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

Salt Stress
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activities (mainly irrigation), as well as the strong negative effects of salinity on agriculture,
saline and alkali soils have been broadly investigated from various aspects, including
their genesis, physical and chemical properties, fertility, management and utilization
(Kamphorst and Bolt, 1976; Bresler et al., 1982; Szabolcs, 1989; Pessarakli and
Szabolcs, 1994).
Natural or primary salinity results from the accumulation of soluble salts in
soils or groundwater over long geological periods, mainly by weathering of parent
minerals, releasing salts of various types, such as chlorides, sulfates, carbonates and
bicarbonates of sodium, magnesium and calcium (Richards, 1954). Besides the naturally-formed
saline and sodic soils, the occurrence of so-called secondary salt-affected
soils is becoming ever more visible, due to application of different agricultural practices,
mainly irrigation.
However, there are human influences, other than irrigation, that lead to adverse
effects of secondary salinization, such as: overgrazing, deforestation in semi
humid and semiarid areas, contamination with chemicals and accumulation of airborne
or waterborne salts (Pessarakli and Szabolcs, 1994).
2. HALOPHYTES VERSUS GLYCOPHYTES
Based on general tolerance to salt stress, all plants can be roughly divided into two
major groups: a) halophytes, that can withstand even 20% of salts in the soil and, in
most cases, successfully grow in conditions with 2-6% of salts (Strogonov, 1964), and
b) non-halophytes or glycophytes, plants that exhibit various degrees of damage and
limited growth in the presence of sodium salts, usually higher than 0.01%. However,
there are great differences in the level of salt stress tolerance within both the halophytes
(Waisel, 1972; Flowers et al., 1977; Munns et al., 1983; Ungar, 1991) and nonhalophytes
(Greenway and Moons, 1980), which include sensitive, moderately tolerant
and very tolerant species. Although halophytes represent only 2% of the terrestrial
plant species, they are present in about half the higher plant families and exhibit a great
diversity of plant forms (Glenn et al., 1999).
Many groups of plants are considered as sensitive (e.g. conifers, ferns,
Orchidaceae, Araceae, Rosaceae, Ericaceae and molds), while particular families comprise
tolerant genera and species, such as Potamogetonaceae, Plumbaginaceae,
Zygophyllaceae, Frankeniaceae, Tamaricaceae, Rhizophoraceae, etc. (Waisel, 1972). It
is interesting that the widespread Chenopods may be designated as halophytes “per
excellence” (Flowers and Yeo, 1988), as half of the total genera successfully grows in
conditions of salinity (Atriplex, Suaeda, Salsola, Camphorosma, Salicornia, etc.).
Regarding the origin of halophytes there are two hypotheses, such species
either migrated form coastal habitats to the inland (Chapman, 1960), or spread from
inland steppes (e.g. Statice ssp.) to the coastal saline regions (van der Pijl, 1969). In any
case, the wide distribution of halophytes throughout the world indicates their polyphyletic
origin (Flowers et al., 1977), while distinct changes in the genome during evo-