Ion Distribution in Salt-stressed MatureZea maysRoots in Relation to Ultrastructure and Retention of Sodium

Abstract

Ultrastructural features and the distribution of soluble ions have been examined in mature roots of Zea mays plants grown in both NaCl and Na₂SO₄ salinities. When the plants were grown in either salt, the Na concentration increased proximally along the root with a concomitant decline in the K concentration. Both trends were reversed in the shoot. X-ray microanalysis of deep-frozen, fully hydrated specimens showed that in salt-treated roots Na, and Cl, or S were distributed about stoichiometrically in the cortex and endodermis. Na was usually less concentrated than the anion in the lumens of the vessels, but was concentrated markedly relative to either Cl or S in the adjoining xylem parenchyma cells. In the older, proximal parts of seminal roots of plants grown both without salt (controls) and in the presence of either NaCl or Na₂SO₄, wall developments occurred in xylem parenchyma cells at the half-bordered pits in which the cell wall became markedly thicker and possessed a loosely packed fibrillar structure. These structures were not comparable with the transfer-cell type of protuberances reported in the roots of other species. In the xylem parenchyma of plants grown in the presence of Na₂SO₄ there were dramatic increases in the quantities of rough endoplasmic reticulum, ribosomes, and mitochondria relative both to controls and NaCl treatments. The results are discussed in relation to the possible function of the xylem parenchyma of the mature root in the reabsorption of Na from the xylem sap, which may mitigate adverse effects of salinity in salt-sensitive glycophytes.