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 (don’t 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?

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.

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 grasseshave stolons (above ground horizontal roots) or rhizomes (similar horizontal growth, but underground).

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.

The fruit, or grain, is single-seeded, dry, and indehiscent (which means it doesn’t 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.

The endosperm is mostly starch, and that is what makes cereal grains so globally important as food.

Now let’s consider the most important cereal grains: wheat, corn, and rice…

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.

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 can’t properly pair up.

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.

Here’s 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.

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.

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.

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% proteinvitamins (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.

However, the production of durum wheat may be declining.

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!)

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.

TaVRT-2

REPRODUCTIVE

TaVRT-1

TaVRT-2

VEGETATIVE

TaVRT-1

TRANSITION

Winter Habit

Spring Habit

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 and spring wheat have different temperature tolerances and requirements for vernalization. Difference represents part of the effect of having different VRN genes.

LT50 (°C)

Days of acclimation

-30

-25

-20

-15

-10

4921 35 63 77 91

Vernalization saturation point

Fre

ezin

g t

ole

ran

ce

Spring wheat

Winter wheat

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. It’s 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, it’s used for cake flour (no rising necessary).

Durum wheat has slightly less protein than the hard wheats, but high gluten content. It’s used for pasta and other noodles (though durum semolina also makes excellent bread).

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).

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.

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.

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.

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

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