Relation of Yield and Petiole Sap Potassium Levels in Tomatoes

Celebrity' tomatoes (Lvcopersicon esculentum Mill.) were grown in peat-perlite under greenhouse conditions with five potassium (K) fertilization concentrations (25, 50, 100, 200 and 300 mg K/liter) in irrigation waters. Two experiments were conducted, one during the winter of 1989 (Expt. 1) and the other during the summer of 1990 (Expt. 2). In both experiments petiole sap K concentrations were monitored on a weekly basis with colorimetric paper test strips. Petiole sap K concentrations (mg K/ml) showed very little variation from plant to plant. However, petiole sap K concentrations varied considerably on a day-to-day basis. The response of petiole sap K concentration to K concentration in the external solutions was guadratic in both experiments. Sap K levels in the summer trial (Expt. 2) were consistently higher than those in the winter trial (Expt. 1) at the same external concentrations. Total and marketable fruit yield increased quadratically with increasing external K concentrations with a plateau occurring at 200 mg K/liter (Expt. 1) and 190 mg K/liter (Expt. 2). The corresponding maximum marketable yields obtained were 2.7 kg/plant and 2.8 kg/plant, respectively. The relationships between total and marketable yields and petiole sap K concentrations were quadratic in Expt. 2, with the maximum marketable yield occurring at 6.1 mg K/ml sap. The corresponding marketable yield was 2.8 kg/plant. Neither the linear nor the quadratic model fit well for yield vs. petiole sap K concentration in Expt. 1 data. However, the similarities of Expt. 1 to Expt. 2 data indicated that this relationship was best interpreted as quadratic. The maximum marketable yield was 2.7 g/plant in Expt. 1, with a corresponding sap K concentration of 5.7 mg K/ml sap. Potassium petiole sap K concentrations between 5.7 to 6.1 mg K/liter appear to provide maximum marketable yield