| Hebei Province is an important winter wheat(Triticum aestivum L.)producing area in China.Its planting area and total production of winter wheat account for 9.8%and 11%of the country respectively,which is of great significance to ensure food security of China.As a traditional productive area of staple food grains in the province,over irrigation and excessive application of nitrogen(N)are widely common in the piedmont plain of Taihang Mountain,which result in inefficient use of N,N surplus in farmland and underground and surface water pollution.Winter wheat in this area is majorly irrigated with groundwater,and the year-by-year decline of groundwater level caused by over depletion has formed serious groundwater funnel areas.The government has issued a series of strict groundwater use policy to solve this problem.It is an inevitable trend to adopt water efficient irrigation strategies with less available water.Therefore,it is badly needed to develop relevant strategies for highly yielding and N efficient winter wheat production under reduced irrigation conditions to contribute to sustainable production of crops in the piedmont plain of Taihang Mountain.This study includes three parts:screening of N efficient varieties,determination of appropriate N application rate under reduced irrigation conditions,and study on molecular mechanism of how N efficient highly yielding varieties resist N stress.In winter wheat seasons during the years of 2018 to 2019 and 2019 to 2020,field experiments were carried out in Gaocheng,Zhaoxian and Xinle,typical highly yielding winter wheat counties in the piedmont plain of Taihang Mountain.A two-factor split plot experiment was conducted,setting varieties of KN199 and JM585 in the main block,and N rates of 0,120,180,240 and 300 kg·hm-2 in split blocks.The effects of N application rates on yield formation process,N accumulation,N source and N use efficiency of winter wheat were systematically studied under the condition of one irrigation during the whole crop season.15N isotope was used to study effects of N rates on the fate of N fertilizer and N balance in soil pool.Transcriptome sequencing was employed to explore the molecular mechanisms of low N tolerance and high-yielding in variety KN199,which has been supposed to be N efficient and productive in low N environment.Major findings are as follows:(1)Treatment N180,180 kg N hm-2,can maintain the stability of winter wheat yield and its components in the piedmont plain by reducing nonproductive tillers and optimizing distribution of assimilation products.Compared with N240 and N300 treatments,the maximum total stem number of N180 decreased by 6.2%,while productive tiller percentage increased by 3.8%,per acreage ear number at maturity decreased by only 3.8%,the kernel number per spike decreased by only 0.8%,and 1000 grain weight increased by 0.3%.In terms of photosynthetic product accumulation and distribution,although the maximum leaf area index(LAI)decreased by 8.0%,canopy biomass at flowering decreased by only 3.2%,and dry matter transferred to grain before anthesis increased by 19.7%and the transferring rate increased by 23.5%.Although aboveground biomass decreased by 4.4%,harvest index(HI)increased by 1.3%.Compared with N240 and N300 treatments,N180 treatment is more stable in acerage ear number,kernel number per ear and 1000-grain weight.In six environments on three experimental sites during the two years,grain yield of N180 treatment was 99.3%of the maximum yield of 7574.5 kg·hm-2.(2)N180 treatment can meet wheat crop N need owing to higher N absorption efficiency both from fertilizer and soil.Compared with N240 and N300 treatments,N absorption efficiency of N180 treatment increased by 41.2%.Although fertilizer N absorption by the wheat crop decreased by 28.2%and contribution of fertilizer N to plant N accumulation decreased by 19.0%,absorption of soil N by the crop increased by 7.1%and the contribution of soil N to plant N accumulation increased by 12.2%.In terms of N accumulation and distribution,although plant N accumulation at anthesis decreased by 5.9%,transferring rate of N accumulated before anthesis to grains increased by 3.5%,and contribution of pre-anthesis transfer to grain increased by 4.7%.Although the N accumulation of plants decreased by 6.9%at maturity,the distribution of N accumulation in vegetative organs decreased by 4.9%and N harvest index increased by 1.8%.(3)The N180 treatment maintained relatively high yield under lower N accumulation with higher N use efficiency.Compared to N240 and N300 treatments,N use efficiency of N180 treatment increased by 4.9%,N production efficiency increased by 46.0%and N agronomic efficiency increased by 37.7%.Compared to N240 and N300 treatments.although N use efficiency increased by 10.5%and N production efficiency increased by 93.5%in N120 treatment.agronomic efficiency of N fertilizer increased only by 15.6%.N120 treatment could not achieve high yield.(4)Results from the 15N tracing experiment showed that N loss and residue in deep soil were significantly decreased in N180 treatment.Compared to N240 and N300 treatments,fertilizer N loss percentage in N180 treatment decreased by 17.6%,N recovery percentage increased by 12.4%,respectively.Percentage of fertilizer N residue in 100 cm soil profile decreased by 2.3%.Compared with N240 and N300 treatments.N loss of N180 treatment decreased by 17.6%and the residual fertilizer N by 2.3%.At the end of the crop season,residual fertilizer N in the 0-40 cm soil accounts for 82%to 91%of that in the 0-100 cm soil profile.Compared to N240 and N300 treatments,residual fertilizer N of N180 treatment in 0-20 cm,20-40 cm and 40-100 cm soil layers decreased by 24.8%,27.8%and 49.5%,respectively.With complete maize(Zea maize L.)residue retention,soil N balance in winter wheat season under N180 treatment was-0.21 kg·hm-2,approximately balanced.(5)Results from the experiment of vermiculite pot planting of 24 varieties in artificial climate chamber showed that low N tolerance indices of grain yield and grain N content of KN199 ranked the 5th among the 24 varieties.In low-N field environment,KN199 showed high N use efficiency and yield under six field environments on three experimental sites during the two years.Compared with JM585,average yield,N accumulation,N use efficiency of KN199 was 7.6%,7.4%and 9.3%higher in NO treatment,respectively.Compared with JM585,average yield and N use efficiency of KN199 was 7.2%and 4.9%higher in N120 treatment,respectively.However,it showed no advantage in treatments of higher N rates.The response mechanism of Winter Wheat under low nitrogen(0 mol·L-1)stress was studied at the transcriptome level using RNA-Seq technology and high-throughput sequencing method.A total of 4260 differentially expressed genes(DEGs)were identified.KEGG enrichment of 4260 DEGs showed that DEGs were mainly distributed in protein processing in endoplasmic reticulum,glutathione metabolism,alanine,aspartate and glutamate metabolism.The expression levels of nitrate transporter(NRT)and ammonium transporter(AMT)coding genes were significantly up-regulated.DEGs involved in glutamate(Glu)metabolism were also significantly up-regulated.MYB,NAC,bHLH and WRKY transcription factors also play a regulatory role in the process of resisting low nitrogen stress.In summary,under the condition of single in-season irrigation and maize residue retention,winter wheat of N180 treatment showed good stability in grain yield and yield components that were resulted from less ineffective tillers and optimized assimilates distribution on the three experimental sites in the piedmont plain of The Taihang Mountain during the two experimental years.The N180 treatment met wheat crop N demand by improving N uptake efficiency and N absorption from the soil.High N use efficiency supported high yield with less plant N accumulation under reduced N application.Also,N loss and deep soil residue decreased significantly and soil N maintained balanced under N180 treatment.We conclude that 180 kg·hm-2 can be recommended as the optimal N application rate of for sustainable intensive winter wheat production under reduced irrigation and residue retention conditions in the piedmont plain of Taihang Mountain. |
