Radial Movement of Cations across Aerenchymatous Roots of Zea mays Measured by Electron Probe X-Ray Microanalysis

The radial movements of Rb⁺ and Sr²⁺ as analogues for K⁺ andCa²⁺ were followed by electron probe X-ray microanalysis inaerenchymatous roots of maize (Zea mays L. cv. LG 11). The objectwas to determine the possible pathways by which ions can reachthe xylem when degeneration and lysis of much of the root cortexhas taken place during formation of numerous gas-filled spacesin aerenchymatous roots. After 1, 6 and 24 h uptake of Rb⁺ andSr²⁺ from a full strength nutrient solution containing K⁺ andCa²⁺, the distribution of these elements was examined. Transverseroot sections, prepared by cryostat sectioning and lyophilization,were used for electron probe X-ray microanalysis. If the cellwalls lining gas-filled spaces become suberized (as reportedin earlier literature), this development might be expected toretard ion movement, but we found that Rb⁺ or Sr²⁺ could migrateas readily in the radial wall residues remaining after corticalcell lysis, as in the walls of intact cortical cells. The distributionsof Rb⁺ and Sr²⁺ differed, however. The lack of a concentrationgradient for Rb⁺ across the root was compatible with its transportmainly in the symplast, constituted by occasional files of intactcortical cells bridging the gas-filled spaces. The evident concentrationgradient for Sr²⁺ was accounted for by its movement to the endodermisby the apoplastic pathway constituted by the walls of intactand lysed cells. Key words: Aeration, aerenchyma, cations, electron probe X-ray microanalysis, Zea mays