RESPONSE TO UV-B RADIATION: WEIGHTING FUNCTIONS AND ACTION SPECTRA - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 83

A prerequisite for the determination of an action spectrum is the definition of a
biological parameter for quantifying the photoresponse, such as, for example, the length
or the weight of a plant, the concentration of a gas or an enzyme, etc..
An action spectrum is the plot of the effectiveness of the light in inducing the
observed photoresponse as a function of wavelength. The effectiveness of the light in
inducing the photoresponse can be quantified, for example, as the reciprocal of the
amount of light required by the system to produce a fixed level of response. A simple
procedure to determine an action spectrum could be to choose a standard level of
response and to measure at each wavelength the radiation intensity inducing that
response. However, a necessary prerequisite of action spectroscopy is the
proportionality between magnitude of the response and intensity of the stimulus;
therefore, to avoid distortion of the action spectrum, a succession of graded responses
should be measured, for each wavelength, to construct an intensity-response curve and
to verify that the chosen standard level of response fall on the linear part of the curve.
Other wavelength-dependent distortions of the action spectrum could be due to
the molecular environment where the target molecule is embedded. In fact, the presence
of other pigments can cause screening or reflection effects, in such a way that the
effectiveness of a particular spectral range on the target molecule could be reduced or
enhanced, respectively. Moreover, in a molecular environment with many scattering
centres, the efficiency in eliciting the photoresponse of the shorter wavelengths can be
apparently decreased, because of the dependence of the diffusion on the inverse of the
wavelength. The sample should also not absorb too much radiation at the tested
wavelengths, so that, at least ideally, every molecule involved in the photoprocess could
be exposed approximately to the same amount of photons and, consequently, have the
same probability of responding.
An important criterion to determine if the intensity-response curves at the
different wavelengths are indicative of the same mechanism of action is to check if they
can be reasonably superimposed, by multiplying each of them by an appropriate factor,
thus showing that they have the same "shape". In other words, given any pair of
intensity-response curves, the ratio of two fluence rates corresponding to the same level
of response should be approximately a constant for any level of response and,
consequently, the difference of their logarithms should be also a constant. This implies
that the curves plotted on a logarithmic abscissa should appear parallel. If a fluence rate-
response curve has been determined with a sufficient number of data points, it can be
fitted with an appropriate mathematical function. This, besides allowing the calculation
of the fluence rates for any standard level of response, makes it possible to check more
precisely if the curves have the same shape.
3. Environmental UV action spectroscopy
Action spectroscopy can also provide significant information in the field of
ecophysiological plant research and, more generally, in the field of environmental
photobiology. In fact, action spectroscopy is an useful tool for investigating the
biological effects of ultraviolet radiation, and, in particular, of the UV-B wavelengths
(280-315 nm), on terrestrial plants and phytoplankton organisms. 6