Elevated carbon dioxide increases nitrate uptake and nitrate reductase activity when tobacco is growing on nitrate, but increases ammonium uptake and inhibits nitrate reductase activity when tobacco is growing on ammonium nitrate. - Matt,P..; Geiger,M..; Walch-Liu,P..; Engels,C..; Krapp,A..; Stitt,M.. (Plant, Cell and Environment, 2001)