Tag Archives: pollen pollination

Come hither, says plant

Posted on 21 February 2013

Flowers: More seductive than ever

Many plants need insects to import pollen to fertilize their eggs and start making seeds. To attract these pollinators, flowers advertise with scents, colors and patterns.

Bee covered in yellow dust, pollen, crouches atop small yellow flower.

Photo: Forest Wander
This honeybee has slathered herself in pollen; some goes back to the hive, and some pollenates the flowers she is visiting.

And now we hear that some plants also use electric billboards to lure their six-legged colleagues.

The plants don’t act deliberately, but their electric field nonetheless communicates with bees, says Daniel Robert, a specialist in insect sensation at the University of Bristol, in the United Kingdom.

Robert, the corresponding author of a study in tomorrow’s Science magazine, says he was intrigued that bees acquire a positive charge during flight, which holds negatively-charged pollen through electrostatic attraction. “A pollen grain needs to stick to the bee, but not too much, so it can be deposited on the next flower,” Robert says. “I thought maybe this electric field was more important than just attraction.”

Pollination is a matter of life and death for many flower species: if insects don’t truck in the pollen, the essential cross-pollination fails, and the plant makes no seeds.

Thus evolution has shaped the flower to be “an extremely manipulative organ when it comes to attracting the bee,” says Robert. Pollen is a protein-rich bee food, and nectar is a sugary delight.

Sending bumblebees to bee school

You maybe didn’t know that bumblebees are trainable. Likewise. But in a series of experiments, Robert found that bumblebees learn to associate the electric field from a fake flower with the presence of sugar.

In soil: Jem Hologram, bee: Lisa Lawley
Bumblebees live the soil, in colonies of a few dozen members. Rollover to see a portrait of the bumblebee.

Visually, the fake flowers were identical, but some emitted no electric field, others had a 10-volt or 30-volt field. Some of the flowers carried a sweet reward — sucrose — while the others delivered super-bitter quinine. “Bees do not like the taste of quinine, and they learned to go to the sucrose, but it took 40 to 50 visits to reach an 80 percent correct rate,” says Robert.
Plants are rooted in the Earth and thus have a neutral or slightly negative charge, while bees accumulate a positive charge as they fly, creating an electric field between plants and bees.
Without the 30-volt cue, the bees could not find the sucrose, “so the information from the electric field was vital.” A 30-volt field is roughly what exists in a flower 30 centimeters tall.

Flower emitting 30 volts quickens bee’s flower-finding by ~25% over flowers with low or no field.

Adapted from Clarke et al. 2013
Fake flowers did not alter bee behavior if they emitted no voltage or 10 volts, but a 30-volt field around the flower led to 80 percent accuracy after 40 bee visits. The stronger field taught the bees which flowers had sugar, and which had bitter quinine.

“It was known before that bees would charge up as they fly through the atmosphere, and that flowers have a negative electric potential compared to the atmosphere, but we are the first to show that bees can detect the field, and can learn to discriminate between different fields,” Robert says.

Water is a great conductor of electric fields, and sharks, skates and other fish use electricity for sensory purposes. “But in terrestrial creatures, this process of recognizing electric fields is unique as far as we know, says Robert.

It’s possible that many insects use electric fields, Robert says. “There is no reason to think that any flying insect that goes through the air will not have this electric potential, because it is physically inevitable that you will accumulate a charge when flying through an ionic medium.”

The economy of flowers

The flowers seem to be an innocent bystander, not taking active charge of their charge, but why did bees evolve a receptor for electric fields, and the neural circuitry necessary to use the sensory input to change their behavior?

The answer resides in evolutionary economics, says Robert. “If a flower attracts too many bees, the nectar that they feed on will run out,” and that could spell disaster for both sides.

“If the flowers start to lie to the bees,” Robert says, “that’s not too good, as the bees are quick to learn which flowers are not good, and then they go back to the hive and say, ‘Let’s go to another place.’”

Two fuchsia horn-shaped flowers ringed with white on petal border.

Photo: Scott Zona
If color, scent and pattern are all signals to pollinators, we don’t know how insects will respond to this beautiful petunia!

And bees can’t waste too much time visiting dry flowers.

Open for business!

But neither flower color nor scent registers the state of the nectar supply, Robert says. So how can flowers tell the bees they need some slack time to produce more nectar?

With the electric field, which gets weaker when a bee lands to gather pollen, and even more when a second bee arrives.

To the bee’s still mysterious electrical detector, Robert says, “some of the flowers look bright, and some are dim, which means they have been visited a few minutes before. And when they are out foraging, this means the bees can avoid a negative reward. It means the advertisement is honest, and it’s changing from minute to minute.”

We learn to rely on ads that accurately reflect conditions, Robert adds. “When you drive in your car, and the motel sign says ‘Vacancy,’ you might stop. When it say ‘No Vacancy, you don’t. You have learned to trust the sign.”

