Study On Differences Of Long-distance Transport And Short-distance Distribution Of NO₃^- Among Two Brassica Napus Genotypes With Different Nitrogen Use Efficiency And Its Impact Mechanism On Nitrogen Use Efficiency

Brassica napus is an important oil-bearing crop, it demands high amounts of nitrogen but characterized by low nitrogen use efficiency (NUE), in general, the nitrogen application was more than 200kg hm-2, but the nitrogen fertilizer use efficiency was less than 40%. Excessive application of nitrogen was not only a waste of resource, but also creates problems for environment. Therefore, it is important to improve the NUE for the development of sustainable agriculture. In this paper, we researched the transcriptome of two Brassica napusgenotypes which have different NUE (H represent the high NUE genotype, and L represents the low NUE genotype), and we explored the differences of the long-distance transport of NO3- and the distribution of NO3- between the cytoplasm and vacuole among the two genotypes. Also, we explored the impact of the differences on the NUE. It reveals the essential cause of the difference of NUE between the two genotypes, and it provides the scientific evidence for improving the utilizing of NO3- and the NUE. The results of the work were as follows:1、We found 1444 genes which expressed significantly difference by exploring the transcriptome of the two Brassica napusgenotypes having different NUE. These genes were significantly enriched in the enzyme regulator activity、pollination、response to external stimulus、and response to stress of the GO database. It showed that there were significantly difference in the amino acid metabolism (Alanine, glutamate,et al)、 metabolism of other amino acids (D-Glutamine, Glutathione,et al、biosynthesis of other secondary metabolites and xenobiotics biodegradation and metabolism between the two genotypes by KEGG metabolic pathway analysis. It provides us important clues for researching the essential cause of the difference of NUE between the two genotypes.2、With nitrogen supply sufficient, the relative expression of NRT1.5 in the H was significantly higher than that in the L, and the relative expression of NRT1.8 in the H was significantly lower than that in the L, besides, the transpiration rate of H was significantly higher than that of the L whether at seedling stage or at flowering stage, it leads more proportion of nitrate was allocated in the shoot of the H than that of the L. The NR activity, GS activity of the root of the H were lower than that of the L, however, The NR activity, GS activity of the shoot of the H were higher than that of the L, moreover, H has higher chlorophyll content and photosynthetic rate, it implied that H has a weaker nitrate assimilation capacity in the root but stronger in the shoot than that of the L. H has a higher NUE due to a more proportion of nitrate transported to the shoot for assimilated than the L. However, with nitrogen supply insufficient, no significantly difference was observed in the long-distance transport of nitrate and the NUE among the two genotypes.3、With nitrogen supplied decreased to some extent, the stomatal conductance and transpiration rate of Brassica napus plants were increased, it promoted nitrate uptake and nitrogen fertilizer use efficiency on one hand, on the other hand, it spurred nitrate transport from root to shoot. And the NR activity and GS activity were not decreased. Therefore, the Brassica napus plants had a higher NUE in the nitrogen insufficientenvironment because of the higher transpiration rate enhanced nitrate allocated in the shoot where it was assimilated. However, the higher transpiration rate in the nitrogen insufficient environment decreased the water use efficiency.4、Because of lower V-ATPase activity and V-PPase activity which due to lower relative expression of vha-a2 and avp1 in the H, there was less NO3- influx and more H+ efflux in the vacuole of the H. There was more NO3- distributed in the cytoplasm in the H, resulting in a higher NUE in the H compared with the L. it showed that there were positive correlation between V-ATPase and NO3- content, and between V-PPase and NO3- content, the correlation between V-ATPase and NO3- content was lower than that between V-PPase and NO3- content, it implied that V-PPase contributed more energy for transporting NO3- from cytoplasm to vacuole.