轻稀土元素对赣南脐橙柠檬酸积累代谢调控的机理研究

The comparably high rare earth content in the soil of Gannan area is an advantage for the development of organic nutrition in navel orange fruits. Compared with the correlation between heavy rare earth element (HREE) the organic nutrition in the fruits, the correlation between LREE and the organic nutrition in the fruits is stronger; while compared with the correlation between total sugar content in fruits and LREE, the correlation between organic acid content and LREE is stronger. Among all organic acids, the content of citric acid accounted for 74%～90%. So citric acid played a decision role on the flavor of navel orange. But the mechanism of LREE affecting the accumulation and metabolism of citric acid in navel orange was not clear, and the mechanism of LREE affecting on the gene expression and degradation pathway of citric acid was not clear either. So this project combined located monitoring technique and controlled experiment, to research the effect of LREE on the accumulation of citric acid in navel oranges at different developing stages and the activities of related metabolic enzymes; Thus to find out the key components of LREE which affect the citric acid accumulation in navel oranges. And we also clarified the physiological effecting mechanism of LREE on the accumulation and metabolism of citric acid in navel orange fruits; the effects of LREE on the differences between citric acid synthesis and its degradation have been discussed as well. After analyzing the relation between the accumulation and metabolism of citric acid and its gene expression, we determined the synthesis and degradation pathway of citric acid in navel orange fruits. It provides the theoretic basis to clarify the effecting mechanisms of LREE on citric acid in Gannan navel orange，and provides a method to further the research of the effects of LREE on the accumulation and metabolism of sugar, the decision flavor ingredient, and mineral substance in navel fruits. And also it is the foundation of fully disclosing of the effects of LREE on the fruits quality of Gannan navel oranges.

赣南土壤稀土含量较高，有利于脐橙果实有机营养物质形成；其中轻稀土元素与有机营养物质含量的相关程度高于重稀土，与有机酸含量的相关程度高于总糖。柠檬酸占脐橙有机酸含量高达74%～90%，是影响脐橙风味的决定性有机酸。然而，轻稀土元素对脐橙柠檬酸积累代谢、及对脐橙柠檬酸关键基因表达和降解途径影响的机制尚不清楚。项目采用定位监测与控制实验结合，研究轻稀土元素对不同生长时期脐橙柠檬酸积累和代谢相关酶活性影响，寻找影响脐橙柠檬酸积累代谢的关键轻稀土元素种类，阐明轻稀土元素对脐橙柠檬酸积累代谢影响的生理机理；探讨轻稀土元素对脐橙柠檬酸合成与降解基因表达差异性，分析柠檬酸积累代谢与基因表达关系，掌握脐橙柠檬酸合成与降解途径。为阐明轻稀土元素对赣南脐橙柠檬酸影响的机制提供理论依据，为深入研究轻稀土元素对影响脐橙果实风味的糖及矿质营养积累代谢提供方法，为全面揭示轻稀土元素对赣南脐橙品质影响奠定基础。

前人研究表明，赣南土壤稀土含量较高，有利于脐橙果实营养物质的形成，其中轻稀土元素对果实营养物质含量的影响大于重稀土，且轻稀土元素对脐橙有机酸含量的影响大于总糖。柠檬酸占脐橙有机酸含量高达74%～90%，是影响脐橙风味的决定性有机酸。因此，项目针对轻稀土元素影响脐橙柠檬酸积累代谢的种类和机制不清楚的问题，以赣南土壤轻稀土背景值较高的‘纽荷尔’脐橙果园为研究对象，首先，通过分析土壤、果实和叶片轻稀土元素含量和柠檬酸含量动态变化，及其相互之间的相关性，表明轻稀土元素从土壤到叶片再到果实的转运分配模式，9月份是轻稀土元素转运的关键时期；轻稀土元素与大量元素间存在协同促进作用，微量元素Cu却抑制轻稀土元素转运，La是影响脐橙品质的主要轻稀土元素。其次，通过分析轻稀土元素对柠檬酸和蔗糖代谢酶活性影响作用，发现轻稀土元素对柠檬酸代谢酶的影响作用明显，轻稀土元素含量能促进果实柠檬酸代谢酶活性和果实柠檬酸含量，La是影响柠檬酸积累代谢的主要轻稀土元素。最后，采用La定量控制研究表明，低浓度的La溶液通过抑制果实中柠檬酸合成关键酶CitPEPCK1和CitPEPC基因、以及果实中柠檬酸降解关键酶CitIDH3和CitGS基因的表达水平来抑制柠檬酸降解。综合表明，轻稀土元素有利于促进脐橙柠檬酸的积累，不仅为研究轻稀土元素对影响脐橙果实其他营养元素的积累代谢提供参考，并为全面解析赣南脐橙果实品质优良提供理论支撑。