Stress responses and nickel and zinc accumulation in different accessions of Stellaria media (L.) Vill. in response to solution pH variation in hydroponic culture

Metal accumulation in cattails cultured in soils flooded with artificial wastewater of varying pH and different levels of metals (Cr, Cd and Zn)

Toxic metals cause risks to the ecological environment. Typha latifolia L. is a good candidate to clean potentially toxic metals contaminated water or soil. However, limited research investigated the impact of environmental factors (e.g., pH, soil substrate, flood duration) on metal accumulations in cattails. In this study, cattails were cultured in soils flooded with artificial wastewater with varying pH (5, 7 or 9) and different levels of Cr, Cd and Zn for four weeks to investigate the interactions between environmental conditions and metal uptake in cattails. The metal content in biomass were measured by an inductively coupled plasma-optical emission spectrometer.

More Zn (>10,000 mg/kg dry biomass) entered plants compared to Cd and Cr (<1,000 mg/kg dry biomass). Approximately 80% of Zn from solutions with 50 mg/L Cd, 25 mg/L Cr, 250 mg/L Zn were removed by cattails. Most Cd and Cr were sorbed onto soils. Cattails exhibited relatively consistent performance in removing metals from wastewater with initial pH of 5, 7 or 9. The pH of all the solutions ended close to neutral after 4 weeks. More research is needed to further understand the influence of environmental conditions on metal uptakes in plants to improve phytoremediation efficiency.

Interaction of nickel with oxidative and antioxidative molecules in Cichorioideae species

The uptake of nickel (Ni) by Asteraceae/Cichorioideae species Cichorium intybus, Leontodon hispidus and Hieracium aurantiacum exposed to Ni (0.3 or 30 μM) over 14 days and subsequent changes of metabolites were compared in order to identify their phytoaccumulation potential. Hieracium contained the most Ni (194 and 1558 μg Ni/g DW at 30 μM Ni in shoots and roots) but had unchanged amount of antioxidants (vitamin C and thiols) in the shoots and an elevated amount in the roots, which may be the reason for the absence of visible damage. On the contrary, Leontodon reacted by a decrease in antioxidants to an excess of Ni, which can be related to enhanced oxidative stress (an increase in ROS and a decrease in nitric oxide detected by fluorescence microscopy). All roots were anatomically in the secondary state and Ni-induced cell wall thickening (i.e. lignin/suberin deposition) was most visible in Hieracium roots, which also contained 2-times more Ni than the other species. Among essential elements, mainly Fe accumulation was affected by Ni excess. The content of soluble phenols increased while organic acids (malic and citric) decreased sometimes extensively (up to 90%) in individual species. PCA analyses showed that especially ascorbic acid, thiols and phenols affect the separation in the shoots especially with regard to applied concentration of Ni, while these metabolites in the roots clearly separated the species (Cichorium from the others). The data show the highest tolerance to Ni in Hieracium, but the highest phytoaccumulation of Ni was found in Cichorium (626 μg Ni/plant or 122 μg Ni/shoot at a dose of 30 μM Ni).

Zinc (Zn) mitigates copper (Cu) toxicity and retrieves yield and quality of lettuce irrigated with Cu and Zn-contaminated simulated wastewater

Owing to a competitive interaction, zinc (Zn) contained in highly Cu-contained wastewater was hypothesized to mitigate Cu toxicity-induced negative effects on the growth and quality of lettuce. Thus, growth, metal accumulation and biochemical responses of lettuce irrigated with simulated wastewater (SW, control), Cu-contaminated SW (CuSW, 20 mg Cu L-1), Zn-contaminated SW (ZnSW, 100 mg Zn L-1) and both Cu- and Zn-contaminated SW (CuZnSW, 20 mg Cu and 100 mg Zn L-1) were evaluated. Results revealed that irrigation with CuSW negatively affected growth (dry matters, root length and plant height) and quality (low mineral concentrations) of lettuce, which were associated with higher Cu uptake. Irrigation with Zn + Cu-contaminated SW retrieved Cu toxicity and improved root and shoot dry matters and root length by 13.5%, 46% and 19%, respectively compared to that with alone Cu-contaminated SW. Moreover, CuZnSW improved lettuce leaf quality compared to CuSW and increased concentrations of Mg (30%), P (15%), Ca (41%), Mn (24%) and Fe (23%). Moreover, compared to CuSW, CuZnSW improved flavonoids (54%), total polyphenolic compounds (1.8-fold), polyphenolic acids (77%) and antiradical activities (16.6%). Most importantly, Zn addition boosted up lettuce Cu tolerance index by 18% under Cu-contaminated SW treatment. Pearson's correlation analysis among various growth and mineral parameters demonstrated that shoot Zn concentration was positively related to elemental concentrations, phytochemical contents and antioxidant activity under Cu-contaminated environment. Thus, it is concluded that Zn supplementation retrieves negative effects of Cu toxicity to lettuce grown with Cu-contaminated wastewater.

