1. Biophysics Group, Department of Physics, Freie Universität Berlin, Arnimallee 14, D-14195 Berlin, Germany, Plant Physiology, Department of Biology, Freie Universität Berlin, Königin-Luise-Strasse 12, D-14195 Berlin, Germany, and Plant Physiology, Department of Biology, Justus-Liebig Universität Giessen, Senckenbergstrasse 3, D-35390 Giessen, Germany