Abstract
Depending on the method of NaCl application, whether gradual or in a single step, plants may experience either salt
stress or salt shock, respectively. The first phase of salt stress is osmotic stress. However, in the event of salt shock,
plants suffer osmotic shock, leading to cell plasmolysis and leakage of osmolytes, phenomena that do not occur with
osmotic stress. Patterns of gene expression are different in response to salt stress and salt shock. Salt stress initiates
relatively smooth changes in gene expression in response to osmotic stress and a more pronounced change in
expression of significant numbers of genes related to the ionic phase of salt stress. There is a considerable time delay
between changes in expression of genes related to the osmotic and ionic phases of salt stress. In contrast, osmotic
shock results in strong, rapid changes in the expression of genes with osmotic function, and fewer changes in ionic-
responsive genes that occur earlier. There are very few studies in which the effects of salt stress and salt shock are
described in parallel experiments. However, the patterns of changes in gene expression observed in these studies
are consistently as described above, despite the use of diverse plant species. It is concluded that gene expression
profiles are very different depending the method of salt application. Imposition of salt stress by gradual exposure to
NaCl rather than salt shock with a single application of a high concentration of NaCl is recommended for genetic and
molecular studies, because this more closely reflects natural incidences of salinity.