by David Fogerty

SINGAPORE - Staples such as cassava on which millions of people depend become more toxic and produce much smaller yields in a world with higher carbon dioxide levels and more drought, Australian scientists say.

The findings, presented on Monday at a conference in Glasgow, Scotland, underscored the need to develop climate-change-resistant cultivars to feed rapidly growing human populations, said Ros Gleadow of the Monash University in Melbourne.

Gleadow's team tested cassava and sorghum under a series of climate change scenarios, with particular focus on different CO2 levels, to study the effect on plant nutritional quality and yield.

Both species belong to a group of plants that produce chemicals called cyanogenic glycosides, which break down to release poisonous cyanide gas if the leaves are crushed or chewed.

Around 10 percent of all plants and 60 percent of crop species produce cyanogenic glycosides.

The team grew cassava and sorghum at three different levels of CO2; just below today's current levels at about 360 parts per million in the atmosphere, at about 550 ppm and about double at 710 pm.

Current levels in the air are just under 390 ppm, around the highest in at least 800,000 years and up by about a third since the start of the Industrial Revolution.

"What we found was the amount of cyanide relative to the amount of protein increases," Gleadow told Reuters from Glasgow, referring to cassava.

At double current CO2 levels, the level of toxin was much higher while protein levels fell.

The ability of people and herbivores, such as cattle, to break down the cyanide depends largely on eating sufficient protein.

Anyone largely reliant on cassava for food, particularly during drought, would be especially at risk of cyanide poisoning.

HARDY STAPLE

While it was possible to use processing techniques to reduce the level of toxin in the cassava leaves, it was the 50 percent or greater drop in the number of tubers that caused most concern, Gleadow said.

About 750 million people in Africa, Asia and Latin America rely on cassava as a staple. The starchy tubers are used to make flour and the plant is ideal in dry regions because of its hardy nature.

The good news was that the levels of toxin in the tuber didn't increase with CO2, unlike the edible leaves.

"The downside of that is that we found the plant didn't grow nearly as well," she said.

"There's been this common assumption that plants will always grow better in a high CO2 world. And we've now found that these plants grew much worse and had smaller tubers."

At the 550 ppm level, the problem was not as serious and this meant scientists had a bit of breathing space.

"We've got 20 to 30 years to develop cultivars, which is going to be absolutely essential because by then about 1 billion people will probably be reliant on cassava."

Gleadow's group looked at a type of sorghum commonly fed to cattle in Australia and Africa and found it became less toxic at the highest CO2 level. But under drought conditions, leaf toxin levels rose.

She said her team was looking at creating mutations to get rid of the toxin response to drought.

"If we're going to adapt in the future to a world with twice today's CO2 we need to understand how plants are working, how they are responding and what cultivars we need to develop."

Her team plans to carry out additional research in Mozambique and study other tropical crops such as taro.

Editing by Alex Richardson