NON-DAMAGING AND POSITIVE EFFECTS OF UV RADIATION ON HIGHER PLANTS - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 153

NON-DAMAGING AND POSITIVE EFFECTS OF UV RADIATION ON
HIGHER PLANTS
M.G. HOLMES
Department of Plant Sciences, University of Cambridge, Cambridge,
CB2 3EA, U.K.
1. Introduction
Excess ultraviolet-B (UV-B; 280-320nm) radiation is detrimental to the growth
and development of higher plants and other organisms, primarily because it damages
DNA. At high levels, UV-B radiation can also damage the photosynthetic system and
membrane lipids. DNA damage resulting from excessive UV-B radiation can occur by
two main ways. The main cause is the direct absorption of photons by DNA. The lesser
cause is by the absorption of photons by other molecules, resulting in the production of
active oxygen species or free radicals which then damage DNA in subsequent reactions.
By either means, the DNA is damaged. Unlike motile organisms which can make
avoidance responses, most plants have to endure ambient UV-B radiation levels and
their only options are to repair damage as efficiently as possible and, preferably, to
minimise that damage by protective mechanisms. Damage effects and their
interpretation are described in detail elsewhere in this volume.
The majority of studies on the responses of plants to UV-B radiation have
concentrated on the potential damage caused by excessive UV-B radiation as a result of
stratospheric ozone depletion. These have reported on the specific damage caused, the
susceptible targets, the mechanisms of damage, and the methods of damage repair. By
contrast, UV radiation also has many non-damaging effects on plants. Separating non-
damaging from damaging effects of UV radiation on plants is not always
straightforward. Morphological and biochemical responses are often used to assess
possible UV-induced damage. There are many responses which have been reported in
the absence of obvious damaging effects; examples include de-etiolation, flavonoid
biosynthesis, dimensions of hypocotyls and leaves, leaf number, branching and,
possibly, cotyledon curling.
The purpose of this chapter is to outline our current understanding of both
non-damaging and positive effects of UV radiation on higher plants. Topics covered
include antioxidants, photoprotection (e.g. flavonoids and phenylpropanoid derivatives).
Indirect effects such as fungal pathogens, phytophagous insects and litter decomposition
are also discussed. Photomorphogenic responses to UV radiation are becoming better
understood. The putative UV-B photoreceptor(s) is discussed. These responses are also
considered in terms of co-action between UV-B and other wavelengths, using
phytochrome and cryptochrome as examples.