Effects Of N-deficiency Supply On Root Growth And Nitrogen Uptake In Wheat Seedlings And Its Physiological Mechanism

Nitrogen(N)fertilizer is commonly considered as one of the most important limiting factors in the agricultural production.As a result,modern wheat production demands large amount of N fertilizer for high yield.Unfortunately,this large amount of N fertilizer input has led to increased plant nitrogen-tolerance and decreased amplitude of yield improvement.Consequently,nitrogen use efficiency(NUE)has been reduced,which results in significant N loss,energy waste and environmental pollution.Therefore,balancing crop yield and nitrogen input is the main obstacle to future agriculture production.Excess basal nitrogen input resulted in a loss of fuel in wheat production.Less nitrogen fertilizer in seedling stage will help to reduce the total nitrogen input,so as to improve nitrogen use efficiency.However,the effect of the root growth and nitrogen uptake in low-nitrate conditions is still unclear.The two wheat cultivars,Zaoyangmai(low-nitrate sensitive genotype)and Yangmai158(low-nitrate tolerant genotype),were supplied with either 0.25 mmol L-1,1 mmol L-1 or 5 mmol L-1 nitrate(control;CK),to investigate the response of growth and photosynthesis to nitrate in hydroponic systems.The relationship between root morphology and endogenous hormone balance was studied to explain the effects of low-nitrate supply on root growth in wheat seedlings.The relationships among N uptake efficiency,iHATS expression,and nitrogen metabolism were demonstrated to explain the effects of low-nitrate supply on root growth and physiological processes.Therefore,theoretical basis was provided for breeding high yield and N high-efficient wheat cultivars.The main results are as follows:1.Low-nitrate enhanced root extension and improved the root nutrient uptake area of wheat seedlings by breaking the balance of endogenous hormones.Low-nitrate decreased shoot dry weight but increased root dry weight of two wheat cultivars.Furthermore,low-nitrate increased root number,total root length,root surface area,and root volume of the two wheat cultivars,and reduced the average root diameter.This showed that low-nitrate induced root extension of wheat seedlings,and increased its uptake area.The increment of root area was significantly higher in Yangmai158 than Zaoyangmai.Low-nitrate treatment increased root elongation region IAA content,and decreased CTK content,thereby broke the balance of plant endogenous hormone.The increase of IAA/CTK ratio resulted in the significant increase in the root length.Additionally,meristem region ABA induced the production of root ROS,which was mainly manifested in the decrease of root GSH/GSSG ratio,the reduction of O2-production rate,and the increase of H2O2 content.ROS accumulation resulted in increased root Ca2+channel activity,increased root intracellular Ca2+-ATPase activity,reduced Na+/K+-ATPase activity,and enhanced Na+/Ca2+ion exchange between intracellular and extracellular space.These physiological changes led to increased influx of Ca2+,increased concentration of intracellular Ca2+,and therefore induced root extension.2.Low-nitrate induced the iHATS upregulation in wheat seedlings to increase its energy supply,followed by improved nitrogen uptake rate.Plant nitrogen content and nitrogen accumulation after low-nitrate treatment decreased less in Yangmai 158 than in Zaoyangmai,suggesting that Yangmai 158 had a stronger growth vigor and N-storage ability under low nitrogen environment.Low-nitrate treatment reduced root nitrate content,followed by the upregulation of root TaNRT1.1,TaNRT2.1 and TaNRT2.in both cultivars.TaNAR2.1 and TaAMT1.1 were subsequently upregulated as well.Additionally,plasma membrane H+-ATPase activity(PM H+-ATPase),which is involved in N-uptake energy supply,increased significantly in correlation with increased TaHAl transcript abundance.Therefore,the maximum uptake rate(Vmax)of both wheat cultivars increased significantly in the low-nitrate condition.It was found that nodal roots increased more than seminal roots,and in Yangmai 158 than Zaoyangmai.Higher iHATS transcript abundance and energy supplication of Yangmai 158 has accelerated N-uptake rate in root.More N was absorbed into cell so as to promote its growth,suggesting that Yangmai 158 has the superior acclimation in N deficient environment.3.Low-nitrate improved N assimilation