Manganese, lying in the centre of the first transition series, exists in several oxidation states. Of the common oxidation states in plants-Mn2+, Mn3+, Mn4+ and Mn5+—the most dominant and stable is manganous (Mn2+). It is readily oxidized to the less stable manganic form (Mn3+). Manganese forms strong bonds with oxygen containing species. It forms ligands with aminoacid residues of the D I protein of the PS II reaction centre and functions as a charge accumulator by undergoing changes in its oxidation states. As a constituent or an activator of enzymes, manganese plays a catalytic role in cellular metabolism including C 02 fixation in C4 and CAM plants and detoxification of oxygen-free radicals. Plant response to inadequate supply of manganese is suggestive of its role in reproductive development, including seed set and development.