| With gorgeous color and luster, beautiful shape .citrus are nourishing fruits and people like it very much. Citrus is the most important fruits and among the most important commodities traded internationally. The annual value of world imports of fresh citrus exceeds USS5 billion. Citrus production and exports generate significant earnings for many countries and the commodity provides a low-cost source of vitamins, thus contributing to nutrition % health and food security. China is the original place of citrus , with abundant species resources, superior ecosystem condition, wide land and immense market capacity . According to the statistics of FAO in 2000, the gross of citrus products in China already reached 11,780,000 ton, only next to apple. In many regions, the value of citrus has occupied 30-10% of the total agriculture production value. Citrus production has increased rapidly in China in recent years and it is likely to lead to higher exports. On the other hand, China is seen by many citrus exporters as a promising market. Imports of citrus fruits into China were valued at USS 188 million in 1999. With the entry into the WTO, China is expected to play an important role in international citrus trade. In general, trends and prospects in the world citrus economy present certain challenges for producers and traders, particularly in regard to international markets. The cultivation and marketing of the crop has become increasingly competitive. But there are some problems that are inhibiting the citrus development of China: irreasonable varieties structure and the distribution,un-advanced culture and management, undeveloped intensive cultivation and low commercial standard,especially too centralized mature time and low quality(excellent variety such as late mature navel, low-acids oranges have not been cultured orpopularized.). So how to increase the yield, at the same time, adjust the species and varieties construction, improve fruit quality, become the problem we need to resolve.The contents of sugars, organic acids in fruit and the ratio of sugar to acid are most important quality index of citrus. Peoples like the fruit with high sugar and low acid. But now, the citrus fruits in some producing areas contain too much content of organic acids and too low ratio of sugar to acid. Therefore, it is very important to study the organic acids metabolism by physiological and molecular means for the control and adjustment of organic acids, for improving the production and quality of the citrus fruits.The heredity of organic acid in citrus fruits is semi-quantitative , so the organic acids metabolism involves many physiological processes and have many factors affecting the content in the fruit. The studies done by many researchers[ Hirai M and Ueno I. (1977) , Sadka A, Artzi L et al. (2000,2001), Yamaki Y T. (1990), Wen Tao (2001) et al] showed that citrate synthase, malate dehydrogenase, NAD-isocitrate dehydrogenase, phosphoenolpyruvatecarboxylase, aconitase could be involve in the accumulation and reduction of organic acids in citrus fruits.However , it was not enough for the research on the organic acids metabolism of citrus fruits in the past, especially investigating many related enzymes activities in many citrus fruits with different contents of organic acids at the same time. Because of the very complicated heredity background, polyembryony, perennation and long juvenile period of citrus plants, the study on the organic acids metabolism by AFLP has not been done before.In this investigation, we chose the high acid species lemon (Citrus limon Burm.F), low acid variety candy orange (C.Sinensis Osbeck) and moderate acid variety Jincheng (C.Sinensis Osbeck) as contrast, laid a strong emphasis on the comparative study of Fengjie navel orange (.C.Sinensis Osbeck) with its low-acid mutant and late mature mutant. The changes in activities of CS, MDH, PEPC, cytosolic aconitase and mitochondria! Aconitase in the juice vesicle tissue of these citrus fruits were studied. Genetic variabilities of the low-acid muta... |
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