The rupee turned sharply weak against the dollar on the interbank foreign exchange market here today on heavy dollar demand.The rupee opened weak at Rs 43.97/99 per dollar against last Friday's close of Rs 43.82/83 and after falling further to a low of Rs 44.0450, ended at Rs 43.92/94, sharply lower than the previous close.Smart rise in dollar against other major world currencies also dampened the rupee sentiment, dealers said.The dollar's rise against the rupee was also triggered by heavy dollar purchases by foreign institutional investors (FIIs) who booked profit at the stock exchange after selling heavily.Shares at the stock exchange fell sharply and the Sensex nosedived by 111.92 points due to late bout of selling spree by FIIs.Meanwhile, the RBI has fixed the reference rate for the dollar at Rs 44.0200 per dollar from the last level of Rs 43.8300. Similarly, the the fixing rate for pound was Rs 82.3966 per pound from Rs 82.2580. The reference rate for euro was fixed at Rs 57.5200 per euro asagainst the previous rate of Rs 57.8100 euro and yen was fixedat rs 42.01 per 100 yen as against the last rate of Rs 41.89.