1. Department for Plant Genetics and Breeding, Centre for Agricultural Research, Caritasstraat 21, B-9090 Melle, Belgium, Centre de recherche TEPRAL, route d'Oberhausbergen 68, F-67037 Strasbourg, France, European Commission, Joint Research Centre, Institute for Health and Consumer Protection, Food Products and Consumer Goods Unit, I-21020 Ispra, (VA) Italy, and Department for Plant Production, Ghent University, Coupure Links 653, B-9000 Gent, Belgium