Earth's sunscreen – the ozone layer
Box 2 | Can plants get sunburn?
Plants are always exposed to UV-A and have mechanisms for coping with UV-induced damage. But high levels of UV-B – far higher than are occurring anywhere at the moment – have been shown to cause great damage. The main effect is on the photosynthetic apparatus – the pigments and enzymes that absorb light and use its energy to process CO2 into sugar.
Tests have shown that plants vary in their sensitivity to UV-B. Most species tested so far have been crops. In experiments subjecting rice plants to a 33 per cent increase in UV, the plants were visibly damaged and the yield of rice-grain fell by 20 per cent. A 33 per cent increase in UV-B at mid-latitudes is not considered likely to occur. However, a 20 per cent increase is a possibility.
Some rice varieties are more resistant to UV than others. As might be expected, these have been grown in high-altitude areas for generations – where the thinner atmosphere ensures naturally higher UV levels. It is quite possible that non-domesticated, wild relatives of food crops may contain valuable genes coding for UV resistance or for repair mechanisms for UV-induced damage. Also, many plants can produce UV-absorbing compounds; and increased exposure to UV stimulates greater production, up to a point. Just as different races of humans can produce different quantities of protective melanin, so plants differ.
In the sea, phytoplankton are also at risk. (Phytoplankton are important because they remove carbon dioxide from the air. They are also at the base of many marine food chains.)
Boxes
Box 1. Meet the ultraviolet family
Box 3. How ozone is lost
Box 4. Australia finds a replacement for methyl bromide
Related site
The effects of ozone depletion (Environmental Protection Agency, USA)
Further reading
Plants in the sun (by Roger Beckmann, Ecos, No. 68, 1991, pages 28-30)
Page updated February 2006.