1. Research Faculty of Agriculture, Hokkaido University, Sapporo, Hokkaido 060-8589, Japan
2. University of Tsukuba, Tsukuba, Ibaraki 305-8518, Japan
3. National Institute of Crop Science, Tsukuba, Ibaraki 305-8518, Japan and
4. National Institute of Agrobiological Sciences, Tsukuba, Ibaraki 305-8602, Japan