Principles of Plant Genetics and Breeding

356 CHAPTER 19
conditions on the harvest index, as previously described.
Sometimes, the breeder selects on the basis of single
plants (e.g., in a space planted, segregating population
in the early part of a breeding program) or on the basis
of families, at some point. Also, sometimes, plants are
evaluated in microplots and at other times in large field
plots at commercial densities. The challenge is for the
breeder to predict yielding ability from one plant
arrangement (isolated plants) to another (field crops).
A much more severe restriction to the practical use of
the harvest index as a selection criterion is the fact that it
is more tedious to measure than grain yield per se.
Selecting for yield per se
As previously indicated, a plant breeder seeking to
improve crop yield affects the trait through manipulating
biomass or partitioning or both. Furthermore,
because yield is a complex trait, an outcome or product
of the interaction of numerous physiological processes,
breeders seek effective and efficient ways of selecting
superior genotypes in a breeding program. As also
previously discussed, biomass and partition are tedious
to estimate. The rationale of yield components as a basis
for selecting for yield has not proven useful because of
the occurrence of compensatory negative correlations
(i.e., an increase in one component produces a decrease
in another). Similarly, certain physiological parameters
(e.g., photosynthetic rate, net assimilation) that have
been proposed as possible indicators of improved
biomass, have not materialized. Breeders have also
resorted to a variety of statistical procedures to help the
selection process to become more efficient and effective
for yield. A notable application is the development of
indices for selection (selection index). Other multivariate
techniques such as path coefficients have been
attempted with little success. Modern molecular technology
is attempting to identify quantitative trait loci
(QTLs) associated with complex traits. In view of the
foregoing, little wonder that many breeders select for
yield per se in their breeding programs. As previously
stated, it is the best measure of the integrated performance
of a plant or crop.
Biological pathways to economic yield
Yield is a very complex trait. A good crop yield reflects
a genotype with high yield potential, growing in a
good environment. It reflects, also, proper growth and
development – processes that are very complex in themselves.
In an effort to manipulate crop yield, plant breeders
attempt to construct the path by which the reproductive,
developmental, and morphological features of plants
in a crop stand contribute to the yield of a specified
product. The pathways to yield are collectively called
yield components. In theory, the total yield can be
increased by increasing one component while holding
the others constant. By breaking down a complex trait
into components, breeders hope to find selection criteria
for improving it.
For grain yield, a model of yield components is:
Yield/unit area = (plants/unit area) × (heads/plant)
× (mean number of seeds/head)
× (mean weight/seed)
Where the plant produces tillers, the model may be
modified as:
Yield/unit area = (plants/unit area) × (mean
number of tillers with ears/plant)
× (mean number of grains/ear)
× (mean grain weight)
These plant characteristics describe yield. They all depend
on energy in a fixed pool that is furnished through
photosynthesis. Plant breeders seek to influence yield
by manipulating its components to positively affect
photosynthesis.
It is important to mention that in interpreting the
correlation between yield and its components, the
breeder should evaluate the components in terms of
relative importance. The seasonal sequence of environmental
conditions that affect plant development should
also be considered. Growing conditions may be ideal in
the early growth and development of the crop, leading
to good initiation of reproductive features. However, if
there is an onset of drought, few pods may complete
their development and be filled with seed, leading to
low correlation between yield and the number of seeds
per pod.
Yield components vary from one species to another in
terms of optimum value relative to other components.
Further, yield components affect each other to varying
degrees. For example, if increasing plant density drastically
reduces the number of pods per plant, the number
of seeds per pod may only be moderately affected
whereas seed size may remain unchanged or only
slightly affected. Whereas a balance among yield components
has great adaptive advantage for the crop,
the components are environmentally labile. High yield