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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