UV-B AND UV-A RADIATION EFFECTS ON PHOTOSYNTHESIS AT THE MOLECULAR LEVEL - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 121

also been observed. One manifestation of the UV-B effect on the acceptor side
components is an apparent resistance against electron transport inhibitors, which act by
replacing the mobile plastoquinone electron acceptor at the Q B binding site. This effect,
which is most likely related to the UV-B induced modification of the Q B binding site,
has been observed not only in oxygen evolving organisms 9,16,21 , but also in isolated
reaction centers of the purple bacterium Rhodobacter sphaeroides 22 , that have
analogous reaction center subunits and quinone acceptors as PSII. Besides the electron
transport components, UV-B light also damages the D1 and D2 reaction center subunits
of PSII, leading eventually to their degradation 17,23-25 (Figure 3), which can be repaired
via de novo protein synthesis in intact cells 26-27 .
In order to obtain more detailed information about the mechanism of UV-B
induced damage of PSII we applied short light flashes to synchronize the water-
oxidizing complex into various oxidation states, called S-states, in isolated thylakoid
membranes. The synchronized samples were then illuminated with monochromatic
UV-B laser flashes of 308 nm. Under the applied experimental conditions 80 repetitions
of the flash protocol was sufficient to cause significant loss of oxygen evolution. The
damage induced by the UV-B flashes shows a clear S-state dependence indicating that
the water-oxidizing complex is most prone to UV damage in the S 2 and S 3 oxidation
0.6
0.5
0.4
0.3
0.2
0.1
0.0
S1 S2 S3 S0
Dominating S-state
Figure 4. The S-state dependence of the UV-B radiation induced damage . The PSII centers were
synchronized into different S-states in isolated spinach thylakoids by using Xe flashes, whose UV
component was filtered out, and the synchronized samples were irradiated by 308 nm laser flashes.
The fraction of damaged PSII centers was calculated from the loss of oxygen evolution using
principal component analysis by taking into account the actual distribution of the different S-states
after synchronization.
states (Figure 4). During the S-state transitions the catalytic Mn cluster of water
oxidation is sequentially oxidized. Mn ions bound to organic ligands (such as amino
acids) have pronounced absorption in the UV-B and UV-A regions in the Mn(III) and
Mn(IV) oxidation states, which dominate the higher S-states, but not in the Mn(II)
oxidation state, which occur in the lower S-states 28 . Thus the high UV sensitivity of