The Bombay Stock Exchange benchmark Sensex on Wednesday fell by 188.51 points, or 1.10 per cent, in opening trade on selling by foreign funds driven by weak trends in the overseas bourses.The 30-share index, which had lost over 860 points in the past five sessions, fell by 188.51 points to 16,591.95 points. Banking, technology and metal sector stocks were under pressure, dragging the Sensex down.The wide-based National Stock Exchange index Nifty dipped below 5,000-point level losing 63.15 points, or 1.26 per cent, to 4,944.75 points.