The Grasses (particularly as cereals)

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The Grasses (particularly as cereals) The
grasses are all members of the Poaceae.
Taxonomically we locate them within the Kingdom
Plantae Division Magnoliophytae
(angiosperms) Class Liliopsida (monocots)
Order Poales (dont ask!) Family
Poaceae (or Gramineae) What you learned earlier
about flower structure applies (with some special
modificatiions) here grasses are all flowering
plants. What are the special differences?
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Grass flowers are small, often greenish, and lack
conspicuous petals. Pollination occurs by wind.
Adaptation to wind pollination has modified the
structure of the flower. Grass flowers have a
single ovary with two feathery stigmas adapted to
the capture of wind-blown pollen. There are three
stamens. Small, swollen scales (called and
labeled lodicules below) swell to open the
flowers when anthers and stigmas are mature. Each
grass flower (a floret) is enclosed within
leaf-like structures called the palea and
lemma. Florets may be borne singly or in clusters
known as spikelets. Each spikelet is enclosed by
two papery glumes, and has a bristle-like
extension called an awn. The fruit of a grass we
call a grain.
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There is also some specialized terminology
related to vegetative characteristics of the
grasses Individual stems are called culms. They
are frequently hollow between nodes (think of
bamboo) and usually without branching. In
addition to fibrous roots, many grasses have
stolons (above ground horizontal roots) or
rhizomes (similar horizontal growth, but
underground).
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Leaves are parallel-veined and attach to the stem
at nodes with a leaf sheath extending from origin
to the point where the blade separates from the
culm.
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The fruit, or grain, is single-seeded, dry, and
indehiscent (which means it doesnt open at
maturity to release the seed). There are a number
of layers surrounding the embryo, and many are
important in considering the grain as food The
glumes, palea and lemma are, in a mature grain,
called chaff. The outer wall of the grain
consists of the fused fruit wall and seed coat.
This is called the bran. Just inside the bran is
a layer of cells called the aleurone layer, which
is rich in protein and the source of enzymes to
break down the endosperm in a germinating and
growing seedling. The embryo and its sheaths is
called the germ.
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The endosperm is mostly starch, and that is what
makes cereal grains so globally important as food.
Now lets consider the most important cereal
grains wheat, corn, and rice
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Wheat
While wheat is grown under varied climates, it
grows best in temperate climates with 30 90 cm
annual rainfall. We know most of those places
midwestern U.S., Canada, the Ukraine, China,
Argentina, but also India, France and South
Africa. Wheat is the oldest domesticated plant
(except possibly barley), and the wild original
types are still found in Iran, Iraq and Turkey.
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There is a fascinating evolutionary history
associated with the evolution of modern wheat
cultivars (durum wheat, Triticum durum, and bread
wheat, T. aestivum). It involves hybridization of
different species and polyploidization. The most
widely accepted version is that Einkorn wheat
randomly and by chance hybridized what your text
calls a goat grass, which was another species
of Triticum. Genetically, the two species were
(and are) quite different , but both have a
diploid number of 14 chromosomes. Assume that we
can describe the genotype of the Einkorn wheat as
AA, and the genotype of the goat grass as BB.
What you would then logically expect is that the
hybrid would have a genotype of AB. One key
problem in meiosis the A and B chromosomes are
not homologous, and cant properly pair up.
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This hybrid would be sterile the end of the
line. However, if there was a failure during
mitosis, with all chromosomes retained in one
cell, or in meiosis, with both copies of A and
both of B were retained in a single gamete, then
offspring produced would be tetraploid, with a
genotype AABB. Such individuals could undergo
normal meiosis and cross with other like
individuals. The new species would have 28
chromosomes, and be instantaneously genetically
isolated from the source species. This type of
polyploid is emmer wheat and durum wheat. Emmer
wheat appeared naturally before domestication,
and was grown agriculturally early in
domestication. One additional hybridization, of
emmer wheat with another goat grass, and bread
wheat has evolved.
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Heres one version of the process. Though the
diagram shows hybridization between two species
with unequal numbers of chromosomes, the
important thing is that they are different sizes
and shapes and could not synapse properly.
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In the formation of bread wheat, once more
polyploidization has to happen for the species to
be able to reproduce. Without becoming polyploid,
the genotype is ABD, and meiosis would be a
mess. If polyploid, it will have 28 14, or 42
chromosomes, and be fully reproductively
competent.
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What are the characteristics of the wild
progenitors and the modern varieties? Wild
einkorn wheat (Triticum boeticum) has hulled
(covered) grains, fragile spikes, and is
shattering. A domesticated einkorn wheat (T.
monococcum) is still found in the Swiss Alps, and
is used to produce a yellow flour and
porridge. Emmer wheat (T. turgidum) is also
hulled, but is non-shattering. Since it is much
more difficult to thresh (remove hulls), it is
rarely grown agriculturally today. We know the
goat grass involved in the second cycle of
polyploidization. It was T. tauschii. The
hexaploid (T. aestivum), arising from the cross
T. turgidum x T. tauschii, bread wheat, appeared
8000 YBP.
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Compared to emmer wheat, bread wheat has a higher
protein content and environmental tolerance. Part
of the enhanced protein content is more gluten.
Gluten gives doughs made with this wheat more
elasticity. Imagine tossing a pizza with an
inelastic dough! The nutritional content of bread
wheat 60 80 starch (carbohydrate) 8 14
protein vitamins (B complex, E) minerals (Ca,
P, Fe, K) Bread wheat accounts for 95 of wheat
crops, durum wheat the remaining 5. Wheat alone
occupies 15 of the arable surface of earth.
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However, the production of durum wheat may be
declining.
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• Wheat is the nutritional staff of life for 35
  of humankind. Its consumption provides
• 20 of all human caloric intake
• 45 of all protein-derived human nutrition
• Advantages of wheat as a dominant plant food
  source
• balanced essential amino acid composition
  lacks only 4
• essential nutrients vitamins A, B , C, and
  iodine
• stores well for several years (if dry!)

