Interactive effects of Cr and Fe treatments on plants growth, nutrition and oxidative status in Zea mays L

Heavy-metal-induced reactive oxygen species: phytotoxicity and physicochemical changes in plants

As a result of the industrial revolution, anthropogenic activities have enhanced there distribution of many toxic heavy metals from the earth's crust to different environmental compartments. Environmental pollution by toxic heavy metals is increasing worldwide, and poses a rising threat to both the environment and to human health.Plants are exposed to heavy metals from various sources: mining and refining of ores, fertilizer and pesticide applications, battery chemicals, disposal of solid wastes(including sewage sludge), irrigation with wastewater, vehicular exhaust emissions and adjacent industrial activity.Heavy metals induce various morphological, physiological, and biochemical dysfunctions in plants, either directly or indirectly, and cause various damaging effects. The most frequently documented and earliest consequence of heavy metal toxicity in plants cells is the overproduction of ROS. Unlike redox-active metals such as iron and copper, heavy metals (e.g, Pb, Cd, Ni, AI, Mn and Zn) cannot generate ROS directly by participating in biological redox reactions such as Haber Weiss/Fenton reactions. However, these metals induce ROS generation via different indirect mechanisms, such as stimulating the activity of NADPH oxidases, displacing essential cations from specific binding sites of enzymes and inhibiting enzymatic activities from their affinity for -SH groups on the enzyme.Under normal conditions, ROS play several essential roles in regulating the expression of different genes. Reactive oxygen species control numerous processes like the cell cycle, plant growth, abiotic stress responses, systemic signalling, programmed cell death, pathogen defence and development. Enhanced generation of these species from heavy metal toxicity deteriorates the intrinsic antioxidant defense system of cells, and causes oxidative stress. Cells with oxidative stress display various chemical,biological and physiological toxic symptoms as a result of the interaction between ROS and biomolecules. Heavy-metal-induced ROS cause lipid peroxidation, membrane dismantling and damage to DNA, protein and carbohydrates. Plants have very well-organized defense systems, consisting of enzymatic and non-enzymatic antioxidation processes. The primary defense mechanism for heavy metal detoxification is the reduced absorption of these metals into plants or their sequestration in root cells.Secondary heavy metal tolerance mechanisms include activation of antioxidant enzymes and the binding of heavy metals by phytochelatins, glutathione and amino acids. These defense systems work in combination to manage the cascades of oxidative stress and to defend plant cells from the toxic effects of ROS.In this review, we summarized the biochemiCal processes involved in the over production of ROS as an aftermath to heavy metal exposure. We also described the ROS scavenging process that is associated with the antioxidant defense machinery.Despite considerable progress in understanding the biochemistry of ROS overproduction and scavenging, we still lack in-depth studies on the parameters associated with heavy metal exclusion and tolerance capacity of plants. For example, data about the role of glutathione-glutaredoxin-thioredoxin system in ROS detoxification in plant cells are scarce. Moreover, how ROS mediate glutathionylation (redox signalling)is still not completely understood. Similarly, induction of glutathione and phytochelatins under oxidative stress is very well reported, but it is still unexplained that some studied compounds are not involved in the detoxification mechanisms. Moreover,although the role of metal transporters and gene expression is well established for a few metals and plants, much more research is needed. Eventually, when results for more metals and plants are available, the mechanism of the biochemical and genetic basis of heavy metal detoxification in plants will be better understood. Moreover, by using recently developed genetic and biotechnological tools it may be possible to produce plants that have traits desirable for imparting heavy metal tolerance.

