Studies On The Physio-ecological Response Of Sugar And Organic Acid Metabolism In Navel Orange Fruit From Different Habitats

In view of the universal key problem that navel fruits contain too much content of organic acidsand too low of sugars, starting with the most important quality index of navel orange that determinedby the content of sugars and organic acids in fruit and the ratio of sugar to acid, and with usingecological physiological methods to analyze sugar and acid metabolism in navel orange, try to explorethe main ecological factors for navel orange ecological divisions and the key enzymes that sugar andacid metabolism in different navel orange varieties fruit from the different habitats. Thus, applying 9to 12-year old Roberson navel orange (Citrus sinensis Osbeck), Newhall navel orange (Citrus sinensisOsbeck) and Harvest navel orange(Citrus sinensis Osbeck) trees grafted on Citrus tangerina from 7representative areas of 3 ecotypes in Sichuan as testing materials, using the methods of combiningfield biological and ecological investigation and indoor analysis, the acid and sugar metabolismduring the development of navel orange fruits were studied systematically, including the activitieschanges of acid-metabolizing enzymes and their relationships with the acid accumulation, thecomparisons of sugar and acid metabolism in different habitats and in different varieties, and theeffects of ecological factors on sugar metabolism, especially sugar and acid accumulation and theirrelated enzymes during the fruit development in navel orange fruit from the different habitats. Themain results showed as follow:1. The activities of CS, PEPC and MDH increased while the organic acid accumulated,however, CS and PEPC played an important role in organic acid accumulation in fruit during theearly stage of the fruit development. When the organic acid content sharply decreased, IDH andcis-ACO IDH activities increased quickly, which indicated IDH and cis-ACO related with organicacid accumulation, but IDH was the key enzyme that cleaved citrate from 120 days after full bloomto the fruit ripening. 2. During the fruit growth, ratio of sugar to acid rose while the activities of synthetic SS and IDHincreasing, and they had a significant positive correlation coefficient, which indicated that syntheticSS and IDH were the key enzymes to regulate the ratio of sugar to acid. Therefore, SPS and cytosolicACO activities had significant positive correlation with ratio of sugar to acid from 105 days after fullbloom to 150 days after full bloom and from 105 days after full bloom to 180 days after full bloom,respectively, which indicated SPS and cytosolic ACO played a part in ratio of sugar to acid to acertain degree. However, CS and PEPC were not the main enzymes for ratio of sugar to acid.3. During the fruit growth, the difference of organic acid content in fruit from the differenthabitats did not depend on organic acid accumulation in the early stage of the fruit growth, butdepended on organic acid cleavage in mid-and late stage of fruit development In different habitats,total sugar, sucrose and fructose content in navel orange fruit had a significant difference, and theyall were highest in south subtropic while lowest in mid-north subtropic. However, glucose contentwas similar in the different habitats Thus, the difference of sucrose and fructose accumulation indifferent habitats led total sugar content difference The difference of ratio of sugar to acid in thedifferent habitats increased gradually while the fruit development, and reaching to the maximum atripping period.4. The activities of CS, MDH and mitochondrial ACO in navel orange fruit showed no obviouschanges in different habitats, and no significant difference between the different habitats. But, PEPCand cytosolic ACO in fruit had significant difference in activities after 120 days after full bloom,which showed organic acid accumulation in difference habitats was influenced to a certain degree bythe PEPC and cytosolic ACO Therefore, IDH activities were a significant difference betweendifferent habitats, which indicated that the differences of organic acid content in different habitatsresulted mainly from activities of IDH changing in every habitat. Moreover, the activities of SPS andSS in the cleavage direction in fruit were similar between different habitats, whereas SS activity in thesynthetic direction changed significantly in the different habitats, which indicated SS in the syntheticdirection was the dominant enzyme that led sugar different accumulation in different habitats.5. The differences of sugar accumulation in different varieties resulted from SPS and SS in thesynthetic direction. In fact, SPS played an important role in sugar accumulation in fruit during thefruit inflating stage, while SS in the synthetic direction was the key enzyme during the fruit middleand late developing stage. 6. The activities of CS, cytosolic ACO, MDH and mitochondrial ACO in navel orange fruitshowed no obvious difference in different varieties, and no significant difference between thedifferent varieties. However, PEPC and IDH activities changed significantly in the different varietiesfruit, which indicated PEPC and IDH were the dominant enzymes that led organic acid differentaccumulation in different fruit.7. Though the grey system analysis of 22 ecological factors in the different habitats to the fruitratio of sugar to acid, the results showed that the grey incidence of temperature and sunshine werehigh, especially Annual light hours, sunshine percentage from 60 days after full bloom to 90 daysafter full bloom and sunshine percentage from 150 days after full bloom to 180 days after full bloom,and their grey incidences all reached to over 0.9. Furthermore, the grey incidences of averagetemperature and sunshine percentage from fruit inflating stage to ripe period almost reached to 0.8. So,the ecological factors which influenced the ratio of sugar to acid were more seriously weretemperature and sunshine. Thus, temperature and sunshine were main ecological factors for the ratioof sugar to acid, and then they became the key indexes for navel orange ecological divisions. Theflavor quality was well in the eco-type habitat that had higher temperature and enough sunshine andquantity of heat with similar cultural managements.