Transfer Cells in Roots of Phaseolus coccineus: Ultrastructure and Possible Function in Exclusion of Sodium from the Shoot

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Abstract
A study was made of ultrastructural aspects and ion distribution in roots of Phaseolus coccineus as affected by NaCl and Na2SO4 salinity. In the proximal region of the root, xylem parenchyma cells are differentiated as transfer cells with well developed wall protuberances adjacent to the half-bordered pits of the vessels. The cytoplasm of these transfer cells contains cisternae of rough endoplasmic reticulum, the number of which was increased greatly when the plants were grown in the presence of NaCl or Na2SO4. The cisternae of the endoplasmic reticulum are often associated closely with the plasmalemma and interconnected with it by fibrillar bridges. Wall protuberances occur also in the exodermis and epidermis of the more apical region of the root. Their function is still unknown. P. coccineus excludes Na, but not Cl, from the leaves by retaining it particularly in the proximal region of the root. X-ray microanalysis of unfixed, frozen, hydrated specimens revealed that the transfer cell-type xylem parenchyma cells in salt-treated roots accumulated Na relative to both the adjoining xylem vessels and the cortical cells and showed very high Na/K and Na/Cl ratios. It is suggested that the xylem parenchyma cells can reabsorb Na from the vessels, probably in exchange for K, and that Na exclusion from the shoot is at least partly mediated by this process. The implication of this for regulation of salt transport in salt sensitive glycophytes is discussed.