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Accumulation of nitrate in vegetables during hydroponic culture has recently been a serious problem. To reduce the nitrate content in water dropwort (Oenanthe stolonifera DC.) grown hydroponically with its exclusive nutrient solution, nutrient solution substitution and photoperiod control for several days before harvest were attempted. When the plant was grown in the nutrient solution, the nitrate content was high ranging from 3,300 to 4,400 ppm in fresh tissues at harvest. The nitrate content was reduced to below 2,000 ppm by the substitution of the nutrient solution with tap water for 7 days before harvest, but the plant exhibited the symptom of nutrient deficiency. However, foliar application of urea was not effective to overcome the deficiency. When nitrogen in the nutrient solution was substituted with 6 me·L^(-1) chloride or with 6 me·L^(-1) chloride plus 3 me·L^(-1) NH₄^+, the nitrate content was reduced to 840 and 2,023 ppm, respectively, without causing any growth retardation of the plant. The nitrate content in the plant was highly negatively correlated with its chloride content, fresh weight, and dry weight during the nutrient substitution period. In addition, photoperiod control to 14 or 20 hours for 5 days before harvest in conjunction with the substitution of chloride for nitrogen had no significant additional effect on reducing the nitrate content, indicating that the nutrient solution substitution had greater effect than the photoperiod control.
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