Guayule and TKS: Two sides to two promising alternatives for hevea

There exists over 2,500 rubber producing plants. Yet, the world relies heavily on just one monoclonal crop, the hevea brasiliensis rubber tree, to support the production of more than 50,000 different products worldwide.

"We use more and more rubber every year because it's such a useful material," Katrina Cornish, professor and Ohio research scholar of bio-emergent materials at the Ohio State University, said in a recent keynote presentation at the ACS Rubber Division's Spring Technical Meeting.

But there are several challenges facing the supply of hevea NR.

For starters, it is highly susceptible to disease as a monoclonal crop. Should South American Leaf Blight (SALB)—a fatal leaf disease endemic to Brazil—"hop the world pond" and infect trees in Southeast Asia, it would bring the "unacceptable risk" of the plant's total collapse, Cornish warned. And with it, the global economy.

As a tropical tree, hevea is grown in limited regions of the world, with China controlling over 80 percent of the global NR supply.

This doesn't bode well for the U.S., particularly, as it has no domestic supply of the critical material despite constantly increasing demand.

Therefore, alternatives are essential to developing biological and geographical diversification of natural rubber, which in turn is necessary for global and U.S. rubber security, according to Cornish.

As it stands today, there are two alternatives to hevea NR that show the most promise for commercialization: the parthenium argentatum desert shrub, commonly known as guayule, and the taraxacum kok-saghyz dandelion, known as TKS.

While both plants bring their own benefits, they also face their own challenges.

As a desert shrub, guayule can be grown in semi-arid conditions and is drought resistant, making it an opportune crop for farmers in the southwest region of the U.S. or other arid regions of the world, according to Cornish. This is a particularly important attribute in the face of worsening climate conditions.

The crop's agronomic production is mature, Cornish added, and because it does not require pesticides while grown in the southwestern region of the U.S., it's beneficial to pollinators.

As for its latex properties, the crop boasts some unique features. Currently, there are no latex allergies associated with GNR, and the material is stronger, softer and stretchier than standard HNR.

In addition, GNR has "exceptionally high polymer filler interactions," Cornish said, with filler loading capacity up to three times higher than hevea.

As for TKS, the crop's rubber properties are most similar to hevea compared to other alternatives.

Its benefits come primarily from its roots, according to Nathaniel King Smith, a Ph.D. student at OSU advised by Cornish who also spoke during the Spring Technical Meeting. In its roots, the plant contains latex and dry rubber, as well as inulin and bagasse, which are beneficial to medical and combustion uses, respectively.

The plant also brings with it numerous byproducts, Smith said. Its leafy greens can be used for human or livestock consumption in salad mixes or as pelletized feed, respectively, and amino acids can be extracted from the plant.

The dandelion can be grown in both colder regions of the world, making it a viable crop for northern regions of the U.S., and in semi-arid conditions, making it a good southern crop for the wintertime, Cornish said.

When harvested in cold conditions, TKS also has shown the potential to double its rubber yield due to the degradation of inulin. This outcome, however, has yet to be controlled, she said.

Both guayule and TKS face similar challenges, however, as both plants are only in the proof-of-concept phase in their development as a commercial alternative to hevea.

To bring these plants up to scale, a significant amount of funding from universities, government entities and related associations is required because the U.S. lacks the infrastructure required to efficiently process the plants and extract rubber.

Both crops also produce significantly lower yields compared to hevea, Cornish said, though genetic modification and molecular breeding is in the works to enhance their yields.

For TKS specifically, challenges are most prevalent in growing and harvesting the crop.

When grown hydroponically using an ebb-and-flow method, the plant yields significantly larger roots, according to Smith's research. And while it produces good quality dry rubber, it does not produce latex when grown in these conditions.

In order to be grown at a commercial scale, TKS must be planted in fields, he said, as hydroponics and indoor greenhouses are not yet viable options for the space required to support rubber product production, particularly for the tire industry.

But as a "weed" itself, the dandelion currently lacks resistance to herbicides, and therefore must be hand-transplanted and weeded.

"How do you kill the dandelions without killing the dandelions?" Cornish asked.