The Nikkei stock index tumbled over 2 percent Tuesday morning on selling of technology shares, after Wall Street declined as rising U.S. long-term yields stirred concern about higher borrowing costs in the world’s largest economy. The 225-issue Nikkei Stock Average fell 827.22 points, or 2.11 percent, from Monday to 38,405.58. The broader Topix index was down 50.00 points, or 1.82 percent, at 2,703.20.