1. Graduate School of Environmental Earth Science, Hokkaido University, Sapporo, Hokkaido 060-0810, Japan
2. Umeå Plant Science Centre, Department of Forest Genetics and Plant Physiology, Swedish University of Agricultural Sciences, 901-83, Umeå, Sweden
3. Graduate School of Agriculture, Hokkaido University, Sapporo, Hokkaido 060-8589, Japan
4. Faculty of Science, Nara Woman's University, Nara 630-8506, Japan