A nutrient solution experiment was done to evaluate effects of different concentrations of nitrogen (N), phosphorus (P) and potassium (K) on leaf mineral concentrations and some enzymes activity of melon seedlings (Cucumismelo var. inodorus subvar. Khatouni). Different levels of these nutrients including 0, 53, 105, 158 and 210 mg L-1 N; 0, 8, 16, 23 and 31 mg L-1 P; 0, 59, 118, 176 and 235 mg L-1 K, all corresponding to 0, 25, 50, 75 and 100% of their concentrations in Hoagland nutrient solution, were applied to plants. The results showed that the highest leaf nitrate reductase (NR) activity was observed at highest N and P levels, whereas the three highest K levels showed the highest NR activity. The highest leaf peroxidase activity was observed at 8 mg L-1 P, 59 mg L-1 K and 158 mg L-1 N. The leaf catalase activity was highest at zero concentration of P, 158 mg L-1 N and 176 mg L-1 K; however, catalase activity was decreased by increasing P levels. Leaf protein content showed an increasing trend with increasing N, P and K levels of nutrient solution, while there was no significant difference between 158 and 210 mg L-1 N. The highest leaf concentrations of N, P, K and Mg were observed at highest nitrogen, potassium and phosphorus levels of nutrient solution, whereas the highest leaf concentration of Ca were obtained at 53 or 105 mg L-1 N, 176 mg L-1 K and 23-31 mg L-1 P. The highest iron concentration of leaves was obtained from 23 to 31 mg L-1 P, 176 mg L-1 K and 210 mg L-1 N.
