| Adrian Higgins |
AZTECS tapped the sap of Hevea brasiliensis much as American Indians bled sugar maples for their sweet syrup. But the hevea rubber tree yielded something quite inedible – a thick, milky liquid we call latex. The Aztecs used it to make a hard, dense ball for playing games.
In the 19th Century, inventors and engineers figured out how to make rubber flex (add sulfur under heat) and to modify it from a solid ring to a tube filled with air.
These breakthroughs led to the world as we know it – on the move on pneumatic tyres. Passenger car tyres are made from a mix of natural and synthetic rubber – mostly the latter and derived from petroleum. Truck tyres need a greater ratio of natural rubber, and aircraft tyres are made wholly of natural rubber.
The successful landing of a wide-bodied jet, with its enormous loads, is in large part because of the skill of the pilot and the existence of a spindly jungle tree native to the Amazon.
If you don’t want to take a jet, you can see a hevea growing in the Tropics, the central conservatory of the United States (US) Botanic Garden at the foot of the US Capitol. Most people pass it by, absorbed by the more imposing palm trees.
On a recent spring one Saturday, the botanic garden was the scene of the announcement of another rubber breakthrough – the successful harvesting of rubber from a scrubby little shrub named guayule (why-YOU-lee).
This modestly attended talk was given by Gene Lester of the Agriculture Department’s Agricultural Research Service (ARS). I got a sense the world was missing something momentous. We’re talking here of the ongoing evolution of the wheel, which is the history of the connecting of human societies, generally considered a good thing.
The world has shrunk. So, too, has our understanding of the plant kingdom and how dependent we are upon it. Where do people think rubber comes from? “The rubber fairy,” said Lester, who directs the research service’s product development programmes.
The hevea tree, which is related to the milky-sapped poinsettia, is king of the rubber world – other plants can yield latex, including dandelions – but hevea is by far the predominant source of commercial natural rubber.
Though the tree is native to the Amazon, most hevea plantations today are in Thailand, Indonesia and Malaysia. For all its market share, hevea has its problems (beyond a leaf blight disease in its original environment).
It grows in a fairly narrow equatorial band, takes almost a decade to reach maturity and requires daily scrapings of its exudations. A rubber plantation requires a lot of labour, though I’m sure they’re working on a robot.
The need for rubber has never been greater. Lester told the audience that the world has more than one billion vehicles along with 39,000 aircraft flying about four billion passengers a year. Lester shared estimates that by 2030, the number of vehicles will double. The number of air travellers will be just shy of eight billion.
A corresponding increase in hevea production would undoubtedly require the clearing of more tropical rain forest at a time when the planet needs every oxygenating jungle it’s got.
Enter guayule. The shrub is native to northern Mexico and the Southwest. As an aside, it has very few close relatives, but one is a great garden plant, a hardy perennial named wild quinine, with clusters of white flowers for weeks in early summer. It does well in difficult, hot and dry sites and draws loads of pollinators. It should be used much more than it is.
Guayule is not as pretty, but it is at least as tough. It grows in hot, dry regions, and if it were to become a major source of rubber, it would flourish in areas of the country where crop options are limited to such things as alfalfa and cotton, both of which require more precious water.
Although the rubber tree is carefully cut and tapped, the guayule is chopped to the ground and its branches mashed up to extract the latex. The plant then regrows and can be harvested several more times before depletion. It also produces valuable byproducts, including fuel.
Under a USD6.9 million government grant, ARS researchers joined other plant scientists and industry experts as part of a consortium looking at the feasibility of guayule production. The five-year effort included the genomic sequencing of Parthenium argentatum for
use in picking plants with desired genetic traits – drought tolerance and high latex yield, for example.
“We think it’s good for the globe and good for farmers, especially in the desert Southwest where water is dear,” said ARS’ lead scientist for domestic natural rubber Colleen McMahan. – The Washington Post