The Indian rupee tumbled by 31 paise to a fresh one-year low of Rs. 46.50 per US dollar in early trade on Friday, extending its downward spiral against the American currency on sustained dollar demand from banks and importers in view of dollar firmness in overseas markets.The rupee resumed lower at Rs. 46.26/27 per US dollar on the Interbank Foreign Exchange on Friday, as against its previous close of Rs. 46.19/20 per dollar, and moved down further to Rs. 46.50 per dollar before quoting at Rs. 46.46/47 per dollar.