— David J. Tenenbaum

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Terry Devitt, editor; S.V. Medaris, designer/illustrator; David J. Tenenbaum, feature writer; Amy Toburen, content development executive; Emily Eggleston, project assistant

Bibliography

  1. Detection and learning of floral electric fields by bumblebees, Dominic Clarke et al, Science, 21 Feb. 2013. ↩
  2. Check out the scientific profile of the petunia ↩
  3. Bees in slow motion! ↩
  4. Bees galore: 50 interesting pics ↩
  5. Find out which flowers attract bumblebees ↩

Cotton pollination

Posted on 25 May 2011

Cluster of 11 spiky balls attached to hundred of finger-like projections
This image shows a very small portion of a cotton flower magnified more than 500 times. The spike-covered orbs are cotton pollen grains stuck to the papillar surface of the stigma, a sticky surface with finger-like projections. The stigma is located at the very top of the pistil, which is the female reproductive structure of the flower.

Cotton can self-pollinate or cross-pollinate with the help of bees that transfer pollen between the flowers of different plants. If conditions are favorable, the pollen grain will germinate after it is stuck to the stigma and form a pollen tube, which extends through the tissues of the pistil. Once the pollen tube reaches the ovary, fertilization can occur.

This image was taken with an Environmental Scanning Electron Microscope (ESEM).

Pollinator crisis ahead

Posted on 5 August 2010

Death of the mastodon

Posted on 19 November 2009

All in the timing: Decline of big beasts triggered ecological chain reaction

All in all, the period since the ice age abated about 15,000 years ago has been pretty interesting. Melting ice raised the oceans, flooding the Bering Strait land bridge across which the Americas were populated. Temperatures rose around the globe, leading to the invention of cities, armies, writing and bacon.

Here’s an enduring question. Why were the giant mammals that made the Americas more zoologically diverse than Africa all exterminated within a few thousand years after the big melt-down? Bye-bye beavers as big as black bears, giant sloths, saber-toothed cats, and the elephant-like mastodon.

[svgallery name="mastadon"]

As Australian paleontologist Christopher Johnson wrote in Science this week, all 10 species of mammals weighing more than a ton had gone extinct in North America by 10,000 years ago.

Why?

Many theories are proposed for the sudden disappearance: An impact of a comet or asteroid around 12,900 years ago. Rapid ecological changes that accompanied the warming. Widespread wildfires. And hunting – the “overkill” hypothesis. Although similar disappearances roughly coincided with the arrival of people in Europe, Eurasia and Australia, and hunger is certainly the ultimate motivation, did people actually lay waste to entire groups of large mammals?

The debate may seem academic, and it has been one of the most brutal and tenacious debates in academia.

Reading the dung calendar

Now we get some solid evidence that the extinction of the mastodon and other large herbivores closely followed the arrival of humans in North America, and that it preceded a pervasive change in type and prevalence of trees.

The new evidence, contained in research by Jacquelyn Gill and Jack Williams of the University of Wisconsin-Madison, and colleagues, was published in Science this week, and although it does not prove the overkill hypothesis, it does usher a new type of evidence into the debate: spores of fungi that grow in herbivore dung.

Between 14,800 and approximately 13,700 years ago, fungal spores of the genus Sporormiella declined by up to 98 percent in sediments found in lakes in Indiana and New York State.

Mastodons eat black ash trees as the last ice age begins to abate. Image courtesy Barry Roal Carlsen, University of Wisconsin-Madison.

Mastodons eat black ash trees as the last ice age begins to abate. Image courtesy Barry Roal Carlsen, University of Wisconsin-Madison.

For decades, students of ancient ecology have been poking through pollen in sediments to see what plants were alive when the sediment was deposited. Pollen are durable structures, but it turns out that Sporormeilla spores are equally tough, and if you have the patience (Why Filers immediately excuse ourselves at this point!) counting spores provides a good gauge of the number of herbivores.

Because the same sample also contains pollen and charcoal, it’s also possible to document the co-existing plant community, and get an idea of the extent of wildfires.

Fungi are a new addition to the paleoecologist’s toolkit, says Gill, first author of the paper, and a graduate student in Williams’s lab. “Only recently have fungal spores been getting any attention; we used to basically ignore them if we counted them at all, but now we realize they are a good source of information about early conditions.”

Being skeptics, we asked whether the decline could simply represent a change in conditions that was less conducive to preservation, but Gill says not. “If so, you would expect other proxies to show similar transitions. Since the same sediment that contains the spores also contains pollen, we’d expect to see pollen disappear, but we don’t.”

The dating game

Having a firm date for the decline of mastodons and other large herbivores is mainly helpful for eliminating some possible explanations, says Gill. The decline started almost 2,000 years before the putative impact of a comet or asteroid. And a change in climate apparently did not cause a broad habitat loss, Gill adds. “The extinction started before the habitat changed; the vegetation is relatively stable until after the extinctions began. We do have evidence of warming taking place, but if climate change is causing the extinctions, it’s not through a loss of food.”

