Nikkei opened sharply lower Thursday and plunged below the 14,000 mark, as falls on Wall Street and a firmer yen added to a sense of caution following recent wide market swings. At 9:15 a.m., the 225-issue Nikkei Stock Average was down 415. 78 points, or 2.90 percent, from Wednesday at 13,910.68, after sliding to as low as 13,880.93, the lowest intraday mark since May 2. The broader Topix index of all First Section issues on the Tokyo Stock Exchange was down 29.60 points, or 2.51 percent, at 1, 149.27.