Researches On Nitrogen Efficiency Evaluation In Rapeseed (Brassica Napus L.) Germplasm And Its Physiological Mechanism

Rapeseed is the main oil crop in China.Rapeseed oil is the largest edible plant in China,accounting for more than 57%of production from domestic oil crops,high and stable yield of rapeseed is of great significance for ensuring food and oil supply safety.Rapeseed has greater demand for nitrogen?N?,irrational application and lower utilization rate of N fertilizer is one of major limitation for development of China's rapeseed industry.It is an important way to solve this problem by investigating the physiological mechanism of high N use efficiency and exploiting the genetic potential for effective N utilization of rapeseed.In this study,hydroponics were applied to evaluate the N efficiency of 350rapeseed germplasm,and two germplasm with extremely difference in N efficiency were screened by repeatedly hydroponics screening and field validation.Using these two extreme materials,the underlying physiological mechanism was preliminary investigated through hydroponics from four aspects,such as root/shoot distubution,root morphological change,root NO₃^-absorption kinetic characteristics and the key enzymes activity in N metabolism.The main results were as follow.1.The results from correlation analysis showed that plant biomass could reflect the status of N in plant?N uptake,N physiological utilization rate?and development of root system.Thus,in this study,biomass and relative biomass were set as main indexes to evaluate the N efficiency of rapeseed at seedling stage.The N efficiency was further divided into four types,including efficient and sensitive,efficient and insensitive,inefficient and sensitive,and inefficient and insensitive.2.Two germplasm with oppsite N efficiency?N-efficient genotype:A294,N-inefficient genotype:A364?were finally screened out.The results showed that the biomass of A294 was significantly higher than that of A364 under low N conditions,while no significant difference in biomass under normal conditions.Two germplasm exhibited the differences in N efficiency and tolerance to N deficiency stress.3.Two extreme germplasm were applied in hydroponics to exploit the physiological mechanism underlying N efficiency.The results showed that the ratio of dry matter and mineral nutrient?N,P,K?which distributed to root was increased at low N,the root dry weight and nutrients concentration of A294 was higher than that of A364.But the dry matter distributed to root of A294 was less than that of A364,which indicated that the N-efficient cultivars A294 could maintain stronger root system and meanwhile could coordinate the distribution of the dry matter from shoot to root.The shoot N concentration of N-inefficient variety was higher but with lower utilation effiency.4.The root P concentration of two germplasm were both decreased under low N condition,while the root P concentration in N-efficient variety A294 was greater than that of A364.In addition,N deficiency significantly increased shoot P concentration of N-inefficient variety A364,but had no influence on the shoot P consentration of A294.The dramatic fluctuation of P concentration in N-inefficient variety A364 indicated that the P metabolism of A364 might be seriously affected by N deficiency.In other word,the relative stable P concentration in N-efficient genotype might imply the less influence in P metabolism by low N stress for N-efficient genotype.5.More developed root system response to N deficiency stress,as indicated by significant increase in root length,root surface area and root tip number of the two germplasm,as well as root surface area produced by per dry weight.Under LN conditions,N-efficient genotype A294 with stronger root system withsignificantly enhance in total root length,root surface area and root tip number.These results indicated that the N-efficient variety not only improve the dry matter utilization efficiency of root system but also increase root dry matter to form stronger root system,wheras the N-inefficient variety only improve the dry matter utilization efficiency of roots,but not for dry matter mass at LN level.6.The NO₃^-absorption rate of rapeseed under control was greater than that of seedlings under LN conditions.There was no significant difference in NO₃^-absorption capacity between two germplasm under same N supply conditions7.The activities of NR and GS showed no significant difference in A294 and A364 under normal condition.However,the NR activity and GS activity of leaves and GS activity of root in N-efficient germplasm A294 were significantly higher than that of N-inefficient germplasm A364.