Transport of radionuclides in a young
tomato plant: The distribution of elements (e.g. micronutrients,
can be visualized by
radioactive labelling followed by an exposure of the dried plants
to X-ray films (autoradiography). The radionuclides can be quantified
by gamma spectrometry or liquid scintillation counting.
|The mobility of nutrients, assimilates and pollutants
in the phloem varies in a wide range. Radionuclides are often used
to investigate transport processes on the whole plant level (see
In cereals, solutes can be selectively removed from the xylem
sap during its acropetal translocation and loaded into the phloem.
Macronutrients (e.g. potassium ions), micronutrients (e.g. zinc
ions), assimilates (e.g. some amino acids) and pollutants (e.g.
radiocesium, cadmium ions) can be transferred from the xylem to
the phloem in the peduncle and in other stem parts of maturing
wheat. Since the phloem is directed to the grains, the relevance
of such xylem-to-phloem transfer processes is obvious.
The mechanisms involved and their regulation are not yet satisfactorily
known and should be further elucidated during the next years.