Abstract
Tomato plants, Lycopersicon esculentum Mill., were decapitated 5 cm above the ground at 1-hr intervals for 24 hr, to investigate the dynamics of mineral transport from root to shoot. Each xylem sap sample was taken for 1 hr after discarding a few initial drops.
Exudation rates increased from dawn (7:00 A. M.), reached the maximum between 9:00 and 12:00, and then decreased gradually.
The concentration of nitrate in the xylem exudate increased slightly from early morning. Phosphate concentration increased rapidly from evening, reached maximum around 10:00 P. M., and then decreased to a minimum at 10:00 A. M. In contrast to phosphate, potassium concentration reached maximum around 10:00 A. M., then fell to a minimum around 8:00 P. M. Calcium and magnesium concentrations tended to increase slightly at night.
The exudation rate decreased for the plants covered with black cheesecloth which reduced light intensity by 70%. Mineral concentrations were slightly higher in the shaded plants than in the control at noon, 3 hr after the treatment. As time lapsed, however, the mineral concentrations except that of phosphate decreased in the shaded plants. Phosphate concentration was always higher in shaded plants than in the control ones.
The concentrations of nitrate, phosphate, calcium and magnesium in the xylem exudate decreased as plants aged, whereas potassium concentration was unaffected by plant age.
