Mitigation of arsenic uptake and accumulation in rice grains by applying husk-derived Si 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 husks could serve as a sustainable strategy mitigating rice As uptake through their shared transport pathway. Open combusted rice husks (CRH) have shown an improved Si release capability, maintaining Si in soil solution at ~27-fold higher level than that in control throughout an entire growing season of rice. Root-applied CRH alone, however, was insufficient to decrease inorganic As (iAs) in polished rice below Chinese food standards (0.2 mg kg-1). To improve this mitigation measure, an aqueous Si solution from CRH was derived for synergistic foliar application, targeting specifically to inhibit rice As uptake over its highest Si-demanding stage (reproduction). On basis of soil-applied CRH, Si supplemented at this critical period 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 soil CRH treatment. In parallel, the expression of OsLis6 gene in the root was downregulated by 91% than that with only root-applied CRH. These changes decreased As influx into root by 56%, and led correspondingly to 41% lower As transfer to the straw, as compared with soil CRH treatment. In node I, the expression of OsLis6 decreased concurrently by 71%, leading ultimately to 28% lower iAs accumulation in grains than with soil CRH alone. With single foliar Si, the mitigation of grain iAs occurred only at lower soil As of 40 mg kg-1 while promoted iAs unloading into grains was determined under higher soil As level (80 mg kg-1) relative to control. We, therefore, conclude that the mitigation of grain iAs accumulation with soil-applied CRH can be strengthened critically by synergistic supply of foliar Si, serving as a more reliable pathway securing rice production in As-contaminated paddy fields.