1. 1Leibniz Institute of Plant Genetics and Crop Plant Research (IPK), Gatersleben, Seeland, Germany; email: huang@ipk-gatersleben.de, schnurbusch@ipk-gatersleben.de
2. 2Faculty of Natural Sciences III, Institute of Agricultural and Nutritional Sciences, Martin Luther University Halle-Wittenberg, Halle, Germany