Genotypic Influence on the Absorption, Use and Toxicity of Manganese by Soybean

Abstract

Plant species and genotypes within species can differ widely in their tolerance to excess manganese (Mn) (Foy et al., 1988) or susceptibility to its deficiency (Graham, 1988) in the soil or other substrate in which they grow. Often these differences are hereditary (Broadley & White, 2005; Pittman, 2005). Although manganese (Mn) toxicity can be a common problem in tropical regions with acid soils, Mn deficiency in soybeans (Glycine max L.) grown in Brazilian Cerrado (savanna) has been recognized as a nutritional disorder frequently related to excessive liming (Tanaka et al., 1992). However, the variations in the concentrations of Mn in plants have been attributed either to genetic factors or to soil fertility conditions (Fageria, 2001). Plant species and genotypes within the same species can differ widely in the tolerance to high Mn (Foy et al., 1988) as well as in susceptibility to the deficiency when grown under conditions of low Mn availability (Graham, 1988). The mechanisms of tolerance to excess Mn have been associated with oxidation of the Mn in the roots, restricted absorption by the roots and translocation of the excess Mn to the leaves, as well as uniform distribution in the tissues, greater internal tolerance and interaction with other elements (mainly K, Ca, Mg, Fe and Si) and defense mechanisms against oxygen reactive species, such as activation of enzymes like ascorbate peroxidase, catalase, Mnsuperoxide dismutase and glutathione peroxidase (Demirevska-Kepova et al., 2004; Morita et al., 2006). On the other hand, better internal utilization or lower requirement, greater redistribution of Mn, increased rate of absorption, exudation of organic acids, acidification of the rhyzosphere and geometry of the root system have been reported as the main mechanisms responsible for lower susceptibility of Mn deficiency (Graham, 1988; Rengel, 1999). The harmful effects of both deficient and excessive Mn on many species have been studied, mainly considering the morphological and anatomical aspects of the ultrastructure and biochemical aspects of the aerial part, where the symptoms are manifested, particularly the leaves (Demirevska-Kepova et al., 2004; Papadakis et al., 2007a,b). However, there are few reports on anatomical and changes in the root ultrastructure in response to variations in the supply of Mn (McQuattie & Schier, 2000) and regarding the effect of genotype (Santandrea et al., 1998a,b; Izaguirre-Mayoral & Sinclair, 2005).