Agriculture Reference
In-Depth Information
modeling of wheat grown at multiple sites and
using over 100 years of historical rainfall data
(Lilley and Kirkegaard 2007). Extraction of mois-
ture from below 1.2 m provided a yield benefi t
of 0.1 to 0.6 t ha
−1
and marginal WUE of 30 to
36 kg ha
−1
mm
−1
. The high marginal WUE
reported in Kirkegaard et al. (2007) was rarely
observed and restricted only to high-rainfall
seasons. The authors conclude there is real value
in development of deep roots in increasing post-
anthesis water use and yield but only in deeper
soils and in seasons where adequate rainfall will
provide subsoil moisture.
Repeatable genotypic variation has been
reported for factors contributing to greater root
length and biomass in wheat (Hurd 1968). In
many cases this variation refl ects adaptation to
hostile subsoils. For example, genes have been
identifi ed and undergone selection for traits
controlling resistance to root disease (Lagudah et
al., 1997) and other soil constraints, including soil
salinity (Byrt et al., 2007) and soil acidity (Delhaize
et al., 2004). Genotypic variation has also been
reported for the ability of wheat roots to penetrate
hard soil pans (Botwright et al., 2007), to produce
secondary branching for increased nitrogen
uptake (Palta et al., 2007), and to access soil mois-
ture at greater depth (Hurd 1968). Selection for
narrower xylem vessel diameter has the potential
to increase hydraulic resistance and delay water
use to later in the season. Richards and Passioura
(1989) backcrossed genes from a Turkish landrace
into two commercial wheat backgrounds to reduce
seminal root xylem diameter from about 65 to 55
μm and increase grain yield between 3% and 11%
in water-limited environments.
Surveys across a range of fi eld studies indicated
wheat roots grow downward an average of 0.8 to
1.1 cm per day, with fi nal rooting depth depending
on the duration of the vegetative phase, soil
wetness, and soil type (Lilley and Kirkegaard
2007). Conceivably then, a 2-week delay in sowing
date has the potential to reduce rooting depth by
10 to 15 cm. Selection for the different fl owering
genes (e.g.,
VRN_
,
PPD_
, and
EPS
) will allow
development of cultivars that can be sown earlier
to extend the duration of the vegetative phase
without changing anthesis date (Richards et al.,
selection (e.g., early vigor to increase T/ET and
CID to increase W; Condon et al., 2004). However,
as indicated in Condon et al. (2004), traits affect-
ing W have the potential to affect other compo-
nents in the framework. Some of these traits will
now be discussed in greater detail.
Increasing water uptake
Increases in crop ET are strongly associated with
increased grain yield in wheat. For example,
across approximately 200 irrigated and dryland
wheat experiments grown in the southern US,
Musick et al. (1994) reported a correlation of ET
and grain yield of 0.87. In turn, increased water
uptake is an obvious means for increasing
productivity in droughted environments. A larger
root system has the potential to access greater soil
moisture, while allowing increased access to soil
nutrients and reducing root lodging through
increased plant anchorage. Smaller root systems,
particularly on coarse-textured soils early in the
growing season, may intercept less moisture and
soluble nutrients, which in turn increases deep
drainage and nutrient leaching, affecting water-
table quality and long-term environmental
sustainability (Passioura 2002).
Despite the potential benefi ts of increased root
growth, few wheat studies have demonstrated a
yield benefi t with increased water uptake in water-
limited environments. Crop rotations, particularly
with the use of
Brassica
species such as canola,
reduce the incidence of soilborne diseases,
increasing water use and yield of the following
wheat crop (Kirkegaard et al., 1994). Using a
simulation model, Manschadi et al. (2006) showed
that delaying water uptake from deep in the soil
profi le increased water availability for postan-
thesis grain growth and improved grain yield. In
carefully managed fi eld studies, Kirkegaard et al.
(2007) showed that extraction of an additional
11 mm of subsoil moisture from soil depths
exceeding 1.3 m contributed an extra 0.6 tonne of
grain per hectare. Marginal WUE of this additional
yield was 59 kg ha
−1
mm
−1
, approximately three
times greater than for water used earlier in the
season. The value of deep rooting in extracting
subsoil moisture was then assessed with simulation
