Agriculture Reference
In-Depth Information
at a depth of about 2 to 3 cm below the soil
surface. If planting is deeper than this, as is often
the case, then the coleoptilar internode will elon-
gate to form the crown at this depth. As the
internodes elongate, the shoot apex moves
from the soil environment (i.e., crown) into the
aerial environment within the canopy. In many
wheat production systems, the danger of cold
temperatures (i.e., frost) is present at this time
and signifi cant damage to the shoot apex spikelet
and fl oret primordia that are forming and differ-
entiating can occur, resulting in yield loss. At
heading, the shoot apex or spike emerges above
the canopy and the glumes, paleas, lemmas, and
awns (if present) are exposed to full sunlight.
Primarily the fl ag leaf and spike photosynthetic
tissues contribute new assimilates for grain fi lling,
with the spike contributing up to about 10% of
the fl ag leaf photosynthetic rate (Weyhrich et al.,
1995).
tion of the seed has occurred, further development
and growth of the existing leaf primordia occurs,
resulting in leaf appearance of L1, L2, and so on.
Coinciding with the appearance of the fi rst leaves
is the initiation of more leaf primordia. Leaf pri-
mordia are initiated up to the stage of double
ridge. At double ridge, the leaf primordium forms
a ridge around the apex and this primordium will
not further differentiate and grow (Fig. 2.1). The
ridge above the leaf primordium is the spikelet
primordium. Primordia formed prior to double
ridge will continue to differentiate and grow
resulting in the continuation of leaf appearance
until the fl ag leaf appears, which is about the
second leaf to appear after the developmental
stage of jointing.
Developing the leaf area index (LAI) of the
canopy is a function of the appearance, growth,
and senescence of leaves on each shoot compris-
ing the canopy. Leaf size increases on a shoot up
to about the 10th leaf, although the fl ag leaf tends
to be slightly smaller than the penultimate leaf
(Gallagher 1979; Hay and Wilson 1982; Rawson
et al., 1983; Kirby et al., 1985b). Under unstressed
conditions, leaves do not begin senescing until
about 6.5 phyllochrons (time interval for appear-
ance of successive leaves) after fi rst appearing,
and abiotic stresses such as water defi cit and low
N availability will enhance the senescence rate
(McMaster et al., 1991; Wilhelm et al., 1993).
Therefore leaves of increasing size accumulate on
a shoot, and leaf senescence begins with the small-
est and oldest leaves. Generally peak LAI is
reached at the time the fl ag leaf completes
growth.
The rate of leaf primordia initiation (plasto-
chron) and appearance (phyllochron) are critical
to many subsequent developmental events. For
instance, until the leaf primordium has been initi-
ated and begins to grow, the axillary bud that can
form a new tiller is not initiated. This is why the
beginning of tiller appearance is delayed from leaf
appearance in Fig. 2.2. The rate of leaf appear-
ance therefore “controls” the window of time that
the tiller can appear. Leaf primordium differen-
tiation and growth also determines the formation
of the node, which is the point where vascular
tissue enters the leaf.
Shoot apex
Developmental events occur at many places
within the plant; however, the shoot apex is the
site of many of the most important developmental
events such as leaf, tiller, and infl orescence pri-
mordia production. As previously mentioned,
these events are not represented in developmental
scales (Fig. 2.1). As with phenology, the shoot
apex of all cultivars and shoots has the same
developmental sequence (Fig. 2.2), but cultivars
and shoots vary in the timing and duration of the
developmental event. Furthermore, this develop-
mental sequence is generally shared by most
grasses, particularly the annual cereal crops such
as barley (
Hordeum vulgare
L.) and rice (Rickman
and Klepper 1995). Many reviews of wheat shoot
apex development are available that provide
considerable detail (e.g., Barnard 1955; Bonnett
1966; Kirby and Appleyard 1984; McMaster
1997).
The developmental sequence of the shoot apex
begins with the embryo within the seed. Wheat
typically has three to four leaf primordia formed
in the embryo (Bonnett 1966; Baker and Galla-
gher 1983a; Hay and Kirby 1991). Once germina-