A major ecological change did follow the elimination of large mammals, however, as documented by pollen representing a new assembly of trees, including ash and ironwood, which had probably been held in check by hungry herbivores, growing along with less nutritious conifers like spruce and larch. Once the grazers left, these trees began to dominate the landscape — and then became fuel for wildfires that burdened younger sediment with charcoal.

mastodon-sedim

Graduate student Jacquelyn Gill holds a sediment jar with a scrap of charcoal being prepared for carbon dating. Photo: The Why Files

Although the sexy “overhunting” hypothesis is sure to get a boost from the Science paper, Gill says one study hardly proves the case. And as Johnson notes in his commentary in Science, the Clovis people who spread across much of North America arrived more than 1,000 years after the decline began. Evidence for earlier North American populations is sketchy and scarce, but it is arising, Johnson added.

A second focus of the Gill paper may be equally important: the effect, rather than the cause, of the extinctions. “What happens when half of the species larger than a German shepherd go extinct in North America?” Gill asks. “Elephants eat 300 pounds of food a day, and when animals like the mastodon are rapidly taken out, you would think the landscape would notice, but that has been absent from the discussion. People were underestimating the power of these fungal spores to tell about the local presence of animals and vegetation.”

– David J. Tenenbaum

Related Why Files

• Revealed: Humans not Such Deadly Hunters
• Extinction: The Danger of Being Big
• Species Reintroductions
• Alpine Iceman: Home at Last!

Bibliography

• Pleistocene Megafaunal Collapse, Novel Plant Communities, and Enhanced Fire Regimes in North America, by Jacquelyn Gill et al, Science, 20 November, 2009.
• Megafaunal Decline and Fall, Christopher Johnson, Science, 20 November, 2009.

Why do flowers smell, and why do plants smell, too?

Posted on 24 July 2007

The luscious aroma of flowers attracts lovers, and the biological role of that smell is similar: to attract pollinators. “Plants need to attract insects, bats and hummingbirds to transfer the pollen and create fertile seeds,” says Hugh Iltis, professor emeritus of botany at UW-Madison.

Pollination is the transfer of pollen (the plant equivalent of sperm) to eggs. Some plants rely on wind or gravity, but many require animals to do the transportation. The smell of the flower alerts pollinators that the plant is ready to be pollinated, and when the animals arrive to collect pollen and/or nectar, pollen gets transferred.

Plants and pollinators often display a long history of mutual evolution, Iltis adds. When Charles Darwin saw a flower with a foot-long tube during the 1800s, he predicted the existence of a moth with an equally long “tongue” that could reach the female parts at the bottom of the tube. This moth was discovered more than a century later!

The minty, oily or sharp smells produced when you crush a leaf or stem play a defensive role, Iltis says. These smells come from chemicals that are often toxic to animals, and thus serve as a one-two punch: they smell (and taste) terrible, and then they make you sick if you ignore your senses and take a bite.

During the long struggle for existence, Iltis says, evolution has shaped every part of plants – including their chemical composition. But pollination is a troublesome subject: many crops are under threat as honeybees succumb to “colony collapse disorder.” Although the cause is unknown, environmental disturbance likely plays a role, Iltis says.

Fallin’ Pollen

Posted on 15 November 2004
Ragweed pollen as seen under a microscope

Ragweed pollen as seen under a microscope

Tissue, please… In honor of the sneezin’ season, this CSI is common ragweed pollen as seen under a microscope. Ragweed pollen is the principal cause of hay fever and can also trigger asthma. But for all the itchy throats and watery eyes, this tough little plant is just trying to survive.

The common ragweed is an annual that grows to about 3.5 feet tall, has hairy stems, divided leaves, and simple greenish-yellow flowers. Arguably not much to look at, the ragweed does not rely on the help of flying creatures to transfer pollen from plant to plant. Thus it lacks the bright, smelly flowers that attract the birds and bees.

The ragweed relies on a process called wind pollination to procreate. The light, powdery pollen forms in the anther, or male part, and breezes through the air to the pistil, or female part, where fertilization occurs. Structurally, the pollen grain is multi-layered. The outer layer, or exine, not only protects the center nuclei responsible for fertilization, but also makes the grain virtually indestructible. A single ragweed plant can generate a million grains of pollen each day during its peak season, mainly late summer and early fall. Not all pollen reaches its intended destination in the pistil and instead lands in the human nose and many geologic sediments. In fact, grains of ragweed pollen have been found 400 miles out to sea and 2 miles high in the air. By studying geologic pollen sediments scientists have been able to make remarkable discoveries about the origin and evolution of plant life.

Pollen image courtesy of NIAID.

Might Have Made You Sneeze…If You Were A Dino!

Posted on 11 January 2000
Triprojectus unicus

Triprojectus unicus

This CSI is a picture of a pollen grain from an extinct group known as triprojectates. This particular beast, Triprojectus unicus, was common about 65 million years ago in the Late Cretaceous, when the dinosaurs still ruled the roost. The nasty-looking recurved spines may have allowed the pollen grains to hitch rides on passing insects. Pollen survives in the fossil record because their external walls were, and are, composed of sporopollenin, a rugged organic polymer that can endure the rigors of the environment and the process of fossilization. It is from little gems like these that scientists learn about the flora — and the larger environment — of the age of the dinosaurs.

Special thanks to Andrew MacRae of the University of Calgary who has a nifty gallery of pollen from the past.