Agriculture Reference
In-Depth Information
merger of both genotype data, which is largely
DNA-based, and phenotype data. The statistical
analysis of these datasets attempts to fi nd associa-
tions between genotype and phenotype and thus
specifi c genetic loci are able to explain signifi cant
amounts of phenotypic variation in a trait (Doerge
2002). The application of QTL analysis in wheat
breeding is widespread and is clearly having a
positive effect on both the precision and speed at
which wheat is improved in wheat breeding
programs.
bacteria, and viruses, and a pest in one area of the
world may be absent in another. Traditionally,
breeding for resistance has been conducted
through
in vivo
screening for resistance to the
pest of segregating populations. Disease incidence
or insect feeding is habitually rated using a scoring
system based on observations of wheat breeding
material in the presence of the pest. Genes for
resistance to pests are derived from other wheat
cultivars, and novel sources of resistance are often
sourced from wheat relatives. Recombinant DNA
technologies now permit the transfer of genes
from nonrelated species. For example, resistance
to glyphosate has been transferred to wheat from
bacteria using particle bombardment of plant
tissue cultured
in vitro
and regeneration of plants
from resistant plant tissue.
Wheat can be used to make many products,
including bread, cookies, cakes, noodles, and
pasta. In some parts of the world, it is also an
important livestock feed and, more recently,
wheat is being used to produce ethanol destined
for industrial purposes such as automobile fuel.
Breeding objectives for end-use quality will
usually focus on a particular trait such as bread-
making quality for hard wheat or cookie quality
for soft wheat. Basic end-use quality tests include
protein content, milling quality, dough rheologi-
cal testing, and baking tests, as elaborated further
in Chapter 20. A list of commonly used wheat
quality tests is given in Table 14.1. While some
of these end-use traits can be measured indirectly
using near-infrared technology, much of the
milling and rheological testing requires laborious,
time-consuming, small-scale milling or wet chem-
istry tests to emulate industrial-scale milling or
baking processes. The rigorous testing is neces-
sary because milling and baking companies
demand consistency in wheat that is targeted to
ongoing industrial end-uses. Large wheat export-
ing nations may have wheat end-use quality stan-
dards that are rather rigid and new cultivars must
adhere to these standards. Quantitative trait locus
analysis and deployment of molecular breeding
has the potential to reduce the time and cost of
some of these laboratory tests (Koebner 2004).
The potential exists to select for good end-use
quality in early generations by genotypic selection
BREEDING BY VISUAL SELECTION
During the development of a wheat cultivar, a
wheat breeder strives to combine multiple traits
into breeding lines, which are subsequently eval-
uated for their potential as new cultivars. Impor-
tant wheat breeding traits can be divided into
three general areas: agronomic, pests, and end-
use quality.
Agronomic traits are generally those of most
direct importance to producers, including matu-
rity, height, lodging, grain yield, and test weight.
Breeding for these characteristics has tradition-
ally been done through visual evaluation in early
generations (i.e., F
2
-F
4
), followed by replicated
fi eld trials in multiple environments. Field obser-
vations on days to heading or days to maturity are
usually recorded in number of days. In cases
where many lines (
50) must be rated per trial,
maturities may be estimated using a relative scale
[i.e., 1 (early) to 5 (late)]. Plant height may be
measured, or as with maturities a relative scale
rating may be given to breeding lines. Lodging is
usually scored using a scale rating (i.e., 1-9).
Grain yield is determined by weighing the grain
harvested from a yield plot of a standardized size
and seeding rate. Test weight is an estimate of
seed packing density, which indicates grain
soundness and can be related to end-use quality.
It is normally measured as weight of a standard-
ized volume (i.e., 0.5 L).
There are numerous pests that can pose impor-
tant wheat breeding objectives depending on the
agro-ecological zone for which the cultivar is des-
tined. Pests can include insects, nematodes, fungi,
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