1. Sugadaira Montane Research Center, University of Tsukuba, Sugadaira, Sanada, Nagano 386-2201, Japan
2. Graduate School of Agricultural Science, Tohoku University, Kawatabi, Naruko, Miyagi 989-6711, Japan
3. Institute of Agriculture and Forestry, University of Tsukuba, Tsukuba, Ibaraki 305-8572, Japan