1. Department of Plant Genetics and Breeding, Agricultural Research Centre, Caritasstraat 21, B-9090 Melle, Belgium, Department of Plant Production, Faculty of Agricultural and Applied Biological Sciences, Ghent University, Coupure 533, B-9000 Ghent, Belgium, and Laboratory of Pharmacognosy and Phytochemistry, Faculty of Pharmaceutical Sciences, Ghent University, Harelbekestraat 72, B-9000 Ghent, Belgium