Blooms on ‘chocolate’ tree are crazy-hard to pollinate | Science News for Students

Blooms on ‘chocolate’ tree are crazy-hard to pollinate

The plant’s reproductive system challenges Mother Nature’s pollen-movers
Feb 8, 2018 — 9:50 am EST
cacao flowers
The seeds that give the world chocolate come from coy, fussy flowers that make pollination very difficult.
Tatters/Flickr (CC BY 2.0)

It’s a wonder chocolate exists. Talk about plants that resist help. Cacao trees provide the seeds from which chocolate is made. But those seeds develop only once the trees’ blooms have been pollinated. The trees’ fruit — known as pods — are created by dime-sized flowers. And those blooms are difficult. They make pollination barely possible.

Growers of other commercial fruits expect 50 to 60 percent of the flowers on their crop plant to make seeds, notes Emily Kearney. And some cacao trees manage those rates. Kearney knows. She works at the University of California, Berkeley. A biologist there, she focuses on the pollination of cacao. The problem: Pollination rates in these plants tends to be much lower — as in closer to 15 to 30 percent. But in the South American country of Ecuador, traditional plantings may contain a mix of species. There,  Kearney has seen cacao pollination rates of just 3 to 5 percent.

The first sight of a blooming cacao tree (Theobroma cacao) can be “disconcerting,” she says. That’s because flowers don’t sprout from branches as in many other trees. Instead, they emerge directly from the trunk. They burst into little pink-and-white constellations of five-pointed starry blooms. Some trunks, Kearney says, “are completely covered with flowers.”

Pretty as they are, these flowers make nothing easy. Each petal curves into a tiny hood. This hood fits down around the plant’s male, pollen-making structure. To reach that pollen, a honeybee would be a useless giant blimp. So tiny flies step up to the task. Each of them is little bigger than a poppy seed. Known as chocolate midges, they are part of a family called biting midges.

After crawling up into the flowers’ hoods, they do — something.

But what? The flower offers those midges no nectar to drink. So far, researchers haven’t even shown that some scent lures in the midges. Some biologists have mused that reddish parts of the flower offer nutritious nibbling for the bugs. But Kearney knows of no tests that have confirmed this.

Another hitch to pollination: One cacao pod (resembling a wrinkled, swollen cucumber in shades of brown, purple or orange) requires from 100 to 250 grains of pollen to fertilize its 40 to 60 seeds. Yet midges typically emerge from a flower hood speckled with just a few to maybe 30 grains of the sticky white pollen. (Kearney says those pollen grains look like “clumpy sugar.”)

Story continues below image.

cacao pods
Pods, here, from Theobroma cacao trees are plump (with dozens of seeds) and vary a lot in color.
E. Kearney

What’s more, the midge can’t just hike over to the female part of the same bloom. The female part sticks up in the very center of the flower, like some white-bristled paintbrush. Yet pollen is useless for any blooms on the tree it came from. That pollen won’t even work for close relatives.

To better understand cacao pollination, Kearney doesn’t suggest looking for answers at cacao farms. She says, “I think it’s the wild individuals that are going to open up the field.”

These trees evolved mostly in the Amazon Basin. There, cacao trees often grow in clusters of siblings that a monkey might have accidentally planted (while sucking pulp from a pod, dropping seeds as it fed).

To Kearney, dot-sized midges seem unlikely to fly the distance from clusters of cacao siblings to unrelated trees where cross-pollination chances would be better. So she wonders: Could the cacao with its elaborate reproductive system have a stealth, strong-flying native pollinator species that has to date escaped scientists’ notice?

Power Words

(for more about Power Words, click here)

basin     (in geology) A low-lying area, often below sea level. It collects water, which then deposits fine silt and other sediment on its bottom. Because it collects these materials, it’s sometimes referred to as a catchment or a drainage basin.

biology     The study of living things. The scientists who study them are known as biologists.

bloom     A flower. Or the term for the emergence of flowers.

bug     The slang term for an insect.

cacao     The name of a tropical tree and of the tree’s seeds (from which chocolate is made).

commercial     (in research and economics) An adjective for something that is ready for sale or already being sold. Commercial goods are those caught or produced for others, and not solely for personal consumption.

constellation     Patterns formed by prominent stars that lie close to each other in the night sky. Or something that has some starlike pattern.

crop    (in agriculture) A type of plant grown intentionally grown and nurtured by farmers, such as corn, coffee or tomatoes. Or the term could apply to the part of the plant harvested and sold by farmers. 

develop     (in biology) To grow as an organism from conception through adulthood, often undergoing changes in chemistry, size and sometimes even shape.

fertilize     (in biology) The merging of a male and a female reproductive cell (egg and sperm) to set in create a new, independent organism.

field     An area of study, as in: Her field of research was biology. Also a term to describe a real-world environment in which some research is conducted, such as at sea, in a forest, on a mountaintop or on a city street. It is the opposite of an artificial setting, such as a research laboratory.

fruit     A seed-containing reproductive organ in a plant.

insect     A type of arthropod that as an adult will have six segmented legs and three body parts: a head, thorax and abdomen. There are hundreds of thousands of insects, which include bees, beetles, flies and moths.

midges     Any of many types of small flies that often live around water. Some are blood-sucking insects; others can derive their energy from eating plants. Frequently mistaken for mosquitoes, midges can transmit disease or move pollutants through an ecosystem.

native     Associated with a particular location; native plants and animals have been found in a particular location since recorded history began. These species also tend to have developed within a region, occurring there naturally (not because they were planted or moved there by people). Most are particularly well adapted to their environment.

nectar     A sugary fluid secreted by plants, especially by flowers. It encourages pollination by insects and other animals. It is collected by bees to make into honey.

pollen     Powdery grains released by the male parts of flowers that can fertilize the female tissue in other flowers. Pollinating insects, such as bees, often pick up pollen that will later be eaten.

pollinator     Something that carries pollen, a plant’s male reproductive cells, to the female parts of a flower, allowing fertilization. Many pollinators are insects such as bees.

pulp     The fibrous inner part of a vegetable or fruit (such as an orange).

sibling     An offspring that shares the same parents (with its brother or sister).

species     A group of similar organisms capable of producing offspring that can survive and reproduce.


Website. U.S. National Park Service. Pollinators — Chocolate midge.