The Relationship Between Citrus Fruit Sugar And Organic Acid Metabolism And Magnesium And Its Mechanism

Sugar and organic acids are two vital roles in determining the terminal fruit quality,which would be affected by many factors during the fruit development.Mineral elements contribute greatly to the accumulation of sugar and organic acids in fruit by participating in photosynthesis,balancing the cell homeostasis,composing the cell carbon and co-factors of enzymes,synthesis and transportation of protein and metabolites,et al.Although numerous investigations had confirmed that leaves sugar transport by loading into phloem could be inhibited by Mg-deficiency,resulted in leaves sucrose accumulation,Mg concentration relating negatively to sucrose but positively to amino in leaves,while it is still uncertain about effects of Mg on fruit sugar and organic acids.Here,we collected different citrus cultivars from a citrus germplasm orchard in Huazhong Agricultural University,Wuhan,Hubei,China,and investigated the relationship between fruit major sugar and organic acid components and nutrients particularly magnesium?Mg?.We further conducted two field experiments,to reveal firstly variations in accumulation characteristics of fruit Mg,sugar and organic acids and key physiological processes between two grapefruit cultivars with obvious difference in ripen fruit taste quality,to discover secondly responses of Satsuma fruit sugar and organic acids to different amounts of Mg fertilizer and key physiological mechanism.We also conducted a hydroponic experiment with tomato as other palnt species to prove the relationship between fruit sugar,organic acids and Mg.Aiming at understanding fruit quality improvement by Mg regulating fruit sugar and organic acid components accumulation,and guiding Mg fertilizer application scientifically to Mg dificient citrus orchard.The main results were as follows:1. In the same citrus germplasm,fruit sugar,organic acid components and mineral nutrients contents were significantly different among cultivars,especially sucrose and citric acid were accumulated dynamically by different models during fruit development,which contributed to the difference in TSS and TA among different citrus cultivars at the ripen stage.The fruit sucrose content was negatively but citric acid content was positively related to Mg concentration significantly respectively,indicated Mg playing important roles in different fruit sugar and organic acids accumulation among different citrus cultivars.2. The fruit Mg content of grapefruit cultivar?Flame?with higher organic acids but lower sugar concentration was higher than Cocktail with lower organic acid but higher sugar contents,the significant higher Mg content in juice sacs of Flame was resulted from higher transport rate of Mg from segment epidermis to juice sacs than Cocktail.The dynamic change of fruit Mg showed also the same as citric acid but the opposite as sucrose,proved our previous findings among different citrus cultivars fruits.The sucrose and citric acid were the maximum accumulated sugar and organic acid components respectively in the fruit of both cultivars.And the quite different accumulation models between them should be the major factors resulting to the apparently different internal fruit taste quality.To the perspective of the difference of juice sac sugar metabolism between two cultivars,the activities of SPS,SS?Synthesis?played crucial roles in sucrose accumulation,as well as the activities of NI and the expressions of AI2 and CWI1contributed to the degradation of sucrose.And the huge difference of citric acid accumulation between two cultivars referred to the activity of Mit-Aco and the expressions of IDH and Cit1,which were important in citric degradation or citric acid transport from vacuole into the cytoplasm.3. Applying Mg decreased Satsuma fruit pulp sucrose content by up regulating the expression of VIN2,CWI1 and SUC1,then further enhanced the activities of sucrose invertase at fruit ripen stage;On the other hand,applying Mg significantly increased Satsuma fruit pulp citric acid contents at fruit enlargement stage,but there were no responses of the enzymes and the genes related to citric acid to different Mg levels,meant the enzymes involved with citic acid metabolism did not contribute to fruit citric acid content accumulation.4. The tomato root,leaf and fruit Mg contents and also the leaf Ca and K contents increased but fruit Ca content decreased significantly by applying higher Mg.Besides,moderate Mg applicaton promoted not only the synthesis of organic acids including citric acid,malic acid,succinic acid,fumaric acid and oxaloacetic acid,involved in the TCA cycle and some amino acids like?-aminobutyric acid,but also the fruit ripening.Additionally,adequate Mg increased obviously leaf pigments contents of chlorophyll and carotenoids,and also accelerated the PS?photochemical efficiency but decreased the heat dissipation of energy and the production of H₂O₂.