Nitrogen Metabolism Of Populus Euramericana And Populus Deltoides And Effect Of Silicon Supply And Phosphorus Deficiency On Nitrogen Metabolism

Nitrogen?N?is an essential plant nutrient,which plays an important role for plant growth and metabolism.Populus is one of the most widely distributed and adaptable tree species in the world.In recent years,due to increasing populations of humans and high demand of fertile soil for agriculture,poplar often grows on marginal lands.In addition,because of the exuberant secondary growth in poplar,the sustainable development of poplar plantation is restricted by the land degradation and the lack of nitrogen and phosphorus in the soil.Many researches have showed that exogenous silicon could alleviate the damage of abiotic stress such as drought and heavy mental pollution to plants and promote the absorption of nitrogen and phosphors nutrition in plants.Therefore,reveal the differences of nitrogen metabolism between poplar species to select new varieties of poplar,and apply exogenous silicon to improve the nitrogen uptake and utilization efficiency of plants could provide a theoretical basis for the application of silicon fertilizer on poplar plantation production.In this study,two Populus nigra species?Populus euramericana,Pe?and?Populus deltoides,Pd?were selected as matirials.We investigated the nitrogen metabolism of two poplar species and the effect of phosphorus deficiency on the nitrogen metabolism with sufficient nitrogen and phosphorus nutrition for comparison and the effect of exogenous silicon on nitrogen metabolism under different nitrogen and phosphosrus supply with non-silicon treatment for comparison.The results were as follows:1.Compared with sufficient nitrogen and phosphorus condition,when the nitrogen was deficient in the soil,the average root diameters of all poplar species were decreased,whereas the root length and number of fine roots were increased obviously.The net photosynthetic rates?A?,biomas,height of plants,ground diameters,the number and surface area of leaves were reduced.Although the NH₄⁺content of roots and leaves,soluble phosphorus and total nitrogen content of two poplar species reduced sharply under the low nitrogen,the nitrogen and phosphorus utilization efficiency were increased obviously.In addition,the activities of enzymes involved in nitrogen metabolism such as GS,GOGAT,GDH especially in roots and stems were inhabited.The NH₄⁺and NO₃^-transporters such as AMT2;1 and NRT2;4B as well as the expression of genes involved in nitrogen metabolism such as NiR,NADH-GOGAT,GDH were also depressed by nitrogen and phosphorus deficiency.2.Compared with sufficient nitrogen and phosphorus condition,when the phosphorus was deficient in the soil,the number and surface area of leaves,biomass,the plant height and ground diameters of all poplar species were decreased,whereas the most of root parameters such as the surface and length of roots and number of fine roots were remained unaltered.Moreover,the nitrogen utilization efficiency in the stems and leaves of Pd and Pe were increased obviously,whereas the the activities of enzymes involved in nitrogen metabolism were promoted or inhabited under the low phosphorus.In addition,The transcriptional levels of NH₄⁺and NO₃^-transporters such as AMT1;6 and NRT2;4B in the leaves were reduced sharply,the NR involved in nitrogen metabolism was increased obviously,while activities of enzymes and the transcriptional levels of genes involved in nitrogen metabolism in the roots were remained unaltered.3.Compared with sufficient nitrogen and phosphorus supply,the physiological parameters and transcriptional levels of genes of two poplar genotypes were reduced in this study when the nitrogen was deficient in the soil.However,compared with Pe,Pd showed larger surface areas in roots and leaves,more number of fine roots,longer root length and higher root-shoot ratio.At the same time,all gas exchange parameters?net photosynthetic rate,transpiration rate,stomatal conductance,intercellular CO₂ concentration?were lower in Pd than in Pe,while the NH₄⁺and NO₃^-content in all organizations,the activities of enzymes related to nitrogen assimilation such as GOGAT,GDH in roots and stems and most of nitrogen transporters?AMTs and NRTs?as well as transcriptional levels of nitrogen metabolism genes in Pd were higher than in Pe.4.When the nitrogen and phosphorus were sufficient in the soil,the surface area of leaves and roots,the length and tips of roots,the number of fine roots of poplar were reduced obviously with exogenous silicon supply,whereas the average root diameter,the leaf stomatal conductance were increased.Additionally,the NO₃^-content in leaves and NH₄⁺content in all organs were enriched,while the activities of enzymes and the transcriptional levels of genes involved in nitrogen metabolism in the roots were remained unaltered.5.When the nitrogen was deficient in the soil,the biomass,plant height and root-shoot ratio were increased with exogenous silicon supply.In addition,the leaf stomatal conductance,intercellular CO₂ concentration and nitrogen use efficiency were improved at the same time,while the rate of transpiration was decreased and the contents of NO₃^-and NH₄⁺in all organs were remained unaltered.Furthermore,the activities of enzymes such as GS,GOGAT,GDH and transcriptional levels of genes involved in nitrogen metabolism in the roots were significantly improved by silicon fertilization in poplar.6.When the phosphorus was deficient in the soil,the surface area of leaves and roots,the length and tips of roots,the numbers of fine roots of poplar were reduced obviously with exogenous silicon supply.Additionally,the biomass,plant height,ground diameters and the rate of transpiration were also decreased,while average diameters of roots,the leaf stomatal conductance and intercellular CO₂ concentration were improved.Although the total nitrogen content was decreased,the nitrogen use efficiency in the leaves was improved when the poplars were exposed to sufficient exogenous silicon.Furthermore,the activities of GDH in the roots of poplar with sufficient fertilization supply,whereas the activities of enzymes such as GS,GOGAT and transcriptional levels of genes involved in nitrogen metabolism were remained unaltered.Take together,compared with Pe,Pd showed stronger resistance to low nitrogen stress.Moreover,the coupled silicon-nitrogen nutrition displayed cross-walk in promoting poplar nitrogen absorption and utilization efficiency.