Environmental Engineering Reference
In-Depth Information
in the decrease of PSII-mediated electron transfer rate, compared with exoge-
nously applied H
2
O
2
and
•
OH stresses (Pandey and Yeo
2008
). Strong illumina-
tion of thylakoid membranes in the absence of an acceptor can results in oxygen
accepting electrons and subsequently producing reactive oxygen species, ROS
(Pandey and Yeo
2008
).
The photoproduction rate of ROS is largely enhanced under conditions
where photon intensity is in excess of that required for the CO
2
assimilation
(Asada
2006
). It has been shown that the quantum yield of PSII is increased
more rapidly than CO
2
assimilation in 20 % O
2
, which can result from the elec-
tron flux through the water-water cycle (Makino et al.
2002
). This flux can
reach a maximum just after illumination, and can rapidly produce non-pho-
toinduced quenching. With increasing CO
2
assimilation, the electron flux of
water-water cycle and the non-photoinduced quenching is decreased (Makino
et al.
2002
). The cyclic electron flow around PSI can produce non-photoinduced
quenching, which remains at elevated levels upon switching to low oxygen
(2 % O
2
) (Makino et al.
2002
). The water-water cycle is thus believed to dis-
sipate the energy of excess photons (Asada
1999
,
2000
,
2006
; Foyer and Noctor
2000
; Osmond
1997
; Osmond and Grace
1995
; ). Such a cycle is defined as the
process of the electron flow from water in PSII to water in PSI (Asada
1999
).
In addition, H
2
O
2
and ROS can directly be produced by excited PSII under pho-
toinhibitory conditions that trigger the turnover of the D1 protein (see also ear-
lier sections) (Aro et al.
1993
; Prasil et al.
1992
; Bradley et al.
1991
). ROS can
influence the outcome of photodamage, primarily via inhibition of translation
of the
psbA
gene, which encodes the precursor of the D1 protein (Nishiyama et
al.
2001
). The rate of photo-damage is proportional to irradiance (Pandey and
Yeo
2008
).
The mechanism behind the high irradiance (or heat stress or high tempera-
ture or drought) effect on higher plant is the similar to that explained before for
cyanobacteria or phytoplankton in aqueous media. However, in higher plants
the reaction centers of PSI and PSII in chloroplast thylakoids are the major ROS
generation site. Photoreduction of O
2
to H
2
O
2
occurs in PSI (Mehler
1951
): the
primary reduced species is the superoxide radical anion
,
and its dispropor-
tionation produces H
2
O
2
and O
2
(Asada et al.
1974
). Correspondingly, ground (tri-
plet) state oxygen
3
O
2
O
2
•−
by the triplet state
of chlorophyll (Hideg et al.
1998
; Telfer et al.
1994
). The mechanism behind the
photoreduction of O
2
in PSI of higher plants according to Asada (Asada
2006
) and
other studies (Lobanov et al.
2008
; Parmon
1985
; Bruskov and Masalimov
2002
)
can be expressed as follows (Eqs.
5.4
-
5.11
):
in PSII is excited to singlet state
1
O
2
(5.4)
P680 or P700
+
h
υ →
e
−
+
P680
+
or P700
+
O
2
(
aq
)
+
e
−
+
h
υ →
O
2
•−
(
PSI
)
(5.5)
2O
2
•−
+
2H
+
→
H
2
O
2
+
O
2
(5.6)
H
2
O
2
+
2AsA
→
2H
2
O
+
2MDA
(5.7)
