1. Department of Plant Physiology, Umeå Plant Science Centre; Umeå University; Umeå SE-90187 Sweden
2. Department of Forest Genetics and Plant Physiology, Umeå Plant Science Centre; Swedish University of Agricultural Sciences; Umeå SE-90183 Sweden
3. Institut Jean-Pierre Bourgin; UMR1318 INRA-AgroParisTech; Versailles Cedex 78026 France