628
K. M. G. Mostofa et al.
with their PSII). This complexation can decrease photosynthesis, either by inducing high photoinduced formation of HO
• from H 2 O 2 , which would damage PSII,
or by blocking the normal function of electron release by the PSII functional
groups. Either effect could alter the normal function of PSII, as extensively discussed in an earlier section (see the “effect of trace metal ions” section). In addition, complexes of trace metal ions in marine waters with autochthonous DOM of
algal/phytoplankton origin and with terrestrial DOM of vascular plant origin can
induce rapid photoinduced excitation of electrons (e − ). Such a process can produce O 2
•−
, H 2 O 2 and HO
• that can subsequently either decompose the proteins
and the functional groups bound to microorganisms, decreasing their photosynthetic efficiency, or transform the DOM components with production of a number
of photoproducts such as CO 2 , DIC, H 2 O 2 and LMW DOM. These products are
directly linked with an enhancement of photosynthesis and might account for algal
blooms, particularly in coastal marine environments.
This mechanism is supported by earlier studies, showing that the inhibition of electron flow on the oxidizing (water) side of PSII causes photoinhibition.
Moreover, photoactivation and dark-inactivation of electron flow on the reducing
side of PSI is completely inhibited at high salinity (Satoh et al. 1983). It is known
that photobleaching of carotenoids and Chl can take place when the oxidizing side
of PSII is inhibited (Yamashita et al. 1969; Katoh 1972). It has also been observed
that inhibition on the reducing side of PSI can give rise to strong reductants, which
can also destroy the reaction centers of both PSI and PSII (Satoh and Fork 1982a,
b). Photobleaching of carotenoids and Chl might be caused by HO
• or other oxidants, generated photolytically by the above mechanism, in analogy with the wellknown phenomena concerning DOM photobleaching in natural waters (see chapters
“Photoinduced Generation of Hydroxyl Radical in Natural Waters”, “Photoinduced
and Microbial Degradation of Dissolved Organic Matter in Natural Waters”,
“Colored and Chromophoric Dissolved Organic Matter in Natural Waters”, and
“Fluorescent Dissolved Organic Matter in Natural Waters” for detailed discussion).
Effects of Salinity on Higher Plants
Salinity of soil or water is one of the key environmental factors that limit plant
growth and productivity, particularly in arid, semi-arid and freshwater land near
coastal regions. Salinity can have a two-fold effect on plants: (i) osmotic stress,
by which salt in the soil can reduce the availability of water to the roots, and
(ii) ionic stress due to the salt taken up by the plant that can be accumulated to
toxic levels in certain tissues (Munns et al. 1995). Reduction of photosynthesis
caused by salt stress has an impact on several physiological responses, such as
inhibition of growth and development, modification of ion balance, mineral nutrition, water status, stomatal behavior, decrease in photosynthetic efficiency and in
chlorophyll content (which leads to a corresponding reduction of light absorption
by leaves), decrease of carotenoids, carbon allocation and utilization, net carbon
dioxide exchange, respiration and protein synthesis, and finally, induction of cell
K. M. G. Mostofa et al.
with their PSII). This complexation can decrease photosynthesis, either by inducing high photoinduced formation of HO
• from H 2 O 2 , which would damage PSII,
or by blocking the normal function of electron release by the PSII functional
groups. Either effect could alter the normal function of PSII, as extensively discussed in an earlier section (see the “effect of trace metal ions” section). In addition, complexes of trace metal ions in marine waters with autochthonous DOM of
algal/phytoplankton origin and with terrestrial DOM of vascular plant origin can
induce rapid photoinduced excitation of electrons (e − ). Such a process can produce O 2
•−
, H 2 O 2 and HO
• that can subsequently either decompose the proteins
and the functional groups bound to microorganisms, decreasing their photosynthetic efficiency, or transform the DOM components with production of a number
of photoproducts such as CO 2 , DIC, H 2 O 2 and LMW DOM. These products are
directly linked with an enhancement of photosynthesis and might account for algal
blooms, particularly in coastal marine environments.
This mechanism is supported by earlier studies, showing that the inhibition of electron flow on the oxidizing (water) side of PSII causes photoinhibition.
Moreover, photoactivation and dark-inactivation of electron flow on the reducing
side of PSI is completely inhibited at high salinity (Satoh et al. 1983). It is known
that photobleaching of carotenoids and Chl can take place when the oxidizing side
of PSII is inhibited (Yamashita et al. 1969; Katoh 1972). It has also been observed
that inhibition on the reducing side of PSI can give rise to strong reductants, which
can also destroy the reaction centers of both PSI and PSII (Satoh and Fork 1982a,
b). Photobleaching of carotenoids and Chl might be caused by HO
• or other oxidants, generated photolytically by the above mechanism, in analogy with the wellknown phenomena concerning DOM photobleaching in natural waters (see chapters
“Photoinduced Generation of Hydroxyl Radical in Natural Waters”, “Photoinduced
and Microbial Degradation of Dissolved Organic Matter in Natural Waters”,
“Colored and Chromophoric Dissolved Organic Matter in Natural Waters”, and
“Fluorescent Dissolved Organic Matter in Natural Waters” for detailed discussion).
Effects of Salinity on Higher Plants
Salinity of soil or water is one of the key environmental factors that limit plant
growth and productivity, particularly in arid, semi-arid and freshwater land near
coastal regions. Salinity can have a two-fold effect on plants: (i) osmotic stress,
by which salt in the soil can reduce the availability of water to the roots, and
(ii) ionic stress due to the salt taken up by the plant that can be accumulated to
toxic levels in certain tissues (Munns et al. 1995). Reduction of photosynthesis
caused by salt stress has an impact on several physiological responses, such as
inhibition of growth and development, modification of ion balance, mineral nutrition, water status, stomatal behavior, decrease in photosynthetic efficiency and in
chlorophyll content (which leads to a corresponding reduction of light absorption
by leaves), decrease of carotenoids, carbon allocation and utilization, net carbon
dioxide exchange, respiration and protein synthesis, and finally, induction of cell