Plants that can be grown for fuel are often touted as a vast, clean energy source - except by those who say precious food is being diverted into gas tanks, and that biofuel crops are using up dwindling land and water. Enter willow, hemp and switchgrass.
Scientists say research into a new generation of biofuel sources could yield cheap energy supplies that do not compete with food crops - or with nature - for water or space. The day may be decades away, but some say plants might even cover a large share of the world's energy needs. Goran Berndes, a researcher at Chalmers University of Technology in Sweden, says the list of possible plants goes far beyond the established crops such as corn, maize and sugar cane that are already grown commercially for fuel uses.
"Bioenergy is much broader," he said. "Most people working in bioenergy expect other crops to dominate in the long term." One promising energy source is the willow, a northern plant used to make baskets and sport bats. Others include hemp, known for its rope-making and mind-altering qualities, and switchgrass, a reedy plant found in the US Midwest.
A new crop that is being used already is jatropha, a resilient, oil-rich, tropical plant that can be grown on waste land and even introduces nutrients to the soil. Its oil is already used in India to power diesel cars and turbines.
Jatropha has grabbed headlines because it avoids the biggest controversy surrounding biofuels: the ethical debate over whether agricultural resources should be used for energy when millions across the planet go hungry. This can mean using up water as well as land - a reminder that biofuel crops themselves can carry severe risks for the environment, especially if hitherto unfarmed land is converted to agriculture with large amounts of fertiliser and irrigation.
The International Water Management Institute, which led a five-year global study on water involving more than 700 researchers, found that if China and India pursued their current biofuel plans, they faced water scarcity by 2030.
Berndes has built models that try to peer even further into the future, assuming that crop yields will continue to climb as agricultural science advances, and new biofuel crops will become more productive.
One scenario - highly optimistic, perhaps, but theoretically possible - suggests that an area of agricultural land twice the size of Mexico could become surplus to current requirements by 2050.
If this were all used to grow biofuels, it could yield 400 exajoules of energy - almost the equivalent of the world's current energy consumption. Of course, such scenarios are hugely complex, and it is not merely a question of finding enough land. The assumed higher crop yields are likely to tax the environment harder by requiring more irrigation and fertilisation. "If you need less land, you cannot be sure you need less water," Berndes says.
Hence the need to ensure that the new generation of biofuel crops are not also hungry for scarce resources - for instance getting their water from rain rather than irrigation.
And they will need to be commercially attractive. "You have many different ways of producing transportation fuels from these new biomass sources that are not there yet commercially," he said.
At least one business sector is prepared to lobby for biofuel crops that do not compete so hard with food production. Nestle, the world's largest food company, says the subsidies being applied to current biofuel crops are distorting the market and pushing up the prices of food crops, and that second-generation biofuels could be an answer.
"If it works, and if it can be made to work economically, that certainly would be - both from an environmental and from an economic point of view - a much better solution than this strong focus on the current first-generation food crop biofuels," said Claus Conzelmann, Nestle vice president for safety, health and the environment.
But there are those who say the entire debate is misguided. Vaclav Smil, a professor at the University of Manitoba, says that with relatively straightforward changes to the cars we drive, we could do without extra energy altogether. "I'm astonished that people even think about biofuel," Smil told a conference in Stockholm. "Do we need more biofuels to feed our cars? We don't."
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