Effects Of Nitrogen And Arbuscular Mycorrhizal Fungi On The Growth Of Populus × Canadensis ‘Neva'

In this study,pot experiments were conducted to investigate the effects of arbuscular mycorrhizal fungi?AMF?Rhizophagus irregularis on the growth,photosynthesis,nutritional status,leaf anatomy traits of Populus×canadensis'Neva'under different nitrogen levels.The effects of AMF on leaf nitrogen allocation,NH₄⁺and NO₃^-absorption kinetics were also investigated.Moreover,the effects of AMF on the expression levels of ammonium transporter genes and nitrate transporter genes in poplar were studied.The main results and conclusions are as follows:1.Effects of nitrogen fertilization and AMF inoculation on the growth,photosynthesis,water status and leaf anatomy of poplarInoculation of AMF in P.×canadensis?Neva‘under different nitrogen levels?0,1,5,10,and 15 mM NH₄NO₃?in unsterilized soil revealed that the effectiveness of AMF was depend on nitrogen fertilization levels.At low nitrogen levels?0,1 mM NH₄NO₃?,inoculation with R.irregularis increased plant height?10.74%¹6.38%?,stem diameter?4.20%⁶.43%?,total biomass?1.17%⁴.36%?,net photosynthetic rate?16.88%¹7.23%?,the maximum photochemical efficiency?Fv/Fm??2.10%².17%?,water use efficiency?29.15%²9.30%?,leaf relative water content?4.61%⁴.83%?,and the average conduit diameter of midribs?11.79%¹1.81%?of P.×canadensis?Neva‘.At high nitrogen levels?10,15 mM NH₄NO₃?,inoculation with R.irregularis increased the Fv/Fm?1.29%¹.65%?,and actual photochemical efficiency??PSII??10.29%³5.45%?of P.×canadensis?Neva‘.The results of correlation analysis showed that there was a significant positive correlation between the average conduit diameter of midribs and plant height,stem diameter,total biomass,net photosynthetic rate,transpiration rate and leaf relative water content.In summary,under low nitrogen levels,AMF promoted poplar biomass accumulation through increasing the average conduit diameter of midribs,leaf water content,water use efficiency and photosynthetic capacity;under high nitrogen levels,inoculation of R.irregularis alleviated the deleterious effects of high nitrogen load on photosystem II?PSII?.2.Effect of nitrogen application and AMF inoculation on nutritional status of poplarThe effects of nitrogen and R.irregularis on the concentrations of macronutrients and micronutrients in the leaves and roots of P.×canadensis?Neva‘were investigated in unsterilized soils.The results showed that nitrogen fertilization increased carbon,nitrogen and copper concentrations in the leaves,as well as nitrogen and iron concentrations in the roots of poplar.Compared with non-fertilized control,phosphorus,calcium,magnesium,iron and manganese concentrations of leaves,and phosphorus,potassium,calcium and zinc concentrations of roots were lower in non-inoculated plants under high nitrogen levels.Under low nitrogen levels,R.irregularis had no significant effect on nutrient concentrations in the leaves and roots of poplar?except for iron,copper,zinc concentrations in leaves,and calcium concentrations in roots?.Under high nitrogen levels,R.irregularis increased leaf carbon,nitrogen,phosphorus,calcium,magnesium,iron,manganese and zinc concentrations,and root phosphorus,iron,manganese,and copper concentrations.The results of correlation analysis showed that calcium and magnesium concentrations in the leaves were significantly positively correlated with Fv/Fm,while phosphorus and zinc concentrations in leaves were significantly positively correlated with?PSII.In summary,R.irregularis may alleviate the nutrient imbalance and PSII damage caused by high nitrogen load by regulating nutrient concentrations in the leaves and roots of P.