in grapevine roots. Root growth, Cu and cation accumulation, reactive oxygen species (ROS) production, and antioxidant activities were examined in grapevine roots grown in nutrient solutions. The experimental setting was divided into three sets; each set contained a check (Hoagland solution only) and four treatments of simultaneous exposure to 15 μM Cu with four cation levels (i.e., Ca set: 0.5, 2.5, 5, and 10 mM Ca; Mg set: 0.2, 2, 4, and 8 mM Mg; K set: 0.6, 2.4, 4.8, and 9.6 mM K). A damage assessment model (DAM)-based approach was then developed to construct the dose-effect relationship between cation levels and the alleviation effects on Cu-induced oxidative stress. Model parameterization was performed by fitting the model to the experimental data using a nonlinear regression estimation. All data were analyzed by a one-way analysis of variance (ANOVA), followed by multiple comparisons using the least significant difference (LSD) test. The results showed that significant inhibitory effects on the elongation of roots occurred in grapevine roots treated with 15 μM Cu. The addition of Ca and Mg significantly mitigated phytotoxicity in root growth, whereas no significant effect of K treatment on root growth was found. With respect to oxidative stress, ROS and malondialdehyde (MDA) contents, as well as antioxidant (superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX)) activities, were stimulated in the roots after exposure to 15 μM Cu for three days. Moreover, H₂O₂ levels decreased significantly as Ca, Mg, and K concentrations increased, indicating that the coexistence of these cations effectively alleviated Cu-induced oxidative stress; however, alleviative effects were not observed in the assessment of the MDA content and antioxidant enzyme activities. Based on the DAM, an exponential decay equation was developed and successfully applied to characterize the alleviative effects of Ca, Mg, and K on the H₂O₂ content induced by Cu in the roots. In addition, compared with Mg and K, Ca was the most effective cation in the alleviation of Cu-induced ROS. Based on the results, it could be concluded that Cu inhibited root growth and Ca and Mg absorption in grapevines, and stimulated the production of ROS, lipid peroxidation, and antioxidant enzymes. Furthermore, the alleviation effects of cations on Cu-induced ROS were well described by the DAM-based approach developed in the present study.

Electrical properties of plant root cell plasma membrane influence the alleviation of Al and Cu phytotoxicity by Ca and Mg cations

Although it is known that cations such as calcium (Ca) and magnesium (Mg) can alleviate metal toxicity in plants, much uncertainty remains regarding the mechanisms by which this alleviation occurs. In this study, three plant species, cowpea (Vigna unguiculata. cv. White Caloona), soybean (Glycine max. cv. Bunya), and wheat (Triticum. cv. Axe), were used to examine growth in nutrient solutions containing a series of cations (Ca and Mg) and toxicants (Al and Cu). The addition of Ca and Mg increased Al³⁺ activities that induced 50% plant root elongation rate decline (EC₅₀) from 4.0, 4.2, 0.34 to 31, 22, 9.4 for cowpea, soybean, wheat respectively. However, when expressed as the Al³⁺ activity at the root-cell plasma membrane (PM) surface, the addition of Ca and Mg increased Al sensitivity. In regression models, plant root elongation rate (RER) had higher R² values with Al³⁺ activity at the PM than with Al³⁺ activity in the bulk solutions (0.60, 0.58, 0.93 for cowpea, soybean, wheat respectively). For Cu, the addition of Ca and Mg alleviated its phytotoxicity when expressed as the Cu²⁺ activity in the bulk-phase solutions for both cowpea and wheat. However, in soybean, the addition of Mg did not alter Cu toxicity when expressed as the Cu²⁺ bulk activity. Generally, the metal activity at the outer PM surface of the root cells is a better predictor for their phytotoxicity than the activity in bulk solution (R²=0.80, 0.80, 0.87 for cowpea, soybean, wheat respectively). These results supported the concept that the strength of alleviation of Ca and Mg cations depends on both the plant species and the toxicants that they alleviate. The different mechanisms should be fully considered in relevant assessments of trace metal toxicity in plants.

Copper toxicity affects phosphorus uptake mechanisms at molecular and physiological levels in Cucumis sativus plants

Due to the deliberate use of cupric fungicides in the last century for crop-defence programs, copper (Cu) has considerably accumulated in the soil. The concentrations of Cu often exceed the safety limits of risk assessment for Cu in soil and this may cause toxicity in plants. Copper toxicity induces nutritional imbalances in plants and constraints to plants growth. These aspects might be of paramount importance in the case of phosphorus (P), which is an essential plant macronutrient. In this work, hydroponically grown cucumber plants were used to investigate the influence of the exposure to different Cu concentrations (0.2, 5, 25 and 50 μM) on i) the phenotypic traits of plants, particularly at root level, ii) the nutrient content in both roots and shoots, and iii) the P uptake mechanisms, considering both the biochemical and molecular aspects. At high Cu concentrations (i.e. above 25 μM), the shoot and root growth resulted stunted and the P influx rate diminished. Furthermore, two P transporter genes (i.e. CsPT1.4 and CsPT1.9) were upregulated at the highest Cu concentration, albeit with different induction kinetics. Overall, these results confirm that high Cu concentrations can limit the root acquisition of P, most likely via a direct action on the uptake mechanisms (e.g. transporters). However, the alteration of root plasma membrane permeability induced by Cu toxicity might also play a pivotal role in the observed phenomenon.

