Hw. Koyro, ULTRASTRUCTURAL AND PHYSIOLOGICAL-CHANGES IN ROOT-CELLS OF SORGHUM PLANTS (SORGHUM-BICOLOR X SORGHUM-SUDANENSIS CV SWEET SIOUX) INDUCED BY NACL, Journal of Experimental Botany, 48(308), 1997, pp. 693-706
The xerophytic, but salt-sensitive Sorghum cultivar 'Sweet Sioux' is k
nown as an ion excluder with a high K/Na selectivity at the plasmalemm
a 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 investigatio
n was carried out with root cells because these cells are most directl
y exposed to the growth medium. Sorghum bicolor x S. sudanensis cv. Sw
eet Sioux plants were grown under steady-state conditions on nutrient
solutions with or without 40 mol m(-3) NaCl. Sorghum sustained this tr
eatment but showed several salt-induced structural and physiological c
hanges which were studied in various cell types of the root tip.(1) Na
Cl salinity led to a shorter growth region and to salt-induced alterat
ions in the chemical and physical properties of the cell walls in the
root tips.(2) Salt treatment also increased the membrane surface in ro
ot cells: root cells showed an increase in the quantity of vesicles in
the epidermis and in the middle cortex cells. Additionally, some of t
he epidermis cells of salt-treated plants revealed a characteristic bu
ild-up of transfer cells, suggesting an increase in membrane surfaces
to increase the uptake and storage of substances. (3) The number of mi
tochondria increased in the epidermal and in the cortex cells after sa
lt stress thus indicating an additional supply of energy for osmotic a
daptation 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 concen
tration. Electron micrographs show an increase in electron optical con
trast within the cytosol and in the matrix of the mitochondria. These
results are discussed with regard to the possibility of influence on t
he 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 incr
ease in vacuolar NaCl concentrations of epidermis and cortex cells are
in the region 2 cm behind the root tip, which is sufficient for an os
motic balance towards the growth medium. Additional solutes are necess
ary 0.5 mm behind the root tip to facilitate osmotic adaptation. The r
esults 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 cyto
plasmic concentrations of such essential elements as Mg, P, S, and Ca
and is thus responsible directly (via energy supply in mitochondria, h
omeostasis, selectivity of K over Na) or indirectly (via enzyme confor
mation, cytoplasmic hydration) for the ultrastructural degradation ind
icated. The salinity-induced multiplicity of structural and functional
changes within cell compartments constitutes a group of indicators fo
r the limited NaCl tolerance of Sorghum.