1. Department of Physics, Norwegian University of Science and Technology, N-7491 Trondheim, Norway, Instituto de Recursos Naturales y Agrobiología de Salamanca (IRNASA-CSIC), Apdo. 257, 37071 Salamanca, Spain, Biological Centre, Academy of Sciences of the Czech Republic and University of South Bohemia, Branišovská 31, 370 05 České Budějovice, Czech Republic