Effects Of Partial Replacement Of Nitrate By Ammonium On Some Physiological Parameters Of Pakchoi (Brassica Campestris L. Ssp. Chinensis (L.))

Previous reports showed that the yield and quality of vegetables can be significantly increased by properly increasing ammonium concentration in the nutrient solution. Thus, enhancing the percentage of ammonium nitrogen in fertilizer application in vegetable production is a potential nitrogen management beneficial to the reduction of environmental risk of nitrate loss, the increase of profit of farmers and the promotion of human health. However, the mechanisms of the effect of enhanced proportion of ammonium on vegetable growth and food quality is little known till now. This thesis for doctor degree is aimed to study the related mechanisms from the view point of plant nutrition and plant physio-biochemistry. Hydroponic experiments were conducted in the greenhouse, Nanjing Agricultural University, P. R. China. Nitrate and ammonium uptake, nitrate reductase activity (NRA), chloroplast protein content, cytokinins content, active iron and total iron contents and leaf areas, et al, were measured. Results obtained were as follows.The biomass and leaf area were mainly influenced by the ratios of NO₃^--N/NH₄⁺-N, other than total nitrogen concentration. Increasing ammonium concentration in the nutrient solution could increase both the biomass and leaf area first then decreased in either the 5 mmol L^-1 total nitrogen solution or 5 mmol L^-1 nitrate solution. As the total nitrogen concentration of the medium solution was fixed at 5 mmol L^-1, the replacement of 1.25 mmol L^-1 nitrate by ammonium increased both the biomass and leaf area by 50%. While properly added some ammonium to the 5 mmol L^-1 nitrate nutrient solution, biomass and leaf area increased by 39.6% and 16.3%, respectively.Ammonium was beneficial to the synthesis of chlorophyll and accumulation of carbon in the plants. The increasing of NH₄⁺-N ratios in the medium solution when the total nitrogen concentration was fixed at 5 mmol L^-1 increased chlorophyll SPAD readings, ammonium (free ammonium) and carbon content., The biomass, leaf area, carbon content, total nitrogen and carbon accumulation were the highest at the ratio of NO₃^--N/NH₄⁺-N, 3.75:1.25 (mmol L^-1) suggesting that 3.75:1.25(mmol L^-1) of NO₃^--N/NH₄⁺-N ratio was the best for the nitrogen uptake, CO₂ fixation and yield formation.Partial replacement of nitrate by ammonium was beneficial to soluble protein accumulation. Significant positive relationships existed between leaf soluble protein content and chlorophyll SPAD readings and ammonium concentration in the nutrient solution when the total nitrogen concentration was 5 mmol L^-1.Chlorophyll SPAD readings, active iron content and chloroplast protein content were all significantly related (p<0.05) with ammonium concentration in the nutrition solution when the nitrate concentration was fixed as 5 mmol L^-1, with the coefficients being 0.914, 0.954 and 0.964. The coefficients of active iron content with chlorophyll SPAD readings and chloroplast protein content were 0.841 and 0.912 respectively. There was no significant relationship between chlorophyll SPAD readings and leaf area. The increased chlorophyll content was not the condensed effect caused by leaf shrink, but the ammonium-promoted chlorophyll synthesis, which was caused by reusage of total iron in the plants, thus increasing active iron content in leaf blades and chloroplast protein content.At 5 mmol L^-1 total nitrogen solution, partial relacement of nitrate by ammonium significantly increased cytokinin content. There was a significant relationship between leaf area and cytokinin content (r =0.970,p<0.05). At the 5 mmol L^-1 total nitrogen and 5 mmol L^-1 nitrate solution, the coefficients of the leaf area of the fully expanded top leaf blade and biomass were 0.966 and 0.941, which were both significant at p<0.01.Genetic differences existed in nitrogen nutrition and biomass.The decreasing order of Shanghaiqing> Luxingqincai>Kangre 605> Liangbaiye 1 was found in the items of biomass, chlorophyll SPAD readings, carbon contents, total carbon and nitrogenaccumulation, whi...