High heat destroys pollen and thus nourishes people. Farmers and researchers are looking for solutions to make agricultural production last

Many of the crops we depend on need pollination to produce food, but extreme heat can destroy pollen. This is a problem in a world facing climate change, so scientists are looking for a solution, according to the BBC.

In June of last year, Aaron Flensburg, a fifth-generation farmer in Washington state (northwest US), felt the sudden rise in temperature and knew what it meant for a crop of canola (a hybrid rapeseed plant). Flensburg canola plants bloom in the cool weeks of early summer. But last year, his crop was exposed to 42 degrees Celsius heat when the flowers opened. “It was unbelievable in our area that temperatures could be like those in June,” he says.

Field canola (rapeseed). Photo: Profimedia Images

The yellow flowers have dried up, reproduction has ceased, and many seeds that have been pressed to obtain canola oil are not formed. Flensburg harvested approximately 272-363 kilograms per acre (almost half a hectare). The previous year, under ideal climatic conditions, her yield was 1,225 kg per acre.

Many factors could have contributed to the poor harvest – heat and drought persisted during the growing season. But there is one aspect that is disturbingly clear to scientists: heat kills pollen.

Even with the right water, heat can damage pollen and stop fertilization in canola and many other crops, including corn, peanuts, and rice. For this reason, many farmers schedule flowering crops before temperatures rise.

But as climate change increases the number of days when temperatures exceed 32°C in different regions of the world, and periods of extreme heat become more frequent, timing with the right timing may become more difficult if not truly impossible.

Researchers are looking for ways to help pollen survive the heat. They’re discovering genes that can create more heat-tolerant varieties and varieties that can survive the winter and also thrive before the onset of heat. They test the fine limits of pollen and even grow pollen on a large scale to be sprayed directly on crops as the weather improves.

Bindweed . Pollen
Pollen grains on the surface of the leaf. Photo: Profimedia Images

Every seed, grain, and fruit we eat is a direct product of pollination

So much of our food is at stake. Every seed, grain, and fruit we eat is a direct product of pollination.

Seed formation begins when a pollen grain leaves the male reproductive organ of the plant (the stamen), lands on the sticky stigma of the female reproductive organ (the pistil) and begins to grow a tube. The tube is made of a single cell that grows in the stigma and through a stem called the pattern until it finally reaches the ovary, where it transfers the genetic material for pollen.

Fruit training, artwork
Fertilize flower and fruit respectively. Photo: Profimedia Images

Pollen tube growth is one of the fastest examples of cell growth in the plant worldsays Mark Westgate, professor emeritus of agricultural engineering at the University of Iowa. “It grows at up to an inch an hour, and it’s incredibly fast,” he says.

32°C – the limit above which pollen disintegrates

Growing at such a rate takes energy. But at temperatures starting around 32 degrees Celsius for many crops, the proteins that drive pollen metabolism begin to break down, Westgate says.

In fact, heat prevents not only the growth of the pollen tube, but also other stages of its development. The result is that pollen may not form at all or may disintegrate, fail to produce that tube that carries the genetic material, or produce a tube that bursts.

Pollen germination, SEM
Pollen germination. Photo: Profimedia Images

Not all types of plants are equally exposed to heat. In fact, researchers are still trying to discover the molecular mechanisms that allow pollen from certain crop varieties to survive while pollen from other species die.

For example, fertilization in many tomato species is notorious for its sensitivity to heat. If the weather is too hot, “the pollen will burn,” says Randall Patterson, president of the North Carolina Tomato Growers Association.

Biological solutions and studies

Patterson synchronizes growing seedlings to thrive during the longest nights below 21°C and on days below 32°C. They usually have a three to five week window in which the weather cooperates in each of the two annual growing seasons.

Gloria Moday of Wake Forest University in North Carolina studies pollen from a mutated tomato plant that may have clues to keeping that window open. In 2018, her team noted that antioxidants known as flavonols play an important role in suppressing highly reactive oxygen-containing molecules, called reactive oxygen species, which can rise to devastating levels when temperatures are high.

Daisy Vaccine, SEM
Daisy vaccine. Photo: Profimedia Images

Researchers at several universities, including Muday, want to discover the molecular mechanisms and genes involved that can help pollen resist during heat waves. The hope is that breeders will incorporate these genes into new, more resistant tomatoes.

Data from her initial study actually helped Muday make tomatoes that produce high levels of flavonols. “They seem to be very good at withstanding heat stress,” she says.

Finally, Muday expects to find that this pathway from heat to pollen death could involve many factors other than flavonols and reactive oxygen species, and thus many potential targets for modification.

Wild honey bee pollination of squash flower, Toronto, Canada - August 12, 2021
A bee pollinates a pumpkin flower. Photo: Profimedia Images

Meanwhile, growers of tomatoes and other crops are already working on developing varieties that can better handle the heat.

Dried bean, pea, lentil and chickpea crops do not require much moisture. But if the temperatures rise too much, the pollen disappears.

Photo: Profimedia Images

Some growers have another strategy – to plant more cold-resistant varieties. Thus, crops that don’t require a lot of moisture, instead of planting them in the spring, farmers in the northern United States are trying to sow in the fall to try to survive the winter and thrive early in the summer.

Rebecca McGee of the USDA Agricultural Research Service in Pullman, Washington, demonstrated such crops are a limited amount of the first three varieties of peas planted in the fall and suitable for food in the region. She says it blooms about two weeks earlier than most spring-planted pea plants with nearly double production.

In the case of fruits, pollen has its temperature limits, being problems detected in blueberry crops, where the threshold at which a plant can be fertilized is 35 ° C, points out Michigan State University researcher Jenna Walters. Many fruits were smaller than normal or not formed at all. In Michigan, where production averages about 45,359 tons of blueberries per year, farmers in 2018 harvested just 29,937 tons.

“Exposure to heat for just a few hours is enough to cause permanent damage” to pollen, Walters says after his simulation.

One solution farmers are adopting is to use water systems to distribute water periodically to crops to cool them. The researcher says that many pathogens are spread by high humidity or water, especially during flowering. And when moisture begins to spread, many pollinators are discouraged from spreading among the flowers.

Walters is also studying whether berry bushes exposed to excessive heat can lead to a decrease in the number of pollinators, such as bees, over time.

Flensburg, a farmer in Washington, doesn’t want to switch to other crops. He still hopes that his breeding efforts will help him continue to grow the canola and other crops his family has grown for generations. However, he is worried about the future. “There is already an overview of climate change that we will have to address and manage in order to continue feeding people,” he said. “There is simply a limit to the heat a plant can tolerate.”

Editor: AC

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