Biology Reference
In-Depth Information
increase and the ion ratios remain constant. In contrast, during salinity stress algal
cells may not only increase ionic concentrations but also undergo changes in ion
ratios owing to selective uptake. This has to be taken into account when comparing
the results obtained with species under salt or desiccation stress.
The primary photosynthetic mechanism is affected at the electron transport stage
between PS I and PS II. The sensitive site in
Porphyra
and
Ulva
species is most
likely between plastoquinone and P 700 (Wiltens et al.
1978
). In
Porphyra
perforata
there seem to be at least three sites in the photosynthetic apparatus that
are inhibited by high salinity, namely the photoactivation of electron flow on the
reducing side of PS I, the electron flow on the water side of PS II and the transfer of
light energy between the pigment complexes (Satoh et al.
1983
). These authors
concluded that a free electron flow at all three sites is essential to avoid
photodamage through chronic photoinhibition, which will occur if only one site is
blocked because of, e.g., the accumulation of highly reactive oxygen species(ROS;
see also Chap.
6
by Bischof and Rautenberger).
More recent data on cyanobacteria indicate a salt-induced inactivation of both
oxygen evolution in PSII and electron transport in PSI (Allakhverdiev and Murata
2008
). The site of inactivation is the electron-donating side of PSII, i.e., the oxygen-
evolving machinery, due to the influx of uncontrolled Na
รพ
and Cl
ions with
resultant dissociation of extrinsic proteins from photosystems (Allakhverdiev and
Murata
2008
).
5.3 Processes of Osmotic Acclimation
Hypersaline conditions in the external medium affect seaweeds in two ways: first,
the water potential is strongly reduced leading to dehydration of cells, and secondly,
high concentrations of some inorganic ions exert toxic effects on cellular metabo-
lism. Similarly, desiccation stress will also result in strong dehydration of cells.
Since both hypersalinity and desiccation affect the internal osmotic potential, the
acclimation responses of seaweeds are comparable.
Water is taken up in all living cells by osmosis, which is driven by the water
potential gradient, i.e., only if the intracellular water potential is lower than in the
external medium there is a water influx. Consequently, marine seaweeds have to
create an internal osmotic potential higher than that of seawater to gain and retain
constant water content of the cells, which is necessary to maintain turgor as the
driving force for growth. Seaweeds typically respond to external salinity changes
with osmotic acclimation processes which involve the control of cytoplasmic and
vacuolar concentrations of osmotically active compounds (Kirst
1990
). Therefore,
osmotic acclimation is a fundamental mechanism of salinity tolerance of these
plants that conserves the stability of the intracellular milieu (homeostasis), and it is
essential for maintaining an efficient functional state in the cells (Kirst
1990
). Most
seaweeds use inorganic ions and small organic osmolytes to create the internal
osmotic potential.
