A Study Of Chemical Substances Effects On Nitrate Regulation In Vegetables

Potential harm of excessive nitrate in vegetables to human bodies has been deeply concerned by the public. However, studies on the measures to regulate nitrate accumulation in vegetables cultivated with organic substrates and corresponded mechanisms were limited. In this thesis, the effects of dicyandiamide (DCD), sodium thiosulfate (STS), 6-benzyladynine (6-BA) and citric acid (CA) applied on the regulation of nitrate accumulation in vegetables such as flowering Chinese cabbage cultivated with bagasse substrates, appropriate quantity and period of DCD applied, a mechanism responsible for the regulation of nitrate accumulation in vegetables were studied to efficiently reduce nitrate accumulation in vegetables. The results indicated that nitrate content in the edible part of flowering Chinese cabbage treated by DCD (equal to 20% total N applied), STS (1.5g-kg-1dry substrates), 6-BA (0.1g-L-1), CA (0.1mmol-L-1) reduced by 31.08%, 2.09%, 12.72%, 13.40%respectively. Application of DCD equal to 10%~25% total N applied at 20 days before harvest reduced the number of Nitrosomonas, the amount of NO3--N and the ratio of NO3--N/NH4+-N in culture substrates, moreover the effect increased with the increasing in apply quantity of DCD. The accumulation of nitrate in flowering Chinese Cabbage decreased with the decrease of the NO3~-N amount and NO3"-N/NH/-N ratio in the substrates after application of DCD, indicating that the decrease of nitrate accumulation in vegetables treated by DCD was resulted from the inhibition of nitrification by DCD applied. Application of DCD equal to 20% total N applied at 20 days before harvest reduced nitrate content by 24.9%, 27.1%, 39.1%, and 41.6%, in flowering Chinese cabbage, spinach, lettuce and Chinese cabbage, respectively, while the yield and the content of vitamin C, water-soluble sugar, chlorophyll of the vegetables were not affected significantly. Moreover, the application of DCD could elevate the level of vitamin C in Chinese cabbage. These results indicated that application of DCD equal to 20% total N applied at 20 days before harvest was an appropriate measure to regulate the accumulation of nitrate in vegetables and maintain yield and quality of vegetables.