Speculation on the Mechanism of Ion Transport in Roots Based Upon Indirect Evidence from Histochemical Studies

Peroxidase distribution in the roots of many plants is characterized by an all-or-none disposition of the enzyme in cells of tangentially adjacent layers of tissue. Concentric bands of cells in the zone of greatest ion-exchange capacity in onion roots exhibit an alternating pattern of peroxidase activity, that is, root cap (erratic, but predominately +); epidermis (- except for weak reaction in outer part); hypodermis (+); cortex (-); endodermis (+); pericycle (-); phloem (+); xylem (-). Intracellular localization of peroxidase traced from root apex to base shifts from the cytoplasm into the walls and outer membranes. Instances of intracellular polarization of peroxidase sites within root apices are of two types: (a) In some plants, Sorghum, for example, the horizontally polar extremes of epidermal cells show a strong peroxidase reaction leaving a negative strip through the middle of the cells. (b) In other plants, including Allium, the peroxidase reaction is typically one-sided. The asymmetry of enzyme activity occurs in epidermal cells at the root apex, the outer portion of each cell being weakly positive for peroxidase. In the case of onion the two tissues that display greatest peroxidase activity in the apical part of the root (i.e., hypodermis and endodermis) subsequently synthesize cell-wall lipids with similar Sudanophilic properties. The role of peroxidase and lipids in the root could relate to absorption processes in terms of the symplast concept of solute translocation.