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To reduce nitrate content and to elevate ascorbic acid content in hydroponically grown water dropwort (Oenanthe stolonifera DC.) and lettuce (Lactuca sativa L.), nitrogen in the nutrient solution was substituted with chloride and ammonium before harvest. Chloride substitution for nitrogen in the nutrient solution for 6 days before harvest caused leaf color of water dropwort to be light green and subsequent depletion of nitrogen deteriorated its visual quality, even though its nitrate content was greatly reduced to the level of 640 ppm as compared with 3,401 ppm of the control. By supplying 0.5 me·L^(-1) NH₄Cl to the nutrient solution, the leaf color of the plant was improved and the nitrate content could be maintained at 27.1% of the control. As the supplying NH₄Cl concentration increased, the nitrate content of the plant increased also. Especially at 2 me·L^(-1) NH₄Cl, its visual quality was deteriorated caused by impaired root growth. Generally, nitrate content was highly negatively correlated with chloride content (r = -0.69). On the other hand, supplying sodium L-ascorbate in addition to the nutrient solution conditioning for 3 days before harvest inhibited the growth of lettuce. Without nitrogen, yield of the plant was reduced due to increasing senescent leaves, although its growth was similar to that of control. The nitrate content in the plant could be reduced from 3,500 ppm to 2,400 ppm by the nutrient solution conditioning and to 2,000 ppm by complete nitrogen depletion. Total ascorbic acid content in the plant tended to be increased by the nutrient solution conditioning for a short period in conjunction with supplying sodium L-ascorbate, but there was no statistically significant effect on increasing ascorbic acid content.
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