This shrub could provide rubber, insect repellent and glue

The sage-green, waist-high shrub guayule might look like any of the many ruffled bushes that dot the hills and valleys of the Chihuahuan Desert in Mexico and the southwestern United States. But its indescribable stems and leaves house an array of botanical treasures, including rubber that indigenous peoples used hundreds of years ago to make playground balls.

For decades, researchers have studied guayule (Parthenium argentatum) like a potential commercial source of natural rubber. The market is currently dominated by rubber from Rubber tree brasiliensis trees in Southeast Asia, and it is difficult for guayule in the United States to compete with these large farms, which have lower labor costs. Farmers in the Southwest tend to favor more lucrative crops, such as alfalfa and cotton.

But two converging developments may be about to change guayule’s outlook. Fungal infestations of monocultures rubber tree trees have caused sharp declines in global natural rubber supplies in recent years. Meanwhile, the southwest’s worst drought in more than a millennium has reduced the water available to farmers, making it harder to grow alfalfa, cotton and other thirsty crops. Guayule, on the other hand, is drought tolerant. And it can be grown and harvested for several years without tilling the ground. This undisturbed soil stores carbon in the soil and prevents erosion.

In an effort to make guayule a more viable crop, academic and industrial researchers are working to increase the plant’s rubber production, while studying uses for the sticky resin and woody material it produces. With guayule, “a farmer could still grow things in the desert and still earn a living while protecting the soil and protecting the water,” says Catherine Brewer, a chemical engineer at New State University. -Mexico. “There aren’t many plants that meet all of these criteria.”

A latex medical glove made from guayule rubber. Credit: Sarah Davis, EnergyEne

Tire maker Bridgestone has operated a demonstration-scale processing facility in central Arizona for the past decade with the goal of showing that guayule can eventually be harvested for natural rubber at scale. commercial. At the end of August, the company announced that it commercialize guayule rubber production by 2030. Although two-thirds of the world’s rubber is now made synthetically from petroleum, rubber produced from natural sources is indispensable for some purposes. Aircraft tires, for example, are made from natural rubber, which is superior to synthetics at handling impacts such as landing on the runway.

Since only a small amount of guayule is currently grown, it could not compete with rubber tree large-scale rubber for a while, says Katrina Cornish, who studies alternative sources of rubber at Ohio State University. This means that companies wishing to use it should start with high-value products, such as the guayule rubber racing tire Bridgestone debuted this year. At his own company, EnergyEne, Cornish focuses on high-end products made from guayule latex, a softer form of rubber that the factory also manufactures. EnergyEne, for example, is developing a medical glove that attenuates radiation. On the consumer side, the company has made some “pretty preservatives,” Cornish adds, including those flavored with Cabernet Sauvignon and Chardonnay. Guayule makes the best latex, compared to other natural and synthetic formulations, because it’s especially strong, stretchy and soft, says Cornish. It is also hypoallergenic, unlike latex derived from rubber tree trees. Jason Quinn, a sustainability researcher at Colorado State University, conducted a study in 2020 that found that guayule rubber grown on a typical-sized farm in Arizona could be cost competitive with rubber tree rubber-although the cost of the guayule product would be towards the upper end of the historical price range rubber tree prices.

At Bridgestone, plant geneticist David Dierig hopes to improve the profitability of guayule by breeding plants that bear more rubber. His team mapped the guayule genome and identified genes associated with higher rubber content. With this information, researchers can select plants with higher rubber-producing potential and cross-breed them before they are fully mature, shortening the time it would normally take to grow new generations. There is a certain limit to the amount of rubber a plant can produce, Dierig says, but it’s hard to know where that limit lies. He says high molecular weight rubber – the type used for tires – initially made up about 2.8% from the factory, and he increased that to 3.8%. “We need to bring the rubber content to around 6% to be directly competitive. [with Hevea]explains Dierig. However, selling other products made from the resin and woody parts of the plant could help guayule compete before it reaches that level of rubber, he adds.

The woody stems provide a less intensive option for developing guayule products to begin creating more of a commercial incentive to grow the plant. In processing guayule, the bushes are crushed and the rubber and resin are extracted using a solvent, leaving behind a dry sawdust-like material called bagasse. This can be pressed into particle board such as that used in furniture or potentially made into biofuels for ships and planes – the latter conversion requires pyrolysis, a process in which bagasse is heated in the absence of oxygen . “There is technology for that. It’s doable, but it takes a bit of energy,” says Kim Ogden, who runs a research project funded by the United States Department of Agriculture which is based at the University of Arizona and works with several partner institutions, including Bridgestone and Colorado State University. “So I’m not sure economically how great it is.”

Ogden and other researchers believe that the real source of silver may be guayule resin. The sticky stuff is made up of many organic compounds, including essential oils that could be used as fragrances and other molecules called guayulines and argentatins that are unique to guayule. Researchers say resin compounds hold promise for a number of potentially lucrative uses. Scientists at the University of Arizona have found that the resin can be used for plant-based adhesives, potentially replacing some formaldehyde-containing products such as wood glue. A recent study also found that argentatin-derived compounds were toxic to three types of cancer cellsopening up the possibility of their use in drug research.

The New Mexico State University brewer is leading a team testing whether the resin compounds could be used as insect repellents. She designed experiments comparing cockroaches’ responses to guayule resin extracts with their reactions to java oil (which is a known cockroach repellent) and acetone (which does not bother cockroaches). Some of the resin compounds, including some specific to guayule, thwarted cockroaches even more than java oil — “which we found quite exciting,” she says. The insects seemed to dislike the smell of the resin, although more research is needed to understand how this possible repellent works. Brewer also begins an experiment with mosquitoes. She thinks a guayule resin extract would probably repel them too. But launching a topical consumer product requires a series of safety tests, and these compounds may cause allergies or irritate the skin.

“Resin contains a huge mixture of compounds, and what’s going to determine how it’s used is how much money it takes to separate it, versus the value of the products you get,” Brewer says. “So there will be a sweet spot of just enough treatment and purification, relative to uses, and we don’t know where that sweet spot is yet.”

Although guayule has some way to go on the road to commercialization, the researchers involved are optimistic. Quinn at Colorado State University leads economic evaluations for many emerging products and technologies, and he says only about one in 10 ends up looking viable. “This,” he said, “is part of it.”