The influence of arsenic speciation (As-III & As-v) and concentration on the growth, uptake and translocation of arsenic in vegetable crops (silverbeet and amaranth): greenhouse study

We examined arsenic (As) uptake by vegetable crops (amaranth, Amaranthus gangeticus, and silverbeet, Beta vulgaris) as affected by As speciation (As-III and As-v) and their concentrations in nutrient solution. Amaranth and silverbeet were grown in a nutrient solution containing four levels of arsenate (Asv): 0, 1, 5, and 25 mg As/l and three levels of arsenite (As-III): 0, 5, 10 mg AM. Both As-v and As-III are phytotoxic to these crops with the latter being five times more toxic. Amaranth treated with As-III exhibited As toxicity symptoms within 48 h of exposure and was close to death within I week. However, As-V treatment did not show clear toxicity symptoms other than wilting and yield reduction at the highest dose rate of 25 mg As-v/l. The main mechanism used by vegetable crops to tolerate As-v is probably avoidance-limiting As transport to shoots and increasing As accumulation in the root system. When As-v was added to the nutrient solution, the uptake of As in shoots increased and, at the highest dose (25 mg As-v/l), 60 mu g As/g DW (3.6 mg/kg FW) accumulated in the edible portion, which exceeds the WHO recommended limit for food stuffs (2 mg/kg FW) as the water contents of the crops were 94%. It is therefore important to determine the nature of the As species and their bio-accessibility. Iron treatment with 0.5 mg NaFe(III)EDTA/l dose decreased silverbeet As uptake by 45% given its affinity to bind As at the root surface or root rhizosphere and so restrict As translocation to the shoots.