Morphological Root Responses and Molecular Regulation of Cation Transporters Are Differently Affected by Copper Toxicity and Cropping System Depending on the Grapevine Rootstock Genotype

The high copper (Cu) concentration in vineyard soils causes the increase of Cu toxicity symptoms in young grapevines. Recently, intercropping of grapevine and oat was shown to reduce Cu toxicity effects, modulating the root ionome. On these bases, the focus of the work was to investigate the impact of Cu toxicity of either monocropped or oat-intercropped grapevine rootstocks plants (196.17 and Fercal), at both phenotypic (i.e., root architecture), and molecular (i.e., expression of transporters) levels. The results showed a different response in terms of root morphology that are both rootstock- and cropping system dependent. Moreover, the expression pattern of transporter genes (i.e., VvCTr, VvNRAMP, and VvIRT1) in monocropped grapevine might resemble a Mn deficiency response induced by the excess of Cu, especially in Fercal plants. The gene expression in intercropped grapevines suggested rootstock-specific response mechanisms, depending on Cu levels. In fact, at low Cu concentrations, Fercal enhanced both root system growth and transporter genes expression; contrarily, 196.17 increased apoplast divalent cations accumulation and transporters expression. At high Cu concentrations, Fercal increased the expression of all bivalent cation transporters and, as previously observed, enhanced the release of root exudates, whereas the 196.17 only modulated transporters. In conclusion, our results might suggest that the different adaptation strategies of the two rootstocks to Cu toxicity could be mainly ascribable to a fine-tuning of bivalent cations transporters expression at root level.

Potential of vermicompost and limestone in reducing copper toxicity in young grapevines grown in Cu-contaminated vineyard soil

Foliar fungicide application in grapevines increases the content of heavy metals such as copper (Cu) in vineyard soils, which may reach phytotoxic levels. The application of soil amendments such as limestone and vermicompost may reduce Cu availability and phytotoxicity. The study aimed to assess growth, physiological status and changes in root morphology in young grapevines grown for 12 months in Cu-contaminated soil with and without the application of soil amendments. Samples of a Typic Hapludalf soil were collected in a vineyard with more than 30 years of cultivation. The following treatments were used: 1) control (without amendment), 2) application of organic vermicompost (86.7 g kg^-1) and 3) application of limestone (3 Mg ha^-1). Grapevines (Paulsen 1103 rootstock) were transplanted and grown for 12 months in PVC soil columns. We assessed parameters of growth, photosynthesis and root morphology. Grapevines grown in soil treated with limestone showed increased growth, dry matter yield and photosynthetic efficiency. The highest Cu concentrations in root tissue were found in grapevines grown in control soil. The application of vermicompost in this study did not alleviate Cu toxicity. Grapevines grown in soil treated with vermicompost showed high manganese (Mn) concentration in shoots before the winter pruning, reflecting the high Mn concentrations in soil solution that caused Mn phytotoxicity, resulting in plant death after the winter pruning. The vermicompost used in this study is not suitable for agronomic use. The use of limestone was an effective strategy to reduce Cu availability and phytotoxicity.

Effects of Copper on Root Morphology, Cations Accumulation, and Oxidative Stress of Grapevine Seedlings

In the present study, a hydroponic experiment was conducted to investigate the oxidative stress and the copper (Cu) accumulation in grapevines exposed to three Cu levels (0, 5, and 15 µM) for 1, 2, and 3 days. The results showed that the root elongation was stunted at the highest-exposure concentration. The Cu accumulation in the grapevines increased with increasing Cu treatments, while the other nutrient elements (Ca, Mg and K) absorbed by the grapevines decreased. Most of the Cu taken up by the grapevines was accumulated in the roots. Compared to the data for 1 day after treatment, the Cu-addition significantly decreased the Mg and K concentration in the roots and leaves, yet increased the superoxide dismutase activity in the leaves after 3 days of treatment. For the reactive oxygen species, the malondialdehyde increased with increasing Cu levels in the roots and leaves; however, both the Cu-addition and exposure duration reduced the H₂O₂ level in the root. Additionally, the Cu-induced accumulation of ^·O₂^- and H₂O₂ in the grapevine leaves can be observed by the histochemical staining of nitroblue tetrazolium and diaminobenzidine, respectively. In conclusion, the present results indicate that excess Cu results in a change of the root morphology and leads to oxidative stress for the grapevine leaves and roots.

