Localization of lead in Allium cepa L. cells by electron microscopy

The study of mechanisms by which plants tolerate lead requires ultrastructural observations of lead distribution in cells. However, the conventional technique used in electron microscopy brings up the problem of lead translocation from tissues during chemical processing. If most of the lead is washed out of tissues, then the method is not suitable for this type of study; if, however, it remains in the tissues, the method can be used. The amount of lead washed out of Allium cepa root tips during successive stages of fixing and dehydrating was determined in this study. Allium cepa plants were treated with Pb(NO₃)₂ in hydroponic cultures. The conventional fixing (GA, OsO₄) and dehydration (alcohol + propylene oxide or acetone) methods used in the preparation of tissues for electron microscopy were then applied to root tips. The lead content in tissues and in reagents was analysed in the successive stages of the procedure. It was shown that 96.2% of the lead taken up was retained in the tissues and was not washed out during fixing and dehydrating. Of the 3.8% of the lead lost in the chemical procedure, about half was washed out during fixing in osmium tetroxide. This reagent is thus the least suitable for this type of study. Our study showed that the conventional electron microscopy preparative technique is appropriate for studying the distribution of lead deposits in A. cepa root cells, owing to the way in which lead is compartmentalized/sequestered in plant cells.