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

44 Z . Dajic
lution have enabled more active regulation of uptake, transport, accumulation and
utilization of ions in order to maintain the water balance and ion homeostasis.
There is no distinct classification of halophytes. Different authors (e.g.
Chapman, 1960; Strogonov, 1973; Waisel, 1972) have tried to distinguish some specific
groups of salt-tolerant species, such as, oligo-, meso- and euhalophytes, obligate and
facultative halophytes, succulent, salt-excreting, non-succulent halophytes, etc. Each
categorization of plant tolerance to salt stress is based upon particular criteria, but
global differences in responses to salinity are mainly dependent on adaptive strategies
gathered during evolution and natural selection. Salt tolerance relies upon mechanisms
at all-organizational levels of the plant. Although numerous similar cellular mechanisms
are present in both halophytes and non-halophytes for growth in saline conditions
(Binzel et al., 1989), the halophytic species seem to be more efficient in utilization of
different salt tolerance mechanisms. Halophytes rely predominately on osmotic adjustment
through utilization of ions as osmolytes with active sequestration into the vacuoles
(e.g. Flowers et al., 1977; Yeo, 1998; Glenn et al., 1999). For example, in the halophytes
Salsola soda L., Suaeda maritima (L.) Dum., and Camphorosma annua Pall.,
contribution of ions (Na + , Cl - , K + and SO 4
2-
) to the osmotic potential ranged from 68.7%
to 80.38% according to survey carried out in the conditions of their natural habitat
(Dajic et al., 1998).
3. MAIN ADAPTATIONS OF PLANTS TO SALT STRESS
Adaptive strategies of plants exposed to salinity are based upon the utilization of one
or more of the following major mechanisms (Levitt, 1972; Munns et al., 1983; Fitter and
Hay, 1989; Niu et al., 1995):
(1) Phenological avoidance (related to plants which complete their cycle of
growth and development in the most favorable period of the vegetation
season);
(2) Salt avoidance through salt exclusion, which can be achieved by low
root permeability for certain ions, especially sodium;
(3) Salt avoidance through secretion, which is dependent on the presence
of special salt glands and bladders;
(4) Dilution of high salt concentration in plant tissues by succulence and
growth, which is, among other things, related to the flexibility of cell
walls;
(5) Active accumulation and compartmentation of salts into the vacuoles;
(6) Biochemical tolerance through adaptations of cell organelles and macromolecular
systems to excess of salt;
(7) Nutritive tolerance (the capacity for metabolic utilization of potassium
and calcium ions in order to mitigate the adverse effects of sodium ions)