NON-DAMAGING AND POSITIVE EFFECTS OF UV RADIATION ON HIGHER PLANTS - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 165

problem in the UV because many molecules exhibit changing absorbance across this
waveband.
Most of the action spectra presented here (Figs. 1, 2, 3a, 4) provide strong
implications of a photoreceptor with an absorption maximum between about 290 and
300nm. Only higher plants are considered in detail here, but it is worth mentioning that
similar action maxima have been found for carotenogenesis in the fungus Verticillium
agaricum 102 and for the induction of sporulation in the ascomycetes Alternaria tomato
and Helminthosporium oryzae 103 . A similar UV peak was shown for anthocyanin
synthesis in the aquatic plant Spirodela oligorhiza 104 , but the existence of another action
maximum in the R waveband and lack of further relevant experiments precludes
substantive conclusions.
There appears to be no spectral evidence for either phytochrome (see also
Mohr 39 ) or flavins (see also Björn 105 ) being specific UV-B photoreceptors. Although
phytochrome and the flavins absorb in the UV-B region, they do not have specific
absorption peaks similar to those shown in Figs. 1, 2, 3a and 4 above. It is also unlikely
that phytochrome or the BAP can act as specific UV-B photoreceptors when natural
daylight contains relatively high levels of R, FR and B radiation (compared to UV-B)
which these photoreceptors absorb strongly compared to UV-B radiation. There is no
current indication that phototropin plays a role. There is some evidence that pterins are a
specific UV-B photoreceptor and in terms of spectral evidence pterins appear to be
likely candidates 105 . Hsaio and Björn 106 suggested this possibility for the photoreceptor
responsible for carotenoid formation in V. agaricum. Furthermore, studies with
Phycomyces mutants indicates an interaction of pterins in photoreception in the near UV
and B spectral regions 106,107 ; however, such detailed studies have not been carried out on
higher plants.
Although virtually nothing is known about the molecular structure of the UV-B
photoreceptor(s), it is likely that current genetic and biochemical approaches will
rapidly advance our understanding.
13. Ozone depletion and the UV-B photoreceptor
Björn 105 has shown that stratospheric ozone depletion with its concomitant
increase in UV-B radiation levels at the Earth's surface will probably have negligible
effect on the estimated perfomance of the UV-B photoreceptor compared to its effects
on DNA. He used the well-established data of Wellmann 36 and Beggs and Wellmann 27
for the induction of flavonoids by UV radiation. Björn calculated radiation
amplification factors (RAF) of 0.58 and 0.42, respectively. The data were calculated for
clear skies in mid-summer at 55.7 o N and compared 300 Dobson units of stratospheric
ozone with 270 units (10% depletion). These RAF values are strikingly low compared
to comparable calculations for damage effects on DNA which are typically in the range
of 1 to 2. Björn convincingly concluded that the effects of stratospheric ozone depletion
on the performance of the UV-B photoreceptor are low compared to the damaging
effects of UV-B radiation on DNA.