Physiological And Biochemical Changes In Plant Growth And Different Plant Enzymes In Response To Lead Stress

Lead (Pb) is one of the most widespread, persistent and toxic heavy metal contaminants in agricultural soil. Though Pb is not an essential metal for plant metabolism, it is taken up primarily by the root system and accumulated in the different plant parts. Because Pb ions accumulate predominantly in roots, root growth is more sensitive to this metal than shoot growth. Growth inhibition due to Pb stress depends on various mechanisms affecting directly (such as reduction of cell division and elongation) or indirectly (such as disorders nutrient uptake, photosynthesis and water uptake) plant growth. After entering the cell, Pb ions can also influence the activity of the key enzymes of different metabolic processes such as antioxidative and photosynthesis. Pb stress might inhibit or induce the activity of these enzymes depending on the plant species, metal type and concentration, and duration of the exposure. The inhibition of enzyme activity by Pb mostly arises from the interaction between the Pb and enzyme sulfhydryl groups. Also, inhibition of metalloenzymes under Pb stress may occur due to the displacement of an essential metal by Pb ion. Furthermore, activities of certain enzymes induced by Pb stress might result from the changes in enzyme synthesis, immobilization of their inhibitors. This chapter reviews from the point of view of physiological and biochemical mechanisms the alterations occurring in growth and the activations of different enzymes in plants due to Pb stress.