POTENTIAL EFFECTS OF UV-B ON PHOTOSYNTHES IS AND PHOTOSYNTHETIC PRODUCTIVITY OF HIGHER PLANTS - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 133

In order for UV-B to have any effects on photosynthetic productivity, UV-B
must penetrate the leaf and be absorbed by chromophores associated with the
photosynthetic apparatus or associated genes and gene products. Cellular components
that absorb UV-B directly include nucleic acids, proteins, lipids and quinones 2 . Leaves
contain water-soluble phenolic pigments, such as flavonoids, which strongly absorb
UV-B radiation whilst not absorbing photosynthetically active radiation. These UV-B
absorbing pigments are present throughout the leaf, but accumulate in adaxial epidermal
cells 3-4 . UV-B radiation stimulates the expression of the genes encoding phenylalanine
ammonia-lyase (PAL), the first stage of the phenylpropanoid pathway, and chalcone
synthase (CHS), the key stage that commits the pathway to flavonoid synthesis (for
reviews see Bornman and Teramura 5 , Beggs and Wellmann 6 , Jenkins et al. 7 ).
In considering how UV-B can potentially affect the photosynthetic productivity
of higher plants a clear distinction has to be made between studies made under
glasshouse conditions at high UV-B doses and field studies using realistic UV-B levels.
In the former, a clear distinction has also to be made between plants grown without
UV-B and suddenly exposed to it, and plants continuously grown and developed under
UV-B.
3. Glasshouse studies
Electron transport and photophosphorylation
Photoinhibitory damage of PSII could be primarily responsible for
UV-B-induced inhibition of photosynthesis in higher plants. In many reviews, PSII
damage has often been implicated as the major potential limitation to photosynthesis in
UV-B irradiated leaves 8-10 , as is the case in the photoinhibition of photosynthesis by
excess photosynthetically-active radiation (400-700 nm) 11 , although it has been
suggested that the mechanism of UV-B induced damage may be different 12-13 .
When pea ( Pisum sativum L. cv. Meteor) plants were grown in a greenhouse and
suddenly irradiated with a high UV-B dose (40 kJ m -2 d -1 weighted with a generalised
plant action spectrum 16 ) during 14 h decreases in CO 2 assimilation occurred (Table I),
which were not accompanied by decreases in F v /F m , the maximum quantum efficiency
of PSII photochemistry 17 . Increased exposure to very high UV-B doses resulted in a
further loss of CO 2 assimilation and decreases in F v /F m , which were accompanied by a
loss of the capacity of thylakoids isolated from the leaves to bind atrazine, but clearly
PSII was not the primary target site involved in the onset of inhibition of photosynthesis
in UV-B-irradiated plants. This results has been recently confirmed in other vascular
plants 18 .
In contrast, when these pea plants were grown throughout their development
under a high irradiance of UV-B in a glasshouse, there was not a significant effect of
UV-B on F v /F m or on relative quantum efficiency of PSII electron transport (I PSII , Table
I) and the only effect was a decrease of adaxial stomatal conductance of the leaves 14 .
It is now widely accepted that PSII is not directly affected by UV-B radiation
and decreases in PSII efficiency are usually attributable to down regulation of electron
transport, due to large reductions in carbon metabolism induced by UV-B radiation.
Decreases in F v /F m usually appeared to be a secondary effect of UV-B radiation.