Serious difficulties are encountered when SIMS analysis is
applied to plant cells because of the cells' basic
organization. In most plant cells, the cytoplasm is present
as a thin layer that surrounds a large central vacuole, and
is surrounded externally by a porous semi-rigid cell wall. Due
to the high internal hydrostatic pressure typical of plant cells,
large-scale solute redistribution may occur when tissues are
excised. Relatively small solute decompartmentation is sufficient
to collapse the native solute gradients between the cytoplasm
and the adjacent compartments, due to the small volume of the
former. For these reasons, most of the SIMS analyses in plant
cells have been performed on elements bound to non-diffusible
structures such as proteins, cell wall polymers, or in dry seeds.
Sample preparation remains a limiting factor when imaging the
distribution of soluble compounds. Cryotechniques have generated
considerable interest to circumvent these problems. Cryofixation
followed by cryosectioning would a priori be the best procedure,
but encouraging results indicate that cryofixation followed
by cryosubstitution is an interesting alternative.