Studies On Nutritious Physiology Of Rapeseed(Brassica Napus L.)Germplasms With High Nitrogen Efficiency

Rapeseed?Brassica napus L.?is one of the important oil crops in China.The demand of nitrogen?N?fertilizer in rapeseed is very high while the utilization rate is extremely low.Exploring the genetic potential of rapeseed and improving its N efficiency is one of the effective ways to reduce fertilizer input and realize the green and sustainable development of rapeseed.However,fully understanding the physiological mechanism of N efficiency in rapeseed is the premise and basis for improving the N efficiency of rapeseed.Based on this background,in this study a pair of rapeseed germplasms with different N efficiency were used as test materials,and used hydroponics experiment to reveal the physiological mechanism of rapeseed's N efficiency from plant phenotype,root morphology and structure,nutrients coordination,N absorption,transport and utilization.The main results are as follows:1.Under normal N condition,the differences between two germplasm with different N efficiency in phenotypes,root morphological structure,plant total N accumulation and total N utilization were small,Low N stress increased the difference between germplasm,the phenotype and physiological performance of the N-efficient rapeseed were better than that of the N-inefficient rapeseed.The results from hydroponic culture experiment showed that low N stress inhibited the growth of rapeseed and significantly reduced the biomass,while low N stress promoted the elongation of main root and lateral root,the root to shoot ratio was increased.The correlation analysis between biomass and root morphological indexes indicated that the accumulation of biomass in rapeseed increased its dependence on total root length,root surface area,root volume,number of lateral roots,main root length and root biomass under low N condition.The results of root morphological index analysis showed that under low N stress,total root length,root surface area,root volume,number of lateral roots and main root length were 30.96%,22.02%,24.13%,28.20%and 40.62%higher than those of low N stress,and the root-shoot ratio of N efficiency gemplasm was more reasonable.A further observation was made on the main root cells found that,the number of cell in meristematic zone and the length of cell in elongation zone of N efficient germplasm under low N stress were more and greater than that of N inefficient germplasm,indicating that N efficient type germplasm under low N stress can promote root cells division and increase the elongation zone length to promote root growth,in order to better adapt to the low N stress,therefore,more developed root system is the basis for N efficiency of rapeseed..2.Under low N stress,the accumulation of nitrogen,phosphorus and potassium in rapeseed was significantly reduced,while the accumulation of nitrogen,phosphorus and potassium in rapeseed germplasm with high N efficiency was slightly reduced and significantly greater than that rapeseed germplasm with low N efficiency,which was 2.02,1.89 and 3.37 times higher than that rapeseed germplasm with low N efficiency.The affinity of rapeseed to NO₃^-and NH₄⁺was significantly increased under low nitrogen stress,and the determination of root absorption kinetic parameters showed that the absorption rates of NO₃^-and NH₄⁺were significantly decreased under nitrogen starvation stress,but the absorption rates of NO₃^-and NH₄⁺in rapeseed with high N efficiency were significantly higher than those in rapeseed with low N efficiency.The absorption capacity of NO₃^-and NH₄⁺in rapeseed under N starvation was determined by ¹⁵N isotope absorption method,and the results showed that rapeseed was more inclined to absorb NH₄⁺.In addition,the absorption capacity of NO₃^-and NH₄⁺of the N efficient rapeseed was significantly greater than that of the N inefficient rapeseed,as well as the transport efficiency of NH₄⁺,but there was no significant difference in the transport of NO₃^-.Therefore,the efficient NO₃^-,NH₄⁺absorption rate and NH₄⁺transport rate are the possible reasons for the high N accumulation capacity of N efficient rapeseed.3.Under low N stress,the physiological N utilization rate of rapeseed plants was significantly increased,while the N efficient type was significantly greater than the N-inefficient type.Similarly,the free amino acid content and GS activity of the plant were also significantly greater than that of the N-inefficient type,and the root NR activity was greater than that of the N inefficient type.Therefore,the high N assimilation capacity provides a favorable material basis for plant morphological construction,and is one of the important reasons for the high N efficiency of rapeseed.