Agriculture Reference
In-Depth Information
or by artificially controlling the natural source-sink
relationship.
For example, in potato, many past breeders have
specifically selected breeding lines, which rarely pro-
duce flowers with the idea that energy put into sexual
reproduction would detract from tuber yield. Flowering
can be induced in some genotypes by planting tubers
under long day conditions and having plants develop to
maturity in shorter days. Enhanced flowering in potato
can also be achieved by 'growing on a brick' where par-
ent tubers are planted on building bricks and covered
with soil. At the stage when tuber initiation occurs the
soil is washed from the mother tuber and newly initiated
tubers are removed, hence offering greater resources for
flower development. A similar effect can be achieved by
grafting potato shoots onto tomato seedlings. Apply-
ing high levels of nitrogen at particular growth stages
can sometimes increase the duration of the flowering
period.
In other crops (and sometimes also in potato) reduced
levels of nutrients cause stress to parental plants which
can induce flowering, which would otherwise not occur
under optimum conditions.
Finally, irrespective of crop or breeding system it is
always desirable to have multiple and sequential plant-
ings of parents that are to be used in crossing designs.
Genetically different parents will of course flower and
dehisce pollen at different times and multiple plant-
ings will increase the possibility of achieving all hybrid
combinations planned.
With many annual (and some biennial) crops it is
possible to grow more than a single generation each year,
therefore greenhouses can be used to reduce generation
times and hence increase the speed to homozygosity.
Single seed descent used in spring barley, where plants
are grown at high density and with low nutrition,
can be used to increase F
1
populations to F
3
popu-
lations within a single year (i.e. three generations in
12 months).
At the advanced stages of a plant breeding scheme,
greenhouse growth can be utilized to increase advanced
selections under controlled conditions prior to produc-
ing breeders' seed. This can be particularly useful in
crops which are grown as true breeding, inbred lines
but in which a relatively high frequency of natural
out-crossing occurs (e.g.
Brassica napus
).
Tissue culture techniques are becoming a routine part
of many plant breeding schemes. Plants rarely can be
transferred directly from
in vitro
growth to field con-
ditions without involving an intermediate greenhouse
stage. Here the greenhouse stage could involve an inter-
mediate operation where plants are weaned from
in vitro
to
in vivo
sterile soil mix, allowed to develop and are later
transplanted to the field. Alternatively the greenhouse
can be used to produce seed (or tubers) from plants that
have previously been grown
in vitro
.
Evaluation of breeding lines
One advantage of growing plants under greenhouse
conditions, rather than field conditions, is related to
environmental control. Control of the environment can
be critical to guarantee epidemics of pests or disease
or to evaluate stress factors, to allow resistance screen-
ing. There have been a number of studies that have
resulted in protocols suitable for evaluating plants under
glasshouse conditions.
Disease and pest testing involves subjecting segre-
gating breeding populations to a disease or insect and
selecting those plants, which show resistance. Examples
include spraying barley seedlings with a suspension of
mildew spores and screening for resistant lines, spraying
potato seedlings with a spore suspension of late blight
or early blight and recovering the seedlings that are not
killed. These tests are often more effective if there is
good environmental control, such as is provided in a
greenhouse. This helps to guarantee that the results
Seed and generation increases
If hybridization is carried out between two homozygous
parents then the F
1
plants will be heterozygous at all the
loci by which those parent lines differ and all plants will
be genetically identical. It is therefore common practice
to go from the F
1
populations to F
2
under glasshouse
conditions. This tends to maximize the use of F
1
seed
because of the high levels of germination and survival
that can be achieved. If F
1
populations are grown under
field conditions it generally requires greater quantities
of hybrid seed. This is disadvantageous since the cost of
producing F
1
seed is usually high, because it involves
emasculation followed by hand-pollination, in relation
to simply bagging the F
1
to allow selfing to produce
the F
2
.
