根系有机酸分泌物对茶树根土界面微域氟有效性的影响机理

Plant root exudates severely influence the physical, chemical and biological properties of nutrients and toxic substances in rhizosphere and root surface, and consequently control the absorption and accumulation of those substances by roots..Tea plant is a super-accumulator.Root-soil interface is a critical approach to absorb fluoride from soils to roots, and thus accumualte of fluoride in tea leaves(＞90%).Low molecular weight organic acids including malic acid, citric acid and oxalic acid, are the most constitutes in root exudate of tea plant, which can significantly impact the speciation and availability of nutrients and contaminants in micro-domain of root-soil interface. However, the reasons are not clear. The purposes of this study are to investigate the impacts of main organic acids in root secretion on the speciation and availablity of fluoride in root-soil interface by the root boxes methods. The competition and chelation of root secreted organic acids with aluminium in soil solution of rhizosphere, and its effects on the composition of Al-F complexes, release and activation of available fluoride will be studied by using HPIC coupled UV-VIS detector and 27Al nuclear magnetic resonance technology. The diffusive gradients in thin-films (DGT) and microelectrode technology will be performed to investigate the influences of root secreted organic acids on the contents of available fluoride in rhizosphere. The effects of different organic acids on the structure and constitute of iron plaque on the tea plant root surface will be studied by using the transmission electron microscopy and ICP methods. The mechanisms of how the different organic acids regulate the speciation and availability of fluoride in root-soil interface, and thereby control the fluoride absorbed by tea roots will be clarified. The results from this study will provide critical facts to explain the micro-mechanisms of soil fluoride absorbed and accumulated by tea roots, and then to elucidate the rhizosphere effects of hyperaccumulation of soil fluoride in tea plants.

茶树是高聚氟植物，根土界面微域是土壤氟进入根系，进而在叶片累积的重要通道。根系分泌物通过影响根际土壤有效态氟的含量，进而调控根系的吸收富集。有机酸作为茶树根系分泌物的主要组分，对根土界面氟形态与有效性的影响及其作用机理尚不清楚。本项目基于前期研究基础，利用根箱装置原位收集根际土壤溶液，采用HPIC-UVVIS及27Al-NMR技术研究茶树有机酸分泌物对土壤氟的溶出释放与氟形态及铝-氟络合物组成的影响；把薄膜梯度扩散技术与微电极技术相结合考察根际土壤有效态氟的释放与活化规律；利用透射电镜与ICP相结合的方法分析有机酸分泌物对茶树根表铁氧化物胶膜的结构、组成及其对根系吸收富集氟的影响；阐明有机酸分泌物影响茶树根土界面微域氟形态转化及有效态氟含量的变化规律，揭示有机酸分泌物调控根际和根表氟有效性的微观机理。该研究为剖析茶树高量富集土壤氟的根际效应，阻控土壤氟向茶树的迁移富集提供理论与实验依据。

茶树是高聚氟植物，根土界面微域是土壤氟进入根系，进而在叶片累积的重要通道。根系分泌物通过影响根际土壤有效态氟的含量，进而调控根系的吸收富集。有机酸作为茶树根系分泌物的主要组分，对根土界面氟形态与有效性的影响及其作用机理尚不清楚。本项目研究了茶树根系主要有机酸分泌物柠檬酸对土壤氟的溶出释放与氟形态及铝—氟络合物组成的影响；分析了有机酸分泌物对茶树根表铁氧化物胶膜的结构、组成及其对根系吸收富集氟的影响；考察根际土壤有效态氟的释放、活化及其在茶树体内的富集规律。结果表明：.1.土壤氟的释放存在快速释放和慢速释放两个阶段，呈现初始溶出浓度较高，而后逐渐减缓的趋势。土壤溶出液中的游离态氟和总氟的释放量随柠檬酸浓度的升高而增加，反应开始前2 h的高浓度柠檬酸溶出液中Al-F络合物的浓度较大，反应开始前1 h的Al-F络合物主要以AlF3为主，占氟溶出总量的90％左右；随着溶出时间的延长，AlF2+的溶出量逐渐增大，并逐渐超过AlF3。.2.茶树根表铁膜含量随Fe2+诱导时间的延长呈现先升高后降低的趋势，随Fe2+ 质量浓度的增加显著升高，与溶液pH、有机酸浓度呈负相关。当铁膜含量为2.40～13.60mg•g-1，根表铁膜含量与茶树吸收、富集氟的能力呈正相关。加Fe2+诱导形成铁膜后，茶树体内氟的含量增加了42.3%～103.7%；有机酸与Fe2+共同作用时，茶树体内氟含量显著增加了101.7%～243.0%。.3.DGT对土壤氟的吸附量(MDGT)、土壤间隙水氟浓度(CF)与茶苗氟含量相关性较好。MDGT、CF与茶苗根氟含量呈显著相关(P<0.05)，相关系数(R2)分别为0.634~0.724和0.609~0.675；MDGT、CF与茶苗地上部分氟含量呈极显著相关(P<0.01)，相关系数分别为0.844~0.896和0.827~0.870。MDGT与茶苗氟含量相关系数大于CF与茶苗氟含量相关系数。.4.氟主要分布在茶树根系和叶片中的细胞溶质部分，占其总富集量的52.3 和 71.8%；富集在叶片原生质体中氟98.1%富集在液泡中。铝主要分布在茶树根系和叶片的细胞壁组分，占其总富集量的69.8 和75.2%，而其中的73.2%被细胞壁的果胶和半纤维素组分所固定。茶树叶片细胞溶质中富集的铝约88.3%储存在液泡中。