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

Salt Stress
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need to rely on sodium recirculation to exclude sodium salts from the shoot. Export of
ions from shoot to root via the phloem has been demonstrated in beans (Greenway and
Munns, 1980), Trifolium alexandrinum (Winter, 1982), maize (Lohaus et al., 2000), barley
(Munns et al., 1986), cotton (Gouia et al., 1994) white lupin (Munns et al., 1988) and
Lycopersicon pennellii (Perez-Alfocea et al., 2000). Phloem export may remove at least
25% of the total Na + intake by the leaf, especially when growth decreases and demands
for ions diminish (Munns et al., 1983).
One possibility for Na + retranslocation is the coupling to inverse H + -gradients
created by H + -ATPases, where Na + /H + antiporters utilize these gradients by exchanging
external H + for internal Na + , i.e., secondary energized Na + export (Gimmler, 2000).
These properties suggest that Na + , which enters the symplast, probably by passive
diffusion from external solution of high concentration, is relatively effectively pumped
from the symplast, either into the external solution or into the stele (Osmond et al.,
1980). Thus, the efflux of sodium ions out of root cells might be associated mainly with
the activity of Na + /H + antiporters (Blumwald et al., 2000).
Besides salt exclusion in the roots, low NaCl levels in leaves can also be
achieved by salt retention in the lower plant parts and also through abscission of old
leaves once they accumulate large quantities of salts (Flowers and Yeo, 1992; Munns,
1993). In beans, ions accumulate in the root or in the basal part of the shoot, from where
they are returned to the root system and excreted back into the medium (Jacoby, 1979).
The mechanism of intra-plant allocation is also characteristic for many halophytes,
which, due to the limited transpiration, can keep excess of salts within their roots and
lower parts of the shoot, thus preventing ion accumulation in the photosynthetic tissues
(e.g. Waisel, 1972; Dajic, 1996).
The significance of ion leaching from the leaf apoplast, through the cuticle, by
rain, fog or dew (Tukey, 1970) is still unclear (Pennewiss et al., 1997). Control of the ion
uptake is certainly achieved through restriction of the transpiration. Plants transpire 30-
70 times more water than they use for cell growth, which means that solutes will be in
the same degree concentrated by the roots of non-excluder species (Munns, 2002). It
was postulated that partial stomatal closure observed in Aster tripolium under high
salinity is induced by the presence of sodium ions in the apoplast surrounding the
guard cells (Kerstiens et al., 2002), causing a reduction in rates of transpiration and
increase of water use efficiency.
4.4. Salt Excretion
Salt excretion is also a very efficient way of preventing excessive concentrations of
salts building up in photosynthetic tissues. This mechanism is typical for species that
have developed special features, mostly localized at the leaf epidermis, known as salt
glands and salt hairs (bladders). One of the most obvious signs of salt excretion is the
salt crust on leaves and shoots of those species with salt glands or salt hairs (Popp,
1995).