Use of Lanthanum to Trace Apoplastic Solute Transport in Intact Plants

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Abstract
This electron microscopic study revealed that solutes enter the apoplasm of root mcristems and move from there to the stele of the root and to the shoots of intact plants. Lanthanum was used as a plasma membrane-impermeable electron-dense marker of apoplastic solute flux in Hordeum vulgare L., Saiicornia virgimca L., Spartina alternflora Loisel. and Zea mays L. The presence of lanthanum in EM specimens was confirmed by X-ray microanalysis. Lanthanum that entered the root apoplasm was also localized in membrane-bound compartments within cells of each plant Lanthanum was localized in vesicles, ER, and vacuoles of root and leaf cells. Following root application, lanthanum was evident in the leaves of the three grass species studied. Lanthanum was rarely observed in S. virginica leaves. Only plants exposed to 23 mol m−3 lanthanum for 24 h or more showed lanthanum in root cell cytosol and this was concluded to be a toxic response.