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 117

UV-B AND UV-A RADIATION EFFECTS ON PHOTOSYNTHESIS AT THE
MOLECULAR LEVEL

COSMIN SICORA, ANDRÁS SZILÁRD, LÁSZLÓ SASS, ENIKė
TURCSÁNYI, ZOLTÁN MÁTÉ AND IMRE VASS
Institute of Plant Biology, Biological Research Center
P.O.Box 521, Szeged, H-6701 Hungary

1. Abstract

Ultraviolet radiation is a well known damaging factor of plant photosynthesis. Here we
studied the mechanism of damage induced by the UV-B and UV-A spectral regions to
the light energy converting Photosystem II (PSII) complex, which is the origin of
electron flow for the whole photosynthetic process. Our results show that the primary
UV damage occurs at the catalytic Mn cluster of water oxidation, which is most likely
sensitized by the UV absorption of Mn(III) and Mn(IV) ions ligated by organic residues.
The presence of visible light enhances the photodamage of PSII, but has no synergistic
interaction with UV radiation. UV-induced damage of PSII can be repaired via de novo
synthesis of the D1 and D2 reaction center protein subunits. This process is facilitated
by low intensity visible light, which thereby can protect against UV-induced
damage.

by visible light at high intensity (above
1000 PEm -2 s -1 ) cancels the protective effect. The protein repair of PSII is also retarded
by the lack of DNA repair as shown

However, the photodamage induced

in a photolyase deficient cyanobacterial mutant.

2. Introduction

Solar radiation is not only the vital source of energy for the biosphere on our planet,
but also acts as an adverse environmental factor for various forms of life. The efficiency of
light-induced damage increases with decreasing wavelength. Thus, from natural sunlight
that reaches the Earth the UV-B (280-315 nm) spectral range has the highest damaging
potential. Therefore, recent reductions in the stratospheric ozone layer, which enhances
UV-B intensity on the surface of Earth and ecologically significant depths of the ocean
have initiated extensive research efforts to elucidate molecular mechanisms regulating
responses of various organisms to UV-B radiation 1 .
Photosynthetic organisms entirely depend on sunlight as the ultimate source of
energy for their survival. As a consequence, their exposure to UV radiation is unavoidable,
which poses a potentially adverse impact leading to decreased oxygen evolution and CO 2
fixation 2-5 , reduction in dry weight, secondary sugars, starch, and total chlorophyll 6-7 . In
cells of non-photosynthetic organisms, DNA absorbs ~50% of the incident ultraviolet
radiation and it is the primary target of UV damage. However, in photosynthetic organisms
chlorophyll and other pigments may contribute significantly to shielding DNA from UV
radiation 8 . Therefore, besides damaging DNA a crucial part of the overall UV-B effect is