Uptake and translocation of inorganic and methylated arsenic species by plants

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 As^V, from 0.9 to 44 for MA^V and from 0.8 to 13 for DMA^V, whereas in shoots the As concentration ranged from 0.10 to 17 for As^V, 0.1 to 13 for MA^V, and 0.2 to 17 for DMA^V. The mean root absorption factor for As^V (1.2 to 95%) was five times higher than for DMA^V (0.8 to 13%) and 2.5 times higher than for MA^V (0.9 to 44%). Although the uptake of arsenic in the form of As^V was significantly higher than that of MA^V and DMA^V, the translocation of the methylated species was more efficient in most plant species studied. Thus, an exposure of plants to DMA^V or MA^V can result in higher arsenic concentrations in the shoots than when exposed to As^V. Shoot-to-root transfer factors (TFs) for all plants varied with plant and arsenic species. While As^V had a median TF of 0.09, the TF of DMA^V was nearly a factor of 10 higher (0.81). The median TF for MA^V was in between (0.30). Although the TF for MA^V correlates well with the TF for DMA^V, the plants can be separated into two groups according to their TF of DMA^V in relation to their TF of As^V. One group can immobilise DMA^V in the roots, while the other group translocates DMA^V very efficiently into the shoot. The reason for this is as yet unknown.