By Joe Cash

BEIJING, Jan 13 (Reuters) - Donald Trump's threat to slap a 25% tariff on countries that trade with Iran risks reopening old wounds with Beijing, Tehran's top partner, challenging Chinese leader Xi Jinping's global image and resolve to defend China's commercial interests.

Iran became a major flashpoint in U.S.-China ties during Trump's first term in the White House, as Washington tightened sanctions on the Islamic Republic and put China's Huawei, accused of selling technology to the country, in its cross-hairs. The arrest of its founder's daughter in Canada at the U.S.'s request sparked bitter recriminations that lingered over the remainder of that administration.

With Iran in his sights once again, a fresh 25% duty would see Chinese shipments to the U.S. incurring levies exceeding 70%, higher than the effective 57.5% tariffs in place before the two leaders struck a deal in October to de-escalate their trade war.

It remains unclear which countries with Iranian business links Trump might target, and he has not named China. The U.S. president has also made offhand remarks that threatened to upend U.S. foreign policy without acting on them before.

"China is........

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