THE EFFECTS OF pH AND CALCIUM ON THE UPTAKE OF RADIOPHOSPHORUS BY FLUE-CURED TOBACCO
The uptake of radiophosphorus by flue-cured tobacco as a function of pH of the culture solution was determined at five different pH values from 4 to 8. When the plants were cultured in a complete nutrient solution containing 240 p.p.m. calcium, P32 uptake increased significantly from pH 4 to a maximum value at pH 5, then decreased with increasing pH to a minimum value of pH 8. At pH values above 5, precipitation occurred as phosphates of calcium, making phosphorus less available to the plant. A reduction in the concentration of calcium in the nutrient solution to 80 p.p.m. and 0, respectively, resulted in a decrease in the amount of phosphorus precipitated and an increase in the P32 uptake at pH 6 to 8. In calcium-free culture solutions, P32 uptake was inhibited by hydrogen ion in the acid region of the pH range and by hydroxyl ion in the alkaline region. Growth differences were not related to variation in P32 uptake.