Environmental Engineering Reference
In-Depth Information
with their PSII). This complexation can decrease photosynthesis, either by induc-
ing 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 dis-
cussed in an earlier section (see the “effect of trace metal ions” section). In addi-
tion, 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 pro-
duce O
2
•
−
•
that can subsequently either decompose the proteins
and the functional groups bound to microorganisms, decreasing their photosyn-
thetic 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 inhibi-
tion 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
, H
2
O
2
and HO
•
or other oxi-
dants, generated photolytically by the above mechanism, in analogy with the well-
known phenomena concerning DOM photobleaching in natural waters (see chapters
“
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 nutri-
tion, 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
