Differences in Fe toxicity response index and associated growth characteristics among rice genotypes

Acid sulfate soils in tropical coastal rice areas contain high Fe levels, and excess ferrous (Fe2+) ions are released during the early growth stage in the wet season. In such environments, Fe toxicity can cause significant yield losses. The objectives of this study were to evaluate the seedling growth of diverse rice germplasm (from japonica, aus and indica groups) under high-Fe conditions in hydroponic culture and to elucidate the growth characteristics of Fe toxicity tolerant genotypes. We calculated an Fe toxicity response index (FTRI) as a selection criterion that reflects both the response of shoot biomass to high Fe and the amount of Fe-induced leaf bronzing damage: higher FTRI values indicate greater tolerance to high Fe levels. We first compared Fe levels to determine the appropriate conditions for genotype selection, and then screened 2- to 3-week-old seedlings grown for 2 more weeks in hydroponic solution containing 500 mg Fe2+ L−1. Sixteen genotypes were screened, and for five of them, the differences in FTRI corresponded with previously reported yield performance in high-Fe fields. We identified Taichung65 (temperate japonica) as a promising donor, as it showed little bronzing damage and high relative (stress/control) biomass under high-Fe conditions, resulting in a high FTRI. Plant nutrient analyses of Taichung65 suggested that maintenance of low Fe concentration and low total Fe content in shoots despite high Fe concentration in the rhizosphere, through prevention of either root Fe uptake or Fe translocation from the root to the shoot, should be an important mechanism of Fe toxicity tolerance.