Effects Of Aluminum Stress On Nitrogen Metabolism Of Citrus Sinensis And Citrus Grandis

Citrus is mostly planted in tropical/subtropical acidic soil areas and is susceptible to aluminum toxicity,which affects tree growth and fruit quality.Previous studies have shown that aluminum stress inhibits citrus growth,reduces the nitrogen content of citrus roots,stems and leaves,and affects plant nitrogen metabolism.Nitrogen metabolism is the most fundamental material metabolism in plants,which not only affects plant growth and development but also plays an important role in plant response to biotic and abiotic stresses.No studies have been reported on the effect of aluminum stress on nitrogen metabolism in different aluminum-tolerant citrus varieties,and it is unknown whether nitrogen metabolism is related to aluminum-tolerance traits in different citrus varieties.Combining the results of existing research in the laboratory,this project uses aluminum-tolerant citrus varieties（Citrus sinensis）and non-aluminum-tolerant citrus varieties（Citrus grandis）as test materials to carry out the following research:（a）Effect of aluminum stress on the growth and development of C.sinensis and C.grandis,（b）Effect of aluminum stress on NH₄⁺-N,NO₃^--N,total nitrogen concentration,nitrogen accumulation,soluble protein and free amino acid content in C.sinensis and C.grandis,（c）Effect of aluminum stress on the activities of enzymes related to nitrogen metabolism in C.sinensis and C.grandis,（d）Effect of aluminum stress on gene expression of enzymes related to nitrogen metabolism in C.sinensis and C.grandis.The following main results were obtained:1.Aluminum treatment inhibited the growth of C.grandis,and significantly reduced the biomass,NH₄⁺-N,total nitrogen concentration,nitrogen accumulation,free amino acids and soluble protein content in all parts of C.grandis;In C.sinensis,aluminum treatment did not affect any of the above indicators except for the reduction of NO₃^--N content in the upper leaves,middle leaves,lower leaves and roots,and NH₄⁺-N content in the lower leaves.Aluminum treatment significantly increased the root DW to shoot DW ratio of C.grandis,but did not affect the root DW to shoot DW ratio of C.sinensis.2.Nitrogen metabolism-related enzyme activities were measured by enzyme kinetic method,and the results showed that the trend of nitrogen metabolism-related enzyme changes caused by aluminum treatment were lower leaves>middle leaves>upper leaves in both C.sinensis and C.grandis leaves.However,the number and magnitude of changes in enzymes related to nitrogen metabolism were greater in C.grandis than in C.sinensis.In C.grandis,aluminum treatment significantly decreased the enzymatic activities of nitrate reductase（NR）,glutamine synthetase（GS）,and glutamate dehydrogenase（GDH）in roots and leaves of all parts,and increased glutamate synthase（GOGAT）activities in leaves of all parts.In C.sinensis,aluminum treatment significantly decreased the enzyme activities of NR in all parts of the leaves and GDH in lower leaves and increased the enzyme activities of GOGAT in all parts of the leaves,without affecting the activities of all enzymes related to root nitrogen metabolism.3.To design gene-specific primer pairs based on the sequences of nitrogen metabolism-related enzymes in the citrus genome and analyze the effects of aluminum treatment on the expression patterns of citrus nitrogen metabolism-related enzyme genes by RT-q PCR.The results showed that aluminum treatment significantly down-regulated the gene expression of NR,GS2,Fd-GOGAT and GDH in leaves and roots of all parts of C.grandis,up-regulated the gene expression of GS1 and NADH-GOGAT in leaves of various parts of C.grandis.Aluminum treatment significantly down-regulated the gene expression of NR and GDH in the middle and lower leaves of C.sinensis and GS2 and GDH in the upper leaves,up-regulated the GS1 gene expression in the leaves of all parts,without affecting the gene expression of Fd-GOGAT and NADH-GOGAT in each part of the leaves and GS2 in the lower and middle leaves.Aluminum treatment down-regulated gene expression of NADH-GOGAT and GS1 in roots of C.grandis,and had no significant effect on the gene expression of NR,GS2,Fd-GOGAT and GDH.Changes in NR,GS,GOGAT and GDH enzyme activities in C.sinensis and C.grandis induced by aluminum treatment may be regulated by NR,GS2,NADH-GOGAT and GDH gene expression,respectively.4.The differences in the regulation of metabolite content and enzyme activities related to nitrogen metabolism and their gene expression patterns in plants under aluminum treatment may be one of the reasons why C.sinensis are more tolerant to aluminum than C.grandis,but the specific regulatory mechanisms involved need to be further investigated.Combining the differences in aluminum tolerance between C.sinensis and C.grandis and the results of correlation and PCA analyses,It was concluded that the increase in the root DW to shoot DW ratio and NO₃^--N content of the middle leaves,upper leaves and roots may be related to the degree of exposure to aluminum treatment,while the up-regulation of GOGAT enzyme activities in leaves may be correlated with the degree of aluminum tolerance in citrus.In summary,this study preliminarily revealed the impact of aluminum stress on nitrogen metabolism and the response of nitrogen metabolism related indicators to aluminum stress in citrus.The results can provide theoretical reference for the application and management of nitrogen fertilizer in citrus orchards under low p H and high aluminum environment,and provide scientific reference for high yield and quality cultivation of citrus grown in acidic soil areas.