EFFECT OF NITROGEN CONCENTRATION IN A HYDROPONIC SOLUTION ON NITROGEN UPTAKE AND GROWTH OF TOMATO PLANTS

In Experiment I, the influence of nitrogen (NO3 -N) level in the hydroponic nutrient solution on the growth of young tomato plants of two cultivars was compared. Simultaneously, nitrogen uptake by the same plants was investigated during a 13-day period while the first cluster was flowering. Plants in Group 1 were supplied with 100 ppm of nitrate-N (100 ppm plot). The concentration was kept nearly constant by adding more N every other day to the culture solution for the duration of the experiment. Plants of Group 2 were set in a culture solution lacking nitrogen for the initial two days. On the third day and every other day thereafter, the amount of N equivalent to that utilized by Group 1 plants was added to the culture solution (0 + alpha-ppm plot). After the initial application of N, the N concentration in the 0 + alpha-ppm plot was kept at less 38 ppm until the experiment was terminated. 1. The increases in fresh weight of plants in the two groups closely approximated each other except that plants of Group 2 maturated two days later. 2. The growth rate of plants in the two treatments were proportional to the rate of N uptake. In Experiment II, 'Kyouryoku-beiju' tomato plants grown hydroponically in a two-truss culture system were administered N at a) constant concentration of 100 ppm; b) 4 g at the beginning of the experiment; or c) 2 g at the beginning of the experiment and 2 g again when the N in the culture solution was exhausted. 1. The growth curves of tomato plants in the three plots were nearly identical until their fruits reached maximum size. 2. The total amount of N absorbed per plant in the 100 ppm plot was about 1.7 times more than those plants which received a total of 4 g N/plant. 3. The yield and nitrogen content of fruits from the 100 ppm plot were almost the same as those from the other two plots. The fresh weight of leaves and stem from the 100 ppm plot was 1.6 times more than those given 4 g N/plant. Thus, the nitrogen efficiency of fruit production (fruit yield per g N) of the 100 ppm plot was lower than those of the other two plots. 4. The effect of N level on the accumulation of P, K, Ca, and Mg was also examined.