Characterization of tissue tolerance to iron by molecular markers in different lines of rice

Ferrous iron (Fe²⁺) toxicity is a major disorder in rice production on acid, flooded soils. Rice (Oryza sativa L.) genotypes differ widely in tolerance to Fe²⁺ toxicity, which makes it possible to breed more tolerant rice varieties. Tissue tolerance to higher iron concentrations in plants has been considered to be important to Fe²⁺ tolerance in rice. Segregation for leaf bronzing and growth reduction due to Fe²⁺ toxicity was observed in a doubled haploid (DH) population with 135 lines derived from a Fe²⁺ tolerant japonica variety, Azucena, and a sensitive indica variety, IR64 in a solution culture with Fe²⁺ stress condition at a Fe²⁺ concentration of 250 mg L⁻¹ at pH 4.5. To better understand the mechanism of tissue tolerance, Leaf Bronzing Index (LBI), total iron concentration in shoot tissue and the enzymes of ascorbate peroxidase (AP), dehydroascorbate reductase (DR) and glutathione reductase (GR), and concentrations of ascorbate (AS) and dehydroascorbate (DHA), which are involved in the ascorbate-specific H₂O₂-scavenging system, were determined for the population under Fe²⁺ stress. A non-normal distribution of LBI was found. About 38 lines showed no bronzing, while the lines with non-zero LBI values ranged from 0.05 to 0.85 and showed a normal distribution. The other parameters measured showed normal distribution. The total iron concentrations in the 38 tolerant lines ranged from 1.76 mg Fe g⁻¹ to 4.12 mg Fe g⁻¹ and was in a similar range as in the non-tolerant genotype (2.04 -4.55 mg Fe g⁻¹). No significant differences in the activities of the enzymes were found between the parents under normal culture, but remarkably higher Fe²⁺ induced enzyme activities were observed in the tolerant parent. AS was similar between the parents under both normal and Fe²⁺ stress, but its concentration was sharply decreased under Fe²⁺ stress. DHA was much lower in the tolerant parent than in the sensitive parent under Fe²⁺ stress. Single locus analysis and interval mapping analysis based on 175 molecular markers revealed that the interval flanked by RG345 and RZ19 on chromosome one was an important location of gene(s) for Fe²⁺ tolerance. The ascorbate-specific system for scavenging Fe²⁺-mediated oxygen free radicals may be an important mechanism for tissue Fe²⁺ tolerance. A gene locus with relative small effect on root ability to exclude Fe²⁺ was also detected.