Agriculture Reference
In-Depth Information
Theoretically, transgenic resistance could be used to increase functional diversity
in crops but it is likely that only few genotypes will be supplied with transgenes and
that they would be marketed to occupy even larger areas per variety than before. The
process would be driven by the current system of patenting and plant varietal
protection which is inappropriate to the development and exploitation of biodiversity
(Busch, 1995). As an example, we may consider the
mlo
-resistance gene of barley to
powdery mildew which has never been overcome by a virulent pathotype in the field
and which has been cloned (Büschges
et al.
, 1997). This gene might be regarded as a
prime candidate for genetic engineering to introduce it into other cereals such as oats
and wheat. We have already pointed out the danger that
mlo-
resistant winter barley
would greatly increase selection on the pathogen, increasing the frequency of the
virulence genes and thus the chances for clones with the appropriate virulences to
emerge (see 10.5). The additional use of the
mlo-
resistance in other cereals would
dramatically increase the genetic uniformity for mildew resistance and might greatly
increase the danger of gene-exchange between the
formae speciales
of powdery
mildew since host species specificity in mildew may be based on only one or a few
genes (Tosa, 1994).
10.7.2 System diversity
Until recently, numerical success in the development of the human population has
been dependent on various systems of ecological agriculture and agroforestry. Major
historical departures from this theme towards monoculture resulted in the loss of many
civilisations (Ponting, 1991). Our recent ability to develop vast monocultures has
depended on major developments in synthetic chemistry and engineering technology.
It is now
clear that these modern applications have many negative impacts; attempts to
overcome these by further technological developments lead often to more and novel
problems. In our view, there is an overwhelming argument to bring the power of
modern biological sciences to bear on the understanding and development of the wide
range of ecological processes and interactions that allowed the human population to
increase without major impacts on the environment as a whole. This means developing
and understanding agricultural systems that are highly diversified at many different
levels. As pointed out earlier, however, the diversification needs to be functional in
relation to the required outcomes from the systems. As also pointed out earlier, the use
of different forms of functional diversity simultaneously can lead to a multiplicative
increase in interactions among the many components.
One example of complex systems, although there are still many deficiencies, is
the development of modern organic agriculture. Organic systems throughout the
world are based on temporal diversity (more or less long rotations) which
necessarily involves a useful degree of crop diversity. The use of composts and
manures represents another form of diversity within these systems by using a range
of crops to generate microbial diversity; many of the microbes involved have
positive effects on diseases (Hoitink and Fahy, 1986
)
and nutrient use efficiency
(Maeder
et al
., 2002). There is also growing interest and application of intercropping
systems, particularly in organic agriculture. Agroforestry systems, integrating tree
