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 i n tolerance to Fe2+ toxicity, which makes it possible to breed more to lerant rice varieties. Tissue tolerance to higher iron concentrations in plants has been considered to be important to Fe2+ tolerance in ric e. Segregation for leaf bronzing and growth reduction due to Fe2+ toxi city was observed in a doubled haploid (DH) population with 135 lines derived from a Fe2+ tolerant japonica variety, Azucena, and a sensitiv e indica variety, IR64 in a solution culture with Fe2+ stress conditio n at a Fe2+ concentration of 250 mg L-1 at pH 4.5. To better understan d the mechanism of tissue tolerance, Leaf Bronzing Index (LBI), total iron concentration in shoot tissue and the enzymes of ascorbate peroxi dase (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, w ere determined for the population under Fe2+ stress. A non-normal dist ribution of LBI was found. About 38 lines showed no bronzing, while th e lines with non-zero LBI values ranged from 0.05 to 0.85 and showed a normal distribution. The other parameters measured showed normal dist ribution. The total iron concentrations in the 38 tolerant lines range d 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 significa nt differences in the activities of the enzymes were found between the parents under normal culture, but remarkably higher Fe2+ induced enzy me activities were observed in the tolerant parent. AS was similar bet ween the parents under both normal and Fe2+ stress, but its concentrat ion was sharply decreased under Fe2+ stress. DHA was much lower in the tolerant parent than in the sensitive parent under Fe2+ stress. Singl e locus analysis and interval mapping analysis based on 175 molecular markers revealed that the interval flanked by RG345 and RZ19 on chromo some one was an important location of gene(s) for Fe2+ tolerance. The ascorbate-specific system for scavenging Fe2+-mediated oxygen free rad icals may be an important mechanism for tissue Fe2+ tolerance. A gene locus with relative small effect on root ability to exclude Fe2+ was a lso detected.