The Physiological Responses Of Stoichiometrics Of Nitrogen And Phosphorus In Populus To The Ratios Of Nitrogen To Phosphorus In Soil

Nitrogen and phosphorus are essential elements for plant growth and development,low or high concentrations of nitrogen and phosphorus in soils may inhibit plant growth.As important economic and greening tree species,Populus widely planted in our country.In recent years,it has also became important tree species as a bioenergy resources.Therefore,studies on the changes of growth,physiological characteristics and the stoichiometrics of nitrogen and phosphorus under different ratios of nitrogen to phosphorus and explore the optimum ratios of nitrogen to phosphorus have important theoretical and practical significance.In this study,we planted the fast-growing Populus species P.× euramericana(Pe)and the slow-growing species Populus simonii(Ps)as materials.Two months later,cultivated with the following nutrition solution of different ratios of nitrogen to phosphorus.When the phosphorus concentration was held constant at 1.33mM,changing nitrogen concentration at 0.6mM,4mM,12mM and 24mM,resulting in four different ratios of nitrogen to phosphorus is 0.2,1.4,4.1,8.1;and the nitrogen concentration was held constant at 12mM,changing phosphorus concentration at 0.04mM,0.133mM,0.53mM and 1.33mM,getting four different ratios of nitrogen to phosphorus is 135.5,40.7,10.2,1.4.After 9 weeks,the plants were harvested.Measuring the physiological parameters and analyzing the expression of related gene,the main results were as follows:(1)When the phosphorus concentration was held constant at 1.33mM,increasing the concentration of nitrogen from 0.6mM to 12mM,leading to the increase of biomass,leaf area and photosynthesis rate of the two poplar species,but excessive nitrogen(24mM)may inhibit their growth;When the nitrogen concentration was held constant at 12mM,the biomass,leaf area and photosynthesis rate increased with the increasing of the concentration of phosphorus from 0.04mM to 1.33mM.(2)Under the same condition,the surface area and volume of the fine root,the photosynthetic rate,ATP content,nitrate reductase(NR)and acid phosphatase(Aps)activities of Pe were higher than those of Ps.These results suggest that Pe possesses higher capacities of nitrogen and phosphorus uptake and assimilation.(3)The distribution of nitrogen in the different tissues of the two poplar species was: leaf > root > bark > wood,the distribution of phosphorus was: root > leaf > wood >bark;The extent of homeostatic regulation in leaves was more sensitive than that in other tissues in the two poplar species,so the ratios of nitrogen to phosphorus in leaves can be used as an indicator to judge whether the plants are nitrogen or phosphorus limitation.(4)In mRNA levels,when the phosphorus concentration was held constant at 1.33mM,the lower concentration of nitrogen(0.6 and 4mM)may promote the expression of genes related to nitrogen absorption and transportation(NRT1.1,NRT2.1)in the fine roots of the two Populus species;When the nitrogen concentration was held constant at 12mM,the higher concentration of phosphorus(1.33mM)may promote the expression of genes related to phosphorus absorption and transportation and(PHT1.5,PHT1.9).(5)Among the seven different nitrogen to phosphorus ratios set in this study,the most suitable ratio of nitrogen to phosphorus for growth in the two poplar species was 4.1(N 12mM P 1.33mM).