Agriculture Reference
In-Depth Information
All cultivars and each shoot go through the
same developmental event (again, this is the
orderliness of plant development), but cultivars
and shoots vary on the timing (beginning and
ending of the developmental event) and duration.
This is a result of both differences in genotype
and the cultivars' responses to environmental
conditions. The period of grain fi lling illustrates
this well. Cultivar trials clearly demonstrate that
within the same environment cultivars vary in the
time of internode elongation, anthesis, and physi-
ological maturity, and that this also affects the
duration of grain fi lling. Synchrony among shoots
in reaching a developmental stage increases as
physiological maturity is approached (Hay and
Kirby 1991).
Clear defi nitions of many developmental stages
are not always reported. Descriptions for the
Feekes (Large 1954) and Zadoks (Zadoks et al.,
1974) scales are provided in Table 2.1. For
instance, the start of internode elongation and
the developmental stage of jointing are two stages
with some ambiguity, particularly for determina-
tion in the fi eld. Both scales only discuss stem
elongation in terms of when nodes are above the
soil surface; therefore the beginning of internode
elongation is earlier than this, as often the fi rst
node is formed below the soil surface. Jointing in
the Feekes scale is defi ned as when the fi rst node
is observed to be one inch (2.54 cm) above the
soil surface, although many practitioners merely
record when the node is fi rst observed at any
distance above the soil surface. The time in which
internode elongation begins prior to jointing is
diffi cult to discern in the fi eld without destruc-
tive sampling; thus internode elongation is some-
times erroneously assumed to begin at jointing.
The concept of fi rst-hollow-stem stage was
created to call attention to the developmental dif-
ference between initiation of internode elonga-
tion and jointing in wheat production systems
with grazing cattle (
Bos taurus
L.). Producers
depend on staging to know the proper time to
remove cattle from wheat pasture just before
jointing so that further development and exten-
sion of the shoot apex is not impaired (Redmon
et al., 1996).
Booting is defi ned as the stage when the spike
can be felt within the whorl of leaf sheaths but is
not visible. Given the ambiguity of measuring
this stage relative to when booting actually begins
and that it is a continuum until heading, a useful
defi nition is to assume booting begins when the
fl ag leaf has formed its ligule and continues until
heading. Heading is defi ned as beginning when
the fi rst spikelet of the spike (i.e., head or ear) fi rst
appears above the ligule of the fl ag leaf at the top
of the canopy. Normally the fi rst spikelet to
appear is the terminal spikelet, and heading is
completed when the basal spikelet (= S1 in the
morphological naming scheme) appears. Gener-
ally awns are ignored in observing the beginning
of heading, and occasionally the spike emerges
“sideways” from the sheath, and many observers
consider this heading.
Physiological maturity is somewhat diffi cult to
quantify and to consistently determine among
different observers. Unlike maize that has a black
layer in the kernel to indicate when maximum dry
weight has been reached (the defi nition of physi-
ological maturity), wheat has no such discernible
trait. The Feekes scale defi nes harvest maturity as
when the kernel is diffi cult to divide along the
crease. Similarly, Zadoks et al. (1974) equate 90%
ripeness of rice (
Oryza sativa
L.) to when the seed
cannot be dented by the fi ngernail. Neither scale
clearly defi nes physiological maturity. An associa-
tion of maturity and maximum dry weight has
been suggested when all glumes, paleas, and
lemmas of the spike have lost all green color; all
leaves will have senesced before this time and
internodes will have lost all green color (Hanft
and Wych 1982). This is a rather obvious time of
maximum dry weight, as all sources of new car-
bohydrates via photosynthesis are gone and
reserves should be allocated to grains or
exhausted.
The location of the shoot apex changes with
developmental stage and has implications for the
environment of the shoot apex and certain man-
agement practices such as cattle grazing men-
tioned previously. Until the time that internode
elongation begins, the shoot apex is located in the
crown of the plant. The crown is normally located