Effect of zinc nanoparticles on the growth and biofortification capability of mungbean (Vigna radiata) seedlings

Zinc insufficiency is a nutritional trouble worldwide, especially in developing countries. In the current study, an experiment was conducted to evaluate the effect of supplementation of MS media culture with different concentrations of ZnO nanoparticles (NPs) (0, 10, 20, 40, 80, and 160 ppm) on growth, nutrient uptake, and some physiological parameters of 7-days-old mung bean seedlings. ZnO NPs enhanced the Zn concentration of mung bean from 106.41 in control to more than 4600 µg/g dry weight in 80 and 160 ppm ZnO NPs treated seedlings. Our results showed that ZnO NPs in the concentration range from 10 to 20 ppm had a positive influence on growth parameters and photosynthetic pigments. Higher levels of ZnO NPs negatively affected seedling's growth by triggering oxidative stress which in turn caused enhancing antioxidative response in seedlings including polyphenol oxidase and peroxidase activity as well as phenolic compounds and anthocyanine contents. Considering the positive effects of ZnO NPs treatment on mungbean seedlings growth, micronutrents, protein and shoot phenolics content, 20 ppm is recommended as the optimal concentration for biofortification. Our findings confirm the capability of ZnO NPs in the remarkable increase of Zn content of mungbean seedlings which can be an efficient way for plant biofortification and dealing with environmental stress.

Supplementary information

The online version contains supplementary material available at 10.1007/s11756-022-01269-3.

Combined Impact of Excess Zinc and Cadmium on Elemental Uptake, Leaf Anatomy and Pigments, Antioxidant Capacity, and Function of Photosynthetic Apparatus in Clary Sage (Salvia sclarea L.)

Clary sage (Salvia sclarea L.) is a medicinal plant that has the potential to be used for phytoextraction of zinc (Zn) and cadmium (Cd) from contaminated soils by accumulating these metals in its tissues. Additionally, it has been found to be more tolerant to excess Zn than to Cd stress alone; however, the interactive effects of the combined treatment with Zn and Cd on this medicinal herb, and the protective strategies of Zn to alleviate Cd toxicity have not yet been established in detail. In this study, clary sage plants grown hydroponically were simultaneously exposed to Zn (900 µM) and Cd (100 μM) for 8 days to obtain more detailed information about the plant responses and the role of excess Zn in mitigating Cd toxicity symptoms. The leaf anatomy, photosynthetic pigments, total phenolic and anthocyanin contents, antioxidant capacity (by DPPH and FRAP analyses), and the uptake and distribution of essential elements were investigated. The results showed that co-exposure to Zn and Cd leads to an increased leaf content of Fe and Mg compared to the control, and to increased leaf Ca, Mn, and Cu contents compared to plants treated with Cd only. This is most likely involved in the defense mechanisms of excess Zn against Cd toxicity to protect the chlorophyll content and the functions of both photosystems and the oxygen-evolving complex. The data also revealed that the leaves of clary sage plants subjected to the combined treatment have an increased antioxidant capacity attributed to the higher content of polyphenolic compounds. Furthermore, light microscopy indicated more alterations in the leaf morphology after Cd-only treatment than after the combined treatment. The present study shows that excess Zn could mitigate Cd toxicity in clary sage plants.

