Chemistry Reference
In-Depth Information
exclude ions from leaves to stems or roots [20, 38]. Some reports [45] described the
avoidance mechanism in the mangroves (
Avicennia marina
), by retaining low internal
salinity in stems and leaves by means of salt excluding mechanisms in the roots. Leaves
of this plant are grey and hairy at the lower surface where most secretion occurs, while
the above surface is shiny green, glabrous with some salt glands. These salt glands play
significant role in the internal balance and regulation of ions [9, 46]. In glycophytes,
there are two examples from Mexican wheat plants, in which exclusion of Cl
-
ions to the
sheaths can be considered as a regulatory mechanism in these plants to avoid Cl
-
accumulation in the leaves. Also, salt sensitive cultivars, like Yecora, failed to exclude
Na
+
to the root systems as compared to salt resistant cultivars, like Cajeme, [22].
Moreover, salt exclusion may occur at the intracellular levels, and salt resistance can be
attributed to the maintenance of ions homeostasis in the cytoplasm [47].
ii.
Salt extrusion: it is an active process to excrete extra salts from the epidermal hairs or
salt glands of leaves of some halophytes such as
Limonium axillare
and
Atriplex
spp.
Some specialized structures called salt glands or salt bladders are found in leaves of
those plants to regulate the extra salts inside the plant body [43]. Salt glands found in
Limonium axillare
are composed of (4-10) cells which are entirely covered by cuticle
which seals the gland from the rest of the plant except for small gaps on the leaf
mesophyll side, salts are deposited on the leaf surface through holes on these glands,
and in mangroves (
Avicennia marina
), salts are excreted from the upper surface of leaves
[46], and the excreted solutions exceed the NaCl concentration of seawater of about 10
times [38, 45, 48]. Salt Bladders are found in
Atriplex
, which are comprised of two cells:
stalk cell and bladder cell [20, 49]. Salt glands are found also in many other halophytes
like
Aeluropus lagopoides
,
Tamarix
spp. and may be in other plant species [9, 50, 51]. These
salt glands could regulate the secretion of salts from leaves to keep their concentrations at
low levels and ultimately to maintain ion homeostasis in the plant body.
iii.
Salt dilution: some halophytes can dilute the accumulated ions in plant tissues to keep
cytoplasmic salinity below toxic levels. Succulence is feature of the whole plant body or
may be confined to stems and / or leaves. These plants are considered as salt includers.
Some succulent halophytes living in Qatar include
Halopeplis
(Fig. 6)
, Suaeda
(Fig. 7) and
Tetraena
(Fig. 3). Halophytes having dilution mechanism are characterized by: (1)
thickening in leaves, (2) elongation of cells, (3) higher elasticity of cell wall, (4) smaller
relative surface areas, (5) decrease in extensive growth, and (6) high water content per
unit of surface areas [38, 52]. In fact, most halophytes accumulate inorganic ions that are
found in abundance in the environment like Na
+
and Cl
-
, and such accumulation is
accompanied with a decline in K
+
content [41]. In addition to the ion accumulation and
succulence phenomenon, shedding of old salt-saturated leaves is found in plants having
dilution mechanism to avoid the damage caused by extra salt accumulation [53].
(b) Tolerance mechanisms
Osmoregulation or osmotic adjustment has been considered as a main secondary
mechanism to tolerate salt stress.
Osmotic adjustment can be defined as maintenance of
positive water balance between soil environment and plant tissues, by lowering their plant
