Agriculture Reference
In-Depth Information
This decrease in calcium content may decline water oxidation at the level of the
water splitting system at the oxidising side of photosystem II (PSII) and decrease
the electron flow needed for photosynthesis (Barry et al.
2005
). Calcium is an es-
sential cofactor for O
2
evolutions and has been shown to play a vital role in stabi-
lizing the protein structure that ligates manganese and plays role in H
2
O oxidation
(Vander Meulen et al.
2004
). It is known that the high depletion of calcium content
in PSII reduces proper photochemical function preventing oxygen evolution. PSII
is believed to play a key role in the responses of leaf photosynthesis to environmen-
tal perturbations. Several heavy metals such as copper, cadmium, zinc, chromium,
have been shown to have their primary targets in the PSII complex (Giardi et al.
2001
). Cr(VI) is a strong oxidant with a high redox potential of 1.38 eV. This may
cause serious oxidative damage to the photosynthetic apparatus reflecting the re-
sults obtained in this study with
L. perenne
. In our studies, we have observed that
chromium caused a slight decrease in the maximal photochemical efficiency of PSII
(Fv/Fm) in
L. perenne
plants at 500 µM chromium. On the contrary, a significant
increase in Fo value was observed from a 100 µM chromium stress. The Fo level is
affected by environmental stresses that cause structural alterations in the pigment-
protein complexes of PSII or when the transfer from antennae to reaction centres is
impeded. These changes affect the ability of the photon harvesting assemblages to
trap photons and transfer the energy to the acceptor of PSII (Ouzounidou
1993
). In
contrast to Fo, Fm is achieved when QA or the plastoquinone pool is fully reduced
by electrons. The decline in Fm, observed from 250 µM chromium and 45 days of
exposure would suggest a change in the ultrastructure of the thylakoid membrane
and indicates irreversible or slowly reversible damage to the photosynthesis sys-
tem which is termed ''photoinhibition'', and the target is mostly localized in PSII.
Furthermore, it has been reported that increasing Fo and decreasing Fm observed
in response to chromium stresses, from 250 µM chromium, can be attributed to the
separation of LHC II from the PSII complex, inactivation of PSII reaction centre,
and perturbation of electron flow within the PQ pool (Yamane et al.
1997
). Up
to 100 µM chromium-treated plants' NPQ coefficients values are higher than in
control plants without changes in qP values. This relationship indicates that non-
radiative dissipative mechanisms are involved in dissipation of the excess excita-
tion energy. There is general agreement that photoinhibition is primarily caused by
an inactivation of the electron transport system in thylakoids. The dominant effect
seems to be an alteration of the reaction centers of PSII leading to a decreased pho-
tochemical efficiency. This photoprotection mechanism is enhanced under stress
conditions when an excess of reducing power (NADPH) is generated at the electron
transport chain. The significant changes in PSII photochemistry in the light adapted
leaves, such as the decreased qP and F
0
v = F
0m
, as well as the increased of NPQ,
can be seen as the regulatory response to down-regulate the quantum yield of PSII
electron transport PSII ¼ F
0
v = F
0m
xqPÞ (Genty et al.
1990
), that would match
with the high decrease of CO
2
assimilation rate. It is suggested that the decay in
PSII of the chromium-stressed plants may be a mechanism to down-regulate the
photosynthetic electron transport so that production of ATP and NADPH would be
in equilibrium with the decreased CO
2
assimilation capacity in stressed plants. The
