RATES OF DIFFUSION INTO ROOTS OF MAIZE

The rate at which solutes may move in the cell wall apoplast of roots was assessed by measuring the diffusivity (D) of a dye, sulphorhodamin e G (SR), in fresh roots of field-grown maize. Pieces of axile and bra nch roots from regions with soil sheaths (with immature late metaxylem ) and without sheaths (with mature late metaxylem) were placed in SR s olution for 5, 20 and 80 min, removed, rinsed and freeze-substituted. The water-soluble SR was retained in position in the tissues during an hydrous embedding and sectioning. The distance of radial penetration o f the dye was measured from micrographs taken in the fluorescence micr oscope. Fick's second law was used to calculate values of D. The Value s are expressed relative to the Value of D in water (D-aq). Mature axi le roots uniformly gave values of D = D-aq/1000. In immature axile roo ts D varied from 0 (no penetration through the hypodermal walls) throu gh D-aq/1000 to D-aq/40 (the maximum value found). This Variation prob ably arises from a developmental sequence, with D high in the region t o 10 cm from the tip, zero in the region 10-25 cm from the tip, and lo w further back (merging with the values for mature roots). In branches of both immature and mature roots also, D was highly variable, from z ero to D-ap/40. SR was found to penetrate through the endodermis of ax es and branches, following sub-microscopic paths around the lining of pits in the secondary walls. It is concluded that soil solutes probabl y enter roots at the outermost layer of cells, and that the cell wall apoplast is most unlikely to be a path for flowing water.