Physiology and Molecular Biology of Stress ... - KHAM PHA MOI

Salt Stress
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Reduced plant growth under saline conditions is at least partly due to high
concentrations of salt building up in the apoplast of growing tissues. Nevertheless,
high ion concentrations in the apoplast may lead to cell and tissue dehydration, as per
Oertli hypothesis (Oertli, 1968). High accumulation of ions in the apoplast has been
reported for salt-sensitive plants, such as rice (Flowers et al., 1991). Under salt treatment,
apoplastic ion concentrations of more tolerant spinach were much lower than in
less tolerant pea (Speer and Kaiser, 1991). However, there was no evidence for high
accumulation of sodium in the leaf apoplast of maize and cotton exposed to salinity
conditions (Mühling and Läuchli, 2002a), suggesting the existence of specific adaptive
responses in different species.
Munns (1993) has suggested that plant growth under salinity is inhibited
through two phases. Initially (phase 1), growth is affected because of cellular responses
to the osmotic effects. In the subsequent phase (phase 2), growth is reduced due to the
toxic effects of accumulated salts.
Time-dependent changes of growth and development of plants exposed to
salinity stress have been reviewed (Munns, 2002). In the first few seconds or minutes,
cells lose water and shrink, whereas over hours cells regain their volume, but the expansion
rates are limited. Over days and weeks, reduced cell elongation and cell division
result in slower leaf appearance and inhibition of shoot growth. Certainly, the ability to
withstand salinity stress over a longer period of time would be dependent on complex
mechanisms of stress tolerance, especially those, which prevent salt reaching toxic
levels in photosynthetic tissues. Thus, the relative rates of appearance of new leaves
and the death of old leaves might be crucial for plants to enter their reproductive period
(Munns, 2002).
Plant growth is direct result of intensive division and expansion of meristematic
cells. The primary response to salinization is associated with the rate of Na + transport
to the shoot apical meristem and other processes in the plant might be affected
before an increase in sodium concentrations within the growing tissue, particularly
sensitive to salinity (Lazof and Bernstein, 1999). Leaf elongation rate was shown to
decline rapidly under salinity conditions, with inhibition of cell extension exerted by
changes in the yield threshold of the cell and not by turgor (Cramer, 1992). Leaf emergence
rate has also been reported to be very sensitive to salinity even in salt-tolerant
species, such as Atriplex amnicola, where the number of emerging leaves decreased
continuously as salinity level increased (Aslam et al., 1986). Complex physiological
changes such as cell wall extensibility and osmotic adjustment are involved in the early
inhibition of growth in expanding plant tissues exposed to osmotic stress (Neumann,
1997).
Alterations in nutritional status under salinity conditions, on the basis of the
concentrations of Na + and K + in growing tissues, disturbed calcium, and the status of
other nutrients in young tissues, as well as, the comparative effects between young
and mature tissues, have been reviewed by Lazof and Bernstein (1999).