Agriculture Reference
In-Depth Information
differentiation of organs and the progression
of stages through which cells, organs, and
plants pass during their life cycle. Often, but
not always, growth and development occur
simultaneously.
J.W. von Goethe recognized the orderly devel-
opment of plants in the late 18th century. Since
that time, extensive research has quantifi ed the
orderly and predictable nature of how the wheat
plant proceeds from germination through its life
cycle to maturity. This orderliness is observed at
all scales of the wheat plant, from the whole
canopy to the whole shoot, as well as within the
shoot. Genes determine the orderly sequence of
wheat development, and the rate and timing
of development respond to environmental
conditions.
Development serves two purposes. First, it
creates the structures such as leaves, roots, and
stems needed to capture resources (e.g., light,
water, nutrients) that are then used to produce a
viable seed. Second, development produces the
infl orescence structures needed to produce a
viable ovule that can be pollinated at anthesis.
Therefore, development is essential in producing
the yield potential and the ultimate realization of
that potential in fi nal yield.
This chapter discusses the development of
the wheat plant. Given the vast research on
wheat development and extensive breadth of
aspects that can be covered, the goal is to
capture and challenge current thinking about
wheat development as it fi ts into the overall
purposes of this topic. This chapter examines,
fi rst, different scales of wheat development, and
second, how a shoot develops since the canopy
is merely a collection of shoots. The underly-
ing building block of a shoot is the phytomer,
and this is discussed to better understand
shoot or canopy development. Environmental
impacts on development are considered, particu-
larly the critical role of temperature. This chapter
presents an overview of simulating wheat devel-
opment and concludes with new information
emerging from molecular biology that provides
better mechanistic understanding of wheat
development.
SCALES OF PLANT DEVELOPMENT
A cursory walk through a wheat fi eld readily
exposes the dynamic nature of wheat develop-
ment, with many different scales to consider
(Masle-Meynard and Sebillotte 1981). Variation
in seedling emergence or among wheat spikes that
are spatially distributed and differing in size and
greenness refl ect variation in the canopy. This
variation likely will be greater in a larger fi eld with
varying soils and topography than within small
areas within a fi eld. Further variation might be
introduced by variation in genes within a plant
community (e.g., single cultivar or multiple cul-
tivars) which control development, though breed-
ers attempt to minimize this variation within
a given cultivar. A higher level of resolution is
represented by the shoots that comprise the
canopy. Examination of individual shoots reveals
other differences, such as height, leaf number and
size, and stage of development, and this is the
result of the basic building block of a shoot, the
phytomer. Therefore, at least three scales of plant
development can be important: the canopy, the
shoot, and the phytomer. Selection of the appro-
priate scale is dependent on the research or
production problem and level of understanding
desired.
Canopies
A wheat fi eld is the collection of plants that form
the canopy. Plants emerge and grow at different
rates, and some die prematurely. Differential pat-
terns of seedling emergence cause some of the
variation in development observed in the fi eld.
The pattern of seedling emergence varies greatly
depending on many nongenetic factors that affect
germination and seedling emergence such as
planting depth and rate, and soil moisture, tem-
perature, and strength. These factors are highly
dependent on the climate, soils, and management
practice; spatial variation even at the microscale
of less than one meter can cause variation in seed-
ling emergence. Genetic factors such as strength
of seed dormancy will infl uence the pattern of
seedling emergence, but even expression of this
