NON-DAMAGING AND POSITIVE EFFECTS OF UV RADIATION ON HIGHER PLANTS - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 162

infection by fungi and viruses 77 . Orth et al 78 found that cucumber ( Cucumis sativus L.)
plants pre-irradiated with UV-B radiation were more susceptible to subsequent infection
by fungal pathogens; interestingly, UV-B irradiation after infection had no effect on the
fungal pathogen load.
Outdoor studies may provide a potentially more realistic analysis of the role of
UV-B radiation in influencing fungal and viral diseases. UV-B exclusion studies on tea
( Camellia sinensis L.) have shown that removal of UV-B radiation results in increased
blister blight 79 . Newsham et al 80 studied the abundance and distribution of phylloplane
fungi on oak ( Quercus robur L.) leaves and compared plants with a 30% elevation
above the ambient level of erythemally-weighted UV-B radiation with control (added
UV-A only) and plants receiving only ambient radiation. Over a four month period they
found that the abundance of these non-phytopathenogenic fungi decreased with time on
the adaxial leaf surface (but not on the abaxial surface) in the plants with added UV-B
radiation compared to either the control or ambient radiation plants. In addition, the
isolation frequency of the fungi decreased with increasing fluence rate and the authors
were able to demonstrate that ambient temperature and rainfall had no influence on the
response. Our own outdoor data (Holmes, unpublished) confirm that general
phylloplane fungi are almost eliminated on the adaxial leaf surfaces of Q. robur using
similar radiation treatments; furthermore, this tendency continued over 7 years and the
plants themselves had a slightly higher dry weight accumulation rate UV-B+A radiation
than under UV-A or ambient treatments. Isoflavonoids, which can be induced by both
stress and UV radiation, have antifungal properties (see above).
Overall, current research on this subject area is inconclusive. However, outdoor
studies, which are more representative than those carried out indoors, tend to indicate
that UV-B radiation has a positive (non-damaging) effect on the plants insofar that
fungal infections are reduced by the UV-B radiation.
10. Insect herbivory
Insect herbivory of plants which have received different amounts of UV-B
radiation has received substantially more attention than the subject of viral and fungal
pathogens. McCloud and Beranbaum 81 reported that caterpillars of the moth
Trichoplusia ni have lower mortality and better growth when fed on leaves of
glasshouse-grown (i.e. a UV-B deficient radiation environment) lemon plant ( Citrus
jambhiri ) leaves than leaves which had received UV-B. The authors ascribed their
observations to the furanocoumarins produced in response to UV-B radiation.
Using growth chamber experiments, Hatcher and Paul 82 found that larvae of the
moth Autographa gamma L. consumed less leaf tissue of pea ( Pisum sativum L.) when
the plants had previously received UV-B radiation. They suggested that the reduced
consumption was caused by the increased nitrogen content of the leaves. A study of
moth larvae ( Operophtera brumata L.) by Lavola et al 83 resulted in a different response.
In this case the larvae preferred to graze on leaves of silver birch ( Betula pendula Roth.)
which had received UV-B radiation rather than on those leaves which had not.
Outdoor exclusion or attenuation experiments in which various wavebands of
natural global UV radiation are attenuated by placing filters above plants have been
used for several decades. UV-B exclusion experiments have generally, but not
exclusively, demonstrated that the removal of UV-B radiation from ambient daylight