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

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Z . Dajic
These structures are common for many halophytic genera such as Cressa
(Convolvulaceae), Frankenia (Frankeniaceae), Spartina, Chloris, Aeluropus (Poaceae),
Atriplex (Chenopodiaceae), Statice, Limonium, Plumbago, Armeria (Plumbaginaceae),
Glaux (Primulaceae), Tamarix, Reamuria (Tamaricaceae), as well as, some mangrove
species, e.g. Avicennia, Aegialitis, Aegiceras and Acanthus (Waisel, 1972; Crawford,
1989; Popp, 1995).
Glandular structures involved in salt excretion vary in structure, position,
physiological mechanism and related ecological significance. The simplest are the twocelled
(Spartina, Aeluropus) and three-celled types (Chloris gayana), while more the
complex are structures composed of 5-9 cells (Avicennia), 8 cells (Tamarix) and, especially,
16 cells in some representatives of the family Plumbaginaceae (Waisel, 1972;
Crawford, 1989). The following basic types of salt excreting structures might be recognized:
two-celled glands of the grasses, multicellular glands of various dicotyledonous
halophytes and bladder hairs in some Chenopods (Thomson et al., 1988). They all
contain one or more subtending cells that are in apoplastic and symplastic connection
with both the adjacent mesophyll and the distal, secreting gland cell (Jacoby, 1994).
Glandular structures are usually spread over the whole surface area of the
shoot, though they are most abundant on the leaves. Differentiation of the salt- excreting
structures is generally completed before differentiation of the entire leaf, indicating
the importance of salt glands in survival during the early developmental stages of the
plant (Waisel, 1972). There is still a dilemma about the salt glands, whether they functions
to excrete, secrete, or recrete (Freitas and Breckle, 1992; Marcum and Murdoch,
1992). There are a lot of similarities in the metabolic principles of ion transport and
functioning of the proton pumps in salt-excreting structures and cells of other tissues.
Features that control free ion transport, analogous to the Casparian strips of the endodermis,
have also been found in salt glands (Breckle et al., 1990). The activity of salt
glands may be induced by an increase of external salinity, which has been shown by a
30% increase in the activity of H + -ATPase isolated from salt glands (Hill and Hill, 1973).
Dschida et al. (1992) identified an energy dependent process in salt excretion, achieved
by plasma membrane H + -ATPase in Avicennia germinans. The chemical composition of
the secretion present in salt glands (inorganic ions and organic compounds, such as
sugars, amino acids and amines) indicates the possibility of leakage from the protoplasm
caused by a disturbed structure of membrane systems of the cell during the
process of excretion (Ziegler and Lüttge, 1967).
Besides the active pumping of ions into the salt glands from the palisade and
water parenchyma cells, it seems that simultaneous vesicular transport of ions leading
to excretion into the extracellular space and direct salt secretion into the subcuticular
area are operating, as shown in Avicennia marina (Ish-Shalom Gordon and Dubinsky,
1990). Although the apoplastic pathway in solute transport to the gland cell prevails,
symplastic transport, documented through the presence of Cl - in the plasmodesmata of
gland cells of Limonium (Ziegler and Lüttge, 1967) also operates. Additionally, the
significance of vesicle-mediated solute transport in salt excretion has been suggested