LOCALIZATION OF CHEMICAL-ELEMENTS AND ISOTOPES IN THE LEAF OF SOYBEAN(GLYCINE-MAX) BY SECONDARY-ION MASS-SPECTROMETRY MICROSCOPY - CRITICAL CHOICE OF SAMPLE PREPARATION PROCEDURE

Secondary ion mass spectrometry (SIMS) is one of the few microscopical methods that potentially can detect and in situ localize the various isotopes of virtually all elements. Recent work with SIMS has demonstr ated the possibility of imaging the distribution of various elements i n plant cell and tissues. However, in these studies, the elements were incorporated in cell macromolecules or associated with structural pol ymers, precipitated or immobilized in dry seeds. The localization of m ineral ions is of particular significance for the physiology of higher plants owing to their quantitative importance and the impact of their cellular distribution on metabolic regulation. Here we analyse the po ssibility of mapping different elements (K, Ca, Mg, P, S, N-15 and N-1 4) present as soluble and/or bound forms in highly vacuolated leaf cel ls. Cryoprocedures to prepare samples for SIMS detection are described and discussed. The quality of the results is assessed at each step of the sample preparation and analysis. Various methodologies are used, including photonic and electronic microscopies, and the agreement of t he observed ion distribution with current knowledge of ion compartment alization in plant cells. The K/Ca emission ratio is proposed as an in dex of the degree of preservation of the natural ion distribution to c ritically evaluate the results and identify where artefacts are likely to occur.