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• There are two different approaches and varieties
  in wheat growth
• Winter wheat planted in the Fall, harvested
  spring/summer
• generally cultivated in regions presenting
  a relatively
• mild winter
• Spring wheat planted in the Spring and
  harvested in the
• Fall more northern climes (up to Arctic
  Circle in NW
• Canada!)
• The difference is controlled by a single genetic
  locus (gene) VRN1 (VRN for vernalization) that
  has been mapped to the chromosome 5 group in
  wheat and barley.

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The TaVRT-1 and TaVRT-2 genes are involved in
(and named for) the transition from vegetative to
reproductive growth. They are part of a complex
of SVT genes that cause a short vegetative phase.
The transition is blocked in winter wheat until
vernalization (cold, winter) has occurred.
Winter Habit
Spring Habit
TaVRT-1
TaVRT-1
TaVRT-2
TaVRT-2
TRANSITION
REPRODUCTIVE
VEGETATIVE
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Winter and spring wheat have different
temperature tolerances and requirements for
vernalization. Difference represents part of the
effect of having different VRN genes.
Vernalization saturation point
LT50 (C)
Spring wheat
-10
-15
Winter wheat
Freezing tolerance
-20
-25
-30
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77
91
49
35
63
Days of acclimation
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There are other differences between hard (mostly
winter) and soft (mostly spring) wheat
varieties Hard wheat has a higher protein
content (and higher gluten) 14 protein (hard
red winter or hard red spring) compared to
somewhat less (and less gluten) in soft red
winter wheat. Its used for bread flour (and
other products that must rise). Soft white wheat
(mostly spring) has even lower protein and gluten
content. As a result, its used for cake flour
(no rising necessary). Durum wheat has slightly
less protein than the hard wheats, but high
gluten content. Its used for pasta and other
noodles (though durum semolina also makes
excellent bread).
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The story says that Marco Polo brought wheat and
millet noodles back from China, as well as seed
to grow wheat. Wheat rapidly became an important
and dominant crop. The expansion of the Roman
Empire may have been driven not by desire for
territory, but control of a greater area to grow
wheat. The growth of wheat in central North
America, and the need to ship it in bulk to
various places around the world, was a key factor
driving the development of the St. Lawrence
Seaway and associated infrastructure (ports,
roads, railways).
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Initially the growth of wheat in western Canada
near Selkirk, using varieties brought with
settlers from England and Scotland was only
marginally successful. What planting began in
1812, and the low success lasted until 1842. In
part this was because the settlers had been
fishermen, not farmers. In part it was due to
locust plagues and to diseases of wheat,
particularly rusts. In 1842 a Mennonite farmer,
David Fife, planted a Ukrainian wheat that
produced a large harvest (36 bushels/acre). That
success was widely publicized. This wheat was
cold tolerant and rust-resistant. It is a hard
spring wheat. Rust resistance was critical to
economic wheat cultivation.
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Today, Marquis wheat has taken up much of the
area for growth of spring hard wheat in both the
U.S. and Canada. It resulted from an experimental
cross in 1906 between red fife and hard red
Calcutta wheat. Marquis wheat ripens about 2
weeks more rapidly than red fife. It is not rust
resistant, but ripens so early it is not heavily
exposed. Today there are even more rapidly
ripening wheat varieties of particular value for
growing in more northerly (colder, shorter
growing season) areas of the Canadian prairies.
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Separately, an oriental wheat, daruma provided
dwarfing characters, had less lodging, and
increased yield. Daruma is a hard winter wheat.
It ripens very rapidly in the spring, and
therefore avoids the hot dry conditions of the
southern (U.S.) prairie summer. Daruma has been
crossed with many other varieties. To produce
large yields with high protein content, varieties
from Brazil produced commercial names Karl The
pedigree also includes Marquis. These wheat
varieties are important, for example, in Kansas,
Nebraska, Missouri and Arkansas.
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• Flour is the product. There are different types
  of flour
• White flour -
• is the result of removing both the germ and bran
• to make white flour the endosperm (high
  carbohydrate) is
• pulverized
• the ground endosperm is bleached of xanthophylls
  (using
• alum, ammonium carbonate, chalk)
• Whole wheat flour
• the bran and germ are retained and milled with
  endosperm
• oils (particularly from the germ) affect
  successful storage
• protein content is about 25 higher, and fat
  content almost
• twice that of white flower
• mineral content much higher (4x P, Fe and K)
• vitamin content similarly much higher

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• Bread
• Made using bread wheat (T. aestivum)
• raised using budding yeast Saccharomyces
  cerevisiae
• different strains of this yeast are also used
  for production of
• beer
• bread holds the CO2 produced by fermentation
  (sugar added
• to start the process) and rises (bubbles up)
  due to sticky
• gluten
• raised (leavened) bread invented in Egypt during
  biblical
• times. Jewish Passover celebrates the need to
  make bread
• rapidly (no time to allow rising) with
  unleavened Matzoh
• bread)
• kneading produces smaller pockets of gas and
  finer texture
• of the product