Floating ability was characterized mainly by two factors, starting time of internodal elongation and the ability to elongate internode along with increasing water depth (Inouye 1987). In most of previous reports on genetics of deepwater rice, however, floating ability was evaluated without separating these two factors. Therefore, QTL analysis for floating ability of rice was performed using two parameters; position of the lowest elongated internode (LEI) (Inouye et al. 1978) and rate of internodal elongation (RIE) (Takahashi and Mochizuki 2000) in this study.

An F₂ population consisting of 155 plants derived from a cross between Taichung 65 (T65), a Japonica non-deepwater rice, and Bhadua, an Indica typical deepwater rice, was used for QTL analysis. All of the plants were grown in pots under 16 h photoperiod conditions. The LEI position was recorded when the internodal elongation started. The internode position corresponds to the leaf number. To evaluate RIE, each plant that started internodal elongation was subjected to raising water level treatment. The water level was daily adjusted by sinking each

plant in a tank with 2 m depth of water. The treatment was continued for 3 weeks. When the plants reached the bottom of the tank in less than 3 weeks, the treatment was ended. For adjustment of water level, one-half of the uppermost fully-expanded leaf blade on main culm in each plant was maintained to be emerged from water. Internode lengths were measured at the start and end of the treatment, and RIE was calculated as follows; RIE (cm d^-1) = (B-A) / C, where A; initial internode length, B; final internode length, C; days for treatment. Genotyping of the F₂ population was done using 93 RFLP and 21 SSR markers. The linkage map was constructed using MAPMAKER/EXP v. 3.0 (Lander et al. 1987), and QTL analysis was performed with QTL Cartographer v. 1.16c (Basten et al. 2002). Both simple interval mapping (SIM) and composite interval mapping (CIM) techniques were used. QTLs for LEI position and RIE were declared significant at the threshold of 1% level with the LOD score higher than 3.4, given by 5000 permutations.

The LEI position of Bhadua was lower than that of T65. RIE of Bhadua was larger than

that of T65. Frequency distributions for LEI position and RIE of the F₂ population were shown in Fig. 1. The linkage map covering the whole genome was constructed with an average marker interval of 15.3 cM (Fig. 2). Two QTLs for LEI position were detected on chromosomes 3 and 12, and explained 79.3% of phenotypic variance. For RIE, two QTLs were detected on chromosomes 1 and 12, and explained 49.7% of phenotypic variance. These QTLs were identified by using SIM. CIM technique did not give further QTL. In all QTLs, Bhadua alleles increased floating ability (Table 1).

In this study, the two factors were investigated together for the first time. Nemoto and Tang (2002) investigated LEI position and detected QTLs on chromosomes 3, 6 and 12. The regions of chromosomes 3 and 12 were almost the same as we detected. Sripongpangkul et al. (2000) detected main-effect QTLs affecting internode elongation on chromosomes 1, 2 and 4. The QTL detected on chromosome 1 almost coincided with one of the QTLs for RIE that we detected. Between LEI position and RIE, there was significant correlation (R=-0.606**). In addition, the QTLs on chromosome 12 were shared by the two traits. Hence, the QTLs on chromosome 12 may be tightly linked or there may be one key gene controlling floating ability. Furthermore, Eiguchi et al. (1993) reported the dw3 gene, which is responsible for internodal elongation. Relation between dw3 and the QTLs detected in this study should be of interest. We are going to do fine mapping and identification of genes for floating ability.

References

Basten, C. J., B. S. Weir and Z. B. Zeng, 2002. QTL Cartographer version 1.16. North Carolina State University, Raleigh, USA.

Eiguchi, M., R. Sano H., Y. Hirano and Y. Sano, 1993. Genetic and developmental bases for phenotypic plasticity in deepwater rice. J. Hered. 25: 201-205.

Inouye, J., T. Kyuragi and V. T. Xuan, 1978. On the growth habits of floating, single- and double-transplanted rice plants in the Mekong Delta. III. Ability for internode elongation of floating rice. Japan. J. Trop. Agr. 22: 67- 70.

Inouye, J., 1987. On floating rice and its ecological traits in Southeast Asia. Southeast Asia Studies. 25: 51-61.

Lander, E. S., P. Green, J. Abrahamson, A. Barlow, M. J. Daly, S. E. Lincoln and L. Newburg, 1987. MAPMAKER: an interactive computer package for constructing primary genetic linkage maps of experimental and natural populations. Genomics 1: 174-181.

Nemoto, K. and D. Q. Tang, 2002. QTL analysis for the early nodal differentiation in floating rice. Jpn. J. Crop Sci. 71 (Extra issue. 1): 120-121.

Sripongpangkul, K., G. B. T. Posa, D. W. Senadhira, D. Brar, N. Huang, G. S. Khush and Z. K. Li, 2000. Genes/QTLs affecting flood tolerance in rice. Theor. Appl. Genet. 101: 1074-1081.

Takahashi, U. and T. Mochizuki, 2000. Varietal differences in internode elongation ability of deepwater rice. Jpn. J. Crop Sci. 69 (Extra issue. 1): 198-199.