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

Salt Stress
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(Echeverria, 2000). The efficacy of salt glands has been established by the presence of
low concentrations of salts in leaves and a high K/Na ratio of species possessing such
structures, even in conditions of rapidly increasing external salinity (Bradley and Morris,
1991).
It seems that in salt-secreting plants the mechanism of salt exclusion is operating
as well, since it was demonstrated that in mangrove plants Aegiceras corniculatum
and Avicennia marina Na + excretion does not keep pace with Na + uptake over a range
of salinities, reducing plant growth at high external salinity (Ball, 1988).
Salt bladders, probably best studied in several species of the genus Atriplex
(Chenopodiaceae), are also of particular importance in the regulation of salt tolerance.
They are composed of two cells: the small, basal and the upper, bladder cell (Osmond et
al., 1980). The latter dies, once it has accumulated a sufficient amount of salt in its
vacuole, leaving a salt crust at the leaf surface (Figure 3).
Figure 3. Leaf surface of the salt-excreting halophyte Atriplex tatarica L. var. diffusa Ten.
(x 1000, Dajic, 1996)
Many Atriplex species are characterized by the presence of salt bladders at the
surface of young leaves only (Breckle et al., 1990), and these, in same cases, may
contain almost the whole leaf sodium (Jeschke and Stelter, 1983). It is considered that
the salt bladders of Atriplex represent a key adaptation to salt tolerance for their natural
habitats (Freitas and Breckle, 1992).
Altogether, the prevention of excessive salt accumulation within the plant body is
achieved through the following modes, such as: a) control of the salt uptake at the root
level and regulation of Na + delivery to the shoot by loading of the xylem and retrieval of
ions from the xylem before reaching the shoot), b) K/Na selectivity, c) recirculation of
salts via the phloem, d) allocation of salts within particular parts of the plant, e) ion
leakage and abscission of organs loaded with salts, f) control of transpiration, and g)
secretion of ions by salt-excreting structures.