Agriculture Reference
In-Depth Information
resulted in a qualitative understanding of response;
however, quantitative characterization of the
response is often lacking and identifi cation of
threshold levels for a factor is unknown.
Regardless of these diffi culties, one general
rule is that growth of an organ is reduced
and abortion or senescence rates are increased
when the environmental factor is limiting. For
instance, leaf, internode, and kernel growth
is reduced and leaf senescence and tiller, spikelet,
and fl oret abortion is increased when water,
nutrients, CO
2
, or light is limiting or under
high salinity conditions (Gifford 1977; Whingwiri
and Kemp 1980; McMaster et al., 1987; Maas
and Grieve 1990; Jedel and Hunt 1990; Maas
et al., 1994).
Although temperature and photoperiod are
the most important factors controlling wheat
phenology, phenological responses to water
availability have been reported (Angus and
Moncur 1977; Bauer et al., 1985; Baker et al.,
1986; Frank et al., 1987; Davidson and Chevalier
1987, 1992). McMaster and Wilhelm (2003)
observed that later developmental stages, particu-
larly anthesis and physiological maturity, were
most sensitive to water defi cits and required less
thermal time to reach the developmental stage
under water defi cits than well-watered condi-
tions. Most notably, the grain fi lling duration
was signifi cantly reduced by water defi cits,
as commonly observed in many experiments and
for most crops (McMaster et al., 2009). In general,
water defi cits must be quite substantial before
phenological responses are noted. Physiological
and genetic mechanisms controlling these pheno-
logical responses are not well understood, but
the likely indirect effect of water defi cits increas-
ing canopy temperature, and therefore thermal
time accumulation, might partly explain the
general response of reaching a developmental
stage earlier under water defi cits (McMaster
et al., 2009).
Occasional responses of some developmental
stages to nutrients have been reported (Frank and
Bauer 1984). At low levels of N, developmental
stages such as double ridge, terminal spikelet, and
anthesis were delayed (Whingwiri and Stern
1982; Longnecker et al., 1993). Nitrogen limita-
tion seems to have the greatest effect on decreas-
ing the duration of grain fi lling, presumably by
increasing leaf senescence. Other factors have a
less reported or consistent effect on phenology
than nutrients. For instance, if ambient CO
2
levels
were doubled (700 ppm), the time required to
reach fl oral induction and anthesis was slightly
shortened (Marc and Gifford 1984), and the
spikelet, fl oret, and grain fi lling development
phases were reduced by elevated CO
2
(550 ppm,
Li et al., 1997, 2000).
Leaf, spikelet, and fl oret primordia initiation
appears to be minimally reduced by factors other
than temperature and light, although leaf appear-
ance and fi nal spikelet and fl oret number are
often reduced by factors other than temperature
and light (Whingwiri and Stern 1982; Long-
necker et al., 1993; Li et al., 1997). Three possible
explanations that are not mutually exclusive
could account for the variable responses noted.
One explanation has been mentioned before:
until a threshold level is reached, little response
to an abiotic factor is observed. An example of
this would be the response of reduced spikelet
primordium initiation and delayed developmen-
tal stages observed only at low levels of N
(Longnecker et al., 1993). Another explanation is
that the duration of the phase is shortened by the
abiotic factor that affects the fi nal number of pri-
mordia initiated. An illustration of this would be
Frank et al. (1987) showing that water stress
mainly affected spikelet primordium initiation by
decreasing the initiation phase. The third expla-
nation for the reduced response of many devel-
opmental processes to environmental factors is
that primordium initiation mainly responds to
hormonal controls and then is driven by cell
cycling time, which is primarily a temperature-
response process that requires little resources in
terms of water and nutrients. If these resources
are limiting to the extent that cell division is
impacted, then in annual plants such as wheat,
the shoot or shoot apex is in danger of death.
However, the differentiation and growth of
these primordia (e.g., leaves, tillers) are strongly
infl uenced by many environmental factors
