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Published on November 10, 2006; 10.1104/pp.106.090845

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Received October 4, 2006
Accepted November 4, 2006

Germanium-68 as an Adequate Tracer for Silicon Transport in Plants. Characterization of Silicon Uptake in Different Crop Species

Miroslav Nikolic *, Nina Nikolic , Yongchao Liang , Ernest A. Kirkby , and Volker Römheld

Department of Natural Resources and Environmental Sciences, Centre for Multidisciplinary Studies, University of Belgrade, Kneza Viseslava 1a, 11030 Belgrade, Serbia
Institute of Soil and Fertilizer, and Ministry of Agriculture Key Laboratory of Plant Nutrition and Nutrient Cycling, Chinese Academy of Agricultural Sciences, Beijing 100081, China
Institute of Integrative and Comparative Biology, Faculty of Biology, University of Leeds, Leeds, LS2 9JT, UK
Institute of Plant Nutrition (330), University of Hohenheim, D-70593 Stuttgart, Germany

* Corresponding author; email: mnikolic{at}cms.bg.ac.yu.

A basic problem in silicon (Si) uptake studies in biology is the lack of an appropriate radioactive isotope. Radioactive germanium (68Ge) has been used previously as a Si tracer in biological materials, but its suitability for the study of Si transport in higher plants is still untested. In the present study we investigated 68Ge-traced Si uptake by four crop species differing widely in uptake capacity for Si including rice (Oryza sativa L.), barley (Hordeum vulgare L.), cucumber (Cucumis sativus L.) and tomato (Lycopersicon esculentum Mill.). Maintenance of a 68Ge/Si molar ratio which was similar in the plant tissues of all four plant species to that supplied in the nutrient solution over a wide range of Si concentrations demonstrated the absence of discrimination between 68Ge and Si. Further, using the 68Ge tracer, a typical Michaelis-Menten uptake kinetics for Si was found in rice, barley and cucumber. Compared to rice, the relative proportion of root-to-shoot translocated Si was lower in barley and cucumber, and especially in tomato (only 30%). Uptake and translocation of Si in rice, barley and cucumber (Si-accumulators) were strongly inhibited by 2,4-dinitrophenol and HgCl2, but in tomato, as a Si-excluding species, both inhibitors produced the opposite effect. In conclusion, our results suggest the use of the 68Ge tracer method as an appropriate choice for future studies of Si transport in plants. Our findings also indicate for the first time that the restriction of Si from symplast to apoplast in the cortex of Si excluders is a metabolically active process.






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