Factors Which Affect the Amount of Inorganic Phosphate, Phosphorylcholine, and Phosphorylethanolamine in Xylem Exudate of Tomato Plants

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Abstract
Phosphate in the xylem exudate of tomato (L. esculentum) plants was 70-98% Pi, 2-30% P-choline, and < 1% P-ethanolamine. Upon adding 32Pi to the nutrient, Pi in xylem exudate had the same specific activity within 4 h. P-choline and P-ethanolamine reached the same specific activity only after 96 h. The amount of Pi in xylem exudate was dependent on Pi concentration in the nutrient and decreased from 1700 to 170 .mu.M when Pi in the nutrient decreased from 50 to 2 .mu.M. The flux of 0.4 nmol organic P/min per g fresh weight root into the xylem exudate was not affected by the Pi concentration in the nutrient solution unless it was below 1 .mu.M. During 7 days of Pi starvation, Pi in the xylem exudate decreased from 1400 to 130 .mu.M while concentrations of the 2 phosphate esters remained unchanged. The concentration of phosphate esters in the xylem exudate was increased by addition of choline or ethanolamine to the nutrient solution, but Pi remained unchanged. Upon adding [14C]choline to the nutrient, 10 times more [14C]P-choline than [14C]choline was in the xylem exudate and 85 to 90% of the ester phosphate was P-choline. When [14C]ethanolamine was added, [14C]P-ethanolamine and [14C]ethanolamine in the xylem sap were equal in amount. P-choline and P-ethanolamine accumulated in leaves of whole plants at the same time and the same proportion as observed for their flux into the xylem exudate. No relationship between the transport of P-choline and Pi in the xylem was established. The amount of choline in xylem exudate and its incorporation into phosphatidylcholine in the leaf suggest that the root is a site of synthesis of P-choline and P-ethanolamine for phospholipid synthesis in tomato leaves.