Ultrastructural and physiological changes in root cells of Sorghum plants (Sorghum bicolor x S. sudanensis cv. Sweet Sioux) induced by NaCl

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Abstract
The xerophytic, but salt-sensitive Sorghum cultivar ‘Sweet Sioux’ is known as an ion excluder with a high K/Na selectivity at the plasmalemma and tonoplast of epidermal root cells. The aim of this study is the correlation of salt-effected changes in physiological parameters with structural and ultrastructural changes in root cells. The investigation was carried out with root cells because these cells are most directly exposed to the growth medium. Sorghum bicolor × S. sudanensis cv. Sweet Sioux plants were grown under steady-state conditions on nutrient solutions with or without 40 mol m−3 NaCl. Sorghum sustained this treatment but showed several salt-induced structural and physiological changes which were studied in various cell types of the root tip. (1) NaCl salinity led to a shorter growth region and to salt-induced alterations in the chemical and physical properties of the cell walls in the root tips. (2) Salt treatment also increased the membrane surface in root cells: root cells showed an increase in the quantity of vesicles in the epidermis and in the middle cortex cells. Additionally, some of the epidermis cells of salt-treated plants revealed a characteristic build-up of transfer cells, suggesting an increase in membrane surfaces to increase the uptake and storage of substances. (3) The number of mitochondria increased in the epidermal and in the cortex cells after salt stress thus indicating an additional supply of energy for osmotic adaptation and for selective uptake and transport processes. (4) In the epidermal cytoplasm NaCl stress led to a significant decrease of the P, K, Ca, and S concentrations accompanied by an increase of Na concentration. Electron micrographs show an increase in electron optical contrast within the cytosol and in the matrix of the mitochondria. These results are discussed with regard to the possibility of influence on the part of metabolic functions. (5) The NaCl concentrations were seen to increase and the K concentrations to decrease during salt stress in the vacuoles of the epidermis and cortex cells. The salt-induced increase in vacuolar NaCl concentrations of epidermis and cortex cells are in the region 2 cm behind the root tip, which is sufficient for an osmotic balance towards the growth medium. Additional solutes are necessary 0.5 mm behind the root tip to facilitate osmotic adaptation. The results show ultrastructural changes caused by an Na-avoiding mechanism characterized by a high level of energy consumption. The exclusion of Na from the symplast seems to lead additionally to a decrease in cytoplasmic concentrations of such essential elements as Mg, P, S, and Ca and is thus responsible directly (via energy supply in mitochondria, homeostasis, selectivity of K over Na) or indirectly (via enzyme conformation, cytoplasmic hydration) for the ultra-structural degradation indicated. The salinity-induced multiplicity of structural and functional changes within cell compartments constitutes a group of indicators for the limited NaCl tolerance of Sorghum.

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