Agriculture Reference
In-Depth Information
benefi t growers in arid regions. Late rains com-
monly delay sowing, leading to reduced aerial
biomass and grain yield (Mahdi et al., 1998).
Often suffi cient moisture for germination is avail-
able deeper in the soil profi le, but the shorter
coleoptile of current semidwarf wheat cultivars
prevents successful establishment (Schillinger
et al., 1998). Deep sowing commonly results in
few, typically later-emerging seedlings, produc-
ing small relative growth rates and leaf area,
reducing seedling biomass (Hadjichristodoulou et
al., 1977; Rebetzke et al., 2007b). In turn, later
emerging plants have lower biomass at anthesis,
fewer spikes, and lower fi nal biomass and yield
(Mahdi et al., 1998; Rebetzke et al., 2007b)
(Fig. 11.17).
Numerous studies (e.g., Schillinger et al., 1998;
Rebetzke et al., 2007b) have demonstrated a posi-
tive association between coleoptile length and
plant number with deep sowing (Fig. 11.17).
Shorter coleoptiles and poor emergence have
commonly been associated with the presence of
Rht-B1b
and
Rht-D1b
dwarfi ng genes (Schillinger
et al., 1998; Rebetzke et al., 2007a,b). Many of the
alternative, gibberellin-sensitive dwarfi ng genes
(e.g.,
Rht4
,
8
,
12
, and
13
) reduce plant height with
little or no effect on coleoptile length (Ellis et al.,
2004). Indeed, studies have demonstrated the
potential of
Rht8
in the development of semi-
dwarf, long-coleoptile wheat targeted at sowing
depths exceeding 110 mm (Schillinger et al.,
1998; Rebetzke et al., 2007b). In addition to
replacement of the
Rht-B1b
and
Rht-D1b
dwarf-
ing genes with alternative dwarfi ng genes, genomic
regions independent of height have been identi-
fi ed to increase coleoptile length in wheat (Table
11.2) (Rebetzke et al., 2001a, 2007b).
Extensive evaluation across irrigated and water-
limited environments has been undertaken for
semidwarf lines containing alternative dwarfi ng
genes and representing a range of genetic back-
grounds (Fig. 11.18). This evaluation has demon-
strated a generally similar yield performance of
alternative dwarfi ng genes compared with exist-
ing
Rht-B1b
and
Rht-D1b
genes. One exception
was the extreme height-reducing
Rht12
, which
despite its high harvest index, showed unstable
performance owing to reduced biomass (data not
shown). The potential also exists in development
of reduced-height wheat that combines
Rht-B1b
or
Rht-D1b
with alternative dwarfi ng genes such
as
Rht8
to increase partitioning to grain and
reduce crop lodging without compromising estab-
lishment. Table 11.3 shows the increased harvest
100
(b) Moombooldool 1998
(a) Condobolin 1998
Rht8
Rht-B1b/Rht-D1b
rht
75
50
25
0
-25
-50
-75
Number
plants
Number
spikes
Spikes/
plant
Kernels/
spike
Kernel
number
Grain
yield
Number
plants
Number
spikes
Spikes/
plant
Kernels/
spike
Kernel
number
Grain
yield
-100
Fig. 11.17
Change (%) in different agronomic characteristics with deep-sowing at 110 mm expressed relative to the shallow
50-mm sowing depth at (a) Condobolin and (b) Moombooldool, Australia, in 1998. Bars represent (left to right)
Rht8,
Rht-B1b/D1b
,
rht
(after Rebetzke et al., 2007b).