Synergism and antagonisms between nutrients induced by copper toxicity in grapevine rootstocks: Monocropping vs. intercropping

The long-term use of Cu-containing fungicides contaminates vineyards soils, which can induce Cu toxicity and nutrient imbalances in several plant species. The aim of this work was to evaluate the effect of Cu toxicity on two grapevine rootstocks, Fercal and 196.17, and to elucidate if intercropping with oat can alleviate grapevine Cu toxicity. Plants were hydroponically-cultivated and treated with different Cu concentrations. At harvest the biomass accumulation, the SPAD index and the symplastic and apoplastic root and leaves ionome were measured to evaluate possible synergistic and/or antagonistic effects on other micro- and macronutrients. The root exudation analysis was correlated with genes expression (VvPEZ-like), whereas PCA analysis performed on the grapevine and oat ionome revealed that both mono- and intercropped 196.17 rootstock display a positive effect on Zn and Mn in the root tissues at high Cu concentrations. An increase of Zn and Mn in roots was also reported for the intercropped Fercal rootstock at high Cu concentrations while an antagonistic relation was reported for root Zn concentration in the monocropped Fercal rootstock. Our results showed that grapevine and oat compete for nutrient uptake and that this phenomenon can possibly alleviate grapevine Cu toxicity. However, Fercal rootstock is able to take advantage from oat, while 196.17 is disadvantaged by the intercropping system. Even though intercropping system seems to be a valuable tool to counteract grapevine Cu toxicity, the application of this agricultural practice has shown to be species dependent and should be evaluated for each rootstock.

Copper phytoextraction by Salvinia cucullata: biochemical and morphological study

The present study investigated the effect of copper on photosynthesis, antioxidant potential, and anatomical response of aquatic fern, Salvinia cucullata, with a view to ascertain its phytoremediation potential. Plants were exposed in hydroponics for 21 days to different Cu concentrations (10, 15, 20, and 30 mg/L). Significant declines in chlorophyll, carotenoids, and soluble proteins, as a function of Cu proportion were observed. Lipid peroxidation was also evident, which implied reactive oxygen species (ROS) generation. However, both root and leaf tissues responded remarkably to the ROS produced, by inducing superoxide dismutase (1.6-6.5 times), catalase (1.5-5.4 times), guaicol peroxidase (1.5-7.2 times), and ascorbyl peroxidase (1.3-4.7 times) over the control. The plant showed best phytoremedial activity within Cu range of 10-15 mg/L, with maximum accumulation of 2956 ± 82.6 μg/g dw., at 15 mg Cu/L and showed efficient root to shoot translocation (translocation factor, TF > 1) at this range, which is the stipulated minimum requirement to be a hyperaccumulator. The capacity of metal extraction from environment to leaf (extraction coefficient, EC) was also high (EC = 73-197). However, at higher doses (20-30 mg/L), the plant resorted to an exclusion strategy, whereby, more metal accumulation was observed in root than in leaf. The plant conferred suitable remediation attributes by showing minimal root and leaf anatomical damages along with high Ca peaks in both the tissues, and rapid leaf stomatal closure, all of which probably helped in the Cu induced stress mitigation. Due to its widespread availability, fast growth, ability to grow in myriads of polluted environment, and having hardy physiology, this plant can be suggested for use as a suitable Cu phytoremediator.

Copper accumulation in vineyard soils: Rhizosphere processes and agronomic practices to limit its toxicity

Viticulture represents an important agricultural practice in many countries worldwide. Yet, the continuous use of fungicides has caused copper (Cu) accumulation in soils, which represent a major environmental and toxicological concern. Despite being an important micronutrient, Cu can be a potential toxicant at high concentrations since it may cause morphological, anatomical and physiological changes in plants, decreasing both food productivity and quality. Rhizosphere processes can, however, actively control the uptake and translocation of Cu in plants. In particular, root exudates affecting the chemical, physical and biological characteristics of the rhizosphere, might reduce the availability of Cu in the soil and hence its absorption. In addition, this review will aim at discussing the advantages and disadvantages of agronomic practices, such as liming, the use of pesticides, the application of organic matter, biochar and coal fly ashes, the inoculation with bacteria and/or mycorrhizal fungi and the intercropping, in alleviating Cu toxicity symptoms.

