1. National Agricultural Research Center for the Hokkaido Region (Memuro), Shinsei, Memuro, Hokkaido 082-0081, Japan.
2. Corresponding author. E-mail: zenta@affrc.go.jp
3. Obihiro University of Agriculture and Veterinary Medicine, Inada-cho, Obihiro, Hokkaido 080-8555, Japan.
4. National Institute for Crop Science, 2-1-18 Kannondai, Tsukuba, Ibaraki 305-8518, Japan.