Foliar-applied sulfate and potassium does not reduce rice grain arsenic concentrations nor straighthead severity

Arsenic (As) is naturally present in all soils and can accumulate to toxic levels in rice (Oryza sativa L.) grains under some production conditions. It can also reduce grain yields, posing a double threat to global food security. Plant genetics can affect the amount of As accumulated in grains. A growing body of evidence suggests that one mechanism plants use to detoxify As and limit As concentrations in grains (grain-As) is to sequester As in vegetative tissues. We evaluated 17 rice cultivars previously shown to have contrastingly high or low grain-As. Cultivars high in grain-As commonly transferred As out of their flag leaves during grain fill, while all eight cultivars having low grain-As retained As in flag leaves, effectively excluding it from transfer to developing grains. Sequestration of As in cell vacuoles involves several sulfur (S)-containing compounds. We hypothesized that enhanced leaf concentration of S via foliar fertilization could further reduce grain-As. Effect of foliar-S on grain-As was studied in field-scale and plot experiments. The plot study included cultivars known to contrastingly retain As in flag leaves during grain fill grown in both As-enriched and Native (non-enriched) soils, and included severity ratings for rice straighthead, an As-induced disorder. Contrary to the hypothesis, while foliar-S did increase grain-S concentrations, it did not prove useful for reducing grain-As, nor did it reduce straighthead severity, possibly because soil was not deficient in S or plants used a biochemical mechanism other than S-related As chelation and sequestration to retain As in leaves.