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

Salt Stress
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tolerance research is to determine those transporters which function in the control of
Na + entry, to find a way to limit Na + influx and thus to improve salinity tolerance (Zhu,
2001).
4.2. Significance of K/Na Selectivity
Uptake and distribution of sodium ions within the root is to a large extent connected
with the effects of potassium, since Na + efflux in root cortex cells is stimulated by K +
influx, which is related to the K/Na root selectivity (Jeschke, 1972). The presence of
potassium (and calcium) ions has been shown to decrease Na + influx into plant cells
(e.g. Lazof and Bernstein, 1999).
In wheat, salt tolerance is related to the enhanced K/Na selectivity (Gorham,
1990) and it has recently been reported that the genus Triticum expresses a range of
genetic variation related to K/Na discrimination. The trait is highly heritable (Munns et
al., 2002). In contrast to dicotyledonous plants, in monocots the maintenance of a lower
Na/K ratio in shoots is of greater significance, because of their lower capacity for
sodium storage and higher requirement for K + and compatible organic solutes (Glenn et
al., 1999).
Time-course measurements of root ion content have indicated the possibility
of a sodium-excluding mechanism in roots of Kosteletzkya virginica through low Na + /
K + ratios brought about not only by exclusion of Na + , but also by a strong K + affinity
which was ascribed to a change of the sterol/phospholipid ratio in salinized roots (Blits
and Gallagher, 1990). A wide range of K + /Na + selectivity and sodium exclusion were
demonstrated for halophytic and non-halophytic Chenopods (Reimann, 1992), with
Na + accumulation being most pronounced in the shoots.
To confer salt stress tolerance, in many species, the achievement of a high K + :
Na + ratio is more important than simply maintaining low concentrations of sodium ions
(Maathuis and Amtmann, 1999). A highly significant correlation between K + and Na + in
leaves and salinity-induced yield reduction was reported for a number of rice genotypes
irrigated with moderately saline water (Asch et al., 2000). The significance of a
high K/Na ratio for maintenance of yield components and fiber characteristic has also
been evaluated in salt-tolerant cotton lines (Ashraf and Ahmad, 2000). The maintenance
of higher K + and Ca 2+ to Na + ratios, especially in young growing and recently
expanded tissues, appeared to be important characteristics of salt tolerance in barley
cultivars (Wei et al., 2003). In a study with four barley cultivars exposed to a range of
salinity concentrations (Figure 1) it was noticed that the highest differences in Na + : K +
ratio, were expressed at 200 mol m -3 NaCl (Dajic, unpublished data). The Na + : K + ratio
seems to be valuable criterion for screening of salt tolerance within salt-sensitive species,
such as maize. It was found that among four maize cultivars differing in drought
tolerance, and tested for their responses to salt stress (Dajic, unpublished data), the
Polj 17, previously characterized for high capacity of ABA accumulation under drought
stress and related tolerance to water deficit (Pekic and Quarrie, 1988) has maintained the
lowest values of Na/K under salinity conditions (Figure 2).