Mineral composition and pH seasonal and diurnal changes of xylem sap were measured in tomato plants, 40 days old, cultivated in nutritive solution under green house conditions in order to evaluate the factors that can regulate it. On a first experiment, the sap was obtained from different plants, same age, always at 9:0 AM, during selected months. On a second experiment, the sap was obtained trough the day, at different times, in a single plant. On the third experiment, the xylem sap was collected from two different positions in the plant body, at 5 cm up from the soil surface, and at the base of the 5th leaf petiole. The pH and Ca+2, K+, Mg+2, NH+4, NO-3, PO(-3)4 and SO-4 from the collected saps were analyzed. The pH varied from 5.3 to 6.4 during the seasonal experiment period. The pH above 6.0 was verified on the months with higher mean temperatures and the pH of 5.4 occurred during the colder months. Potassium and nitrate were the ions found in higher concentrations in the sap. On the diurnal changes experiment, higher values of nitrate, cation/anion ratio and pH were found during the afternoon readings. On the seasonal experiment the nitrate concentration and cation/anion ratio of the sap were higher during the months with lower mean temperatures and lower pH values. The pH of the sap exudates from the stem at 5 cm above the soil surface showed a less acid value (pH 6.13) than the sap collected from the petiole base (pH 5.64). The results obtained suggest that the factors regulating the sap pH affect in different ways the seasonal and diurnal variations of the pH and mineral concentration in tomato plants.
Lycopersicon esculentum; pH; nutrient; xylem sap
