Effects Of Nitrogen Application On Seedling Growth In Camellia Oleifera Abel.

Camellia oleifera Abel.is a unique woody edible oil tree species in China,mainly distributed in red soil area.Nitrogen is one of the important factors limiting the growth of C.oleifera.Nitrate nitrogen and ammonium nitrogen are the main nitrogen forms which could be absorbed and utilized from soil by plant.In this study,1-year-old seedlings of C.oleifera 'Xianglin1','Xianglin27' and 'Xianglin210' were used as materials.The effects of different nitrogen levels(0 mmol·L-1,8 mmol·L-1,16 mmol·L-1)and different nitrogen form(NO3-:NH4+=0:0,10:0,7:3,5:5,3:7,0:10)are set by analytical pure(NH4)2SO4 and NaNO3 on the nutrient growth,nitrogen absorption and utilization and photosynthetic characteristics of C.oleifera seedlings.This study revealed the characteristics of nitrogen absorption by roots of C.oleifera seedlings,and elucidated the physiological and molecular mechanisms of nitrogen promoting seedling growth.The research results can provide a theoretical basis for scientific and reasonable fertilization of C.oleifera seedlings and improving nitrogen use efficiency.The main results are as follows:1.Different nitrogen levels and forms had significant effects on the vegetative growth of C oleifera seedlings.When the total nitrogen was 8 mmol·L-1,it was more conducive to the vegetative growth of C.oleifera seedlings.The results showed that mixed nitrogen forms could significantly increase the height increment,ground diameter increment and total biomass of C.oleifera seedlings,among which NO3-:NH4+=5:5 exerted the highest promotion effect in height increment and ground diameter increment.Mixed nitrogen form significantly increased the root development of C.oleifera seedlings.However,an inhibitory effect on the growth of the root system was found when single ammonium nitrogen source was added.2.Absorption kinetics method was used to study the nitrogen absorption characteristics of C.oleifera seedlings roots,and it was found that Vmax ammonium>Vmax nitrate and Km nitrate>Km ammonium,indicating that C.oleifera preferred ammonium nitrogen,and its affinity to ammonium nitrogen was greater than that of nitrate nitrogen.The utilization and distribution characteristics of NO3--15N and NH4+-15N were studied using 15N isotope tracer technology,and the results showed that different organs(root,stem and leaf)had higher collection capacity of NH4+-15N than NO3--15N,with the the strongest collection capacity in root system.At different stages after fertilization,the uptake and utilization rate of NH4+-15N by the whole plant were significantly higher than that of NO3--15N,and the average utilization rate of NH4+-15N(15.656%)was 1.51 times than that of NO3--15N(10.361%).The highest distribution rate of two nitrogen forms was found in root system,followed by leaf and stem.After fertilization,the nitrogen direction followed the rule of nitrogen loss amount>substrate residue amount>seedling absorption amount.3.The mixture of nitrogen could greatly improve the photosynthetic performance of C.oleifera and contribute to the accumulation of photosynthetic products.When the NO3-:NH4+=5:5,the soluble sugar content of leaves was significantly higher than other treatments(5.68%),which was related to the strong photosynthetic capacity.When the NO3-:NH4=5:5,C.oleifera possessed higher photosynthetic gas exchange parameters,such as net photosynthetic rate(5.62 ?mol·m-2·s-1),transpiration rate(3.17 mmol·m-2·s-1)and stomatal conductance(0.36 mol·m-2·s-1),higher carboxylation efficiency(0.0 148),chlorophyll content(1.763 mg·g-1)and soluble protein content(14.27 mg·g-1),bigger photosynthetic effective area(14.32 cm2),higher capacity to utilize low light(optical compensation point 4.41?mol·m-2·s-1)and resistance to light stress(Fv/Fm 0.665)when NO3-:NH4+=5:5.Correlation analysis showed that net photosynthetic rate and soluble sugar content,transpiration rate,stomatal conductance,carboxylation efficiency,soluble protein content,chlorophyll b content were highly significantly positively correlated,the net photosynthetic rate and leaf total nitrogen content,content of chlorophyll a,total chlorophyll content,leaf length,leaf width,leaf area were significantly positively related,and negatively related with the light compensation point.The photosynthetic characteristic,fluorescence characteristic,photosynthetic physiological index,photosynthetic product accumulation in C.oleifera leaves were significantly relevant,suggesting that these traits could influence and restrict each other.4.The results of principal component analysis and membership function analysis showed that NO3-:NH4+=5:5 was the most beneficial to the growth of C.oleifera seedlings,followed by NO3-:NH4+=3:7,suggesting that the mixture of nitrogen(equal ratio of ammonium and nitrate)was more favorable for the growth of the seedlings of C.oleifera.5.Transcriptomic sequencing was performed on the leaves and roots of C.oleifera seedlings treated with different nitrogen forms and ratios(NO3-:NH4+=0:10,3:7,5:5,7:3,10:0 and no nitrogen as control),and 3 561 and 4 024 differentially expressed genes were identified in the leaves and roots respectively.The GO function annotation found that the differential genes in leaves and roots were significantly enriched in the stress response process in the biological process item,the catalysis and binding process in the molecular function item,and the cell connection and membrane process in the cell component item.The results of KEGG cluster analysis showed that the differential expressed genes in leaves were located in metabolic pathways such as signal transduction,sugar metabolism,energy metabolism and lipid metabolism.The differential expressed genes in roots were located in metabolic pathways such as sugar metabolism,lipid metabolism,and signal transduction.Preliminary screening obtained some differentially expressed genes of key enzymes in nitrogen nutrition metabolic pathways under different nitrogen forms,and the changes of gene expression pattern were analyzed.Results showed that the nitrate reductase gene and nitrite reductase gene in root were significantly up-expressed under NO3-:NH4+=5:5 ratio,which might be associated with the growth advantage of C.oleifera seedling.In addition,appropriate nitrogen ratio could promote the glutamine synthase and glutamate synthase gene expression levels,thus improving the efficiency of nitrogen assimilation and utilization.