Heavy metals are important environmental pollutants and their toxicity is a problem of increasing significance for ecological, evolutionary, nutritional, and environmental reasons. Plants posses homeostatic cellular mechanisms to regulate the concentration of metal ions inside the cell to minimize the potential damage that could result from the exposure to nonessential metal ions. This paper summarizes present knowledge in the field of higher plant responses to cadmium, an important environmental pollutant. Knowledge concerning metal toxicity, including mechanisms of cadmium homeostasis, uptake, transport and accumulation are evaluated. The role of the cell wall, the plasma membrane and the mycorrhizas, as the main barriers against cadmium entrance to the cell, as well as some aspects related to phytochelatin-based sequestration and compartmentalization processes are also reviewed. Cadmium-induced oxidative stress was also considered as one of the most studied topics of cadmium toxicity.

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A relationship between the antioxidant defence system and salt tolerance in two clones of potato (Solanum tuberosum L.) differing in salt sensitivity was studied. The antioxidant defence system of the sensitive clone responded differently to 100 and 150 mM NaCl. At 100 mM NaCl, growth, dehydroascorbate reductase and catalase activities remained unaltered, but chlorophyll and reduced glutathione content decreased (23% and 35%, respectively), while ascorbate content and superoxide dismutase activity were increased 34% and 63%, with respect to the control (0 mM NaCl). The superoxide dismutase increment was higher under 150 mM NaCl treatment, while a general decrease (except for dehydroascorbate reductase and catalase activities) in all the antioxidant parameters studied was observed in the sensitive clone. Reduced glutathione and ascorbate, the main antioxidant soluble defences, and all antioxidant enzymes (except catalase) were significantly elevated in the tolerant clone compared to the sensitive one when both were grown in the absence of NaCl. Under 100 and 150 mM NaCl treatments, no changes in the antioxidant stress parameters were detected in the tolerant clone. These results suggest a relationship between salt tolerance and the antioxidant defence system in the two clones.

The nitrogen metabolism of soybean (Glycine max L.) nodules and roots was studied in plants subjected to two different concentrations (50 and 200 μM) of CdCl2. Nitrogenase activity was decreased in nodules treated with 200 μM Cd2+. In 50 μM Cd2+-treated plants, NH4+ content showed similar values to controls in nodules, but increased by 55% in roots. However, after treatment with 200 μM Cd2+, NH4+ levels increased in both tissues. Glutamate (Glu) and protein contents remained unaltered in nodules treated with 50 μM Cd2+, while at the higher Cd2+ concentration both were decreased. Nevertheless, polyamine content was increased at the two Cd2+ concentrations. In roots, Glu, polyamine and protein levels were significantly diminished at 50 and 200 μM CdCl2. For nitrogen-assimilation enzymes, glutamate dehydrogenase activity was moderately increased in nodules and roots following the lower Cd2+ treatment, though at the higher Cd2+ concentration root enzyme activity returned to control levels. An impressive increase in enzyme activity was found in nodules. In roots, the glutamine synthetase / glutamate synthase pathway was decreased at the two Cd2+ concentrations, though in nodules it was diminished only at 200 μM Cd2+. No changes in protease activity were found in the two tissues treated with 50�μM�Cd2+. However, at 200 μM Cd2+, nodule and root protease activities decreased and increased, respectively. These results suggest that, in general, treatment with Cd2+ affects nitrogen assimilation and metabolism to a greater extent in soybean roots than in nodules.

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