Radial Movement of Cations across Aerenchymatous Roots ofZea maysMeasured by Electron Probe X-Ray Microanalysis

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Abstract
The radial movements of Rb+ and Sr2+ as analogues for K+ and Ca2+ were followed by electron probe X-ray microanalysis in aerenchymatous roots of maize (Zea mays L. cv. LG 11). The object was to determine the possible pathways by which ions can reach the xylem when degeneration and lysis of much of the root cortex has taken place during formation of numerous gas-filled spaces in aerenchymatous roots. After 1, 6 and 24 h uptake of Rb+ and Sr2+ from a full strength nutrient solution containing K+ and Ca2+, the distribution of these elements was examined. Transverse root sections, prepared by cryostat sectioning and lyophilization, were used for electron probe X-ray microanalysis. If the cell walls lining gas-filled spaces become suberized (as reported in earlier literature), this development might be expected to retard ion movement, but we found that Rb+ or Sr2+ could migrate as readily in the radial wall residues remaining after cortical cell lysis, as in the walls of intact cortical cells. The distributions of Rb+ and Sr2+ differed, however. The lack of a concentration gradient for Rb+ across the root was compatible with its transport mainly in the symplast, constituted by occasional files of intact cortical cells bridging the gas-filled spaces. The evident concentration gradient for Sr2+ was accounted for by its movement to the endodermis by the apoplastic pathway constituted by the walls of intact and lysed cells.