Identification, separation and component analysis of reddish brown and non-reddish brown iron plaque on rice (Oryza sativa) root surface

Background and aim Iron plaque (IP) on rice (Oryza sativa) root surface consists of reddish brown IP (RIP) and non-reddish brown IP (NRIP), however, their extraction and characterization need further studies. Methods A reliable method combining chemical and X-ray diffraction (XRD) analysis was introduced to discriminate RIP and NRIP on root surface of rice plants subjected to different phosphate (P) treatments. Results RIP appeared only on P-deficient rice root surface, and NRIP occurred on both P-deficient and P-sufficient rice root surface. Both RIP and NRIP could be extracted by dithionite-citrate-bicarbonate while only NRIP could be extracted by 0.1 M tetrasodium pyrophosphate (Na₄P₂O₇) specifically. NRIP on P-sufficient root surface was 2.42 times ofthat on P-deficient plants. Iron speciation analysis showed that the total, amorphous and crystalline iron on P-deficient root surface were 1.47-, 1.50- and 1.35-times of those of P-sufficient plants, respectively. XRD analysis further confirmed that IP on both P-sufficient and P-deficient root surface presented as amorphous features. Dominant minerals of NRIP were iron phosphates, while those of RIP were ferric (hydr)oxides. The specific removal effect of 0.1 M Na₄P₂O₇ for NRIP was also verified by XRD. Conclusion In this study, phosphate levels in nutrient solution significantly influence the formation of RIP and NRIP on rice root surface. Main components of RIP and NRIP are ferric (hydr)oxides and iron phosphates, respectively. They can be separated by 0.1 M Na₄P₂O₇.