Gleyic paddy soils are widely distributed in People's Republic of China, SriLanka, India, Indonesia, Sierra Leone,Libya, Nigeria, Columbia and Philippines (Mahadavappa et al. 1979, Yoshida 1981). In the People's Republic of China, gleyic paddy soils are estimated to cover 7.5 to 8.0 million hectares, where ferrous iron toxicity in the soils reduces rice yield by 10-20% depending on the intensity of toxicity and tolerance of the rice cultivar (Li et al. 1991).

Ninety-six backcross Inbred lines (BILs) derived from Nipponbare (japonica) /Kasalath (indica)//Nipponbare were used for molecular mapping of QTL associated with ferrous iron tolerance. RFLP map and molecular data obtained from the Rice Genome Project, Japan were used for QTL mapping. BILs, along with their parents and control cultivar IR26, IR64, IR74, Mahsuri and Suakoko8 were screened for ferrous iron tolerance at Nanjing Agricultural University, People's Republic of China in 2001. Leaf bronzing index (LBI = bronzing leaf number/ total leaf number), stem dry weight (SDW), tiller number (TN) and root dry weight (RDW) were measured after 4weeks of solution culture experiment.

Frequency distributions of leaf bronzing index, stem dry weight, tiller number and root dry weight of the BILs are shown in Fig. 1. There is a clear difference between Nipponbare and Kasalath based on all traits measured. BILs showed average phenotypic values.

We identified a total of eight QTLs associated with ferrous iron tolerance using the computer package Mapmaker/QTL(Lincoln et al. 1993) with threshold LOD > 3.0. One putative QTL was detected for leaf bronzing index on chromosome 1 (Table 1, Fig. 2). It explained 20.5% of the variation for leaf bronzing index observed among BILs. Of the two QTLs mapped

for stem dry weight, the one on chromosome 1 coincided with QTL for leaf bronzing index (Fig. 2). Three putative QTLs were detected for root dry weight on chromosome 1 and 3. The QTL on chromosome 1 coincided with QTL for leaf bronzing index, while the other QTL on chromosome 3 coincided with QTL for stem dry weight on chromosome 3 (Fig. 2). Of the two QTLs mapped for tiller number located on chromosome 1, one QTL located at the region of C955-C885 on chromosome 1 also coincided with QTL for leaf bronzing index.

Breeders will be able to utilize the QTLs found in this study in marker-aided selection to transfer genes for ferrous iron tolerance from Japonica cultivar like Nipponbare into elite breeding materials with a higher harvest index, to increase productivity of rice grown in soils with ferrous iron toxicity above present levels.

Acknowledgments

We thank Dr. M. Yano at Rice Genome Research Program, National Institute of Agrobiological Resources, Tsukuba, Japan for providing backcross inbred lines (BILs) and Wang Chunming, Su Changchao at Nanjing Agricultural University in People's Republic of China for

helping to calculate QTLs.

References

Li, D. M., J. J. Tang, J. L. Zhou, and S. Q. Li, 1991. The eco-physiological mechanism of rice tolerance for gleyic soil stress and the breeding of varieties tolerance for soil-related stress. Rice Review and Abstracts. 10: 1-4.

Lincoln, S. E., M. J. Daly, E. S. Lander, 1993. Mapping genes controlling quantitative traits using MAPMAKER/ QTL version 1.1: a tutorial and reference manual. 2^nd ed. Cambridge Mass: Whitehead Institute for Biometrical Research.

Ahadavappa M. 1979. IRRPS, NO. 43. IRRI.

Yoshida, S. 1981. Fundamentals of Rice Crop Science. IRRI.