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Saturday, December 10, 2022

Pollen and heat: a deadly mix for global agriculture

This story was originally published by Yale Environment 360 and appears here as part of the Climate Desk collaboration.

Last June, Aaron Flensberg felt the temperature rise and knew what that meant for his canola crop. A fifth-generation grower in Washington state, Flansburgh prunes his canola plantings to get blooms in the cool weeks of early summer. But his fields were hit by 108 F Fahrenheit heat as soon as the flowers opened last year. “It’s almost unheard of in our region to have temperatures like this in June,” he says.

Yellow flowers bloomed, reproduction halted, and many of the seeds that were pressed for canola oil never formed. Flansburgh yields about 600 to 800 pounds per acre. Last year, in ideal weather conditions, that reached 2,700.

Several factors contributed to this poor harvest – heat and drought persisted throughout the growing season. But one point is becoming alarmingly clear to scientists: Heat is a pollen killer. Even with enough water, heat can damage pollen and prevent fertilization in canola and many other crops, including corn, peanuts and rice.

For this reason, many growers aim for crops to bloom before temperatures rise. But as climate change increases the number of days over 90 F in regions around the world, and multi-day portions of extreme heat become more common, getting that timing right can be challenging, if not impossible.

Facing a warming future, researchers are exploring ways to help pollen beat the heat. They are searching for genes that could lead to more heat-tolerant varieties and breeding varieties that can survive winter and flower before heat strikes. They are investigating the exact limits of pollen and are even harvesting pollen on a large scale to spray directly on crops when the weather improves.

There is a lot of our diet at stake. Every seed, grain, and fruit we eat is a direct product of pollination, explains biochemist Gloria Muday of Wake Forest University in North Carolina. “The critical parameter during breeding is the maximum temperature,” she says.

Seed formation begins when a pollen grain leaves the anther of the plant’s male reproductive organ (stamen), descends on the sticky stigma of the female reproductive organ (pistil), and prepares to develop a tube. This tube is formed by a single cell that grows through the stigma and down a stalk called the stye until it eventually reaches the ovary, where it delivers the pollen grain’s genetic material. Pollen tube development is one of the fastest examples of cellular evolution in the plant world, says Mark Westgate, an emeritus professor of agronomy at Iowa State University. “It moves up to a centimeter per hour, which is incredibly fast,” he says.

To grow on such a clip requires energy. But for many crops, starting around 90 F, the proteins that power pollen grain metabolism begin to break down, Westgate says.

In fact, heat hinders not only tube development but also other stages of pollen development. Consequences: A pollen grain may never form, or may burst, fail to produce a tube, or produce a tube that bursts.

There is a lot of our diet at stake. Every seed, grain, and fruit we eat is a direct product of pollination, explains biochemist Gloria Muday of Wake Forest University in North Carolina. #Agriculture #Global Food Supply

Not all varieties are equally sensitive to heat. In fact, researchers are still working on the molecular mechanism that allows pollen from some crop varieties to survive while pollen from others dies.

For example, fertilization in many tomato varieties is notoriously sensitive to heat—a crop that covered 274,000 acres of open fields in the United States alone in 2021. If the weather gets too hot, “the pollen will burn,” says Randall Patterson, president of the North Carolina Tomato Growers Association. Patterson multiplies his tomato plantings below 70 F and below 90 F to flower during the longest periods of days. Typically, it has a window of three to five weeks in which the weather cooperates for each of its two annual growing seasons. “If it gets hot, and if we have nights over 70 degrees,” he says, “it’s going to close our window.”

Muday studies pollen from a mutant tomato plant that may hold clues to keeping that window open. In 2018, his team reported that antioxidants known as flavonols play a key role in suppressing molecules called ROS, which otherwise rise to destructive levels at high temperatures.

With funding from the National Science Foundation, Muday is now part of a multi-university team that aims to uncover the molecular mechanisms and underlying genes that may help tomato pollen weather summer. The hope is that breeders can incorporate these genes into new, more resilient tomatoes.

Insights from his initial studies helped Muday develop a tomato that produced particularly high levels of flavonols. “They seem extra good at dealing with high temperature stress,” she says. Ultimately, Muday hopes they will find that the pathway from heat to pollen death involves many players beyond flavonols and ROS, and potentially many targets for improvement.

Meanwhile, breeders of tomatoes and other crops are already working to develop varieties that can better handle the heat. “If farmers in the Pacific Northwest or the mountain states or the high plains are going to grow peas, and the climate is going to warm, we have to have peas with a greater heat tolerance,” says pulse crop breeder and plant Pullman, Geneticist Rebecca McGee of the USDA Agricultural Research Service in Wash.

Pulse crops—hence the name of the Latin “pulse”, meaning thick soup—include dried beans, peas, lentils and chickpeas. These plants do not require much moisture. But if temperatures get too hot, the pollen takes off, says Todd Scholz, vice president of research for the USA Dry Pea and Lentil Council. The same heat wave scorched the Flansburgh crop last year, which destroyed pulses. Crops of lentils and dried peas fell to almost half of the average production, while gram fell by more than 60 percent.

