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Environmental and Stress Physiology

Rhizosphere Acidification by Iron Deficient Bean Plants: The Role of Trace Amounts of Divalent Metal Ions

A Study on Roots of Intact Plants with the Use of ¹¹C- and ³¹P-NMR

Plantenfysiologisch Laboratorium, Universiteit van Amsterdam, Kruislaan 318, 1098 SM Amsterdam, The Netherlands, Nationaal Instituut voor Kernfysica en Hoge-energiefysica, sectie K, Kruislaan 317, 1098 SM Amsterdam, The Netherlands, Laboratorium voor Fysische Chemie, Universiteit van Groningen, Nijenborch 16, 9747 AG Groningen, The Netherlands

Rhizosphere acidification by Fe-deficient bean (Phaseolus vulgaris L.) plants was induced by trace amounts of divalent metal ions (Zn, Mn). The induction of this Fe-efficiency reaction was studied by ¹⁴CO₂ and ¹¹CO₂ fixation experiments, and with ³¹P-NMR on roots of whole plants. The starting and ending of an acidification cycle was closely coupled to parallel changes in CO₂ fixation, within the maximal resolution capacity of 20 min. ³¹P-NMR experiments on intact root systems showed one peak which was ascribed to vacuolar free phosphate. At the onset of proton extrusion this peak shifted, indicating increase of pH in the cells. Proton extrusion was inhibited, with a lag period of 2 hours, by the protein synthesis inhibitors cycloheximide and hygromycin. It is assumed that Zn and Mn induce proton extrusion in Fe-deficient bean roots by activating the synthesis of a short-living polypeptide; the NMR data suggest a role for this peptide in the functioning of a proton pumping ATPase in the plasma membrane.

² Present address: International Institute for Hydraulic and Environmental Engineering, Oude Delft 95, Delft, The Netherlands.

³ Present address: Organisch-chemisch Laboratorium, Universiteit van Utrecht, Transitorium 3, 3584 CH Utrecht, The Netherlands.

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