Heavy metal stress induced alterations in the activities of several representatives of the enzymatic antioxidant defense system such as guiacol peroxidase (POD), catalase (CAT) and ascorbate peroxidase (APX) were comparatively studied in barley seedlings treated by Zn and Cd. Although roots were the main sites of metal accumulation, the oxidative damage was detrimental mainly in the leaves. Our experiments clearly show that the investigated heavy metals have quite different effects on plant metabolism and induce the protective enzymatic system in different ways. It is inevitable that we should cope with the environmental damages induced by increased concentration of heavy metals. One of the common characteristics of these events is the oxidative stress generating active oxygen species (AOS) (Salin 1987). Plants have evolved various protective mechanisms to eliminate or reduce AOS species. One of them is the enzymatic antioxidant system, including SOD, POD, CAT and APX. Each of these enzymes has physiological function under non-stressed conditions, but their activity/or quantity is increased under oxidative stress. The comparison of the the antioxidative enzymatic system in seedlings exposed to two essentially different heavy metals, namely Zn and Cd should reveal essential elements of this defense system. While Zn is an essential microelement that is indispensable for normal plant growth at low concentration and is toxic only at high concentration, Cd has no vital function in plants developing under “natural” conditions.
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