Physiological insensitivity to an increase in leaf Fe constitutes a tissue-tolerance mechanism against Fe toxicity in rice

Abstract Aims Iron (Fe) toxicity is a major nutrient stress that severely limits lowland rice yield worldwide. While tolerant rice varieties have been identified, their specific physiological factors underlying the tolerance remain unclear. This study aims to clarify the physiological mechanisms that contribute to field-based Fe stress tolerance. Methods Field trials were conducted at Fe-stressed alongside control sites in Indonesia. Three contrasting lowland Indica rice genotypes were used: Ciherang (sensitive), IR42 (tolerant), and a breeding line KA-28 (tolerant) derived from a cross between Ciherang and IR42. A hydroponic study was conducted to investigate responses of photosynthesis and gene expression. Results In the field experiment, Ciherang exhibited the most severe leaf bronzing, biomass reduction, and > 50% yield loss, whereas IR42 and KA-28 maintained > 60% of control yield with milder symptoms. Sensitive Ciherang and tolerant KA-28 had similar shoot Fe concentrations (> 700 ppm), indicating that tolerance was due to a tissue-based mechanism rather than the exclusion of excess Fe at the root. The hydroponic study confirmed the “tissue-based” tolerance mechanism of KA-28 under Fe toxicity, with less impact of unit Fe on leaf symptoms compared to Ciherang. Reductions in photosynthetic rates and changes in a Fe-responsive vacuolar Fe transporter gene, OsVIT2 , per unit increase in Fe concentrations were more pronounced in Ciherang than in KA-28. Conclusions These findings collectively suggest that the physiological insensitivity to increased Fe levels is the basis for the tissue-tolerance mechanism observed in KA-28 in the field. This study provides valuable insights for breeding Fe-tolerant rice varieties.