Agriculture Reference
In-Depth Information
index, but that grain yield was positively corre-
lated at the 20/15 ºC regime. They concluded
that heat tolerance was associated with low yield
in the absence of high temperature and that the
synthetics may be useful for improving wheat
in regions where stress from high temperature
occurs frequently. The synthetics could, there-
fore, provide useful variation for breeders target-
ing the 7 million hectares of wheat limited by
consistently high temperatures (Reynolds et al.,
1994).
Freezing temperatures can also limit wheat
yield. However, it appears that the potential of
synthetic wheat as a source of variation is limited.
Although the synthetics do not appear to offer
signifi cant new variation for cold tolerance in the
vegetative stage (Limin and Fowler 1982, 1993),
they may contain useful variation for tolerance to
frost at fl owering (Maes et al., 2001). Maes et al.
(2001) found that fl oret death under freezing tem-
peratures was delayed in primary synthetics with
pubescent glumes by up to 4 minutes. Although
statistically signifi cant, the biological signifi cance
of these results is not clear.
Grain quality attributes
Wheat processing and product quality are impor-
tant breeding objectives in many parts of the
world, and the defi nition of quality varies among
regions. Three general determinants of wheat
quality are grain hardness or endosperm texture,
protein content, and protein quality. Hard-
grained wheat is required for processing leavened
and fl at breads and some noodle products, as
milling induces starch damage which, in turn,
increases water absorption, an important criterion
for the development of these products. The
Ae.
tauschii
accessions are generally soft-grained, as
are the primary synthetics produced from them.
The hard endosperm trait arose from a mutation
at the
Ha
or hardness locus on chromosome 5D
(Lillemo et al., 2006). This locus confers produc-
tion of puroindoline a (gene
Pina
) and puroin-
doline b (gene
Pinb
). The two linked genes confer
soft endosperm when in their wild-type allelic
state (
Pina-D1a
/
Pinb-D1a
). Synthetic wheat
introduces seven new alleles of
Pina
and six alleles
of
Pinb
from
Ae. tauschii
, all of which confer soft
grain (Lillemo et al., 2006). Hence it is necessary
to cross primary synthetics with hard-grained
sources if the associated array of hard-grained
products is expected.
Wheat protein content is largely infl uenced by
the environment and can be inversely related to
yield (Peña et al., 2002), although a gene from
T.
dicoccoides
has been reported to improve protein
content without adversely affecting yield (Davies
et al., 2006). Protein quality is important in
determining dough extensibility and is largely
controlled by high- and low-molecular-weight
glutenins and the gliadins. The
Glu-D1
locus of
Ae. tauschii
contributes alleles not found in culti-
vated bread wheat (William et al., 1993; Peña et
al., 1995; Pfl uger et al., 2001). Peña et al. (1995)
found that synthetics derived from a common
durum wheat had better overall quality and bread
loaf volume when they possessed the allelic vari-
ants 5
Preharvest sprouting
Preharvest sprouting is a signifi cant problem in
areas where rainfall occurs during harvest. The
grain sprouts in the head, reducing both grain
yield and economic value of the grain. Seed dor-
mancy is the primary mechanism of tolerance in
wheat, and red-grained wheat is generally more
tolerant than white-grained types (Gale 1989).
Gatford et al. (2002) screened
Ae. tauschii
accessions for preharvest sprouting tolerance
and the synthetic hexaploids derived from them.
They concluded that tolerance from the D-
genome donor was expressed, primarily as
seed dormancy, in the resultant synthetic. It was
later shown that this source of preharvest sprout-
ing tolerance is different from that previously
identifi ed in the wheat gene pool (Ogbonnaya
et al., 2007b). The contribution of this dormancy
source to the improvement of preharvest
sprouting in white-grained wheat is yet to be
confi rmed.
10 than when they had any
other
Glu-D1
encoded glutenin subunit. Simi-
larly Nelson et al. (2006), in an analysis of quality
characteristics in the ITMI population, found
recombinant inbred lines with quality superior to
+
12 or 1·5
+
