Genetic variation in leaf bronzing and dry matter production of rice varieties as indicators of tolerance to iron toxicity

Iron toxicity, caused by excessive iron uptake, can reduce biomass and yield in rice (Oryza sativa L.). In this report, we present a wide genetic variation among 91 varieties, including 18 upland NERICAs, in terms of leaf bronzing score (LBS), dry weight under iron toxicity (Iron-DW), dry weight under control conditions (Control-DW), and relative dry weight (RDW = Iron-DW/Control-DW), using agar nutrient solution at rice seedling stage. We found no correlations between LBS and the other three. These were classified into three clusters: A, B1 and B2, based on these trait variations. Cluster A consists mainly of lowland Japonica and Indica Groups, exhibiting the lowest LBS and RDW that were intermediate between B1 and B2. Clusters B1 and B2 included both Japonica and Indica Groups’ varieties, as well as the 18 upland NERICAs. B1 displayed the highest RDW and a LBS that was intermediate between A and B2. Conversely, B2 had the highest LBS and the lowest RDW. Both the Indica and Japonica Groups displayed extensive variation in tolerance to iron toxicity. Moreover, LBS and RDW are controlled by different genetic mechanisms. In our association analysis using NERICAs, we identified two quantitative trait loci (QTLs): one for LBS on chromosome 9, which is a novel discovery, and another for Control-DW, which enhances tolerance through O. glaberrima alleles.