Agriculture Reference
In-Depth Information
of these growth factors are quite variable
among cultivars, but in most instances, the
duration of kernel growth is more important
than rate of kernel growth in determining fi nal
kernel size (Herzog 1986; McMaster 1997).
A management consideration in heat-stressed
environments is managing when grain fi lling
occurs as a means of avoiding adverse high
temperatures.
Figure 2.2 presents a time series of the pres-
ence of sources and sinks, and indirectly source
and sink activity. Although not shown, root
growth is the fi rst main signifi cant plant compo-
nent early in the life cycle. As leaves and tillers
begin to appear, they are initially an equal sink
(for growth) as roots. At the time of the initiation
of internode elongation, leaf appearance has
nearly stopped, and stem tissue is the main sink
component for carbohydrates and nutrients.
Internode growth is occurring normally at peak
canopy LAI and photosynthetic capacity, and
this is often a period when carbohydrate reserves
are stored in the stem material. When grain
fi lling starts, internode elongation has nearly
ceased, and the seeds are the main sink compo-
nent. Although components overlap somewhat in
their primary growth period, a general rule is
that only one component is primarily growing
at a time.
predict the rates have not been very successful
(McMaster and Wilhelm 1995).
Phytomers (or the subcomponents of a phy-
tomer) appear, grow, and senesce or abort on a
shoot (Fig. 2.2). Some phenological stages of the
shoot may be defi ned by the state of a specifi c
phytomer. For instance, the beginning of the boot
stage is when the leaf in the last vegetative phy-
tomer (i.e., the fl ag leaf) has completed growth on
the shoot. Often the phenological stage is the
result of a collection of phytomers on the shoot.
Illustrations of this are: the tillering stage, when
axillary buds of different vegetative phytomers
differentiate and grow; jointing, which occurs
when one or more internodes of vegetative phy-
tomers have elongated so that the fi rst node is
elevated above the soil surface; and fl owering,
which occurs when several reproductive phy-
tomers have produced anthers.
ENVIRONMENTAL FACTORS
INFLUENCING SHOOT DEVELOPMENT
The relationship between plant development and
the environment was undoubtedly recognized by
prehistoric civilizations. While the orderliness of
wheat development at various scales outlined
above occurs regardless of the environment,
abiotic and biotic factors and some management
practices change developmental rates and the
beginning and ending of the process (and there-
fore duration). Abiotic factors are used to predict
development. The mechanisms of action of man-
agement practices and biotic factors are best
viewed by how they alter the predictive nature of
abiotic factors.
Abiotic factors such as temperature, light,
water, fertility, and CO
2
control wheat develop-
ment, though temperature is viewed as the most
important factor (Klepper et al., 1982; Baker and
Gallagher 1983a,b; Bauer et al., 1985, 1986; Frank
et al., 1987; Porter and Delecolle 1988; Masle
et al., 1989; McMaster 2005). Temperature
strongly impacts cell cycling time, reaction rates,
and progression through the life cycle. Light can
strongly infl uence some developmental events,
but the other abiotic factors are considered of
Integrating phenology, the shoot apex,
and phytomers
The orderly development of the wheat plant
begins with the developmental events occurring
at the shoot apex. By producing the leaf and
spikelet primordia on a shoot, the basic building
blocks of the phytomers are added to comprise
the shoot. The rate of appearance of vegetative
phytomers can be determined by rate of leaf pri-
mordium initiation (plastochron) or leaf appear-
ance (phyllochron). Similarly, reproductive
phytomers in the spike are determined by the rate
of spikelet primordium initiation and fl oret pri-
mordium differentiation in the spikelet. While a
great number of studies have examined leaf,
spikelet, and fl oret primordium initiation and
subsequent growth, equations to accurately
