Agriculture Reference
In-Depth Information
under low and high nitrogen conditions. If the yield
performance of the two lines follows the pattern where
entry A is higher yielding than entry B at both nitrogen
levels, it is said that there is no interaction. If, however,
entry A is highest yielding at high nitrogen levels but
entry B is highest yielding at low nitrogen levels then
there is said to be interaction between genotypes and
nitrogen levels. The significance of interaction is tested
from analysis of variance. It should be noted that in
testing genotype
GREENHOUSE MANAGEMENT
A large proportion of the tasks that are necessary in the
early parts of a plant breeding programme can often
be carried out in a greenhouse. An integrated green-
house system is not essential for a successful varietal
development programme. However, many of the oper-
ations can be carried out more effectively if a greenhouse
system is conveniently available.
Greenhouses come in many different shapes and sizes
and can be constructed from many different materials
including wood, aluminium, glass, plastic, polythene.
The actual design of these systems will not, however, be
covered here. A greenhouse can simply be considered
as a relatively large area where there is some control
of environmental conditions such as soil type, irriga-
tion management, nutrient management, lighting and
temperature.
The operations, which can be carried out in a green-
house with regard to a plant breeding programme,
include:
×
treatment interactions if there are
no changes in genotype ranking then although formally
an interaction will be detected the implication for plant
breeding is minimal, unless other treatments (maybe
outside the range tested) are envisaged as being likely.
Split-plot designs
In some cases a breeder is interested in estimating the
difference between the effect of one factor and the inter-
action of that factor with a second factor while having
lesser interest in variation within the second factor in
its own right. In plant breeding for example, it is well
established that higher nitrogen (within limits) applied
to cereal crops will result in higher yield. Genotypes
•
Artificial hybridization
×
nitrogen studies are routinely carried out, not to deter-
mine if there is an average yield increase with increased
nitrogen (as this has already been established). The
primary goal is to determine the differences between
genotype variation and the interaction between geno-
types and nitrogen levels. In these cases a special type of
factorial design called a
split-plot design
is commonly
used.
Split-plot designs divide the total area of a test into
main blocks
,
sub-blocks, sub-sub-blocks
, etc. First,
the main blocks are arranged at random, and then fac-
tors within sub-blocks are arranged at random within
the main blocks, factors of sub-sub-blocks are arranged
at random within sub-blocks etc. A simple split-plot
design is illustrated in Figure 9.4 (b). The numbers 1,
2, 3 and 4 are main blocks and the letters a, b, c and d
are sub-blocks. Only a single replicate is shown in the
figure, but as before other replicates would be similar
and would always be blocked.
The analysis of variance produces two errors for
use in F tests. The error #1 is estimated by the sub-
blocks within main blocks
•
Seed increases of breeding lines, including progress-
ing to homozygosity
•
Evaluation of characters, which are difficult to control
under field conditions
Artificial hybridization
It is possible to carry out artificial hybridization under
field conditions, however, many breeding schemes use
greenhouse facilities for this task because it is easier
to control the conditions necessary to ensure cross-
pollination between chosen parents (Figure 9.5). Also, it
can often be possible to achieve cross-pollination out of
season in greenhouses and usually it is easier to prevent
unwanted illegitimate cross-pollination.
The method used for cross-pollination will be depen-
dent on the crop species involved, whether the crop
is out-crossing or self-pollinating. The major goal in
artificial hybridization is to ensure that the seed pro-
duced is in fact from the particular, desired paired parent
combination. Therefore steps must be taken to make
sure that seed has not resulted from an unwanted self-
pollination or from an accidental cross-pollination that
is not intended by the breeder.
×
replicates effect while the
interaction effects would be tested against the between
main blocks
×
replicates interaction.