Arsenic and selenium interactive effect on alga Desmodesmus quadricauda

Substances known to be toxic in one-component solutions often exhibit unexpected effects when present in mixtures. Only a few efforts have been made to assess the effect of As-Se mixture in algae or plants in general. Due to the lack of information on this topic, the aim of this study was to examine the As-Se interactive effect in the alga species Desmodesmus quadricauda. The initial density of algal cells was 1.9×10(4), cultures were permanently illuminated (70μEm(-2)s(-1)) and As and Se adverse effect was expressed as EC (effective concentration) value. For all experiments three EC (EC(10), EC(20), EC(50)) values for both metalloids were used: for As 26.20, 29.05, 35.38mg L(-1) and for Se 1.93, 3.65, 12.24mg L(-1), respectively. During this study algal biomass growth, lipid peroxidation and protein-bound thiol content parameters were used to assess the As-Se interactions. The reciprocal effect of the elements on their uptake by the alga was also determined. The As-treated algae supplemented with Se exhibited impaired growth indicating a synergistic interaction between the two elements. In samples treated with As-Se mixture, the total algal As content showed marked increase depending on the Se concentration in the mixture. Se uptake was also positively affected by rising As concentrations in the mixture. Consequently, the As-Se-treated algae experienced greater damage to membranes, evidenced by marked elevation of the TBARS (thiobarbituric acid reactive substances) content. The TBARS content increased to a maximum level by 29.05mg L(-1) of As and 3.65mg L(-1) of Se, which was around 70 percent higher than that of the control. The thiol content was very close to that of the control treatment over the entire concentration range and for all As and Se combinations tested. Possible explanation for the synergism observed in D. quadricauda, is that the elevated uptake of As and Se upon their interaction and impaired antioxidant system, has added to the toxicity of the elements.

Chromium as an Environmental Pollutant: Insights on Induced Plant Toxicity

In the past decades the increased use of chromium (Cr) in several anthropogenic activities and consequent contamination of soil and water have become an increasing concern. Cr exists in several oxidation states but the most stable and common forms are Cr(0), Cr(III) and Cr(VI) species. Cr toxicity in plants depends on its valence state. Cr(VI) as being highly mobile is toxic, while Cr(III) as less mobile is less toxic. Cr is taken up by plants through carriers of essential ions such as sulphate. Cr uptake, translocation, and accumulation depend on its speciation, which also conditions its toxicity to plants. Symptoms of Cr toxicity in plants are diverse and include decrease of seed germination, reduction of growth, decrease of yield, inhibition of enzymatic activities, impairment of photosynthesis, nutrient and oxidative imbalances, and mutagenesis.

Combined effect of copper and cadmium on heavy metal ion bioaccumulation and antioxidant enzymes induction in Chlorella vulgaris

The relationships between metal uptake and antioxidant enzyme activities or a response to membrane lipid peroxidation (i.e., malondialdehyde production) in Chlorella vulgaris exposed to Cu and Cd compounds singly and in combination were investigated. The results showed that bioaccumulation of a single metal was influenced by the presence of the other metal. The activities of superoxide dismutase and peroxidase increased to more than fivefold of the control after exposure to Cu(1.5 μM) alone or to Cu(1.5 μM) with Cd mixtures. Malondialdehyde levels in C. vulgaris also increased to approximately twofold of the control after exposure to high concentration of Cu(1.5 μM) alone or to Cu and Cd mixtures. However, Cd alone did not significantly increase the levels of antioxidant enzymes or malondialdehyde.

Metal accumulation, growth, antioxidants and oil yield of Brassica juncea L. exposed to different metals

In agricultural fields, heavy metal contamination is responsible for limiting the crop productivity and quality. This study reports that the plants of Brassica juncea L. cv. Pusa bold grown on contaminated substrates [Cu, Cr(VI), As(III), As(V)] under simulated field conditions have shown translocation of metals to the upper part and its sequestration in the leaves without significantly affecting on oil yield, except for Cr and higher concentration of As(V), compared to control. Decrease in the oil content in As(V) treated plants was observed in a dose dependent manner; however, maximum decrease was recorded in Cr treated plants. Among all the metal treatments, Cr was the most toxic as evident from the decrease in oil content, growth parameters and antioxidants. The accumulation of metals was below the detection limit in the seeds grown on 10 and 30 mg kg(-1) As(III) and Cr(VI); 10 mg kg(-1) As(V)) and thus can be recommended only for oil cultivation.