Nickel uptake in hydroponics and elemental profile in relation to cultivation reveal variability in three Hypericum species

The Hypericum species (H. perforatum, H. olympicum, and H. orientale) were cultured in hydroponics with excess nickel (Ni, 1 or 100 μM Ni) to compare the metallic and metabolite content. Identical species were collected outdoor to assess the same parameters (including uranium and lanthanides) with total of 53 elements. The results showed that Ni was less accumulated in shoots in hydroponics (translocation factor of 0.01-0.25) and the highest absolute amount was detected in H. olympicum. Essential elements were typically depleted by Ni excess, but Co and Na increased. Soluble phenols, sum of flavonols and catechin rather increased in response to Ni but quercetin glycosides and free amino acids decreased in the shoots of H. olympicum mainly. Comparison of laboratory and outdoor growing plants showed more phenols in outdoor samples but not in H. olympicum and individual metabolites differed too. Plants cultured in hydroponics contained lower amount of non-essential, toxic and rare earth elements (30-100-fold) and shoot bioaccumulation factor in outdoor samples was low for most elements (<0.01) but not for Cd and Pt. Data reveal that H. olympicum is a potent source of phenolic metabolites whereas H. orientale accumulates many elements (38 out of 53 elements).

Zinc in soil-plant-human system: A data-analysis review

Zinc (Zn) plays an important role in the physiology and biochemistry of plants due to its established essentiality and toxicity for living beings at certain Zn concentration i.e., deficient or toxic over the optimum range. Being a vital cofactor of important enzymes, Zn participates in plant metabolic processes therefore, alters the biophysicochemical processes mediated by Zn-related enzymes/proteins. Excess Zn can provoke oxidative damage by enhancing the levels of reactive radicals. Hence, it is imperative to monitor Zn levels and associated biophysicochemical roles, essential or toxic, in the soil-plant interactions. This data-analysis review has critically summarized the recent literature of (i) Zn mobility/phytoavailability in soil (ii) molecular understanding of Zn phytouptake, (iii) uptake and distribution in the plants, (iv) essential roles in plants, (v) phyto-deficiency and phytotoxicity, (vi) detoxification processes to scavenge Zn phytotoxicity inside plants, and (vii) associated health hazards. The review especially compares the essential, deficient and toxic roles of Zn in biophysicochemical and detoxification processes inside the plants. To conclude, this review recommends some Zn-related research perspectives. Overall, this review reveals a thorough representation of Zn bio-geo-physicochemical interactions in soil-plant system using recent data.

Physiological, photochemical, and antioxidant responses of wild and cultivated Carthamus species exposed to nickel toxicity and evaluation of their usage potential in phytoremediation

The impacts of Ni toxicity on growth behaviors, photochemical, and antioxidant enzymes activities of wild (Carthamus oxyacantha M. Bieb.) and cultivated (Carthamus tinctorius L.) safflower species were investigated in this study. Fourteen-day-old seedlings were treated with excessive Ni levels [control, 0.5, 0.75, and 1.0 mM NiCl2·6H2O] for 7 days. The results of chlorophyll a fluorescence indicated that toxic nickel exposure led to changes in specific, phenomenological energy fluxes and quantum yields in thylakoid membranes, and activities of donor and acceptor sides of photosystems. These changes resulted in a significant decrease in the photosynthetic activities by about 50% in both species, but these negative effects of Ni were not in a level to destroy the functionality of the photosystems. At the same time, toxic Ni affected membrane integrity and the amount of photosynthetic pigments in the antenna and active reaction centers. Additionally, the accumulation of Ni was higher in roots than in stem and leaves for both species. Depending on Ni accumulation, a significant reduction in dry biomass of root by approx. 64.8 and 45.7% and shoot by 41 and 24.7% were observed in wild and cultivated species, respectively. Two species could probably withstand deleterious Ni toxicity with better upregulating own protective defense systems such as antioxidant enzymes and phenolic compounds. Among of them, SOD and POD activities were increased with increasing Ni concentrations. The POD activities of both species were most prominent and consistently increased (approx. 2 folds in roots and 6 folds in leaves) in highly toxic Ni levels and may be protected them from damaging effect of H2O2. When all results are evaluated as a whole, Carthamus species produced similar responses to toxicity and also both species have bioconcentration (BCF) and bioaccumulation factor (BF) > 1 and translocation factor < 1 under Ni toxicity may be regarded a good indication of Ni tolerance. Furthermore, it is possible to use the Carthamus species as phytostabilizers of soils contaminated with nickel, because of their roots accumulating more nickel.

