Japan stocks tumbled Monday, hit by mounting concerns about the future of the US auto industry. The benchmark Nikkei 225 stock average lost 390.89 points, or 4.5 percent, to 8,236.08. The broader Topix index fell 4.2 percent to 789.54. Negative sentiment pummeled shares of Japanese automakers after the White House refused to give General Motors Corp. or Chrysler LLC more bailout money. Instead, Chrysler will get up to 30 days to complete an alliance with Italian automaker Fiat SpA, with the government dangling a $6 billion carrot if they can negotiate a deal before time runs out.