Effects of nutrient solution substitution and photoperiod control before harvest on reduction of nitrate content in hydroponically grown water dropwort (Oenanthe stolonifera DC.).

摘要

To reduce the accumulation of nitrate which causes serious problems in hydroponic culture, the effects of nutrient solution substitution and photoperiod control several days before harvest were determined in water dropwort (O. stolonifera [O. javanica]) grown hydroponically. The nitrate content in fresh tissues of O. stolonifera, which was 3300-4400 ppm at harvest, was reduced to below 2000 ppm by substituting the nutrient solution with tap water for 7 days before harvest. The plant, however, exhibited nutrient deficiency, and foliar application of urea did not overcome the deficiency. When nitrogen in the solution was substituted with 6 mEq chloride/litre alone or in combination with 3 mEq NH4+/litre, the nitrate content was reduced to 840 and 2023 ppm, respectively, without causing growth retardation. The nitrate content was negatively correlated with chloride content, and fresh and dry weights during the nutrient substitution period. Photoperiod control (14 or 20 h) for 5 days before harvest +substitution of nitrogen with chloride had no significant additional effect on reducing the nitrate content, indicating that the nutrient solution substitution had a greater effect than the photoperiod control.