Agriculture Reference
In-Depth Information
Hard white wheat has utility in several domes-
tic markets and is the preferred wheat of com-
merce in a number of international markets. The
favorable end-use characteristics of HW wheat
differ from HRW and are particularly well suited
for whole-wheat products, as well as replacing
HRW wheat in pan breads, tortillas, and oriental
products (e.g., noodles and steamed bread).
Whole wheat pan bread made from HW wheat is
not only lighter colored, but also less bitter than
bread made from HRW wheat at the same milling
extraction, because white wheat bran contains
fewer bitter components associated with a strong
fl avor. Tortillas are traditional Mexican fl at breads
made from either maize (
Zea mays
L.) or wheat
fl our. Tortillas are used for many traditional
Mexican dishes and, increasingly, for a variety
of non-Mexican foods such as “wraps.” Wheat
tortillas are twice as popular as corn tortillas in
the US.
In Asia, wheat is consumed primarily in
noodles and steamed bread. Wheat quality attri-
butes (including functional protein characteris-
tics), initial product color, color stability, and
mellow gluten (Kruger 1996) are critical for
these applications. Australian white wheat is
known for its capability to produce noodles
with a stable white or bright yellow color. This
desirable color must be maintained for at least
24 hours to meet requirements of the Asian
market. Enzymatic activity of polyphenol oxidase
(PPO) is responsible for noodle discoloration
(Corke and Bhattacharya 2001), and HW culti-
vars with very low PPO, such as Lakin and
Platte, have been developed (Lin and Vocke
2004). Research at the Wheat Marketing Center
in Portland, Oregon, has helped identify US
wheat cultivars that perform best in noodle
applications.
As learned from HW wheat production in the
Great Plains, the grain-handling infrastructure
needs to achieve and maintain segregation
throughout the value chain. Capturing the value
of any trait differentiated from commodity-based
wheat will require segregation during storage and
transportation, and ultimately, any higher value
trait will face similar challenges.
LOW POLYPHENOL OXIDASE WHEAT
High levels of grain PPO have been associated
with discoloration in a variety of fresh wheat
products, including Asian noodles (Baik et al.,
1995). Cultivated durum (
T. durum
L.) wheat
has none to trace amounts of grain PPO activity.
Among US HW bread wheat cultivars, Platte
and Lakin have low levels of grain PPO but still
produce more PPO than typical durum wheats
(Sayaslan et al., 2005). The PPO activity of
Platte was found to still be 3 times as high as that
of the durum cultivars Ben and Renville (Bettge
2004). Characterization of the PPO levels of a
given genotype is easily accomplished via a
quantitative assay using
L
-dopa as a substrate
(Anderson and Morris 2001). Such assays are,
however, reliable in genetic studies only if con-
ducted on samples from multiple locations or
growing seasons; cultural environment will
infl uence levels of PPO detected (Anderson and
Morris 2001). DNA markers recently have been
identifi ed and linked to the low PPO trait in a
number of genetic backgrounds (Raman et al.,
2005; Sun et al., 2005). Either by DNA markers
or by simple whole-seed assays of grain PPO
using tyrosine or
L
-dopa substrates, selection for
low PPO is readily accomplished in breeding
programs.
Genetic studies in common hexaploid
(
T. aestivum
L.) wheat (Demeke et al., 2001)
have demonstrated PPO levels to be infl uenced
by multiple genes arising from several chromo-
somes, including 2A, 2B, 3B, 3D, and 6B. The
complex nature of the inheritance of PPO activ-
ity suggests that genetic complementation of low
PPO mutants might provide a path toward
development of more hexaploid wheat germ-
plasm with nil grain PPO. Bernier and Howes
(1994), using a tyrosine assay, found these hexa-
ploid cultivars with grain PPO levels approach-
ing that of durum wheat: 'Bihar 124' (from
India), 'Little Club', 'Cadet', and 'Fielder' (US),
'Cook' and 'Tincurrin' (Australia), and 'Pitic 62'
(Mexico). Intermatings of such low PPO lines
might generate segregates with nil levels of the
enzyme.
