1. NARO Institute of Crop Science, 1-18, Kannondai 2-chome, Tsukuba, Ibaraki 305-8518, Japan.
2. Corresponding author. Phone: +81-29-838-8503. Fax: +81-29-838-7488.
3. National Food Research Institute, 1-12, Kannondai 2-chome, Tsukuba, Ibaraki 305-8642, Japan.