The Sun is responsible for the development and continued existence of life on Earth. We are warmed by the Sun's infrared rays and we can see with eyes which respond to the visible part of the Sun's terrestrial spectrum. More importantly, visible light is an essential component of photosynthesis, the process whereby plants, which are necessary for man's nutrition, derive their energy. However, the deleterious effects of sunlight on biological systems are due almost entirely to radiation within the ultraviolet spectrum of the Sun's emission.
The ultraviolet region of the electromagnetic spectrum is subdivided into three bands termed UVA, UVB and UVC. The subdivisions are arbitrary and differ somewhat depending on the discipline involved. Environmental photobiologists normally define the wavelength regions as: UVA, 400-320 nm; UVB, 320-290 nm; and UVC, 290-200 nm. The division between UVB and UVC is chosen as 290 nm since ultraviolet radiation (UVR) at shorter wavelengths is unlikely to be present in terrestrial sunlight, except at high altitudes (Henderson 1977). The choice of 320 nm as the division between UVB and UVA is perhaps more arbitrary. Although radiation at wavelengths shorter than 320 nm is generally more photobiologically active than longer wavelength UVR, recent advances in molecular photobiology indicate that a subdivision at 330-340 nm may be more appropriate (Peak and Peak 1986).
4. Effects of solar UVR on plants
Twenty years ago virtually nothing was known about the effects of UVR on plants (Teramura 1986). Even today knowledge is principally limited to the effects on agricultural crops; little is known of the effects of UVR in other natural ecosystems such as forests, meadows, savannas, tundra and alpine areas (Tevini and Teramura 1989).
There are over 350 000 species of crop plants in the world and although 80 000 of these are edible, only about 3000 are harvested by man and used for food (Miller 1982). Of the 80 or so plant species that have been domesticated (Ehrlich et al 1977), just 15 species supply nearly all the food calories and three-quarters of the world's protein consumption (table 5).
The responses of plants to UV irradiation include physiological, biochemical, morphological and anatomical changes (table 6). In general UVR deleteriously affects plant growth, reducing leaf size and limiting the area available for energy capture. These findings have been achieved mainly through studies in greenhouses and exposure to artificial sources of ultraviolet radiation. Extrapolation to changes on crop yield as a result of increases in terrestrial solar UVR is difficult, and in those few field trials conducted outdoors the results were variable (Tevini and Teramura 1989). Furthermore, the effects of natural UVR on plants will be influenced by other stresses such as water shortage, mineral deficiency and increased concentrations of carbon dioxide. It is of note that increased ambient levels of CO2 (the greenhouse effect) have a beneficial effect on plants (Lemon 1983) but this may not necessarily compensate for the anticipated deleterious effects of increased ambient UVB as a consequence of ozone depletion.
Clearly more information is needed before a reliable assessment can be made on whether changes in ambient UVB are likely to affect significantly global crop productivity.
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