Phosphorus and Sulfur Compounds in Plant Xylem Sap

Abstract

The xylem exudate of plants contained inorganic phosphate and two unknown P32 labeled compounds. Sulfur was present only as inorganic sulfate. As much as 20% of the total radioactive phosphorus in the exudate on decapitated tomato stems was in the unknown present in largest amounts. This unknown was found also in guttate fluid but not in exudate from squash peduncle. The two unknowns are normal constituents of the xylem exudate rather than wound compounds on the cut surface or products formed by radiation injury from the p32. The percentage of P32 in the unknowns reached a maximum value about 4 hours after the addition of P32 to the nutrient solution bathing the roots. This was interpreted as indicative of the rate of incorporation of phosphate into the compounds. Both the seeds of seedling and the roots of older plants contribute to the formation of the two unknown phosphorus-containing substances.