Effects of chloramines concentration in nutrient solution and exposure time on plant growth in hydroponically cultured lettuce

The effects of the concentration of and exposure time to chloramine, the product of the reaction between hypochlorous acid and NH4+, on root browning and growth of lettuce (Lactuca sativa L.) cultured hydroponically were investigated. Addition of hypochlorous acid in the form of sodium hypochlorite at 0.3 mg Cl/l to the nutrient solution containing 0.67 mM NH4+ significantly inhibited plant growth 10 days after the start of treatment. Exposure to chloramine for even 1 h by the addition of hypochlorous acid at 0.5 mg Cl/l to the nutrient solution containing 0.67 mM NH4+ also significantly inhibited plant growth. Furthermore, in the nutrient solution containing NH4+ at more than x1/256 unit of Enshi standard, 5.2 × 10−3 mM, plant growth was significantly inhibited following the addition of 0.5 mg Cl/l hypochlorous acid. It was estimated that the critical chloramine amount at which lettuce plant growth was significantly inhibited was 0.18 mg Cl/g root FW.