Agriculture Reference
In-Depth Information
of increasing plot sizes associated with advancing gen-
erations in the selection scheme. Towards the most
advanced stages, a few breeding lines may be grown
in farm-scale tests.
Irrespective of the crop species involved, most plant
breeders have been successful by selecting for yield
per
se
of the plant part of importance, rather than by
selecting of modified assimilate partition within a new
crop cultivar, albeit that selection for the first resulted
in a difference in the second. A number of differ-
ent plant breeders have selected for yield components
(i.e. attributes which contribute to total yield such as
number of ears per plant, number of seeds per ear and
seed weight in the case of cereals or number of tubers per
plant and tuber weight in potato) rather than for yield
itself. In most of these cases, however, there has been
little achieved in respect of increasing overall yield. The
reasons behind this failure are complex but one factor
is related to the negative relationships between many
yield components. Therefore positive selection for one
component is counter-balanced by a negative response
in another.
It is the actual yield obtained by the farmer that
is clearly an important criterion, and therefore factors
such as '
harvestability
' come to the fore. Mechanical
harvesters now carry out many harvest operations. If a
given genotype is not suited to mechanical harvest its
usefulness can be greatly limited. Therefore characters
such as plant lodging, precocious sprouting, seed shat-
tering or fruit drop are all factors which will reduce the
harvestable yield.
In many instances the uniformity of morphological
characters (seed/tuber/fruit - size, shape, colour etc.)
have a great effect on '
useable yield
'. Obviously, the
end-user has a demand for a product which has a certain
size, shape or colour and any deviation (either genetic
or environmental) from this appearance will reduce use-
able yield. Similarly, if user product of a crop is prone
to develop defects when processed, or in storage, this
will affect useable yield as the defects will not meet
the required legislative standards, or customer expecta-
tions, and will need to be culled out. A secondary factor
regarding defective products is related to the cost that
is incurred in having the defective fruits/tubers/seeds
removed. Uniformity of yield is more difficult to evalu-
ate than yield itself. Often it is not possible to evaluate
product uniformity with any accuracy in small plot trails
and therefore many potentially highly uniform breeding
lines may be wrongfully discarded in the early stages of
selection.
Research by crop physiologists has provided a great
deal of knowledge regarding plant growth models for
yield, and we have developed the ability to predict actual
yield from a wide range of different physiological mea-
surements. In the latter half of the last century, many
plant breeders believed that input from crop physiolo-
gists and physiological biotype models of our crop plants
would assist plant breeders to identify superior cultivars.
Crop physiologists believed that photosynthetic or net
assimilation rates could be used as selection tools to
increase plant productivity and hence increase yield of
crop plants. Some exceptions do exist and an example is
afforded by lupins. Physiologically based research and
modelling led to the proposal it would be beneficial to
aid the development of the crop into Northern Europe
to breed for a particular crop architecture, using geno-
types with a determinate growth habit. Suitable mutants
were found and indeed proved to be a marked improve-
ment on the traditional lupin types in the new target
environments. Despite the success in lupin, however,
the impacts of physiological biotype models in plant
breeding are rare.
INCREASING END-USE QUALITY
Irrespective of the yielding potential of a newly devel-
oped cultivar, success in agriculture will be determined
by the end-use quality of the saleable product. Demand
for sale of year-round fruits and vegetables has resulted
in food products being shipped greater and greater dis-
tances to arrive fresh almost on a daily basis. In addition,
greater emphasis is now, and will continue, to be placed
on storage of perishable agricultural products to make
them available at times of shortage of local supplies or,
as noted above, to simply make them available on a
year-round basis.
The are two main types of end-use quality:
•
Organoleptic
- consumer acceptance or preference of
taste, size, texture and colour. Although many peo-
ple differ in their preference, there is often agreement
within taste panels as to general preference towards
certain levels of expression of these attributes, thus
'liking' some genotypes over others (even disregard-
ing 'off-tastes'). Similarly, the visual appearance of
