In recent decades, people have blindly pursued increased yield; the excessive use of fertilizer not only causes the widespread waste of resources but also puts great pressure on environmental protection. In this study, to find out the optimum nitrogen application to endive crops under hydroponic conditions, this experiment was conducted to investigate the changes in nitrate reductase (NR), nitrite reductase (NiR), glutamine synthetase (GS), and glutamate dehydrogenase (GDH) activities under different nitrogen supply levels, and to fit the equations between nitrogen supply levels and aboveground dry matter accumulation, the aboveground nitrogen accumulation, and the yield of endive crops. The results showed that the activities of the key enzymes of nitrogen metabolism were higher at nitrogen supply concentrations of 8 and 11 mmol·L−1. The dry matter and nitrogen accumulation of endive at different nitrogen supply levels were analyzed with the logistic model; the theoretical yield was found to be the highest at 9.935~11.448 mmol·L−1 of nitrogen application in the two different fertility trials by function fitting.

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