Functions and strategies for enhancing zinc availability in plants for sustainable agriculture

Zinc (Zn), which is regarded as a crucial micronutrient for plants, and is considered to be a vital micronutrient for plants. Zn has a significant role in the biochemistry and metabolism of plants owing to its significance and toxicity for biological systems at specific Zn concentrations, i.e., insufficient or harmful above the optimal range. It contributes to several cellular and physiological activities of plants and promotes plant growth, development, and yield. Zn is an important structural, enzymatic, and regulatory component of many proteins and enzymes. Consequently, it is essential to understand the interplay and chemistry of Zn in soil, its absorption, transport, and the response of plants to Zn deficiency, as well as to develop sustainable strategies for Zn deficiency in plants. Zn deficiency appears to be a widespread and prevalent issue in crops across the world, resulting in severe production losses that compromise nutritional quality. Considering this, enhancing Zn usage efficiency is the most effective strategy, which entails improving the architecture of the root system, absorption of Zn complexes by organic acids, and Zn uptake and translocation mechanisms in plants. Here, we provide an overview of various biotechnological techniques to improve Zn utilization efficiency and ensure the quality of crop. In light of the current status, an effort has been made to further dissect the absorption, transport, assimilation, function, deficiency, and toxicity symptoms caused by Zn in plants. As a result, we have described the potential information on diverse solutions, such as root structure alteration, the use of biostimulators, and nanomaterials, that may be used efficiently for Zn uptake, thereby assuring sustainable agriculture.

Induction of hormesis in plants by urban trace metal pollution

Hormesis is a dose-response phenomenon observed in numerous living organisms, caused by low levels of a large number of stressors, among which metal ions. In cities, metal levels are usually below toxicity limits for most plant species, however, it is of primary importance to understand whether urban metal pollution can threaten plant survival, or, conversely, be beneficial by triggering hormesis. The effects of Cd, Cr and Pb urban concentrations were tested in hydroponics on three annual plants, Cardamine hirsuta L., Poa annua L. and Stellaria media (L.) Vill., commonly growing in cities. Results highlighted for the first time that average urban trace metal concentrations do not hinder plant growth but cause instead hormesis, leading to a considerable increase in plant performance (e.g., two to five-fold higher shoot biomass with Cd and Cr). The present findings, show that city habitats are more suitable for plants than previously assumed, and that what is generally considered to be detrimental to plants, such as trace metals, could instead be exactly the plus factor allowing urban plants to thrive.

Production of Antioxidant Molecules in Polygonum aviculare (L.) and Senecio vulgaris (L.) under Metal Stress: A Possible Tool in the Evaluation of Plant Metal Tolerance

Plants growing on heavy metal (HM)-polluted soils show toxicity symptoms, such as chlorosis and growth reduction, and undergo oxidative stress due to the formation of reactive oxygen species (ROS). Plants overcome oxidative stress by producing a wide range of antioxidant molecules, such as polyphenols and flavonoids. The aim of the present work was to study the accumulation of these molecules in response to increasing concentrations of Cd, Cr, Cu, Ni, Pb and Zn and to assess whether they can be used as a tool in assessing metal-related stress in Polygonum aviculare and Senecio vulgaris. On average, P. aviculare shoots accumulated lower amounts of metals than S. vulgaris shoots. The uptake of all six elements was correlated and proportional to their concentration in the nutrient solution (ρ > 0.9), with the bioaccumulation factor (BAF) being >1 for most of them. The present research demonstrated that 82% of the samples showed a good correlation (|ρ| > 0.5) between the level of polyphenols, flavonoids and antioxidant activity and the metal concentration in plant shoots, confirming that the metal stress level and production of phenolic compounds having antioxidant activity were strictly connected. Nonetheless, the mere quantification of these molecules cannot identify the type of metal that caused the oxidative stress, neither determine the concentration of the stressors. The five tested populations of each species did not show any specific adaptation to the environment of origin.