Agriculture Reference
In-Depth Information
sequence of spikelet and fl oret primordia initia-
tion is that, in the mature spike, central spikelets
begin differentiation fi rst and thus have the
longest period for initiating fl orets and longest
time for those fl orets to fully develop. This is why
central spikelets have the most kernels and kernel
number declines acropetally and basipetally from
central spikelets.
Yield potential has been determined by the time
of fl owering, as all fl orets have been produced. Pol-
lination of fully developed fl orets is critical in
determining grain set and fi nal yield. Environmen-
tal conditions determine the success of grain set,
with temperature and water stress normally the
most important factors. In many semiarid wheat
production regions, hot and dry conditions can
signifi cantly reduce grain set both through loss of
pollen viability and early seed abortion.
Wheat is self-pollinated, and pollination follows
the pattern of fl oret primordia initiation within
the spike discussed previously. This has implica-
tions for fi nal kernel size. Earlier fertilization
events allow greater time for seed development
and growth, and potentially larger fi nal kernel
size. Seed development and growth proceeds
through a clear set of steps, and extensive research
has been published on that subject [Herzog
(1986) and McMaster (1997) cite many refer-
ences]. Ontogeny of the seed mainly consists of
development of the embryo and the endosperm
tissue. Embryo development is completed prior
to endosperm maturity, with the loss of seed
moisture in the endosperm that is necessary for
combine harvesting (usually about 120 g kg
−1
moisture).
Growth of all kernels is characterized by a sig-
moidal pattern that is frequently divided into
three phases: lag phase, linear phase, and matura-
tion phase (Herzog 1986). The lag phase primar-
ily consists of cell division, the linear phase is
driven by maximum cell expansion growth rates,
and the maturation phase is mostly the loss of
seed moisture. Abiotic and biotic factors strongly
infl uence fi nal kernel size, consummated both
by the duration of the kernel growth (primarily
the length of the linear phase) and by the rate
of growth (the slope of the linear phase). Both
nalization requirement. Temperatures within the
range of about 2-7 ºC seem most effective for ver-
nalizing, with decreasing effectiveness below or
above this range (McMaster et al., 2008). High
temperatures (>30 ºC) can also “undo” some of
the vernalization (i.e., devernalize) that had previ-
ously occurred.
Photoperiod sensitivity varies considerably
among genotypes and is the result of which
photoperiod (
PPD
) genes are present (also dis-
cussed in Chapter 3). The conceptual model is
that photoperiod or day length must reach a certain
threshold to produce the signal that induces the
primordium switch. Cultivars with low photope-
riod sensitivity tend to have a low day length
requirement so that it is met under most environ-
ments. Some uncertainty has existed on how to
combine the vernalization and photoperiod
responses in developmental models, with the view
often held that vernalization requirements must be
at least largely met before the photoperiod response
can occur (McMaster et al., 2008).
It is diffi cult to precisely determine the vernal-
ization response and photoperiod sensitivity of a
genotype, but efforts to estimate these from loci
present in the genotype appear promising (White
et al., 2008). Genes for frost tolerance can also
complicate the relationship (Prasil et al., 2004).
Until recently, the mechanisms and genetic path-
ways for the developmental switch were not known.
Chapter 3 provides an excellent summary of the
genetic pathway as currently understood and
explains the variation observed among genotypes.
Once spikelet primordium initiation begins at
double ridge, spikelet primordia are produced
until near the time that internode elongation
begins, when the terminal spikelet is initiated
(Fig. 2.2). Shortly after a spikelet primordium is
initiated, fl oret primordia are initiated acropetally
within the spikelet. Floret primordium differen-
tiation then occurs to produce the structures of
the fl oret. As many as 10 fl oret primordia may be
initiated within a spikelet, and initiation occurs
until about when the fl ag leaf appears. After this
time, fl oret primordium abortion tends to occur
basipetally within a spikelet until the time of
anthesis. The end result of the overlapping