Incorporating bioavailability into toxicity assessment of Cu-Ni, Cu-Cd, and Ni-Cd mixtures with the extended biotic ligand model and the WHAM-F(tox) approach

There are only a limited number of studies that have developed appropriate models which incorporate bioavailability to estimate mixture toxicity. Here, we explored the applicability of the extended biotic ligand model (BLM) and the WHAM-F(tox) approach for predicting and interpreting mixture toxicity, with the assumption that interactions between metal ions obey the BLM theory. Seedlings of lettuce Lactuca sativa were exposed to metal mixtures (Cu-Ni, Cu-Cd, and Ni-Cd) contained in hydroponic solutions for 4 days. Inhibition to root elongation was the endpoint used to quantify the toxic response. Assuming that metal ions compete with each other for binding at a single biotic ligand, the extended BLM succeeded in predicting toxicity of three mixtures to lettuce, with more than 82% of toxicity variation explained. There were no significant differences in the values of f(mix50) (i.e., the overall amounts of metal ions bound to the biotic ligand inducing 50% effect) for the three mixture combinations, showing the possibility of extrapolating these values to other binary metal combinations. The WHAM-F(tox) approach showed a similar level of precision in estimating mixture toxicity while requiring fewer parameters than the BLM-f(mix) model. External validation of the WHAM-F(tox) approach using literature data showed its applicability for other species and other mixtures. The WHAM-F(tox) model is suitable for delineating mixture effects where the extended BLM also applies. Therefore, in case of lower data availability, we recommend the lower parameterized WHAM-F(tox) as an effective approach to incorporate bioavailability in quantifying mixture toxicity.

Reduction of copper phytotoxicity by liming: A study of the root anatomy of young vines (Vitis labrusca L.)

Frequent applications of copper (Cu)-based fungicides on vines causes the accumulation of this metal in vineyard soils, which can cause toxicity in young vines. However, liming may reduce these toxic effects. The present study aimed to evaluate the effects of Cu toxicity on the root anatomy of young vines and the alleviation of Cu toxicity by lime applications to contaminated sandy soil. The treatments consisted of the addition of lime (0.0, 1.5 and 3.0 Mg ha(-1)) and two Cu concentrations (0 and 50 mg kg(-1)) to Typic Hapludalf soil. Young vines 'Niágara Branca' (Vitis labrusca L.) were obtained by micropropagation and cultivated for 70 days. The young vines grown with Cu and without liming presented a disorganized root structure; reduced root cap size; increased diameter (47%), cortex area (128%), vascular cylinder area (93%), and number of cortical layers and cells containing phenolic compounds (132%); and reduced root (41%), stem (44%) and leaf dry mass (21%) and height increase (55%). Moreover, Cu exposure reduced Ca concentrations (13%) and increased Cu concentrations (371%) in the roots. Liming, primarily with the highest tested dose, increased the soil pH (from 4.4 to 5.4-6.1), decreased the Cu concentration in the soil (extracted by CaCl2), increased the calcium (Ca) and magnesium (Mg) uptake by plants, prevented root anatomical changes and benefited young vine growth in soil with higher Cu concentrations.

Magnesium alleviates plant toxicity of aluminium and heavy metals

Magnesium (Mg) is an essential nutrient that can alleviate soilborne toxicity of many ions. This review paper critically assesses the literature on interactions and mechanisms influencing Mg alleviation of aluminium (Al) and heavy metal toxicity. Hydrated radii of Mg2+ and Al3+ are similar; therefore, these two ions compete for binding to ion transporters and other important biological molecules. In monocotyledonous species such as rice and wheat, millimolar concentrations of Mg alleviate Al toxicity, mainly by decreasing Al saturation and activity at cell wall and plasma membrane binding sites. In dicotyledonous legume species such as soybean (Glycine max), rice bean (Vigna umbellata) and broad bean (Vicia faba), micromolar concentrations of Mg may enhance biosynthesis of organic ligands and thus underpin alleviation of Al toxicity. Resistance to Al may be enhanced by increased expression of the genes coding for Mg transporters, as well as by upregulation of activity of Mg-transport proteins; intracellular Mg2+ activity may thus be increased under Al stress, which may increase the activity of H+-ATPases. In Vicia faba, Mg-related enhancement in the activity of plasma membrane H+-ATPase under Al stress was found to be due to post-translational modification (increased phosphorylation of the penultimate threonine as well as association with regulatory 14-3-3 proteins), resulting in increased resistance to Al stress. Magnesium can alleviate heavy metal stress by decreasing negative electrical potential and thus metal ion activities at the plasma membrane surface (physico-chemical competition), by enhancing activities of enzymes involved in biosynthesis of organic ligands, and by increasing vacuolar sequestration of heavy metals via increasing H+-pumping activity at the tonoplast. Future work should concentrate on characterising the role of intracellular Mg2+ homeostasis and Mg transporters in alleviating metal stress as well as in transcriptional, translational and post-translational regulation of H+-pumps and enzymes involved in biosynthesis and exudation of organic ligands.