Mechanical Understanding Of Nitrogen Utilization Efficiency In Different Genotypes Of Oilseed Rape (Brassica Napus L.)

Since the last century,the application rate of nitrogen(N)fertilizers in many regions exceeds crop demand,resulting in both higher production costs and a greater risk of environmental hazards.Thus,breeding N-efficient crop genotypes and increasing N use efficiency(NUE)could be an important strategy to meet the sustainability of agroecosystem with lower intensity input.Among the major crops,oilseed rape(Brassica napus L.)has a higher N requirement,but the NUE is low because of the low NUtE.Based on this,improving NUtE is the key to improve NUE,reduce N application and stabilize yield in oilseed rape production.Understanding the differences in NUtE of contrasting rape genotypes and clarifying the biological mechanism of efficient N utilization are the basis for cultivating new N-efficient genotypes.However,current researches in this area were quite limited.Herein,A comprehensive study of Nt-responder,Nt-nonresponder,Nt-efficient and Nt-inefficient genotypes was conducted under field,pot,hydroponic and vermiculite experiments.The specific objectives of the present study were to determine the plant traits closely related to NUtE and explore the main reasons for NUtE differences of contrasting oilseed rape genotypes.With this aim in view,we hoped that our results would be helpful for the researchers in revealing the biological mechanism of efficient N utilization and providing reliable theoretical basis for breeding high NUtE oilseed rape genotypes.The main conclusions were listed below:(1)Seed yield and seed N accumulation are two important plant traits for evaluating of NUtE in oilseed rape genotypes.Under both a field and pot experiments,we found that the high NUtE genotypes displayed significantly higher NUE(36.5%)and NUtE(42.0%),along with greater harvest index(34.0%),N harvest index(42.0%),seed yield(39.5%),seeds per silique(31.5%)and seed N accumulation(30.5%)than the low NUtE genotypes.The further analysis indicated that the NUtE was positively correlated with seed yield and seed N accumulation,but there were no significant differences in shoot biomass and shoot N accumulation among the contrasting oilseed rape genotypes.(2)Nitrogen utilization-efficient oilseed rape genotypes exhibit stronger growth attributes at flowering stage.Results from a field experiment and a hydroponic culture showed that the high NUtE genotypes displayed lower growth activity from seedling to flowering stages,compared to the low NUtE genotypes.However,flowering was the critical period,when the reverse growth habit occurred between high and low NUtE genotypes.The high NUtE genotypes displayed 12.4%higher root biomass and 13.5%greater root N concentration,along with larger 16.4%root GS and 17.3%greater GOGAT activity;16.0%higher shoot biomass and 20.4%larger shoot N concentration,accompanied by 52.2%higher leaf net photosynthetic rate and 15.0%greater transpiration rate than the low NUtE genotypes.(3)Pollen grain number and viability are important factors that determine the seeds per silique,seed yield and NUtE among oilseed rape genotypes.We selected 18 oilseed rape genotypes with contrasting NUtE to measure pollen grain number,pollen vigor,initial ovule number at flowering stage,and observed the dynamic changes of silique traits from pre-embryo,globular,heart,torpedo to maturity stages,and finally measured the ovule abortion rate at the mature stage.Our results showed that no significant differences in initial ovule numbers were found among the contrasting oilseed rape genotypes.Compared to the low NUtE genotypes,the high NUtE genotypes displayed 44.1%higher pollen grain number,23.5%greater pollen viability,and 39.3%lower ovule abortion rate.Additionally,the heart stage was the critical ovule development period for delineating the variability of the seeds per silique among contrasting NUtE oilseed rape genotypes,when the high NUtE genotypes displayed higher silique net photosynthetic rate,surface area,biomass,and RNA expression levels.(4)Post-flowering N uptake led to the genotypic variation in seed N accumulation between the high and low NUtE genotypes.In this study,four contrasting NUtE oilseed rape genotypes were fed with labeled ¹⁵N at the early stage of flowering under hydroponic culture,and the biomass and N concentration of each plant organ were measured both at the early stage of flowering and the maturity stage.Our results indicated that the post-flowering N uptake for the high NUtE genotypes was31.0%higher than for the low NUtE genotypes,while no major differences in post-flowering N remobilization between the high and low NUtE genotypes were recorded.At the same time,the contribution of post-flowering N uptake to seed N accumulation for the high NUtE genotypes was 79.5%higher than that of the low NUtE oilseed rape,while there was no significant difference in N remobilization between the high and low NUtE genotypes.However,the contribution of post-flowering N remobilization among different oilseed rape genotypes to seed N accumulation at maturity stage was 2-fold than that of post-flowering N uptake.Further study found that the contribution of N accumulation in vegetative organs to the seed N was ranked as follows:leaf(39.3%)>stem(37.3%)>root(23.4%).(5)Difference in root physiological characteristics result in genotypic variation in post-flowering N uptake of the oilseed rape genotypes.The root morphological and physiological indexes among the contrasting oilseed rape genotypes at 10 days,20 days,30 days and 40 days after flowering were measured under hydroponic and vermiculite culture.Results found that compared with the low NUtE genotypes,the high NUtE genotypes displayed 24.1%higher root length,61.9%larger root surface area,98.5%greater root biomass,38.8%higher root N concentration,along with 68.4%greater root nitrate reductase(NR),85.4%larger glutamine synthetase(GS)and 75.5%higher glutamate synthetase(GOGAT),86.2%superoxide dismutase(SOD)and 72.5%greater glutathione(GSH)activity,compared to the low NUtE genotypes.In summary,the physiological mechanism of N efficient utilization in oilseed rape genotypes is that the high NUtE oilseed rape genotypes exhibit stronger growth attributes at flowering stage;higher pollen grain number and viability for the high NUtE genotypes reduced the ovule abortion rate during ovule development stage,resulting in more seeds per silique,seed yield and harvest index,leading to the greater NUtE.On the other hand,higher post-flowering root characteristics of the high NUtE genotypes contributed to the stronger post-flowering N uptake potential,and thus uptake more N stored in the seed,so that increased seed N accumulation and N harvest index,resulting in higher NUtE.