Response To Replacemnt Of Nitrate By Ammonium Of Chinese Cabbage

This dissertation is aimed to clarify the mechanism of NH₄⁺ enhancement on pakchoi growth from the aspects of physiology and biochemistry. Hydroponic experiments were carried out to study the effect of different forms of N on photosynthetic characteristics, root growth and development, uptake of N, P and K, as well as the yield and quality of pakchoi.The results obtained are listed as follows.1. Five cultivars of pakchoi, very popularly cultivated in south-east China, such as Qf 1, Szq, Shq and Ry 2, responded in a significantly different way to different ratios of NH₄⁺-N : NO₃^--N. Biomasses of roots and shoots of these cultivars increased with the increasing of NH₄⁺-N:NO₃^--N ratio, and obtained the greatest value at 25:75 of NH₄⁺-N:NO₃^--N. Therefore, different ratios of NH₄⁺-N:NO₃^--N in nutrient solution could influence the biomass of pakchoi, and the best ratio of NH₄⁺-N:NO₃^--N for pakchoi growth and development is 25 : 75. Different ratio of NH₄⁺-N:NO₃^--N in nutrient solution could affect the ratio of root and shoot, which was increased with the increased application of NH₄⁺-N, and then decreased with the further increasing of applied NH₄⁺-N.2. Different ratios of NH₄⁺-N:NO₃^--N in nutrient solution significantly affected the contents of NO₃^- in shoots of four cultivars. NO₃^- contents of four cultivars decreased with the increasing of the percentage of NH₄⁺-N in total N applied.3. Different cultivar of pakchoi responded differently to different NH₄⁺-N:NO₃^--N ratios in nutrient solution. Szq was a mostly sensitive cultivar and Ry 2 was a mostly insensitive cultivar to different ratios of NH₄⁺-N:NO₃^--N in terms of biomass production and NO₃^- content among the cultivars tested in this experiment.4. There aren't significant effect of different NH₄⁺-N:NO₃^--N ratios in nutrient solution on nitrogen content of pakchoi. But there are significant effect of different NH₄⁺-N:NO₃^--N ratios in nutrient solution on P and K content of pakchoi. The photosynthetic characteristics of cultivars, Szq and Ry 2, responded in a significantly different way to different ratios of NH₄⁺-N:NO₃^--N. The SPAD readings, net photosynthesis rate (Pn) and stomatol conductance (Cond) of Szq and Ry 2 cultivars increased with the increasing of NH₄⁺-N:NO₃^--N ratio from 0:100 to 25:75, and they were found to be the highest in 25:75 for NH₄⁺-N:NO₃^--N. However, they decreased with the increasing of NH₄⁺-N:NO₃^--N ratio further.5. Results from the photosynthesis experiments showed that the cultivar named as Szq was a more sensitive genotype to different ratios of NH₄⁺-N:NO₃^--N than the cultivar named as Ry 2 in this experiment. Therefore, Szq was a sensitive genotype and Ry 2 was a insensitive genotype to different ratios of NH₄⁺-N:NO₃^--N.6. Different ratios of NH₄⁺-N:NO₃^--N in nutrient solution could greatly affect the root growth of Szq and Ry 2. Total root surface area, root volume, total root length and root number increased with increased application of NH₄⁺-N, percentage from 0% to 25%, They were found to be the highest in 25:75 of NH₄⁺-N:NO₃^--N, and then decreased quickly with the further increase NH₄⁺ amount. This result indicated that moderate NH₄⁺ addition could improve the development of root growth of pakchoi.7. In terms of the effects of different ratios of NH₄⁺-N:NO₃^--N on root architectures of Szq and Ry 2 cultivars, Szq had greater response and be more sensitive than that of Ry 2 under moderate ratio of NH₄⁺-N:NO₃^--N.