Will petunias and poppies need sunscreen?

Plants can't slather protective lotion on their delicate parts, and that may pose a problem. A laboratory simulation of ozone-layer thinning that allows extra ultraviolet-B (UV-B) radiation to reach Earth has made pollen sluggish in more than half the species tested.

Pollen from 34 species got mock suntans in the broadest survey yet of UV-B effects on flowering plants. In these experiments, run by Javad Torabinejad and his colleagues at Utah State University in Logan, the low UV-B doses mimicked current losses in Earth's protective ozone layer and high doses simulated an extreme scenario of 15 percent less ozone.

Pollen from 19 of the tested species developed significantly more slowly than pollen that wasn't exposed to UV-B radiation, the group reports in the March American Journal of Botany. If the irradiated pollen performs as feebly in fertilizing real flowers as it did in the tests, plants might set fewer seeds.

However, Torabinejad emphasizes that no one knows how the simulations translate into real-world effects. "I don't want to say this is going to be a devastation," he says.

The UV-B-sensitive pollen came from sweet corn, rye, pears, pistachios, Montmorency cherries, California poppies, the Ultra Pink petunia, and a range of other plants. Begonias and tobacco did not suffer significantly from the radiation.

Only a small fraction of previous research on UV-B effects has examined plant reproduction, notes study coauthor Stephan D. Flint. Some of Flint's work has shown that plant structures protect female reproductive systems -- and even pollen at certain stages. However, pollen must face the elements as it hitchhikes between plants.

Pollen grains are exposed to sunlight when a bee or other pollinator brushes them up from their parent flower and sets off to forage. Pollen grains that finally reach other flowers often lie in the sun while germinating and sprouting the tube that penetrates the female tissue.

In the test, exposure to UV-B radiation significantly reduced germination in only five species, yet it slowed tube growth in a wide variety of others.

For practical reasons, Torabinejad and his coworkers measured tube growth on artificial culture media instead of on flower tissue. Even so, the experiment was arduous, requiring more than 30,000 measurements under a microscope. "We hired people who quit within the first hour," Torabinejad remembers.

The results suggest that monocots, a large group of plants that includes grasses and lilies, tend to be more sensitive to UV-B radiation than other species. Torabinejad speculates that plants with slow-growing pollen cells that contain three nuclei, like the ornamental cargopteris, may also be particularly vulnerable.

Ecologist Jeffrey K. Conner of the Kellogg Biological Station at Michigan State University in East Lansing calls the wide range of plants in the new study "a real strength." His own studies of Brassicas found little cause for concern in the greenhouse but worrisome losses of seed quality and number in a field experiment. He hesitates to guess what the new study means for plants in the field. but he does say it presents "potential cause for concern."