Biology Reference
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documented a salt-induced upregulation of various antioxidant enzymes such as
superoxide dismutase, ascorbate peroxidase, glutathione reductase, and catalase,
which efficiently detoxified ROS.
5.3.5 Ultrastructural Changes
In many seaweeds, the separation of the plasmalemma from the cell wall rather than
cell volume changes under hypersaline conditions is the most damaging aspect of
plasmolysis (Reed
1990
). However, in some species such as the red alga
Caloglossa
leprieurii
, the elastic cell walls accommodated marked shrinkage of the protoplasm
when treated with hypersaline media resulting in a concomitant swelling and hence
prevention of plasmolysis (Fig.
5.2
, Mostaert and King
1993
). These authors
suggested that the enormous flexibility of the cell wall thickness may be a protec-
tive mechanism in response to fluctuating salinities.
Besides the adverse ionic influences on particular sites of the photosynthetic
machinery, the cellular ultrastructure may also be affected by salt stress resulting in
reduced photosynthesis and respiration. In
Porphyra umbilicalis
changes in the
thylakoid structure of the chloroplasts after salt treatment have been described
(Wiencke
1982
). Hypersaline conditions typically result in cell shrinkage due to
water loss with thylakoids and membranes appressed, and under hyposaline
conditions cellular organelles are typically swollen. This disturbance of fine struc-
ture may cause a disruption of energy transfer in photosynthetic electron flow
(Satoh et al.
1983
). In contrast to chloroplasts, mitochondrial cristae ultrastructure
is less affected by salinity changes (Kirst
1990
), and this well explains the better
functioning of respiration compared to photosynthesis under the respective stress.
Another ultrastructural observation is the formation of vacuoles after hypo- or
hypersaline stress in seaweeds such as
Porphyra umbilicalis
, which under normal
conditions contain only very few of these organelles (Wiencke and Lauchli
1980
).
The vacuoles formed under hypersaline conditions may serve as compartments to
sequester metabolically toxic ions, mainly Na
þ
and CI
(Wiencke et al.
1983
).
Concomitantly, the fine structure of the tonoplast changes is probably related to the
intensity of ion transport across it (Wiencke and L
auchli
1983
).
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5.3.6 Self Protection
In addition to active processes, which compensate for osmotic stress,
Laminaria
blades form multiple-layered, mat-like canopies at neap tides, protecting the indi-
vidual thalli against desiccation and salinity changes (L
uning
1990
). Macroalgal
canopies of
Ulva
sp. in the upper littoral zone in southern Spain form under emersed
conditions sheet-like, multiple-layered structures in which the top layer usually
bleaches due to strong insolation, desiccation, and other abiotic stresses, thereby
providing photoprotection and moisture for subcanopy thalli (Bischof et al.
2002
).
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