Arsenic toxicity and accumulation in radish as affected by arsenic chemical speciation

Arsenic (As) uptake by Rhapanus sativus L. (radish), cv. Nueva Orleans, gro wing in soil-less culture conditions was studied in relation to the chemica l form and concentration of As. A 4 x 3 factorial experiment was conducted with treatments consisting of four As chemical forms [As(III), As(V), MMAA, DMAA] and three As concentrations (1.0, 2.0, and 5.0 mg As L-1). None of t he As treatments were clearly phytotoxic to this radish cultivar. Arsenic p hytoavailability was primarily determined by the As chemical form present i n the nutrient solution and followed the trend DMAA less than or equal to A s(V) less than or equal to As(III) << MMAA. Root and shoot As concentration s significantly increased with increasing As application rates. Monomethyl arsonic acid treatments caused the highest As accumulation in both roots an d shoots, and this organic arsenical showed a higher uptake rate than the o ther As compounds. Inner root As concentrations were, in general, within th e normal range for As contents in food crops but root skin As levels were c lose or above the maximum threshold set for As content in edible fruit, cro ps and vegetables. The statement that toxicity limits plant As uptake to sa fe levels was not confirmed in our study. If radish plants are exposed to a large pulse of As, as growth on contaminated nutrient solutions, they may accumulate residues which are unacceptable for animal and human consumption without exhibiting symptoms of phytotoxicity.