Agriculture Reference
In-Depth Information
western Asia, and parts of India. A resistance gene
designated
Cre3
was found in a primary synthetic
and subsequently transferred to adapted wheat
(Eastwood et al., 1991). Molecular markers are
now available for
Cre3
and routinely used in wheat
breeding (Martin et al., 2004). Root lesion nema-
tode (
Pratylenchus thornei
Sher and Allen and
P.
neglectus
Rensch) limits wheat yields in Australia
and parts of North Africa and western Asia. Zwart
et al. (2005) found QTL associated with resis-
tance to both
P. thornei
and
P. neglectus
in a cross
with a primary synthetic.
In perhaps the most comprehensive evaluation
of primary synthetic wheat, Ogbonnaya et al.
(2008) considered 253 synthetics representing
192 unique
Ae. tauschii
accessions and 39 durum
wheat genotypes for resistance to cereal cyst nem-
atode, root lesion nematode (both
P. thornei
and
neglectus
),
S. tritici
and
nodorum
, tan spot, leaf
rust, stem rust, and stripe rust. Table 16.1 sum-
marizes their fi ndings. Sources of resistance for
all of these diseases were found, ranging from
only 1% of tested materials for
P. neglectus
up to
73% for
S. tritici
. Although the uniqueness of
genes conferring resistance in the primary syn-
thetics has in many instances yet to be established,
signifi cant variation is clearly present for reaction
to many of the important pathogens and pests
affecting wheat in the synthetic wheat gene
pool.
New genetic variability for tolerance to
abiotic stress
Abiotic stresses severely limit wheat productivity
and product quality in many environments world-
wide. Limited available moisture is the most sig-
nifi cant of all stresses. It affects crops in most
wheat-growing regions and is expected to increase
in intensity with climate change. Even the high-
yielding river valleys of China and India are pro-
jected to suffer moisture defi cit as water tables fall
and water for irrigation becomes scarce (Reeves
et al., 2001). Populations of
Ae. tauschii
and the
wild and cultivated emmers have evolved over
thousands of years in some of the harshest envi-
ronments on earth across North Africa and
western Asia. Natural selection has in all proba-
bility skewed gene frequency in favor of abiotic
stress-related gene complexes that, when com-
bined to reconstitute synthetic wheat, produce
genetic variation previously unseen in the hexa-
ploid wheat gene pool.
Drought
Although the database of derived synthetic per-
formance under drought is expanding, along with
anecdotal evidence of the superior performance of
primary synthetic wheat, there remains little pub-
lished evidence. This probably refl ects the diffi -
culty of assessing grain yield of agronomically
poor, primary materials that are diffi cult to thresh.
Nevertheless, Villareal et al. (1998) and Villareal
and Mujeeb-Kazi (1999) assessed primary syn-
thetics under managed postanthesis drought
stress in the Sonoran Desert in northwestern
Mexico. They concluded that many synthetics
were superior in drought adaptation to their
durum progenitors and the adapted hexaploid
wheat check cultivar, Seri 82.
Table 16.1
Percentage of lines resistant to various diseases
among 253 primary synthetic wheat accessions.
Percentage of
Resistant Lines
Disease
Cereal cyst nematode
10
Root lesion nematode (
P. neglectus
)
1
Root lesion nematode (
P. thornei
)
21
Septoria nodorum
10
Septoria tritici
73
Salinity and waterlogging
There is more evidence of salt tolerance in
primary synthetic wheat, largely related to the
ease with which controlled-environment assays
can be applied. Salinity reduces crop yields
Tan spot
10
Leaf rust
15
Stem rust
40
Stripe rust
24
Source:
Summarized from Ogbonnaya et al. (2008).
