The influence of arsenic speciation (AsIII & AsV) 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 (As(V)): 0, 1, 5, and 25 mg As/l and three levels of arsenite (As(III)): 0, 5, 10 mg As/l. 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 1 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 microg 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.