** Sweetener maker PureCircle's shares down 12 pctpct, worst day since listing last October, after the U.S. seizeslow-calorie sweetener stevia imported from China
** U.S. Customs and Border Protection says it obtainedinformation that stevia was produced by convict labour
** PureCircle disputes allegations, but says depending onduration of detention order sales and profit could be impacted
** Shares at 5-mth lows (venkateshasoumithri.mamidipudi.thomsonreuters.com@reuters.net)