Valores de prevenção para fitotoxidez por cromo em solos de Minas Gerais

Abstract

Chromium (Cr) has no metabolic function in plants and can be toxic in high concentrations. In the environment, it can be found in oxidation states III or VI, the second being more toxic. In plants, Cr appears to be more toxic to roots rather than to the shoot, severely reducing root growth. The establishment of prevention values (PV’s) protects from changes in soil quality and is calculated using ecotoxicological tests with living organisms. In this sense, the objective of this research was to obtain information in order to determine the prevention value for Cr in soils in the state of Minas Gerais, Brazil, by means of phytotoxicity tests. The test was performed with four types of plant species: two monocots (Sorghum bicolor and Zea mays), and two dicots (Phaseolus vulgaris and Raphanus sativus). Two soils (LVAd and CXdb) were artificially contaminated with seven concentrations of Cr (5, 10, 20, 45, 90, 200 and 400 mg kg-1 of dry soil) using dichromate K. We evaluated shoot height, root length, shoot dry weight, root dry weight, stem diameter, number of leaves, gas exchange and nutrient content to establish the effects of the treatments over the plant species. The data were subjected to non-linear analysis using the GraphPad Prism Version 6.0 models for EC's 50. We observed toxic effect of Cr for both soils, with plants cultivated in LVAd displaying greater sensitivity. The study provides basis for determining the PV for Cr in the state of Minas Gerais. Using EC's 50 on both soil types, we obtained a PV (179 mg kg-1) higher than that demanded in legislation (75 mg kg-1). Due to the different behavior of Cr in both soils, different values are necessary for each soil class or according to soil attributes such as clay content, effective cation exchange capacity (t) and organic matter. Changing the current prevention value for the one found in this study, would allow the soils with naturally high PV to fit the legislation.