Upbeat research news helped two London-listed agri-science groups sidestep the slide in UK equities prompted by economic concerns.

London stocks closed down 2.8% after Standard & Poor's cut its ratings outlook on UK government debt to negative from stable, citing fears over the size of the country's debt burden.

However, shares in FuturaGene, a genetically modified crops company, added 2.6% to 58.5p after it said that its technology was to be tested by US-based Targeted Growth in field trials of modified corn.

Targeted Growth, a biofuels group, has signed a licence to test FuturaGene research which, through modifying cell walls, is claimed to offer improvements such as stronger plant growth and easier crop processing.

FuturaGene shares have doubled this year, helped also by a £1.5m investment from the Delaware-based International Institute of BioScience Research and Development.

'Commercial advantage'

Meanwhile, shares in Plant Impact closed up 3.6% at 43.5p – their highest finish since July – after the group said that research had shown its technology improved vegetable production and longevity.

"Crucially, shelf life was extended by 4-10 days compared to rival technologies, giving producers a commercial advantage in supplying quality produce to the markets," said the company, whose research focuses on calcium uptake.

"Highlights of the improvements to crop quality included extra weight, increased number of leaves and better post harvest storage in lettuces."

Plant Impact shares have been one of London's best performers in 2009, which they entered at 10p.