Development of the Wheat Plant - Wheat Science and Trade - page 31

prior to the beginning of stem elongation, each
phytomer has a minimal internode. The node is
the region where the leaf attaches to the shoot and
the vascular tissue connects the shoot and leaf; the
nodes are closely stacked just below the shoot
apex located in the crown of the plant. Once the
signal to begin internode elongation occurs, the
intercalary meristem of the most recently formed
node at the apical region of the node begins cell
division and expansion that extends the shoot
apex above the crown and ultimately to the top of
the canopy. Similarly, the leaf appears, grows,
and ultimately senesces. The axillary bud can
produce either another shoot, as in wheat, or an
infl orescence structure (e.g., either a shoot or the
ear of maize, Zea mays L.).
For plants such as wheat that can produce
nodal roots, the phytomer concept should be
expanded to include this component (Forster et
al., 2007). Given this defi nition, generally the
phytomer has been viewed as a vegetative build-
ing block, yet it is clear that the basic unit is
repeated throughout the grass infl orescence
(Forster et al., 2007).
The phytomer concept has proven to be a
useful botanical abstraction for providing a foun-
dation to understand plant development and
architecture. It is the appearance, growth, and
senescence of these phytomers that determine the
characteristics of individual shoots. This dynamic
interplay of phytomers can be considered analo-
gous to a composition of music called a canon (a
familiar simple form being a round), where indi-
vidual phytomers repeat a part against and with
other phytomers as do the melodies of a canon
(Hargreaves and McMaster 2008). Similarly, the
integration of these phytomers within a plant has
been outlined in a generic pattern by Rickman
and Klepper (1995) for a variety of grasses. This
is an excellent demonstration of the consistent
and orderly development of the grass shoot
apex.

tainly they aid in communicating precisely what
part of the plant is being measured. Naming
schemes also can provide insight into how the
plant perceives its environment and the effi cacy
of management practices. Although various
nomenclatures have been proposed and modifi ed,
most are quite similar.

Leaves
A simple leaf-naming scheme has been proposed
by Jewiss (1972) and Klepper et al. (1982, 1983a),
in which true leaves (not including the coleoptilar
leaves) are numbered acropetally for each culm.
The fi rst leaf is designated L1, with successive
leaves L2, L3, up to the last leaf formed on the
shoot (i.e., the fl ag leaf). A numerical leaf staging
system was proposed by Haun (1973):

Haun stage = ( n − 1) + L n / L n −1 , (0 < L n / L n −1 ≤ 1)

in which n is the number of leaves that have
appeared on the culm, L n −1 is the blade length of
the penultimate leaf, and L n is the blade length of
the youngest visible leaf extending from the
sheath of the penultimate leaf.
One increasingly important application of
knowing leaf number in crop management is that
many pesticides are to be applied at certain leaf
stages. For instance, the herbicide imazamox is to
be applied at the fourth-leaf stage for control of
many grass weeds when growing the cultivar
Above.

Tillers
The system fi rst proposed by Jewiss (1972) for
naming tillers has been modifi ed and extended
(Masle-Meynard and Sebillotte 1981; Fraser et
al., 1982; Kirby et al., 1985a; Masle 1985). The
modifi ed system proposed by Klepper et al. (1982,
1983a) is increasingly being adopted. In this
system, the leaf axil and parent culm are used to
name the tiller. The fi rst culm to emerge from the
seed is the main stem (MS), with subsequent new
culms being tillers that can be considered primary,
secondary, tertiary, and so on, based on the parent
culm. Tillers appearing from axillary buds in the

MORPHOLOGICAL NAMING SCHEMES

Naming schemes which uniquely identify each
part of the plant can have many advantages. Cer-