Pollen wars: Plants compete for space on pollinators

New research by scientists from South Africa and Brazil reveals that rival plants may compete for space on pollinators, with pollen grains vying for the chance to be carried to the next flower. 

The study, published in the journal The American Naturalist, suggests that plants have developed sophisticated mechanisms to manipulate pollen placement on pollinators, similar to how some animals manipulate sperm during reproduction.

Plants and competitive pollen removal 

Unlike animals, where mating involves direct contact, plants rely on pollinators such as birds and insects to transport pollen from one flower to another. In this context, plants need to find ways to ensure their pollen makes it to the pollinator and, eventually, to the next flower. 

According to the study’s lead author, professor Bruce Anderson of Stellenbosch University, the strategy plants have developed can be compared to male-male competition in animals.

“Flowers visited by hummingbirds deposit their pollen on the hummingbird bills, but there is very little place for the pollen to be deposited,” Anderson explained. 

“Flowers have evolved a catapult mechanism where pollen is shot at the bill of the hummingbird. The force of the ballistic grains dislodges previously deposited grains from rival plants, allowing the flower to place its own grains onto a cleaner bill, thus increasing its chances of reproductive success.”

This finding supports the idea of competitive pollen removal, where flowers actively remove rival pollen from pollinators before securing a spot for their own pollen.

Explosive pollen placement in action

To demonstrate this mechanism, the researchers conducted experiments with Hypenea macrantha, a red flower native to Brazil. Using slow-motion video footage, they captured the flower’s explosive pollen placement mechanism in action. 

The flower essentially “catapults” its pollen onto the bill of a hummingbird, dislodging the pollen of competing plants in the process.

This mechanism, known as explosive pollen placement, is not new in the plant kingdom, but this study marks the first time researchers have provided empirical evidence of its effectiveness. 

By labeling pollen with quantum dots and counting how much was displaced by the explosion, the team was able to confirm the success of the mechanism.

A new perspective on floral competition

Traditionally, floral explosion has been viewed as a strategy to help plants efficiently place their pollen on pollinators or to startle pollinators into flying to other plants, thereby dispersing the pollen over greater distances. 

However, this new research shows that explosive pollen placement may serve an additional function: removing rival pollen to improve a plant’s reproductive success.

“Our data suggest, however, that this mechanism may actually displace pollen from previous flowers, enhancing male reproductive success by increasing competition for space on the pollinators’ bodies,” said co-author Vinícius Brito, a botanist at the Federal University of Uberlândia in Brazil.

Plant reproduction and pollen rivalry

The research highlights a previously unrecognized form of competition between plants. In contrast to animals, where males may use physical structures to remove rival sperm from females, plants must manipulate the placement of their pollen on pollinators before it reaches the next flower.

As Anderson explained, “until recently, no one has ever thought of looking for these kinds of structures in plants.”

This form of competition occurs on the pollinator’s body. When a pollinator, such as a hummingbird, arrives at a flower, it may already be covered in pollen from previously visited plants. 

This makes it challenging for the new flower to place its pollen, as space is limited. Even if the flower does manage to place its pollen, it must then compete with rival pollen grains for access to the ovules of the next flower.

Implications for understanding plant evolution

This discovery offers new insights into how plants may compete with one another for reproductive success in ways that were previously unimagined. Anderson and his colleagues suggest that plants may evolve strategies akin to those in animals, where physical adaptations aid in removing rival sperm.

As the researchers continue to explore this phenomenon, their findings could reshape our understanding of how plants have evolved complex mechanisms to increase their chances of reproductive success. 

By manipulating pollen on the bodies of pollinators, flowers may be competing in a hidden but highly effective battle for space and, ultimately, for the continuation of their species.

In conclusion, this study adds a fascinating new dimension to our understanding of plant reproduction and competition. As Brito points out, these discoveries highlight the importance of further research into how plants interact with their environment, pollinators, and even each other in their quest for survival.

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