**8. Influence of iron plaque on Rice roots and its effect on arsenic uptake**

Aquatic plants frequently show accumulations of iron and manganese coatings (Fe-plaque) on root systems, commonly attributed to more oxidized soil conditions in the root rhizosphere leading to ferrous to ferric ion production and subsequent hydrolysis to a Fe-Mn oxyhydroxide status. Many researchers have investigated whether Fe-plaque on rice root systems act as a preferential adsorption site for arsenic, thus limiting the potential for arsenic accumulation in plant organs

[12, 43, 50–52]. Most authors acknowledge that the degree of arsenic adsorption by Fe-plaque and the protection afforded towards limiting arsenic accumulation in plant tissue is dependent on (i) soil pH, (ii) the soils oxidation oxidation–reduction status of the bulk soil and the rhizosphere, (iii) the microbial composition, (iv) the quantity of Fe-plaque present on the rice roots, (v) the stage of growth of the rice plant, (vi) the arsenic flux towards the root system and (vii) the presence of competing anionic species in the adsorption processes.

Dong et al. [50] observed that Fe-Mn plaque formation on rice roots was increased because of inoculation with Fe/Mn-oxidizing bacterial strains. The activity of bacterial strains, in combination with exogenous ferrous iron, significantly decreased As and Cd uptake in rice. Interestingly the untreated check showed the following rice plant arsenic concentrations: 354 mg kg−1 for roots, 14.2 mg kg−1 for stem (culm), 24.4 mg kg−1 for leaf, and 0.81 mg kg−1 for brown rice. Conversely, the bacterial strains plus exogenous Fe(II) showed the following rice plant organ arsenic concentrations: 259 mg kg−1 for roots, 13.0 mg kg−1 for stem (culm), 19.2 mg kg−1 for leaf, and 0.72 mg kg−1 for brown rice.
