Chemical Forms of Manganese and Zinc in Phloem Exudates

Investigations were performed to study the chemical form in which manganese and zinc are transported in sieve tubes. As a test plant Ricinus communis was used. From this plant phloem sap can be obtained in a rather pure state. The plants received labelled manganese or zinc. In the experiments on manganese translocation a radioactive phosphorus isotope was added to the nutrient solution to test if complex compounds of the cation contain also phosphorus. Components of the phloem exudate were determined by means of physicochemical separation methods. Almost all zinc but only a part of the manganese were bound to organic compounds. The major part of the manganese was in ionic form. The molecular weight of the complexing com‐pound(s) of manganese was estimated to be between 1000 and 5000 and of zinc between 1000 and 1500. The complexes probably contain some phosphorus. The charge of the zinc complex is negative.