Being a toxic metalloid and group I carcinogen, Arsenic (As) poses a threat to plants, especially to crops which are consumed by human beings, and sooner or later results in hyper/hypopigmentation and skin cancer. It is a well‐known fact that South‐East Asia is suffering from groundwater As contamination, and according to a recent report, the contamination has been found also in Hungary, Mexico, Argentina, Australia, United States, etc. Thus, As contamination has become a global problem. As is toxic even at low concentration because it has no known function as nutrients. Arsenite (III) and arsenate (V) are the main phytoavailable forms of inorganic arsenic. Being analogous to phosphate, As(V) is transported by a phosphate‐cotransport system in plants, whereas As(III) is transported through ‘OsNIP2.1’ (member of aquaporin superfamily) in rice. Besides, ‘AsSe1’ (As‐accumulation gene), ‘AsTol’ (As‐tolerance gene) and ‘OsACR2.1’ (an arsenate reductase gene) have also been identified. The production of phytochelatins (PCs), a metal‐binding thiol peptide, in response to As stress may hold a way of proper As tolerance in plants but still needs a thorough study. However, with the proper knowledge of arsenic speciation, transportation, accumulation, overexpression in crop plants may result in ways to develop arsenic tolerant transgenics.

The effects of increasing concentrations of copper on the growth, ultra-structure and on certain biochemical parameters of water lettuce (Pistia stratiotes L.) were investigated under controlled conditions in the nutrient solutions containing increased copper sulfate concentrations ranging from 0 to 100 µM. Copper treatment for 12, 18 or 24 h resulted in inhibition of roots and leaves dry biomass. Atomic absorption spectrometry analysis of roots and leaves showed that copper accumulation increased with increase in concentration and duration of metal treatment. It is seen that copper resulted in increased production of hydrogen peroxide and superoxide radical in both roots and leave cells, showed a significant change after 24 h of treatment. Also, the significant decrease in the contents of total protein and photosynthetic pigments was observed. The antioxidant enzymes, viz., peroxidase (POX, E.C.1.11.1.7), catalase (CAT, E.C.1.11.1.6) and superoxide dismutase (SOD, E.C.1.15.1.1) showed significant variation with the increase in lipid peroxidation. Increasing trends was observed in levels of ascorbate and glutathione. The rapid inducibility of some of these enzymes are useful early and sensitive indicators of heavy metal toxicity. The results demonstrated that exposure to elevated concentration of Cu had a remarkable effect on the biochemistry and physiology, induced oxidative stress in water lettuce characterized by the initiation of lipid peroxidation that inhibited growth and disintegration of major antioxidant systems.

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Lemna minor L. roots were treated with different concentrations of NaCl. Lipid peroxidation was investigated histochemically and biochemically. At higher NaCl concentrations an increase in staining was observed in the root apices as compared to control for lipid peroxidation and loss of membrane integrity as well as an increase in contents of thiobarbituric acid reactive substance and peroxide. Both the non-enzymic antioxidants, ascorbate and glutathione increased with the NaCl concentration in the roots. Whereas an increase in superoxide dismutase, guaiacol peroxidase, and glutathione reductase activities were marked, catalase activity decreased in the roots under NaCl stress.

The physiological responses to NaCl salinity were investigated in two floating aquatic macrophytes, Pistia stratiotes L. and Salvinia molesta L. With the increasing NaCl concentration a decrease in chlorophyll and carotenoid contents was recorded in Salvinia as compared to Pistia. Also a greater increase in H 2 O 2 accumulation and lipid peroxidation was observed in the shoot and root tissues of Salvinia as compared to Pistia. The superoxide dismutase, glutathione reductase, catalase and guaiacol peroxidase activities, and ascorbate and glutathione contents increased in Salvinia and Pistia shoot and root tissues in response to NaCl.Additional key words: catalase, glutathione reductase, NaCl-salinity, peroxidase, superoxide dismutase, thiobarbituric acid reactive substance. ⎯⎯⎯⎯An inevitable product of aerobic cellular metabolism is generation of reactive oxygen species (ROS) and abiotic stresses are known to act as catalyst in producing free radical reactions causing oxidative stress in plants where reactive oxygen species (ROS), i.e., superoxide radicals, hydroxyl radicals, alkoxyl radicals, and hydrogen peroxide are produced (for details see Scandalios 2002). These ROS cause lipid peroxidation and consequent membrane damage, protein and nucleic acid degradation and pigment bleaching (for details see Hendry and Crawford 1994). Plants are well equipped with both enzymatic [superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), guaiacol peroxidase (GPX), dehydro-ascorbate reductase (DHAR), glutathione reductase (GR)] and non-enzymatic (carotenoid, ascorbate, glutathione, α-tocopherol) antioxidants to overcome the oxidative stress.Pistia stratiotes L., an angiosperm and Salvinia molesta L., a pteridophyte, are two fast growing aquatic floating macrophytes and are important components of the natural ecosystems which are badly affected now-adays with the ever-changing anthropogenic activities (Arber 1963). To test the hypothesis that NaCl-salinity induces oxidative stress in aquatic macrophytes and the possible involvement of antioxidant regulation in the differential salt tolerance of both the aquatic floating macrophytes, the present investigation was undertaken.Floating macrophytes of two types (Pistia stratiotes L. and Salvinia molesta L.) were collected from the uncontaminated pond nearby university (90° 40' E longitude and 20° 04' N latitude) and grown under laboratory condition. The plants were washed with double distilled water several times and soaked dry without damaging the tissues. Plants were then transferred to Petri plates with different concentrations (0, 50, 100 and 200 mM) of NaCl solution with three replicates each. The Petri plates were incubated under white fluorescent tubes (Philips 36 Watt TLD, Bombay, India) giving a photon flux density of 52 µmol m -2 s -1 , temperature of 29 °C and relative humidity of 67 % for 3 d. After the treatments, the shoot and root were separated out, soaked dry and sampled for various biochemical and enzymic estimations. ⎯⎯⎯⎯