cordingly, Becerril et al. [51] reported that the inhibition was partly restored by photosystem II–specific
electron donors such as hydroxylamine and MnCl 2 but not by diphenylcarbazide.
A site at the level of the water-oxidizing complex was confirmed using photosystem II submembrane
fractions isolated from spinach. It was shown that Ca^2 and Cl, which are essential cofactors for oxy-
gen evolution, could protect against lead inhibition, indicating that the metal competes for binding near
the calcium and chloride binding sites in the water-oxidizing complex [109]. In those preparations,
diphenylcarbazide was able to restore the inhibition presumably because of the loss of the Mn cluster of
the oxygen-evolving complex as this electron donor was reported to be efficient only when the water-ox-
idizing complex is depleted of its Mn [109]. Another argument in favor of an inhibitory site at the oxy-
gen-evolving complex is the loss of the extrinsic polypeptides of 17 and 24 kDa in lead-treated photo-
system II submembrane fractions, a loss that can be prevented by added Ca^2 [72]. A study using a
combination of fluorescence and photoacoustic spectroscopy in isolated photosystem II submembrane
fractions indicated that this metal may affect only the donor side of the photosystem with no effect on the
acceptor side [19]. Lead was also shown to form metal-protein complexes by association with CBO and
CMN groups of amino acids in photosystem II submembrane fractions [65]. These bindings may be in-
volved in the inhibitory mode of action.
F. Nickel
Nickel is readily absorbed by plant roots and translocated to the leaves, where it clearly affects electron
transport [97,110,111]. A significant reduction of grana structures has been observed in nickel-treated
cabbage plants [112,113]. The pigment content was also shown to be reduced following exposure to this
metal in various photosynthetic organisms, also depending on growth stage [97,114–116]. A possible
mode of action would be the peroxidation of membrane lipids due to the induction of free radical reac-
tions by nickel [112]. As with other metals, nickel was reported to affect both photosystems. It induced a
decrease of variable chlorophyll fluorescence and an inhibition of the reduction of photosystem II artifi-
cial electron acceptors [15], indicating an inhibitory site on the donor side of photosystem II. More bio-
chemical studies will be appropriate to clarify further the mode(s) and site(s) of action of this metal.
III. CONCLUSIONS
When toxic metal cations reach the photosynthetic apparatus, they clearly affect photosynthetic elec-
tron transport at the level of photosystems I and II. Except for mercury, whose inhibitory action at the
level of photosystem I is well documented to be significant, the metals mostly affect photosystem II.
Multiple inhibitory active sites were reported for most of the metals. However, a general consensus has
been reached indicating that they mainly affect the oxygen-evolving complex with the loss of all or part
of the manganese cluster together with some of the extrinsic polypeptides associated with the water ox-
idation mechanism. Their mode of action probably includes binding or modification of some membrane
protein groups such as SH groups. Their presence in the environment makes them part of the several
stresses imposed on plants. It can thus be expected that their action involves an enhancement of pho-
toinhibition together with an increased turnover of the D1 protein, which are now though to be part of
a more general protective response against environmental stresses [117,118]. Further research will
probably confirm this hypothesis.
REFERENCES
- AJM Baker. Accumulators and excluders: strategies in the response of plants to heavy metals. J Plant Nutr
3:643–654, 1981. - A Trebst. Inhibitors in electron flow: tools for the functional and structural localization of carriers and energy
conservation sites. Methods Enzymol 69:675–715, 1980. - ES Holdsworth, JH Arshad. A manganese-copper-pigment-protein complex isolated from the photosystem II
ofPhaeodactylum tricornutum. Arch Biochem Biophy 183:361–373, 1977. - JB Arellano, M Baron, A Chueca, M Lachica. Determination of copper in different chloroplast preparations.
Plant Soil 154:7–11, 1993. - M Baron, JB Arellano, W Schröder, M Lachica, A Chueca. Copper binding sites associated with photosystem
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768 CARPENTIER