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which arises from the S
2
state of the water-oxidizing complex is lost much faster than
the EPR signal which arises from the interaction of Q
A
-
with the non-heme Fe
2+
or from
Tyr-D
+
. Thus, the immediate cause for the loss of oxygen evolution is the inactivation of
electron transport between the catalytic Mn cluster and the tyrosine electron donors.
However, the loss of Q
A
function points to additional UV-A induced alteration of PSII
acceptor side components. This observation is in agreement with flash-induced
thermoluminescence and chlorophyll fluorescence measurements which showed that the
Q
B
-binding pocket on the acceptor side is also modified
31
. Comparison of the
characteristics of PSII damage induced by UV-A and UV-B radiation, shows that the
two spectral ranges inhibit PSII by very similar or identical mechanisms, which target
primarily the water-oxidizing complex. Although the damaging efficiency of UV-A is
much smaller than that of UV-B, due to its higher intensity the UV-A component of
sunlight appears to have the same overall potential to inactivate the light reactions of
photosynthesis as UV-B.
The comparison of action spectra with the absorption of putative UV targets is
often useful approach in identifying the critical target sites. The data shown in Figure 6
demonstrate that absorption by Mn (III) and Mn (IV) ions matches well the action
spectrum of UV-induced inactivation of PSII in the whole UV-A and UV-B range, in
contrast to PQ
-
and Tyr-Z
+
, which give only limited match. However, in the UV-C
region additional targets, most likely PQ and/or Tyr-Z
+
, are also involved.
PQ
Action spectrum
Mn(III) + Mn(IV)
+
-Y
z
Y
z
PQ
-
UV-B
UV-C
UV-A
200
240
280
320
360
400
Wavelength (nm)
Figure 6.
The action spectrum of UV damage in comparison with the absorption spectra of the
main UV targets in PSII
. Action spectrum of UV damage (thick solid line
2
). Absorption spectra: PQ
(dotted line
32
) PQ
-
(dashed-dotted line
32
), oxidized minus reduced Tyr-Z (dashed line
33
), Mn(III) +
Mn(IV) bound to an organic ligand (thin solid line
28
).
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