Chromium is an important toxic environmental pollutant. Chromium pollution results largely from industrial activities, but other natural and anthropogenic sources also contribute to the problem. Plants that are exposed to environmental contamination by chromium are affected in diverse ways, including a tendency to suffer metabolic stress. The stress imposed by Cr exposure also extends to oxidative metabolic stress in plants that leads to the generation of active toxic oxygen free radicals. Such active free radicals degrade essential biomolecules and distort plant biological membranes. In this chapter, we describe sources of environmental chromium contamination, and provide information about the toxic impact of chromium on plant growth and metabolism. In addition, we address different phytoremediation processes that are being studied for use worldwide, in contaminated regions, to address and mitigate Cr pollution. There has been a long history of attempts to successfully mitigate the toxic effects of chromium-contaminated soil on plants and other organisms. One common approach, the shifting of polluted soil to landfills, is expensive and imposes environmental risks and health hazards of its own. Therefore, alternative eco-friendly bioremediation approaches are much in demand for cleaning chromium-polluted areas. To achieve its cleaning effects, bioremediation utilizes living organisms (bacteria, algae, fungi, and plants) that are capable of absorbing and processing chromium residues in ways which amend or eliminate it. Phytoremediation (bioremediation with plants) techniques are increasingly being used to reduce heavy metal contamination and to minimize the hazards of heavy metal toxicity. To achieve this, several processes, viz., rhizofiltration, phytoextraction, phytodetoxification, phytostabilization, and phytovolatilization, have been developed and are showing utility in practice, or promise. Sources of new native hyperaccumulator plants for use at contaminated sites are needed and constitute a key goal of ongoing phytoremediation research programs. Such new plants are needed to enhance the attractiveness of phytoremediation as an effective, affordable, and eco-friendly technique to achieve successful clean-up of metal-contaminated sites worldwide.

No abstract

In the present investigation, we studied the possible potentiating effect of salicylic acid (SA) under Cd toxicity in Oryza sativa L. leaves. Cd treatments for 24 h reduced the shoot length, dry biomass and total chlorophyll content followed by high Cd accumulation in shoots. About 16 h presoaking with SA resulted in partial protection against Cd, as observed by minor changes in length, biomass and total chlorophyll. SA priming resulted in low Cd accumulation. Enhanced thiobarbituric acid reactive substances (TBARS), hydrogen peroxide (H 2 O 2 ) and superoxide anion (O 2 -) content were seen when Cd was applied alone, while under SA priming the extent of TBARS, H 2 O 2 and O 2 -were significantly low, suggesting SA-regulated protection against oxidative stress. The antioxidant enzymes like Catalase (CAT), guaiacol peroxidase (GPx), glutathione reductase (GR) and superoxide dismutase (SOD) showed varied activities under Cd alone. CAT activity increased after Cd treatment, followed by a decline in GPX and GR activity. SOD also declined at the highest concentrations with an initial increase. Under SA-priming conditions, the efficiency of the antioxidant enzymes was significantly elevated. GPx and SOD activity showed significant increase in activity. The ascorbate activity increased after Cd treatment, followed by a decline in glutathione under SA-free condition. SA priming showed gradual increase in these non-enzymic antioxidants. Our results indicate that Cd-induced oxidative stress can be regulated by SA.

No abstract

Inorganic pyrophosphatase, peroxidase, and polyphenoloxidase activities were studied as the function of leaf insertion level in eight monocotyledonous and eight dicotyledonous species. Alkaline inorganic pyrophosphatase shows a declining activity toward the end of senescence whereas no regular drift in either peroxidase or polyphenoloxidase activities was noticed during senescence of attached leaves. In the primary leaves of rice, peroxidase and polyphenoloxidase activities were high in the senescent leaves and there exists a correlation between chlorophyll content and peroxidase activity though not with polyphenoloxidase activity. Upon detachment leaves exhibit increasing peroxidase and polyphenoloxidase activities with time. The distribution of the enzyme activities during senescence of attached leaves is suggested to be species-specific, and an increase in peroxidase and polyphenoloxidase activities cannot be taken as an indicator of leaf senescence.A number of reports are available regarding the increase (1, 2, 4, 7-10, 14, 16, 19) and/or decrease (4,10,12,19,20) in activities of several hydrolyzing and oxidative enzymes during senescence of leaves. Differences in the senescence pattern of attached leaves compared to detached leaves or leaf discs have been reported (13,14,24, 25).Both acid and alkaline inorganic pyrophosphatase (EC 3.6.1.1) occur in higher plants (17). It has been suggested that the alkaline inorganic pyrophosphatase, having extremely high affinity for pyrophosphate, is associated with the anabolic process in the leaf, while acid inorganic pyrophosphatase participates in the catabolic process (7,21,23 This investigation was undertaken to study the distribution pattern of inorganic pyrophosphatase, peroxidase, and polyphenoloxidase activities in attached leaves of different physiological ages and to determine if the changes in peroxidase and polyphenoloxidase activities during leaf senescence can be considered to be species-specific. MATERIALS AND METHODSThe following species were investigated: monocotyledons: Amomum aromaticum Roxb., Eleusine corocana Gaertn., Hordeum vulgare L., Oryza sativa L., Pennisetum typhoideum Rich., Sorghum vulgare Pers., Triticum vulgare Villars., and Zea mays L.; dicotyledons: Arachis hypogaea L., Boerhaavia diffusa L., Chenopodium album L., Crotolaria striata DC., Hibiscus micranthus L., Nicotiana plumbaginifolia Viv., Raphanus sativus L., and Tabernaemontana coronaria Br.Healthy plant samples were collected on sunny days at 10 AM and the approximate age of the organs determined by the method of Kornilov et al. (11). The samples were transported to the laboratory in polyethylene bags covered with crushed ice. The leaves were numbered from the apex downward so as to obtain subsamples representative of various developmental stages. The leaves were excised within 15 min of collection, washed thoroughly with tap water, and then in distilled H20. The leaves were blotted, weighed, and stored in a refrigerator at 8 C for 30 min.Enzyme Extraction and Assay. Enz...