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American Association for the Advancement of Science

The good and bad smells of flowers

Archilochus alexandri (hummingbird).

Plants rely on pollinating insects to visit their flowers and spread their pollen to other flowers. That's how they reproduce -- with help from visiting insects. But a plant must first look and smell good enough to attract a pollinating insect to its flowers. Then, when an insect arrives, the flowers must make sure it doesn't drink too much nectar or harm them in any way. So life as a flower might not be as easy as it seems.

Xylocopa (carpenter bee).

Danny Kessler and a team of German researchers recently performed a genetic experiment to find out exactly how flowers do it. They knew that the colorful patterns on most flowers are used to attract pollinating insects. But they also wondered how the flowers' smells affected their ability to reproduce.

What they found was a bit surprising. In order for flowers to balance the needs of attracting pollinators while also defending themselves, flowers seem to use different scents -- some that attract insects and others that repel them away. Flowers seem to balance their good smells with their bad smells so that insects will visit, but not stay too long.

Kessler and his colleagues looked at tobacco plants, and found that a chemical in their flowers called benzyl acetone (BA) was the main attractant used by the plant. They also observed that the chemical nicotine was used by the tobacco plants as a repellant. So, with that in mind, the researchers altered the DNA in a large group of tobacco plants so that some only had BA as a scent, some others only had nicotine as a scent, and even some others had different levels of both chemicals.

The researchers quickly saw that the BA scent increased visits from pollinators to the flowers, but without nicotine, the insects would stay for a long time and drink too much nectar. Pollinating insects would still visit flowers with only nicotine, but they would not stay long at all. So it appears that a balance must exist between the BA and nicotine scents in order for the plant to reproduce to the best of their abilities. The BA brings insects to the pollen, but the nicotine makes them leave and spread their pollen to other flowers.

These findings were published in the 29 August issue of the journal Science.