Agriculture Reference
In-Depth Information
endosperm to reduce cell and starch granule
number and decrease kernel size (Nicolas et al.,
1985). A lack of water can also reduce grain weight
through reductions in current photosynthesis and
reduced leaf area (via canopy desiccation and
senescence), which in turn reduce transpiration
and accelerate the rate of grain fi lling, partly via
increased crop temperature (Brooks et al., 1982).
Collectively these effects decrease kernel size
unless kernel number has already been greatly
reduced by earlier stress. Reductions in kernel
size contribute to an increase in the proportion
of shriveled kernels, lower harvest index and
grain yield, and decreased grain quality (van
Herwaarden and Richards 2002; Ruuska et al.,
2006). Agronomic factors such as excessive nitro-
gen can contribute to premature crop senescence
where excessive leaf area can exhaust available soil
water, leaving little water for grain fi lling (van
Herwaarden et al., 1998).
and accumulating biomass under cool conditions
of low radiation load. Moisture necessary for
growth up to anthesis is provided either through
irrigation or rainfall, including that stored in the
soil prior to sowing (Fig. 11.1). Vapor pressure
defi cits are typically low during the vegetative
period, although transient water defi cits may arise
if available soil water becomes depleted. After
fl owering, rising temperature and radiation
increase VPD and evaporative demand (Fig. 11.1).
In many parts of the world and particularly in
Mediterranean environments (e.g., North Africa-
southern Europe, Southern Australia, and the US
Central Plains), the period from anthesis to crop
maturity coincides with a reduction in rainfall.
The wheat crop is increasingly reliant on irrigation
or moisture stored and available in the soil profi le
(e.g., locations Geraldton and Minnipa in Fig.
11.1). If transpiration demand of the crop and
evaporation from the soil surface (i.e., ET) together
exceed potential soil water supply, then the crop
experiences drought stress (Nix 1975).
Even when water is freely available across the
season, crops typically experience short periods of
transient water defi cit. These small-scale events
lower leaf conductance and photosynthesis, and
slow leaf expansion. But these perturbations may
contribute little to change in fi nal performance if
adequate water is available around key periods for
growth and yield formation (e.g., around pollen
meiosis and grain set) (Passioura 2002). Perhaps
the greatest effect on grain number and yield
arises for water defi cit around fl owering. For
example, Fischer (1973) initiated water defi cit at
different times commencing 20 days before fl ow-
ering. He found a 60% to 80% reduction in kernel
number per spike with water stress at pollen
meiosis (approx. 10 days before anthesis) com-
pared with a reduction of only 20% to 40% with
stress at fl owering. Similarly, Saini and Aspinall
(1981) demonstrated greater sensitivity to water
stress in the male gametophyte (especially pollen
mother cell meiosis), with male sterility in about
40% of fl orets but with little effect on female
fertility.
The infl uence of postanthesis water defi cit on
grain yield refl ects direct and indirect changes on
kernel size. Drought infl uences cell division in the
WATER-LIMITED YIELD POTENTIAL
Factors during the season can impact effective
water use by the growing wheat crop. In water-
limited environments, fi nal biomass and yield are
often related to total crop ET (Taylor et al., 1983
and references therein). French and Schultz
(1984) fi rst identifi ed that few wheat crops in
southern Australia ever achieved potential grain
yield for a given growing season ET. This obser-
vation has recently been extended to a meta-
analysis of almost 700 experimental and farmer's
wheat crops worldwide (Sadras and Angus 2006)
(Fig. 11.2). Among all these crops, the yields of
approximately 97% of them lay below the water-
limited grain yield potential of 22 kg ha
−1
mm
−1
or
aboveground biomass of 55 kg ha
−1
mm
−1
, assum-
ing a water-limited harvest index of 0.40. The
value of 22 kg ha
−1
mm
−1
represents a “boundary”
or “upper limit” to grain yield under rainfed
conditions.
Sadras and Rodriguez (2007) have since
extended the model of French and Schultz (1984)
to demonstrate how other climate factors, such as
increased VPD and decreased fraction of diffuse
