Geoscience Reference
In-Depth Information
act as an alternative electron acceptor; photosynthetic electron
transport may also consume oxygen. Even under normal condi-
tion, up to 5-10% of the photosynthetic electrons that are gen-
erated by PSI may react with molecular oxygen rather than with
NADP
+
. This has important functional consequence for active
chloroplasts. The photo-reduction of oxygen by PSI is called
the Mehler reaction and results in the production of another
toxic, reactive oxygen species known as a super-oxide radical.
To counteract the accumulation of this radical, photosynthetic
organisms have evolved mechanism to protect themselves from
excess light and the potential ravages of O
2
. An effective sys-
tem for the removal of super oxide is the ubiquitous enzyme
superoxide dismutase. SOD is found in several cellular com-
partments including the chloroplast. It is able to scavenge and
inactivate superoxide radicals by forming hydrogen peroxide
and molecular oxygen.
At the molecular level, the negative effect of high-
temperature stress on leaves may be partly a consequence of
the oxidative damage to important molecules as a result of the
imbalance between production of activated O
2
and antioxidant
defences (Foyer et al. 1994). This hypothesis is very plausi-
ble because chloroplasts are a major source of activated O
2
in
plants (Asada et al. 1998), and because antioxidants, which
may play a critical role in preventing oxidative damage, are
greatly affected by environmental stresses (Bowler et al. 1994).
In chloroplasts, the superoxide radical (O
2
•-) is produced by
photo-reduction of O
2
at PSI and PSII, and singlet O
2
is formed
by energy transfer to O
2
from triplet excited-state chlorophyll
(Asada and Takahashi 1987). H
2
O
2
can originate, in turn, from
the spontaneous or enzyme-catalysed dis-mutation of O
2
•-.
Fortunately, in optimal conditions leaves are rich in antioxi-
dant enzymes and metabolites and can cope with activated O
2
,
thus minimising oxidative damage. An increase of the active O
2
forms in plant tissue has been found at high-temperature stress
(Foyer et al. 1997; Dat et al. 1998). High temperatures can
also influence the antioxidant enzymes: superoxide dismutase
(EC 1.15.1.1), the first enzyme in the detoxifying process, con-
verts O
2
•- radicals to H
2
O
2
. In chloroplasts, H
2
O
2
is reduced
by ascorbate peroxidase (EC 1.11.1.11) using ascorbate as an
electron donor. Oxidised ascorbate is then reduced by reac-
tions that are catalysed by monodehydroascorbate reductase
(EC 1.8.5.1) and glutathione reductase (EC 1.6.4.2) in a series
of reactions known as the Halliwell-Asada pathway (Bowler
et al. 1992).
