1. a Institute of Experimental Botany, Department of Plant Biotechnology , Sokolovská 6, CZ-772 00, Olomouc , Czech Republic
2. b ENEA, C.R.E. Casaccia, Department of Agricultural Research and Development , C.P. 2400, 1-00 100, Rome , Italy
3. c Institute of Plant Genetics and Crop Plant Research, Department of Cytogenetics , D-0 64 66 Gatersleben, Germany