Translocation of nickel in xylem exudate of plants

Topped plants of tomato (Lycopersicon esculentum), cucumber (Cucumis sativus), corn (Zea mays), carrot (Daucus carota), and peanut (Arachis hypogaea) were treated with 0.5 to 50 micromolar Ni (containing ⁶³Ni) in nutrient solutions. Xylem exudate was collected for 10 hours or, in the case of corn, for 20 hours at 5-hour intervals. Electrophoresis of nutrient solution distributed all Ni cathodically as inorganic Ni²⁺. Low concentrations of Ni in tomato exudate migrated anodically, presumably bound to organic anion (carrier). However, this carrier became saturated at about 2 micromolar Ni in exudate, and excess Ni ran cathodically. Most of the Ni in cucumber, corn, carrot, and peanut exudate ran anodically, and its migration rate was identical for all exudates. Peanut root sap contained 14 to 735 micromolar Ni. The anodic Ni carriers in root sap and exudate appear identical. The carrier in root sap became saturated near 100 micromolar Ni, as shown by cathodic streaking of Ni exceeding that concentration. It appears that all five species translocate low concentrations of Ni in the same anionic form.