1. iThemba LABS, Materials Research Department, National Research Foundation, P. O. Box 722 Somerset West 7129, South Africa
2. UNESCO-UNISA Africa Chair in Nanosciences/Nanotechnology, College of Graduate Studies, University of South Africa, Muckleneuk Ridge, P. O. Box 392, Pretoria 0001, South Africa
3. Council for Scientific & Industrial Research, Biosciences, P. O. Box 395, Pretoria 0001, South Africa
4. Institute of Plant Biology, Biological Research Center, P. O. Box 521, Szeged H-6701, Hungary