WASHINGTON, March 29— The White House said today that in 1978 Hillary Rodham Clinton invested $1,000 in commodities futures and that the investment grew in 10 months of trading in the notoriously volatile market into a gain of nearly $100,000. Seeking to dispel suggestions that the trades were risk-free and improperly arranged by an Arkansas lawyer who represents one of the state's most powerful companies, the White House issued a statement this afternoon that said the First Lady had put up her own money and that she bore all of the financial risks in a marketplace where three out of...