1. Faculty of Agriculture, Ibaraki University, Ami-machi, Ibaraki 300-0393, Japan
2. Ibaraki Horticultural Research Institute, Minori-machi, Ibaraki 319-02, Japan
3. Kyushu National Agricultural Experiment Station, Nishigoshi, Kumamoto 861-11, Japan
4. Faculty of Agriculture, The University of Tokyo, Bunkyo-ku, Tokyo 113, Japan
5. Faculty of Agriculture, Ibaraki University, Ami-machi, Ibaraki 300-03, Japan