RESPONSE TO UV-B RADIATION: WEIGHTING FUNCTIONS AND ACTION SPECTRA - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 85

response of the organism; the examined UV effects should be tested on whole
organisms, because in vitro studies can have limitations for extrapolation to the natural
performance of the whole organism; the presence of reciprocity effects should be
carefully tested, when performing experiments with different duration at different
wavelengths. 7
An important aspect to consider is the use of irradiation conditions not too far
from present levels of natural radiation or those under predictable ozone depletion
scenarios. Using extraterrestrial or so high irradiances that the organisms are
irreversibly damaged certainly produce a clear effect, but not so meaningful for a
realistic assessment of ozone depletion consequences.
Also the spectral composition of the radiation is crucial. Monochromatic action
spectra can provide indications on the effects of UV-B without interference due to
processes induced by other radiation bands and are very useful to demonstrate a direct
photochemical effect on a specific target. For example, the action spectrum for DNA
cyclobutyl pyrimidine dimer formation in alfalfa seedlings shows that the damage
extends well into UV-A up to about 370 nm. 8 However, studies conducted using
polychromatic irradiation allow for photoregulated compensatory mechanisms which
occur in nature, such as photorepair 9 and the induction of protective UV-absorbing
compounds. 10,11 A complementary approach, which takes into account information from
both monochromatic and polychromatic irradiation conditions, seems to be most
adequate to describe the complex biological responses to UV of whole, intact
organisms.
Sun simulators allow to reproduce at any time irradiation conditions and climatic
factors and to exclude disturbing agents. They are therefore very promising tools to
overcome the difficulties of outdoor studies. 12-14 The employment of a wide range of
modern lamp types and different filter combinations allow to obtain the spectral quality
and the intensity of natural global radiation and the appropriate shaping of the short-
wave cut-off. 15,16
Notwithstanding the complexity of experiments under natural radiation, various
spectral studies have been performed either removing increasingly larger portions of the
solar UV spectrum (natural or simulated) by means of different combinations of long-
pass filters (see Fig. 2), 17-19 or supplementing UV radiation by means of appropriate
fluorescent lamps. 20,21 Recently, a sophisticated irradiation system has been described,
which uses both filters and supplementary UV lamps and can provide computer-
controlled modulated changes in any radiation band between about 250 and 730 nm. 22-24
Two different approaches has been proposed for extracting an action spectrum
from experimental data obtained with polychromatic exposures in which UV irradiance
is varied by means of filter combinations with progressively decreasing cut-off
wavelength. Differential action spectroscopy (DAS) takes into account the spectral
bands given by the difference in irradiance of two filter combinations with successive
cut-off wavelength. Assuming that different spectral bands supplement one another in
producing the observed effect, it is possible to estimate the weighted contribution of
each band by dividing the differences in response to the treatment by the corresponding
differential irradiances. Finally, the action spectrum is derived by plotting the weighted
effects as a function of the average wavelength of the corresponding band. As UV
action spectra tends to exponentially increase with decreasing wavelength, the main