Biology Reference
In-Depth Information
suppression of the de novo synthesis of proteins and, in particular, of the D1 protein
that are required for the repair of PSII (Nishiyama et al.
2006
; Murata et al.
2007
).
In addition, H
2
O
2
can inhibit CO
2
fixation by 50% because of the oxidation of the
thiolmodulated enzymes of the Calvin cycle even at low concentrations (e.g.,
10 mM) (Foyer and Shigeoka
2011
). A charge recombination between the PSII
primary electron acceptor in its semiquinone state and the oxidized chlorophyll
donor lead to accumulation of excited triple chlorophylls (Eberhard et al.
2008
).
These triplet chlorophyll molecules can react with oxygen and generates harmful
singlet oxygen, causing damage to PSII and other cell constituents. At the photo-
system I acceptor side the redox potential under strong supersaturating light is high
enough to reduce molecular oxygen to form O
2
-superoxide radicals. Quenching of
these radicals by carotenoids and ascorbate can protect the photosynthetic apparatus
against such damage.
Nultsch et al. (
1987
) showed that, depending on the duration and the fluence
rate of the excessive irradiation, a decrease in both the photosynthetic efficiency
and capacity in the brown alga
Dictyota dichotoma
was caused by photoinhibition.
Recovery in a subsequent dim light period was very fast so that in this species the
photosynthetic efficiency recovered from inhibition of 80% already within 1 h.
Moreover, the action spectrum of photoinhibition revealed that the photoinhibitory
radiation was absorbed by all photosynthetic pigments, and that photosystem II is
the main site of the photoinhibitory process. According to the definition by Franklin
et al. (
2003
) this species shows a high potential for photoprotection.
Similiar results were presented with the red alga
Polyneura hilliae
(Nultsch et al.
1990
). However, in contrast to
Dictyota
, the red algal species recovered very slowly
after strong photoinhibition. In this red alga only a partial recovery was observed
in complete darkness, and the recovery phase is clearly biphasic. A first fast phase
occurs already in darkness, whereas the subsequent slow phase requires dim
light. The potential for photoprotection in the red algal species was low compared
to
Dictyota
. The above described kinetics indicates that apparently two different
molecular mechanisms are involved in photoinhibition. The discrimination of
dynamic and chronic photoinhibition recommended by Osmond (
1994
) is inge-
nious, according to photoprotection and photoinactivation. Studies of the kinetics
revealed that especially during a fast light acclimation the reaction can be described
combining two different processes of a slow fraction (
P
slow
) with a slow rate
constant (
k
slow
) and a fast fraction (
P
fast
) with a fast rate constant (
k
fast
) at a given
time (
t
).
1. The phase of inhibition is described by:
y
Inh
¼
e
ðk
fast
t
e
ðk
slow
t
Þ
Þ
P
fast
þ
P
slow
at which
F
v
/
F
m
¼
P
fast
þ
P
slow
at
t
¼
0
2. The phase of recovery is described by:
y
Rec
¼
e
ðk
fast
tÞ
þ
e
ðk
slow
tÞ
Þ
F
v
=
F
m
P
fast
P
slow
