The uptake of a polyvalent cation and its distribution in the root apices of Allium cepa: Tracer and autoradiographic studies

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Abstract
Observations on the inhibition of root elongation and cell division in Allium cepa showed that the toxic effects of scandium and aluminium were very similar. Tracer uptake studies using 46Sc indicated that the rate of uptake in the apical 3.0 mm of the axis was more rapid than elsewhere in the root and proceeded in two distinct phases; Phase 1, probably superficial adsorption, was characterised by a rapid initial rate which was little affected by low temperature, the rate of Phase 2 was slower but remained constant for 24 hours and was highly dependent on temperature. Autoradiographs from roots treated for 30 min with 46Sc showed that most of the isotope in the root tip was concentrated in a peripheral belt corresponding with the mucigel layer of the root cap and it is suggested that this is the site of Phase 1 adsorption. The underlying root cap and epidermal cells retained little scandium but interior to them some isotope was associated with dividing cells; this increased steadily over 6 hour to an estimated concentration of 30 mM, and possibly represents Phase 2 uptake. Differentiation and secondary wall formation in the cortex restricted the rate of radial penetration of scandium. The primary endodermis restricted the entry of scandium into the stele at a very early stage in its development, which leads to the conclusion that migration of the ion across the root is primarily in the free space. Scandium enters the dividing cells in advance of observable effects on cell division, a situation compatible with the direct involvement of this ion in the inhibition of the mitotic cycle. Suggestions are made on the mechanisms by which polyvalent cations might disturb cell division and extension.