Orchids Profit From False Advertising, Study Says
for National Geographic News
|March 8, 2004|
Scientists working on a Swedish island have helped illuminate the bizarre sexual antics of orchids. Their new study, they say, reveals why some orchids have plenty to gain from false appearances.
Unlike most flowers, roughly a third of the approximately 30,000 orchid species in the world don't rely on the lure of food to achieve pollination. Instead, the orchids trick insects into performing the task without reward.
Most of these orchids are "food deceptive." Using bright colors and sweet perfumes, they falsely advertise a free meal of pollen and nectar to attract bees, beetles, butterflies, and other pollinators. (Other of these orchids are even more elaborate in their cunning, mimicking the appearance and sex pheromones of female insects to lure males looking for a mate.)
Scientists have long been mystified by these strategies, since studies have shown that plants that actually provide nectar and other food-rewards attract more pollinating insects. The same is true for orchids
So why do some orchids continue to dupe insects in order to reproduce? One leading theory held that the strategy reduces the chances of self-pollination, making the orchids more efficient in dispersing their pollen to other plants.
The South African and Swedish scientists behind the new study say their research is the first solid evidence that appears to confirm the theory. "[It] has been out there for some while but without empirical evidence. So this is a big step," said Martyn Powell, an orchid expert at the Royal Botanic Gardens in London, England. Powell was not involved in the study.
The study was published late last month in Proceedings: Biological Sciences, a journal of the Royal Society, the U.K.'s national academy of science. The study's findings confirm an observation made by Charles Darwin, who wrote that orchids employ a "beautiful contrivance" to avoid self-pollination.
Like Darwin, the research team focused on the green-veined, or green-winged, orchid (Anacamptis morio). This food-deceptive species is found throughout much of Europe.
Scientists conducted their research on the Baltic Sea Island of Öland, off the southeast coast of Sweden. Orchids there use their purplish pink flowers and strong scent to dupe queen bumblebees (the orchids' main pollinators in Sweden) into thinking the plants harbor nectar.
As with other orchids, Anacamptis morio pollen is bound in packages, called pollinaria. Roughly three millimeters (one-tenth of an inch) in length, pollinaria protect their pollen holdings from hungry insects, while acting as shipping containers that, in the case of probing bumblebees, stick to the insects' heads. The bees transport pollinaria to other plants.
To learn the effect food-rewards play in pollen dispersal, researchers attached orchids to canes that were stuck in the ground, among the foraging bumblebees. Nectar was added to some orchids but not others. For all the test flowers, color-coded dye was added their pollen to help track its movement.
As expected, researchers found that nectar significantly influenced the bumblebees' behavior. Bees probed flowers with nectar 2.3 times more often than plants without it. Bees also spent five times as long in plants with nectar than those without it, and the bees removed far more pollen in the process.
In addition, the experiments revealed that nectar-enriched orchids were subject to high levels of self-pollination (a process that degrades plant health). The findings highlight the benefit gained by green-veined orchids in their reproductive strategy, which significantly reduces the risk of self-fertilization by enticing bumblebees with a fake meal.
Steven Johnsona researcher in the School of Botany and Zoology at South Africa's University of KwaZulu-Natallead the study. He says that because orchids are hermaphroditic, or able to mate both as males and as females, they can self-pollinate. However, genetic variety is often lost when this occurs, leading to less-fit offspring, Johnson said. The process is known as inbreeding depression.
Previous studies by Johnson suggest inbred green-veined-orchid embryos are twice as likely to abort as those arising from cross-fertilized flowers.
Self-pollination can also signal lost opportunities to mate with other plants. "This boils down to a plant having less pollen to export as a consequence of some of its pollen being deposited on its own stigmas," Johnson said. The stigma is the part of the orchid on which pollen grains germinate.
Beyond false ads for nonexistent nectar and pollen, orchids employ another clever trick to guard against self-pollination. In the 1870s, Charles Darwin highlighted a phenomenon known as pollinarium bending. The term refers to the manner in which orchid pollinaria start to bend after an 18- to 20-second interval after attaching to a visiting insect.
"The bending mechanism ensures there is a time delay before pollen can be transferred from a freshly removed pollinarium to a stigma," Johnson said. "By this time, bees have usually left the plant, and self-pollination is avoided."
In their recent study, the Swedish and South African scientists demonstrated that Darwin was right about the purpose of this ingenious mechanism. They found that during visits lasting fewer than 18 seconds, bumblebees triggered orchid self-pollination only 5.4 percent of the time. But in longer, nectar-induced visits, orchids self-pollinated 63.6 percent of the time.
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