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

Authors: Pinson, Shannon; Heuschele, Deborah - Jo; ISBELL, CHRIS - Isbell Farms; SMITH, AARON - Louisiana State University; LI, JIFENG - Louisiana State University; VANDAL, MATTHEW - Louisiana State University

Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/8/2023
Publication Date: 2/22/2023
Citation: Pinson, S.R., Heuschele, D.J., Isbell, C., Smith, A.P., Li, J., Vandal, M.P. 2023. Foliar-applied sulfate and potassium does not reduce rice grain arsenic concentrations nor straighthead severity. Crop Science. https://doi.org/10.1002/csc2.20945.
DOI: https://doi.org/10.1002/csc2.20945

Interpretive Summary: There is global concern about the amount of arsenic contained in rice grains. While a survey by the USDA, FDA of more than 1000 samples of rice and rice products produced in the USA determined that U.S.-produced rice is safe for human consumption, only a portion of their samples met the more stringent, lower limit on arsenic required for baby foods. Arsenic is of higher concern in rice than other grain crops because rice is typically produced under flooded field conditions, creating an environment that makes some compounds, including arsenic, more available for plant uptake. Arsenic is also toxic to plants and causes a yield-reducing disorder as rice straighthead. One mechanism that can use to detoxify As is to sequester, or trap, it in vacuoles of roots, stem, or leaf cells. The first step to sequestering a toxic metal, such as arsenic, is to chelate it, or bind it chemically. Because plants use sulfur-containing compounds for this chelation process, it has been hypothesized that increasing availability of sulfur in plant roots, stems and leaves can increase the sequestration process, thereby reducing grain arsenic accumulation and/or straighthead severity. In our first study, we showed that rice cultivars known to produce grains low in arsenic content did indeed retain more arsenic in their flag leaves during grain fill than cultivars with higher grain arsenic levels. One mechanism for leaf retention of arsenic would be increased vacuolar sequestration. In contrast, cultivars with higher grain arsenic reduced the amount of arsenic in their leaves, and the arsenic transported out of leaves during grain fill was available for transport to grains. We then sprayed rice plants with sulfur fertilizer, using both a large field-scale study, as well as a replicated small-plot study, anticipating that increased leaf sequestration would reduce grain arsenic concentrations. Contrary to our hypothesis, we saw neither a reduction in straighthead severity nor in grain arsenic concentrations in sulfur-sprayed plants. The fact that grains increased in sulfur content demonstrated that the sulfur sprayed onto the leaves did get into the plants and was available for use and transport to grains. It is possible that the plants in this study were already obtaining sufficient amounts of sulfur from the soil. Soil analyses did show sufficient sulfur for rice production, but we anticipated that the plants would use and therefore need more sulfur than typically recommended when challenged with As-amended soil. It is further possible that the plants are retaining arsenic in their leaves due to a mechanism other than sulfur-requiring chelation and vacuolar sequestration.

Technical Abstract: Arsenic can accumulate in toxic levels in rice grains under some production conditions, and can reduce grain yields as well, posing a double threat to global food security. The amount of arsenic accumulated in rice depends on both plant genetics and the production environment. There is an accumulating body of evidence suggesting that one mechanism plants use to limit grain arsenic concentrations is to sequester arsenic in vegetative tissues. Here we evaluated 17 rice cultivars that were shown from prior data to have contrastingly high or low grain-arsenic and shown that cultivars high in grain-arsenic commonly transfer arsenic out of their flag leaves during grain fill, while all eight cultivars selected having low grain-arsenic were found to retain arsenic in their flag leaves, effectively excluding it from transfer to the developing grain. Because sequestration of arsenic in cell vacuoles involves several sulfur containing compounds, this led us to hypothesize that enhanced leaf concentration of S via foliar fertilization could further reduce grain-arsenic. We studied the effect of foliar-sulfur on grain-arsenic using both field-scale and small plot experiments, with the small plot study using arsenic-enriched and Native (non-enriched) soils and included evaluation of rice straighthead severity, an arsenic-induced disorder. We evaluated effects in cultivars known to contrastingly retain arsenic in flag leaves, or not, during grain fill. Contrary to our hypothesis, while foliar-sulfur did increase grain-sulfur concentrations, it did not prove useful for reducing grain-arsenic, nor did it reduce straighthead severity, possibly because soil sulfur was not deficient, or use of a different means for leaf arsenic retention.