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 123

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 ).