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Published in the six Nashoba Publishing papers on Friday, 5 July 2002

When July arrives, I slow down. It's not just the heat; the sunlight is too intense. Even with a hat and sunglasses, I get headaches if I work in the garden in the middle of the day. Of course I'm hypersensitive; but most people have learned to cover up or risk skin and eye damage from ultraviolet radiation (UV).

As the ozone layer thins, and the Earth loses its UV screen, plants too are affected. They're built to take in light, it's their food. (Fertilizer is their vitamins.) Animals in turn depend on nutrients that plants manufacture from light. Even the oxygen we breathe is created by photosynthesis.

Increase in UV changes the life of plants. The immediate effect is clear: plants' metabolisms are slowed when UV bleaches out chlorophyll -- reducing size and productivity. But UV also knocks electrons off the DNA of cells -- including reproductive cells, which transmit these mutations to offspring. Such changes are more complex.

For example changes in shape, developmental timing, and how nutrients are distributed within tissues: these can influence how plants compete with other plants, as well as how they respond to diseases and pests, both of which have their own developmental cycles. A pest in a voracious stage can wipe out a whole crop of a plant just sprouting.

UV reduces legumes' ability to take nitrogen from the air and fix it in the soil: bean production falls -- and soil fertility, too. Quality also declines in other crops, including varieties of tomatoes, potatoes, sugar beets, rice, corn, winter wheat, and cotton.

Single-celled phytoplankton algae not only form the bottom of the planet's food chain; they also recycle about half the world's carbon, removing carbon dioxide and creating oxygen. Recent research on the antarctic "ozone hole" shows UV damage to these tiny plants.

Meanwhile my garden's had weeks of cloudy skies, and most of the plants are happy to see more sun. They respond by growing fast. I swear I can see a difference from day to day in the Apple of Peru, a wacky annual that by September will look like a three-foot-tall tree, with arching branches six feet across.

And the weeds! I need to get at them, if I do nothing else. Here's my strategy: stay in the shade, and work before 8 am or after 5 pm, when it's not only cooler, but the sunlight is not so direct. When it slants through the atmosphere, it goes through more ozone.

Next Date

For more information

  • "Significant changes in UVR climatology have occurred at all latitudes, except the tropics, during the last 20 years, with potentially significant consequences for coastal oceanography." Marine Science Institute abstracts: Barbara B. Prézelin NSF OCE96-18390 Patterns and Sources of Variability in Biological Weighting Functions for UVR Inhibition of Primary Production in California Coastal Waters.
  • Ultraviolet Reflections: Life under a Thinning Ozone Layer — by Annika Nilsson. A book I think I'll read on this subject.
  • Ozone Depletion — United States Environmental Protection Agency. "Indirect changes caused by UVB (such as changes in plant form, how nutrients are distributed within the plant, timing of developmental phases and secondary metabolism) may be equally, or sometimes more, important than damaging effects of UVB."
  • The Ozone Hole — This site also documents other UV health hazards to humans.
  • Ultraviolet Light and Phytoplankton Growth
  • Photosynthesis: Pathway of Carbon Fixation
  • The Storage Battery of Life — shows how oxygen is a byproduct of the cellular respiration of plants, which in turn uses the glucose produced in photosynthesis.

© Copyright 2002 Catherine Holmes Clark