×canadensis?Neva‘.3.Effect of AMF inoculation on nitrogen distribution in poplar leavesA pot experiment was conducted to investigate the effects of AM fungi on nitrogen allocation to the water-soluble proteins,membrane-bound proteins,cell wall proteins and photosynthesis?carboxylation?P_C?,electron transport?P_B?and light-harvesting processes?of P.×canadensis?Neva‘in sterilized soils.The results showed that AMF decreased leaf mass per area and the allocation of leaf nitrogen to cell wall proteins,increased allocation of leaf nitrogen to water-soluble and membrane-bound proteins,indicating that R.irregularis allocated more nitrogen for metabolism by reducing the distribution of nitrogen into structure.Under low nitrogen levels,R.irregularis increased plant height?18.76%³6.08%?,shoot biomass?50.09%⁶3.97%?,total biomass?43.42%⁵2.29%?,the maximum net photosynthetic rate(P_max)?16.83%²0.11%?,photosynthetic nitrogen use efficiency?PNUE??40.01%⁴3.14%?,P_C?33.52%³8.54%?and P_B?28.98%³3.90%?,but had no significant effect on leaf nitrogen content per unit area and leaf nitrogen content per unit mass.Meanwhile,the P_max was positively correlated with P_C and P_B;PNUE was positively correlated with PC,PB and P_max,suggesting that R.irregularis increased P_max and PNUE by increasing P_C and P_B,and further plant growth.In summary,AMF may help plant adapt to low nitrogen environment by regulating leaf nitrogen allocation strategies.4.Effects of nitrogen fertilization and inoculation of R.irregularis on poplar NH₄⁺and NO₃^-absorption kineticsThis study investigated the effects of R.irregularis on total absorption area,active absorption area,NH₄⁺and NO₃^-absorption kinetics of poplar.The results showed that inoculation of R.irregularis increased the proportion of active absorption area in the roots of poplar.Under the concentration of no more than 1 mM nitrogen,the NH₄⁺and NO₃^-absorption curves met the characteristics of the Michaelis-Menten equation,the absorption rate?V?=the maximum absorption rate(V_max)×substrate concentration?S?/?K_m+S?.According to the formula,inoculation of R.irregularis increased the maximum absorption rate(V_max)of NH₄⁺and NO₃^-of poplar.The results suggested that the formation of symbiotic relationship with R.irregularis enhanced the ability of poplar to absorb NH₄⁺and NO₃^-.5.Effects of nitrogen and inoculation of R.irregularis on the expression of ammonium and nitrate transporter genes in poplarThe effects of nitrogen and R.irregularis on the expression of ammonium transporter genes and nitrate transporter genes were studied using qRT-PCR technology.The results showed that in roots,nitrogen fertilization up-regulated the expression of AMT1;2,AMT2;1,NRT2;4B,NRT2;4C,NRT3;1A,NRT3;1B and NRT3;1C in non-inoculated plants;in leaves,nitrogen fertilization down-regulated the expression of AMT1;2,AMT1;6,AMT2;1,NRT1;1,NRT2;4B in non-inoculated plants,suggesting that the expression of key transporters in roots increased to improve the ability of poplar to obtain nitrogen,and the expression of transporters in leaves was inhibited due to nitrogen shortage under low nitrogen deficiency.Under low nitrogen levels,R.irregularis down-regulated the expression of NRT1;2,NRT2;4B,NRT2;4C,NRT3;1A,NRT3;1B and NRT3;1C in roots,and up-regulated NRT2;4B expression in leaves,indicating mycorrhizal poplar may down-regulate the expression of root nitrogen transport related genes due to their ability to absorb nitrogen through the mycorrhizal pathway.Under high nitrogen levels,R.irregularis up-regulated the expression of AMT1;6,AMT2;1and NRT1;1 in roots,down-regulated the expression of AMT1;2,AMT1;6,AMT2;1,NRT1;1,NRT1;2 and NRT2;4B in leaves.These results suggested that AMF can regulate plant nitrogen uptake and translocation by regulating the expression of plant ammonium and nitrate transporter genes.