1. International Potato Center; Av. La Molina, 1895 P.O. Box, 1558 Lima 12 Peru
2. Brazilian Agric. Research Corp. (Embrapa) - Agric. Instrumentation Cntr.; P.O. Box. 741 13560-970 São Carlos SP Brazil
3. Department of Physics; Ponta Grossa State University; Av. Carlos Cavalcanti, 4748 84030-000 Ponta Grossa PR Brazil