in wheat seedlings by raising the photosynthetic capacity,N metabolism and organic acid metabolism.More biomass was redistributed to the root,which led to decreased root N content and provided substance basis and feedback signal for N-uptake.Low-nitrate increased the photosynthetic rate(Pn)and stomatal conductance(Gs)of both two wheat cultivars shortly,with higher Pn and Gs found in Yangmai 158 than Zaoyangmai.Sucrose synthase(SS)and sucrose phosphate synthase(SPS)activity were improved significantly in low-nitrate treatment.This phenomenon was consistent with the increased soluble sugar and sucrose content.Higher soluble sugar and sucrose content in roots than in leaves showed that carbohydrates produced by photosynthesis were distributed more in roots,which could provide substance basis for root extension,and reduce the concentration of photosynthetic product in leaves followed by lifting the inhibition of photosynthesis.The increase of nitrate reductase(NR)and glutamine synthetase(GS)activities in leaves and roots were induced by the upregulation of TaNRA2.1 and TaGS1/TaGS2,respectively,showing that N assimilation ability of the two wheat cultivars was improved in low-nitrate conditions.The higher NR and GS activities were found in Yangmail58 than in Zaoyangmai.Low-nitrate increased citrate synthase(CS),succinate dehydrogenase(SDH)and PEPC carboxylase activity,decreased malate dehydrogenase(MDH)activity.Although low-nitrate reduced parts of the organic acid content in root system,small molecular weight organic acids can be transported within the cell,such as citric acid,malic acid and oxaloacetic acid content increased significantly.The increased small molecular weight organic acids were transported up to the N assimilation position to clear the released hydroxyl(-OH)during the assimilation process.Part of those organic acids were downward transported and secreted into the rhizosphere environment,which were then used to neutralize exocytosis OH' ions by NO3-uptake,balanced the charge between environment and cell,and broke NO3-uptake inhibition caused by the rise of pH.4.After nitrate restore supply treatment,the growth rate of wheat seedlings was accelerated,and N uptake and assimilation were further improved,which were greater in Yangmai158 than Zaoyangmai.After 120-hour nitrate restore treatment,two wheat cultivars growths were gradually restored for plant height,dry weight,leaf area and N content.Yangmai158 resumed growth faster than Zaoyangmai.Root extension rate was lower but the root uptake area continued to increase,mainly in nodal roots.Root number,total root length,root volume and root surface area increased,whereas root average diameter reduced.Increased root NO3-content decreased iHATS transcript abundance significantly.The relative expression of TaHA1 decreased.PM H+-ATPase activities still remains at a high level to provide energy for N-uptake in the root system.NR/GS activities and TaNRA2.1 with TaGS1/TaGS2 relative expression of leaves and roots were significantly improved,free amino acids and soluble protein content gradually restored either,showing that the gap was narrowing.Results suggested that Yangmai158 had the higher N metabolism recovery ability,NR/GS activity and transcript abundance,and there was no significant difference compared to plant morphology grew in N sufficient conditions.Photosynthetic capacity increased significantly by improving photosynthetic rate(Pn),stomatal conductance(Gs)and the chloroplast Mg2+-ATPase activity,while the increase of soluble sugar and sucrose content offer more material base for plant growth.It was found that the growth and physiological activity rate of Yangmai 158 were significantly higher than Zaoyangmai after nitrate recovery treatment,which suggested that Yangmai158 had better recovery ability.To sum up,the response to nitrate deficiency has significant genotypic differences in both two wheat cultivars,mainly manifested in root morphology,N-uptake rate,photosynthetic capacity and N metabolism activity,etc.Low-nitrate tolerant genotype maintained a higher N-uptake ability to alleviate its growth inhibition by root extension and N-uptake rate increased in the nitrate deficiency conditions.After nitrate retore supply,low-nitrate tolerant genotype showed stronger environmental acclimation,by maintaining the higher N-uptake ability together with the growth rate and photosynthetic capacity.