Mitigation of arsenic uptake and accumulation in rice grains by applying husk-derived silicon in a synergistic way: Evidence from pot and field trials

Arsenic (As) contamination in paddy soils has posed a prominent threat to rice production in Asia. Recycling of silicon (Si) from Si-rich combusted rice husk (CRH) could serve as a sustainable strategy to mitigate rice As uptake through their shared transport pathway. Root (soil) application of CRH alone, however, was insufficient to decrease inorganic As (iAs) in polished rice below Chinese food standards (0.2 mg kg^-1). In this study, an aqueous Si solution derived from CRH was used for synergistic foliar application over the highest Si-demanding stage (reproductive stage) of rice, following root application of Si, to investigate rice As uptake in both pot and field experiments. In the pot experiment, on the basis of root application of CRH, Si supplementation before the reproductive stage of rice led to a 51% decrease in As concentration on root surface along with a prominent reduction of Fe plaque due to enhanced root suberization, relative to single root application of CRH treatment. In parallel, the expression of OsLis6 gene in the root was downregulated by 91% than that with only root application of CRH. These changes decreased As influx into root by 56% and led correspondingly to 41% lower As transfer to the straw, as compared with root application of CRH treatment. In node I, the expression of OsLis6 decreased concurrently by 71%, leading ultimately to 28% lower iAs accumulation in grains than that with root application of CRH alone. In the field experiment, with single foliar Si, the mitigation of grain iAs occurred only at lower soil As level of 40 mg kg^-1, while promoted iAs unloading into grains was determined under higher soil As level (80 mg kg^-1) relative to the control without Si application. It was, therefore, concluded that the mitigation of grain iAs accumulation with soil application of CRH can be strengthened critically by synergistic supply of foliar Si, serving as a more reliable pathway to secure rice production in As-contaminated paddy fields.