Iron transport in soybeans (Glycine max [L] Merr.) and tomatoes (Lycopersicum esculentum) is controlled by factors that are altered manyfold as the plant experiences an iron stress (deficiency). Depending on their response to an Fe stress, plants in this study are classed (a) Fe-inefficient or (b) Fe-efficient. The Fe-efficient plants transport more Fe and concomitantly more citrate than the Fe-inefficient plants.An available supply of Fe added to the plant root will facilitate the release of citrate to the stem exudate in both soybeans and tomatoes. Also, when the iron supply is decreased by trapping Fe(2+) at the root, Fe transport decreases with a concomitant decrease of citrate in the stem exudate. Factors other than citrate appear to affect movement of Fe from the external solution into root cells where Fe is chelated with citrate and moves thereafter as Fe citrate. This makes some of the citrate that is transported in the stem exudate dependent on the amount of Fe made available at the root.