1. Institute of Plant Sciences, Swiss Federal Institute of Technology (ETH), Universitätstrasse 2, CH–8092 Zurich, Switzerland;
2. Present address: CRINA SA, Chemin de la Combe 15, CH–1196 Gland, Switzerland;
3. Present address: Swiss Federal Research Station for Agroecology and Agriculture (FAL), Reckenholzstrasse 191, CH–8046 Zurich, Switzerland