Nikkei opened 0.72 percent lower Monday. At 9:15 a.m., the 225-issue Nikkei Stock Average was down 91. 43 points, or 0.72 percent, from Friday at 12,595.09. The broader Topix index of all First Section issues on the Tokyo Stock Exchange was down 1.29 points, or 0.12 percent, at 1, 055.16.