The impact of sodium chloride on root growth, cell division, and interphase silver-stained nucleolar organizer regions (agnors) in root tip cells of allium cepa l.

1. Introduction
Environmental stresses include high metal concentration in
soil, the presence of air pollutants, high or low temperature, water
deficits, and high soil salinity. Salinity is asignificant stressor that
reduces plant productivity of several horticultural crops. Most of
the salt stresses in nature arise from sodium chloride salts (Levitt,
1972). When plants experience salinitystress, they develop
different coping mechanisms that allow them to tolerate, avoid,
or escape the stressor. These responses have been investigated at
the morphological, anatomical, physiologicaland cellular levels.
Furthermore, environmental stresses also impact cell cycle
regulation by altering the endoreduplication process and changing
cell proliferation (Granier et al., 2007).Cytological approaches are
used for analyzing the changes in the proportion of mitotic phases
and rate of cell division in response to environmental conditions.
Cytological parameters are used to identifythe effect of salt
stresses on cell division in root tips to evaluate these effects.
Nucleolar organizer regions (NORs) contain tandemrepeat genes
called rDNA transcribed into ribosomal RNAsnecessary for all
cellular protein synthesis (Sumner, 2003).NOR argyphilia is due to a
set of nucleolar proteins which are selectively stained by silver
methods leading to silver-stained nucleolarorganizer regions
(AgNORs). After silver staining, NORs can be visualized as black
(Ploton et al., 1986). During interphase, NORs are located in the
regions of thenucleolus knownas thefibrillarcentre,andthese areas
retainnucleolinandproteinB23,neither ofwhichisdirectlyinvolved
in the transcription of ribosomal genes (Sumner, 2003). SilverstainingofNORsisarapidmethodfor
visualizationofribosomalgene
activities to evaluate protein synthesis rate. Previous investigations
have shown that the amount of AgNOR protein is a good marker of
cell proliferation activity (Trere` et al., 1989;...