By Samuel Shen and Ankur BanerjeeSHANGHAI/SINGAPORE (Reuters) - China's stock exchanges and central bank scurried to defend a tumbling yuan and falling stock markets on Monday, trying to soothe investors concerned about Donald Trump's return to the White House and Beijing's ability to revive the economy.With two weeks before Trump begins a second U.S. presidency, his threats of big tariffs on Chinese imports have rattled the yuan, driven mainland bond yields down and got stocks off to a rough start to 2025.On Monday, China's tightly controlled yuan weakened to its lowest in 16 months while the blue-chip stock index touched its weakest level since the end of September, down as much as 0.8% on the day. The index fell 5% last week to clock its biggest weekly loss in more than two years.The Shanghai and Shenzhen stock exchanges recently held meetings with foreign institutions, both bourses said on Sunday, assuring investors they would continue to open up China's capital markets.The People's Bank of China could issue more yuan bills in Hong Kong in January, state-owned news outlet Yicai reported on Monday, in a sign authorities want to absorb currency to dampen speculation. Financial News, a central bank publication, said the PBOC has the tools and the experience to react to yuan depreciation."The decision to allow the yuan to weaken last week has heightened concerns about capital outflows, further dampening investor sentiment," said Charu Chanana, chief investment strategist at Saxo."Preventing a sharp decline of the yuan will be crucial for China's recovery. Any tactical recovery this year will need more than just stimulus measures, particularly whether China can negotiate a deal with President-elect Trump."investing.com