is the development ofmultiline varieties, which contain a mixture of resistance
types. Another method is to “pyramid” or “stack” multiple sources of resistance
into a single variety.3.3 Objectives for Plant Breeding
The overall value of a plant variety is determined by many small and subtle characteristics
that are quantitative and polygenic in addition to a few major characteristics that may be
qualitative and monogenic. It is not difficult to draw a parallel in human traits. Think of
someone whom you admire; you might like this person because you have a fondness for
a certain eye color, hair color, height, or other characteristic, but most likely it would be
because of a combination of many subtle traits (involving both appearance and personality)
that are controlled by numerous genes but also influenced by environment. Beauty is in the
eye of the beholder, or in this case, in the eye of the plant breeder.
Many breeding objectives fall into two general categories of traits. We often categorize
certain traits asagronomic traitsorinput traits, because they relate to production practices
and to the amount of raw material that can be harvested. Such traits include crop yield, pest
resistance, height, flowering time, susceptibility to lodging (falling down), seed vigor, and
seed dormancy. Crop yield includes many component traits, such as seed size, seeds per
pod, pods per branch, and branches per plant. Some breeders prefer to select according
to component traits rather than on final yield, but the value of a plant variety is almost
always judged for its potential to produce high yields per unit area. The second general cat-
egory is described asoutput traits, which include anything related to the composition or
quality of what gets harvested. Examples are the composition and content of protein or
oil; the relative proportions of oil, starch, and protein; and the composition of secondary
compounds that may have value relative to human health and industrial use. Many
output traits are extremely complex. These include traits related to the use of a plant
product in processing.
For every cultivated plant species, there are different breeding objectives. Often there are
several different sets of objectives, sometimes conflicting, for the same species. An example
of this is a barley, which may be used for animal feed or malt production. A major objective
for animal feed is high protein content, whereas malt production requires low protein
content. The breeding objectives for crops such as malting barley or bread wheat are
dependent on markets that have evolved very specific industrial processes that require
dependable and uniform grain. Thus, a variety that is merely high-yielding and pest-resist-
ant will not suffice. Other crops have highly diverse objectives that may be driven by many
different markets. Soybean, for example, is used in the manufacture of many different food
products, and each product has very specific requirements such as taste, texture, chemical
composition, seed size, and seed coat color.
Horticultural crops can have many interesting and diverse objectives. The market for gar-
dening varieties is driven by diversity because gardeners like to try new things. In contrast,
the market for commercial varieties of horticultural crops is driven by the need for confor-
mity and uniformity, and for improvement of specific high-value characteristics. For
example, processing tomatoes need to deliver maximum amounts of soluble solids, but
there would be little tolerance for a variety that had a different taste or color. The market
for fresh shipping tomatoes has requirements for produce that is both durable in transit
and attractive in appearance (and, some would argue, tasteless).
62 PLANT BREEDING