III. ADAPTATIONS OF SALINE PLANTS
Not only are halophytes in their saline environments exposed to salt stress, but the root may also be ex-
posed to osmotic water stress and low oxygen pressure stress [36]. They must adjust their tissue water po-
tentials to a level that is lower than that of the soil water potential in the habitat where they are growing;
enabling the plants to absorb water. Without sufficient moisture, halophytes can be stunted, and repro-
duction becomes very limited [37]. Halophytes take up ions to increase the osmotic levels in their tissues,
which permits moisture to move from the soil into the tissues. On the other hand, excess salt ions can pro-
duce a toxic effect to the plant cells. Some of the mechanisms used by halophytes to counter the potential
toxic effect of high concentration of ions involve exclusion of salts by the roots, dilution of the ions
through succulence, synthesis of organic osmotic compounds that can reduce the need for salt ions, and
compartmentalization of the excess salt ions into tissues, organs, or cell vacuoles.
Plants have evolved two very different strategies in adapting to high levels of sodium salts in their
environment. One strategy is to exclude the salts from the interior of the leaf cells, and the other is to in-
clude the salts within the leaf cells but sequester most of them in the cell vacuoles of those cells. In both
cases, the end result is to maintain a relatively low cytoplasmic sodium concentration [38]. These two
broad categories of plants are referred to as salt secreting and salt accumulating. Those that exclude salts
from the leaf cells are able to tolerate high levels of the salts in the root environment but at the expense
of reduced growth. Most of them avoid salinity, some evade it, and a few others tolerate it. Most plants
avoid salinity by limiting reproduction, growth, and germination during specific parts of the year, by lim-
iting the uptake of salt, and by allowing roots to penetrate into nonsaline soils. Evasion of salt has been
achieved through the accumulation of salts into certain specific cells and trichomes or secretion of excess
salts through especially mechanized salt-secreting glands [12,32,39]. Secretion of ions by special salt
glands or bladder hairs, release through the cuticle or in the guttation fluid, and retransportation via the
phloem are examples of these mechanisms.
The exclusion of ions by roots can be a factor in salt tolerance, but some type of osmotic com-
pound needs to be produced in the plant for it continue to absorb water from the saline soil. In dicot
halophytes, root exclusion is not an effective mechanism, although there may be some ion regulation
at the root level [40].
Salt resistance and salt tolerance on the cellular level as well as the formative effects of salinity
producing halosucculent leaves and/or stems also need to be taken into consideration. The genetic back-
ground regulating compartmentalization of solutes and formation of compatible solutes has to be re-
garded in connection with the adaptation on the higher levels of complexity. There are still several
questions open concerning the growth and development of halophytes, e.g., root architecture in saline
habitats and formation of mycorrhizae, hormonal balance and growth regulation, mineral uptake, and
selectivity [41].
Xerosucculents are characterized by a thick cuticle and a cover of waxy layers, such as displayed in
Suaeda fruticosa,Salsola baryosma, and Haloxylon recurvum. Cuticle and waxy layers have also been
reported on the leaf surfaces of Cressa cretica,Aeluropus lagopoides,Sporobolus helvolus, and Chloris
virgata. Some halophytes, such as C. cretica,C. virgata,S. helvolus, and A. lagopoides, show an addi-
tional mode of adaptation to their habitat. The leaves and stems of these plants remain covered with hairs
(trichomes), giving the plant a grayish appearance. Their effectiveness in reducing water loss is small, but
they are able to protect the leaf surface against dust [12].
Another approach for accumulating ions and at the same time preventing them from becoming toxic
to the photosynthetic cells is through the accumulation of excess ions in tissues that eventually die. The
dead tissues containing the excess salts act as a storage region for the salts. In the case of Heliotropium
curassavicum, during the dry conditions, more and more salts are accumulated in their fleshy leaves and
these salt-saturated leaves dry up to keep the osmotic level of the plant balanced. New leaves sprouts, and
this process continues during the whole life span of the plant [24].
Rajput [26] reported that in Atriplexspecies with increased duration of leaching, the amount of ions
leached from the leaves also increased and the maximum value was observed after 72 hr. Maximum val-
ues of leacheable Naand Clwere observed in young leaves of A. halimusandA. nummularia, respec-
tively, during the summer, and a minimum value of Nawas observed in young leaves of A. argentina
during the rainy season, probably because of leaching of the salts from the leaves by rainwater.
BIOLOGY AND PHYSIOLOGY OF SALINE PLANTS 565