1. Département de Biologie Végétale, Université de Genève, 1211 Genève 4, Switzerland
2. Laboratory of Growth Regulators, Faculty of Science, Palacky University, CZ-78371 Olomouc, Czech Republic
3. Swedish University of Agricultural Science, Umea Plant Science Center, Department of Forest Genetics and Plant Physiology, Umea 901 83, Sweden