Agriculture Reference
In-Depth Information
trait is not completely independent of environ-
mental infl uences.
The importance is that the pattern of seedling
emergence is refl ected in variation in the develop-
ment observed within a fi eld throughout the
growing season. For instance, seedlings that
emerge earliest normally are bigger, reach devel-
opmental stages earlier, and are higher yielding
than those seedlings that emerge later (Gan and
Stobbe 1995). Interestingly, although seedlings
that emerge later may reach developmental stages
later, their development rate often is faster.
However, the increased development rate is insuf-
fi cient to offset their delayed emergence; thus
they may still reach maturity slightly later
(Nuttonson 1948; Angus et al., 1981; O'Leary et
al., 1985) or, if simultaneously, then at the cost of
reduced growth.
should focus on producing stands of uniculm
plants (i.e., only main shoots). Producing tillers,
particularly those that abort before physiological
maturity, merely “waste” resources. This
approach seems most viable for high-production
environments that are less variable in precipita-
tion and extreme events. In highly variable
environments, such as many semiarid wheat pro-
duction regions, tillering can partially adjust for
winter kill and other causes of plant loss, and it
can provide an adjustment for planting rates nor-
mally used that are less than optimal for favorable
years (McMaster et al., 2002). Further, when
planting at high densities and reducing tiller
spikes, main-stem spikes usually will be less
productive, so little gain is obtained by the
uniculm approach.
Just as later emerging plants usually require
less thermal time (i.e., fewer growing degree-
days) to reach a given developmental stage, the
same has been observed for shoots that appear
later, as well as the variation among the main stem
and different tillers in reaching a developmental
stage being reduced as the plant approaches phys-
iological maturity (Hay and Kirby 1991). This
increasing synchrony among shoots results in less
canopy variation over time.
Despite differences in developmental stage
among shoots on the plant, the pattern of devel-
opment is the same for each shoot. Further, the
response to environmental factors is similar
among shoots, although specifi c responses of
individual shoots will vary because they are in
different environments within the canopy and the
vascular connections and root systems of each
shoot will be different. This will be discussed in
greater detail later in the chapter.
Shoots or tillers
The canopy is a collection of shoots or tillers
which appear, grow, abort, and ultimately senesce,
albeit at variable rates among individuals within
the canopy. The cumulative number of shoots
that appear is dependent on many factors, includ-
ing the density of plants, the genotype, the
environment, and management (Darwinkel 1978;
Masle-Meynard and Sebillote 1981; Fraser et al.,
1982; Masle 1985). Seedling emergence is impor-
tant not only in determining the number of plants,
but also in determining the number of shoots that
a plant produces that will likely survive to produce
a spike. This is because a specifi c axillary bud that
produces a tiller has a window of time during
which it can appear (Klepper et al., 1982), and
once this window passes, that axillary bud will not
further differentiate and grow. In most instances,
tillers that appear the earliest and from axillary
buds on the main stem will be the last ones to
abort. This has implications for fi nal yield predic-
tion, as these tillers and the main shoot (i.e., the
fi rst shoot to emerge from the seed) are the
primary yield-producing shoots (Power and Alessi
1978; McMaster et al., 1994).
Some propose that because the main shoot
is usually the most productive, management
Phytomers
The shoot is composed of subunits, or building
blocks, called phytomers (Gray 1879; Bateson
1894). The phytomer has generally been defi ned
as the leaf, node, internode above the node, and
the axillary bud (Wilhelm and McMaster 1995).
Each phytomer component can appear, change
over time, and/or abort or senesce. In wheat,
