Effects of Different Concentrations and Form Ratios of Nitrogen on Chlorophyll Biosynthesis,Photosynthesis, and Biomass Partitioning in Fraxinus mandshurica Seedlings

Abstract

Fraxinus mandshurica, native to northeastern China, is an important timber tree. The tree often suffers from nitrogen (N) deficiency because long winter results in slow decomposition rate of litter. In the present study, effects of different concentrations and form ratios of nitrogen on chlorophyll biosynthesis, total leaf N, soluble protein, and photosynthesis were studied. During the N concentration treatment, biosynthesis rate of 5_aminolevulinic acid (ALA) and activity of porphobilinogen (PBG) synthase showed irregular changes, but total chlorophyll content increased when N concentrations varied from 1 to 8 mmol·L -1, and activity of PBG synthase did not change significantly. Total leaf N, soluble leaf proteins, and photosynthesis rate also increased when N concentrations increased from 1 to 8 mmol·L -1. These results suggested that N concentrations did not affect biosynthesis rates of ALA and activities of PBG synthase and 8 mmol·L -1 of N most increased total leaf N, soluble proteins in leaves, and photosynthesis rate. During the N form treatment, biosynthesis of ALA increased when the ratio of NO - 3_N increased from 0 to 50%, but activity of PBG synthase, total chlorophyll concentrations, total leaf N and soluble leaf proteins increased when the ratio of NO - 3_N increased from 0 to 75%. Photosynthesis rate also increased when the ratio of NO - 3_N increased from 0 to 75%. During the two types of treatments, there were significant relationships between total leaf N, soluble protein concentrations and net photosynthesis rates. These results suggested that F. mandshurica seedlings showed a preference for NO - 3_N, but the ratio of 75% NO - 3_N to 25% NH + 4_N promoted an increase in chlorophyll biosynthesis, accumulation of N and soluble proteins in leaves, and net photosynthesis rate. Total biomass increased as exogenous nitrogen concentrations increased from 1 to 8 mmol·L -1. Biomass partitioning to roots decreased, and biomass partitioning to shoots increased. As ratios of NO - 3_N to NH + 4_N increased from 0 to 75%, total biomass increased, but biomass partitioning to roots increased as ratios of NO - 3_N to NH + 4_N increased from 0 to 100%. When both nitrogen concentration treatment and form ratio treatment were carried out, biomass accumulation was closely related to photosynthesis rate, and the relationship between biomass accumulation and photosynthesis was related to nitrogen partitioning in leaves. It is proposed that 8 mmol·L -1 and 75% NO - 3_N∶25%NH + 4_N should be used in practical cultivation of F. mandshurica seedlings.