¿ìè¶ÌÊÓƵ

CEREAL CROPS which can ‘fix’ their own nitrogen from the environment
have become one of the holy grails of scientists working in agricultural
biotechnology. If genetic engineers could alter the DNA of staple crops
such as wheat and rice in this way, then farmers may no longer need to spread
tonne after tonne of nitrogen fertiliser on their fields.

But by concentrating on nitrogen fixation the biotechnology industry
is not promising to solve any food crises. Reducing the use of fertilisers
would almost certainly cut costs for farmers, and may even help to protect
the environment. But it is only in developing countries, where farmers cannot
afford to use fertilisers today, that yields of crops could be expected
to rise.

The biotechnology community is divided over whether crops which fix
their own nitrogen would produce any more grain than those grown elsewhere
in the developed world with an abundant supply of fertiliser. Many scientists
believe that natural limitations on the photosynthesis which creates energy
in plants will prevent the new technology from raising the yields of such
crops.

Leguminous plants are well adapted to exploit nitrogen from the soil
and air. They to have a symbiotic relationship with bacteria which convert
nitrogen into a form the plants can use. Barry Smith, from the AFRC’s Nitrogen
Fixation Laboratory at the University of Sussex, is trying to shift the
20 genes in these bacteria responsible for nitrogen fixation into the genetic
blueprint of other crops.

This is very difficult. Biotechnologists can usually shift only one
or two genes into new hosts. And, to date, the standard techniques of genetic
engineering have failed to incorporate any foreign genes at all into the
cereal crops. Smith believes it will be at least 20 years before his work
will produce a commercially useful crop. Smith and his colleagues also acknowledge
that these engineered crops will still ‘decide’ for themselves the amount
of solar energy they should convert into a useful form, by photosynthesis.
This ‘photosynthetic rate’ will limit the amount of energy available to
the plant as a whole, because the crop will use some of the energy it converts
to express the genes for nitrogen fixing. This energy will not therefore
be available for swelling the seeds and increasing the yield of the crop.

Colin Law, head of the AFRC’s Cambridge laboratory for plant breeding,
believes that nitrogen fixing may only be one part of the puzzle of increasing
yields. It may also be necessary, he says, to insert foreign genes to alter
the photosynthetic rate of plants. ‘At the moment it is not clear what genes
you need to put into plants to increase their yields. There are lots of
³Ü²Ô°ì²Ô´Ç·É²Ô²õ.’

Law is more optimistic about other ways in which biotechnology might
help to increase yields. One example would be the use of molecular probes
to select the best plants to breed from. This might help farmers to overcome
‘plateaus’ in their yields – when no matter how much extra fertiliser they
put onto their fields the yields of their crops stubbornly refuse to increase.
Law also believes that biotechnology may help to increase the resistance
of crops to their common pests and diseases.

John North, from the Department of Land Economy at Cambridge University
and former head of the government’s agricultural advice service, is adamant
that attempts to add genes conferring the ability to fix nitrogen will never
increase yields from crops. North believes that in countries where there
is no limit on the amount of nitrogen fertiliser available the crop will
stubbornly photosynthesise only the energy it decides it needs.