Principles of Plant Genetics and Breeding

water utilization, photoperiod, harvest index, tolerance
to environmental stresses (drought, cold, salt, heat), and
mineral nutrition. Some of the significant achievements
with breeding for physiological traits have resulted in
the modification of plant architecture, specifically short
stature (semidwarf) in cereals (e.g., rice, wheat), with all
the advantages that such a plant architecture brings.
Photoperiod response is discussed in this chapter because
of its association with maturity and plant stature.
What is yield?
Yield is a generic term used by crop producers to
describe the amount of the part of a crop plant of interest
that is harvested from a given area at the end of the
cropping season or within a given period. The plant
part of interest is that for which the crop producer grows
the crop. It could be the leaves, fruits, stems, roots, or
flowers, or any other morphological part. It could also
be the chemical content of the plant such as oil, sugar, or
latex. In certain industrial crops such as cotton, the plant
part of economic interest to the producer is the fiber,
while for the producer of tea or tobacco the part of
interest is the leaf. It should be added that a producer
might harvest multiple parts of the plant (e.g., grain and
leaf) for use or sale (i.e., multiple economic parts). Plant
breeders seldom select solely on yield basis, without
some attention to other morphological features of the
plants. Yield is the best measure of the integrated performance
of a plant.
Biological yield may be measured by breeding for
physiological and morphological traits. All crop production
ultimately depends on photosynthesis (as well as
other physiological processes, for example respiration
and translocation). Over the years, various researchers
have attempted to improve biological yield by: (i)
increasing the photosynthetic capacity of the individual
leaf; (ii) improving the light interception characteristics
of plants; and (iii) reducing wasteful respiration. In addition
to increasing plant biomass, the goals of breeding
for physiological and morphological traits include the
redistribution of assimilates to the economic products
within the plant as well as alleviating or avoiding the
effects of adverse environmental conditions.
The term biomass is used by scientists to describe the
amount or mass of organic matter in a prescribed area at
a given point in time. This measure of biological matter
includes material formed above and below ground. Yields
of liquid products (e.g., latex, syrup) are measured
by quantifying the volume of the product harvested.
BREEDING FOR PHYSIOLOGICAL AND MORPHOLOGICAL TRAITS 353
Depending on the type of product and the purpose of
producing it, harvesting may be undertaken at various
stages of maturity for various product quality preferences,
as demanded by the targeted market. Plant breeders
may breed certain crops for early harvesting (for the
fresh market) and others for dry grain. The yields at
various stages of harvesting will differ between premature
and fully mature products. Sometimes, scientists
eliminate the moisture factor by measuring the weight
of the harvested product on a dry matter basis after drying
the product in an oven prior to being weighed.
Biological versus economic yield
Yield may be divided into two types:
1 Biological yield. This may be defined as the total
dry matter produced per plant or per unit area (i.e.,
biomass). Researchers use this measurement of yield
in agronomic, physiological, and plant breeding
research to indicate dry matter accumulation by
plants. All yield is at first biological yield.
2 Economic yield. The economic yield represents the
total weight per unit area of a specified plant product
that is of marketable value or other use to the
producer. The producer determines the product of
economic value. A producer of corn for grain is interested
in the grain; a producer of corn for silage is
interested in the young, fresh stems and leaves. All
yield is biological yield, but all biological yield is not
necessarily economic yield. For example, the aboveground
parts of corn may be totally useful in one way
or another (e.g., the grain for food or feed, and the
remainder also for feed or crafts). The roots are of
no practical or economic use. However, in certain
root crops such as sugar beet, the total plant is of
economic value (root for sugar extraction and the
leafy tops for livestock feed).
Yield depends on biomass and how it is partitioned.
To increase yield, the breeder may breed for increased
biomass and efficient partitioning of assimilates. The
potential biomass of a crop is determined by factors
including genotype, local environment (soil, weather),
and the agronomic practices used to grow it. N. W.
Simmonds identified three strategies for enhancing
biomass:
1 Seasonal adaptation. The objective of this strategy
is to optimally exploit the growing season by sowing
early and harvesting late to maximize biomass