Photo By Ali Graney/Flickr (Cc By 2.0)
A mutant tomato plant provides clues to producing more heat-tolerant pollen. Photo by Ali Graney/Flickr (CC BY 2.0)

McGee is breeding some of its peas and lentils to make them more resilient to high temperatures. But as with other projects, she’s taking a different and somewhat counterintuitive approach: growing crops that can withstand the cold.

In the northern United States, growers typically harvest pulses in the spring. McGee is breeding peas, lentils and chickpeas that are sown in autumn instead. The idea is that these varieties will survive the winter and then jump-start when they flower in early summer—giving them a fighting chance to successfully pollinate before a heat wave.

Last year, McGee released seed growers a limited quantity of the first three autumn-sown, food-quality pea varieties for their region. She says they flower about two weeks earlier than most spring-sown peas—and double the yield. Of course, these crops aren’t guaranteed to flower before high summer arrives, McGee says, “but you don’t have to worry as much.”

At Michigan State University, Jenna Walters is studying how temperature affects pollen and pollinators in fruit crops. Over 2018 Memorial Day weekend, temperatures hit 95 F in southwestern Michigan while bees buzzed among clusters of delicate white flowers on blueberry bushes. Come harvest, many fruits were smaller than usual or had failed to form fully. In a state that produces an average of about 100 million pounds of blueberries annually, growers harvest just 66 million.

Walters – a PhD candidate who earned a dual degree in entomology and ecology, evolution and behavior – is investigating what really went wrong. She began by pinpointing the heat limit of the blueberry pollen grain — exposing the pollen to a range of temperatures in a Petri dish and monitoring the pollen for 24 hours. Her results, which have not yet been published, suggest that at temperatures above 95 F, pollen tubes fail to grow.

Walters simulated an intense heat wave by exposing the pollen grains to 99.5 F heat for four hours and then lowering the temperature to 77 F for another 20 hours. “Basically there’s no return,” Walters says. ,[Heat] Exposure for just four hours is enough to cause permanent damage.”

She is now confirming these results in actual blueberry bushes in growth chambers set at different temperatures. If the findings hold, she says, 95 F could trigger growers to periodically flip over cold fields on their haze systems. But producers have to consider tradeoffs. “A lot of pathogens are spread through high humidity or water, especially during that flower-opening period,” she says. And when misting machines are on, most pollinators are unlikely to arrive.

Walters says it’s possible that warming blueberry bushes over time could reduce the number of blueberry pollinators. He and his colleagues are comparing the nutritional content of heat-stressed and unstressed pollen, finding differences in protein, carbohydrate and other factors that may be important to bee health.

This year, she’ll fill eight 6-by-12-foot mesh-walled cages with more than two dozen potted blueberry bushes, plus a few female Blue Orchard bees — one of several bee species that pollinate blueberry flowers. Huh. For four hours a day, over the course of four or five weeks, she would sit inside her cages and watch the bees lay eggs and forage for pollen on bushes that, in half the cages, were exposed to early heat stress. . Their bloom

The worry, Walters says, is that if heat is destroying pollen, nutritional stress can cause females to produce more male eggs, which require less pollen to produce. But male blue orchard bees are less useful to the blueberry grower, as only the females pollinate and lay eggs to start the next generation. To compensate for the loss of pollen, Walters says, growers may consider planting strips of wildflowers that are more heat tolerant and can provide pollinators with additional nutrients.

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Bumble bee on Blue Berry Blossom. Photo by John Flannery/Flickr (CC BY-ND 2.0)

And then there are technofixes. Mark Westgate of Iowa State is chief science officer at Powerpollen, an Iowa-based ag tech company focused on improving pollination for growers of hybrid corn seeds—a crop in which pollen grows at temperatures above 104 F. fails.

Using a tassel-shaking collection device attached to a tractor, the company collects large amounts of ripe pollen in fields, then stores those live pollen grains in a controlled environment. PowerPollen returns to applying that pollen when weather conditions favor fertilization—usually no later than five days after collection. The window seems small, but it enables farmers to dodge a particularly hot day. The company is working on extending this deadline and applying its technology to other crops.

For some, a simple solution may be to replace crops altogether. “There are pulses that grow in tropical climates, so you might want to choose a different variety,” says Scholz of the Dry Peas and Lentils Council. But some pulses that stand up to heat, he notes, such as fava beans and black-eyed peas, require more moisture than the dryland farmers of the Pacific Northwest supply.

Flensburg in Washington, doesn’t want to switch. He hopes the breeding efforts will help him grow canola and other crops that his family has cultivated for generations. Still, he worries about the future.

“There is an overall picture of a changing climate that we have to address and if we are going to be able to continue to feed people,” he says. “There is a limit to how much heat a plant can take in.”

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