USE AND EVALUATION OF BIOLOGICAL SPECTRAL UV WEIGHTING FUNCTIONS FOR THE OZONE REDUCTION ISSUE - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 75

In general, a BSWF that falls more steeply with increasing wavelength will yield
a greater RAF. But, in experiments involving fluorescent lamp systems designed to
supply added UV in a manner simulating reduced ozone, a steeper BSWF results in less
supplemental UV than does a less steep BSWF.
2. Testing BSWF in polychromatic radiation
2.1. RATIONALE FOR TESTING
The BSWF for the ozone reduction problem are used in the context of polychromatic
radiation. Yet, most of these BSWF are derived from action spectra developed with
monochromatic radiation under laboratory conditions, and often for purposes other than
as BSWF. There are several reasons to question how applicable such BSWF may be
with polychromatic radiation, especially with solar radiation in the field. With
monochromatic radiation in the laboratory during development of the action spectrum,
the absence of broadband radiation, especially at longer wavelengths, may deprive the
organism of opportunity for repair of damage caused by the UV-B radiation. In sunlight,
there is a high flux of UV-A and visible radiation relative to solar UV-B. This longer
wavelength radiation may contribute to several pathways of repair and adaptation to
UV-B. Mitigation of UV-B response by visible (especially blue light) and UV-A flux is
best documented in photoreactivation (PR) of damage to DNA since this radiation
drives the photolyase enzyme. For higher plants, peak effectiveness tends to occur at the
longer UV-A wavelengths (375-400 nm) [12,13]. In experiments with light-grown
higher plants, both supplemental UV-A and blue light can aid in ameliorating UV-B-
caused chlorosis [14]; this may or may not involve PR.
In general, for plant growth and morphological response to UV, several
phenomena may interact: DNA damage [13, 15, 16], free radical formation [7, 17, 18],
and the action of photoreceptors such as flavins [19,20,21]. The manner in which UV-A
and PFD interact with UV-B can also involve several possibilities. For example, UV-A
and blue light can drive the photolyase enzyme to repair a common lesion of DNA
caused by UV-B [22,23], in the process of photoreactivation (PR); UV-B can induce PR
and other DNA repair systems [13, 24]; UV-A can cause growth delay (at least in
bacteria) allowing more time for dark DNA repair [25], but UV-B and UV-A at high
flux rates can damage DNA repair systems [26]. For free radical damage, UV-A can
stimulate carotenoid quenching [26]. The blue light receptor appears to be important in
mitigating UV-B damage [27]. Interactions between the UV-B, UV-A/blue, and
phytochrome receptors have also been demonstrated [28, 29]. Thus, there are many
potential pathways by which radiation of different wavelengths may interact. In the
laboratory with monochromatic radiation the plant is exposed to radiation of only one
wavelength at a time and this removes the possibility of the various wavelength
interactions that might be expected for plants exposed to sunlight in nature. Also, the
time frame of exposures during action spectrum development with monochromatic
radiation in the laboratory usually involves hours, or even fractions of an hour. Yet,