Our nearest star, the sun, emits short wavelength radiation that is incident on the earth’s atmosphere. Most of the radiation in the atmosphere is infrared radiation (700-3000 nm, 67% of the photons) and visible light (400-700 nm, 28%; Nobel, 1983). Ultraviolet radiation (UV, 200-400 nm), on the other hand, reaches the atmosphere in smaller amounts (5% of the photons). The biologically most hazardous part of UV radiation, i.e. UV-C (200-280 nm) and UV-B (280-320 nm) below 290 nm, are completely absorbed by the stratospheric ozone (O3) layer and by other oxygen molecules in the atmosphere (Frederick, 1993). In addition, the ozone layer absorbs some longer-wave UV-B and UV-A radiation (320-400 nm) (Fig. 1). Consequently, of the photons at the earth’s surface, only about 2% are in the ultraviolet range (Nobel, 1983). However, of the total solar energy reaching the earth’s surface, UV-B radiation comprises about 1.5% and UV-A radiation about 6.4% (Frederick et al., 1989). The intensity of UV-B radiation, in particular, is affected by the thickness of the ozone layer, which in turn varies periodically as a consequence of natural processes such as seasons, winds and solar cycles. In addition, latitude, time of year and time of day determine the length of the path of a UV-B photon through the absorptive ozone layer (Caldwell et al., 1980).
Effect of UV-B radiation in plants
Fig. 3. Effects of UV-B radiation on plant system.
Signal transduction and gene expression
Frederick, J.E. 1993. Ultraviolet sunlight reaching the Earth’s surface: a review of recent research. Photochemistry and Photobiology 57:175-178.
Nobel, P.S. 1983. Biophysical Plant Physiology and Ecology. Freeman & Co., New York, pp.185-238.
Jordan, B.R. 1996. The effects of ultraviolet-B radiation on plants: a molecular perspective. In: Callow, J.A. (editor). Advances in Botanical Research, Vol 22. Academic Press, New York, pp. 97-162.
Jordan, B.R., P. James and S.A-H. Mackerness. (1998). Factors affecting UV-B induced changes in Arabidopsis thaliana gene expression: role of development, protective pigments and the chloroplast signal. Plant & Cell Physiolog, 39: 769-778.
Jordan, B.R., J. He, W.S. Chow and J.M. Anderson. Changes in mRNA levels and polypeptide subunits of ribulose bisphosphate carboxylase in response to supplemental UVB radiation. Plant, Cell and Environment 15: 91-98, 1992.