Iron (Fe) Amendments Mitigate Acidic Saline Effects on Rice Seedlings via Tissue-Specific Na Sequestration in Leaf Sheaths

Soil salinity and iron (Fe) imbalance commonly occur together in acidic paddy soils, but their combined effects on Na transport regulation in rice are not well understood. This study investigated how exogenous Fe supplementation influences growth, ionic balance, oxidative stress, and Na transporter gene expression in roots, leaf sheaths (LS), and leaf blades (LB) under NaCl (salt stress) at low pH. NaCl treatment increased Na concentration and disrupted ionic homeostasis across all organs. Fe supplementation restored ionic balance in aerial tissues, reducing Na concentration in LS and LB relative to NaCl treatment, while root Na concentration remained elevated, suggesting that Fe influenced Na partitioning during translocation rather than exclusion at the root level. Fe application reduced H 2 O 2 concentrations in roots and LB and lowered root MDA; however, MDA concentrations remained elevated in LS and LB under Fe + Na treatment, consistent with organ-level redistribution of oxidative burden rather than a uniform reduction in oxidative damage. Antioxidant enzyme activities were enhanced across organs. Transcript-level analysis indicated contrasting organ-specific patterns: OsSOS1 , OsHKT1;5 , and OsNHX1 expression was normalized in roots; OsHKT1;4 and OsNHX1 were strongly upregulated in LS; and OsNHX1 was attenuated in LB. These transcriptional patterns are consistent with a spatial reorganization of Na sequestration centered on LS buffering, though a causal relationship cannot be inferred from transcript data alone. Principal component analysis integrated physiological, ionic, and molecular responses into a distinct Fe-associated tolerance profile. Overall, exogenous Fe supplementation appears to enhance salt tolerance under acidic conditions in an organ–specific manner, with LS emerging as a proposed central site of Na interception and possible sequestration, potentially protecting photosynthetically active LB tissues.