Uptake and translocation of inorganic and methylated arsenic species by plants

Abstract

Environmental context. The molecular occurrence of arsenic in soils can vary as a result of soil conditions and/or application of arsenic-containing herbicides or fertiliser. Although large amounts of As-containing herbicides are used for different crops, there is still a lack of understanding as to how the molecular form of As determines the uptake of arsenic into plants and, in particular, the translocation into shoot and grain. Abstract. The uptake and translocation into shoots of arsenate, methylarsonate (MA), and dimethylarsinate (DMA) by 46 different plant species were studied. The plants (n = 3 per As species) were exposed for 24 h to 1 mg of As per litre under identical conditions. Total arsenic was measured in the roots and the shoots by acid digestion and inductively coupled plasma mass spectrometry from which, besides total As values, root absorption factors and shoot-to-root transfer factors were calculated. As uptake into the root for the different plant species ranged from 1.2 to 95 (µg of As per g of dry weight) for AsV, from 0.9 to 44 for MAV and from 0.8 to 13 for DMAV, whereas in shoots the As concentration ranged from 0.10 to 17 for AsV, 0.1 to 13 for MAV, and 0.2 to 17 for DMAV. The mean root absorption factor for AsV (1.2 to 95%) was five times higher than for DMAV (0.8 to 13%) and 2.5 times higher than for MAV (0.9 to 44%). Although the uptake of arsenic in the form of AsV was significantly higher than that of MAV and DMAV, the translocation of the methylated species was more efficient in most plant species studied. Thus, an exposure of plants to DMAV or MAV can result in higher arsenic concentrations in the shoots than when exposed to AsV. Shoot-to-root transfer factors (TFs) for all plants varied with plant and arsenic species. While AsV had a median TF of 0.09, the TF of DMAV was nearly a factor of 10 higher (0.81). The median TF for MAV was in between (0.30). Although the TF for MAV correlates well with the TF for DMAV, the plants can be separated into two groups according to their TF of DMAV in relation to their TF of AsV. One group can immobilise DMAV in the roots, while the other group translocates DMAV very efficiently into the shoot. The reason for this is as yet unknown.