Response And Metabolic Regulation Of Citrate In Leaves Of Navel Orange Plants To Boron Deficiency

Boron(B)is one of the essential micronutrients necessary for the normal growth and development of higher plants.Most of the citrus growing areas in China have low boron content in the soil.The boron deficiency of citrus is common,and boron deficiency has severely restricted the yield and output of citrus in China.Therefore,for the citrus industry,it is of great significance to select citrus varieties with strong tolerance to boron deficiency and to provide the theoretical basis for the application of corresponding cultivation measures for boron-deficient soils.In the early stage of this research group,we studied the physiological and metabolic reactions of Newhall navel orange after boron deficiency,and found that the content of organic acids in leaves of different parts and different levels of boron deficiency was obviously different,but it is not yet clear what role boron plays in the metabolism of organic acids.To this end,in this paper,the trifoliate-orange-grafted plants was used as the test material to study the citric acid changes of navel orange leaves under boron deficiency and the relationship with related enzyme activities,and to study the changes of metabolic spectrum in leaves of navel orange under the condition of boron deficiency.Finally,the response and metabolic regulation of organic acids in navel orange leaves to boron deficiency stress were deeply understood.The main results obtained are as follows:1.After boron deficiency,the root growth of navel orange was inhibited first,showing symptoms such as a reduction in the number of lateral roots,short root enlargement,etc.;there was no obvious symptom of boron deficiency 3 weeks navel orange plant above ground,but boron deficiency 12 weeks navel orange plant top growth point stop pumping new shoots.The boron content and accumulation of navel orange leaves and roots were significantly reduced at 3 and 12 weeks of boron deficiency.2.Compared with normal treatment,the citric acid content of navel orange leaves with boron deficiency for 3 weeks decreased significantly,while the citric acid content with boron deficiency for 12 weeks increased significantly.Compared with the control treatment,the ACO enzyme activity of navel orange leaves with boron deficiency for 3 weeks was significantly increased,while the enzyme activities of PEPC and CS related to citric acid synthesis did not change significantly,indicating that the decrease in boron deficiency for 3 weeks was mainly due to the citric acid caused by increased degradation.Enzyme activities of CS,PEPC and NADP-IDH in navel orange leaves increased significantly at 12 weeks of boron deficiency,but there was no significant change in ACO enzyme activity,indicating that the increase in citric acid content at 12 weeks of boron deficiency was mainly due to the increased synthesis of citric acid.3.The total sugar,soluble sugar,fructose,reducing sugar,sucrose and glucose content ofnavel orange leaves at 3 weeks of boron deficiency were significantly reduced;compared with the control treatment,the contents of total sugar,soluble sugar,sucrose and fructose in leaves of boron deficiency for 12 weeks increased significantly,and the glucose content decreased significantly.The content of glycine,threonine,cystine,tyrosine and serine in navel orange leaves decreased significantly during 3 weeks of boron deficiency,while the content of methionine increased significantly,and the content of other amino acids did not change significantly.The contents of alanine,methionine,leucine,lysine and proline in navel orange leaves increased significantly during 12 weeks of boron deficiency,while the contents of aspartic acid,serine,valine,tyrosine and phenylalanine were significantly decreased,and there was no significant change in the content of other amino acids.4.Exogenous spraying of citric acid treatment when the nutrient solution is normally supplied with boron(BAFC),compared with the normal supply of boron treatment(BA),the content of citric acid in the leaves is significantly increased,the activity of PEPC enzyme is significantly increased,but the activities of ACO,CS and NADP-IDH enzyme showed no significant changes.Exogenous spraying of citric acid treatment(BDFC)under nutrient solution boron deficiency compared with nutrient solution boron deficiency treatment(BD),the content of citric acid in the leaves is significantly increased,the activity of PEPC enzyme is significantly decreased,but the activities of ACO,CS and NADP-IDH enzyme showed no significant changes.It shows that exogenous citric acid can regulate PEPC activity under different boron supply conditions.5.Exogenous spraying of citric acid(BDFC)under nutrient solution boron deficiency compared to nutrient solution boron deficiency(BD)treatment,the relative content of metabolites such as cellobiose,maltose,inositol,tyrosine,α-ketoglutarate and phytol in navel orange leaves increased significantly,while glucose-1-phosphate,serine,glycine,pyruvate and the relative content of metabolites such as citraconic acid decreased significantly.When the nutrient solution is normally supplied with boron,exogenous spraying of citric acid(BAFC)compared to the nutrient solution with normal supply of boron(BA),the changes of the above metabolites in navel orange leaves are completely opposite.This shows that the effects of exogenous citric acid spraying on leaf metabolites and metabolic pathways under the conditions of boron deficiency and normal boron supply are significantly different.6.Metabolomics results showed that the metabolic spectrum of navel orange leaves under boron deficiency stress changed significantly.Spraying boric acid after a period of boron deficiency can reverse the changes of most of the metabolites in the leaves caused by boron deficiency.The physiological mechanism of boron supply for plant growth relief.Exogenousspraying of citric acid under boron deficiency also reversed most of the metabolite changes caused by boron deficiency,and was similar to the effect of boron acid spraying after boron deficiency to restore metabolites,indicating that the changes in metabolites caused by boron deficiency A large part may be caused by the decrease in citric acid content under boron deficiency.Therefore,spraying citric acid after boron deficiency will restore many metabolites to normal levels.