Poaceae (Gramineae) Organic Seed Spring 2004

Poaceae §One of most important from human food §500 genera and 8,000 species

Grass structure

Inflorescence Spikelet of Poaceae

Generalized grass flower

Determinate /F_Organization/307notes07.html

Grass flower diagram

Indeterminate

Important cereals A. Oats (Avena sativa); B. Barley (Hordeum vulgare); C. Bread Wheat (Triticum aestivum); D. Rye (Secale cereale).

A. Bread wheat (Triticum aestivum); B. Rye (Secale cereale); C. Triticale (Triticosecale). Wheat and rye are crossed together to produce the hybrid triticale.

Rice (Oryza sativa) A. Grain-bearing spikelet showing a pair of slender basal bracts (glumes) and the stalk (pedicel). B. An empty spikelet with the lemma and palea slightly separated from each other. These two leathery bracts enclosed the grain or caryopsis. C. A grain (caryopsis) removed from spikelet

Maize or Corn (Zea mays) Strawberry corn, popping corn, Indian corn and pod corn. Progenitor: teosinte.

I. ORIGIN A. S. Mexico ‑ 5000 BC Developed from teosinte (UWis- Hugh Iltis) Modern- increased apical dominance of tassel-- only 1 or 2 ears and few tillers

B. Ohio valley by 800's AD C. Columbus to Spain 1493; Africa & Asia within 50 years D. Corn belt dent (northern flint x southern dent) spread in 1850's

Teosinte (Zea mexicana) A. Male Inflorescence (Tassel) B. Female Inflorescence (Ear) With Outer Husk & Silk C. Ear With Husk Pulled Away Exposing A Row Of Grains D. A Leaf Pulled Back Exposing Two Ears E. Leaf (Blade)

Teosinte (Zea mexicana) showing pollen-bearing tassel (left) and a female ear with silk (right). This is truly a miniature version of the modern corn plant (Zea mays). Compare this tiny ear with the following image of a modern ear of corn.

Modern corn Silk: red, threadlike styles and the green, leaflike husk enclosing numerous ovaries of female flowers which develop into the grains.

II. BOTANY (OVER 300 RACES) A. Monoecious (tassel- male; cob-female) (that and being x- pollinated greatly facilitates hybrid seed production) B. 25 million pollen grains/plant (50,000 for each one that produces a grain) C. Husks are modified leaves (allow harvest in rainy season)

Corn female flower

Corn male flower

The seed spike of teosinte consists of a single row of grains. Each grain is enclosed in a hard fruit case (called a cupule) and a pair of papery glumes. This spike = ear of modern corn. Tunicate Mutation: The grains are enclosed in a pair of husklike glumes (without the hard outer cupule) from which they can be threshed easily. Further selection by pre-Columbian farmers could have reduced the size of the tunicate glumes and resulted in ears with many rows.

Close-up view of pod corn showing papery glumes enclosing the grains

The reddish streaks on these corn grains are caused by transposons. When a transposon moves to different positions within cells of the corn kernel, the coloration gene is "turned on" or "turned off" depending on whether it lands in a position adjacent to the pigmentation gene. Transposons may also have a profound effect on embryonic development and tumor formation in animal cells. Oncogenes (genes that cause tumors) may be activated by the random reshuffling of transposons to a position adjacent to the oncogene. Transposons may also be useful in genetic engineering with eukaryotic cells, by splicing in transposons to activate certain genes. The implications from Barbara McClintock's discovery of transposons may be far-reaching and as significant as Watson and Crick's discovery of the structure of DNA. Federoff, N.V "Transposable Genetic Elements in Maize." Scientific American Volume 250 (Number 6):

§Grains of Indian corn come in different colors, such as purple, yellow and white. Sometimes the individual grains are purple with white streaks or mottling. This mottling effect defies Mendel's basic principles of genetics because individual grains may be multicolored rather than a single color. The movement of transposons on chromosomes may result in colored, non-colored and variegated grains that do not fit traditional Mendelian ratios based solely on chromosome assortment during meiosis and random combination of gametes. The explanation for this phenomenon involves "jumping genes" or transposons, and earned Dr. Barbara McClintock the prestigious Nobel Prize in Medicine in 1983 for her life-long research on corn genetics. §Transposons are genes that move from one location to another on a chromosome. In the pigmented aleurone layer of corn grains, the position of transposons may inhibit or block pigment production in some cells. For example, if the transposon moves to a position adjacent to a pigment-producing gene, the cells are unable to produce the purple pigment. This results in white streaks or mottling rather than a solid purple grain. The duration of a transposon in this "turned off" position affects the degree of mottling. If the pigmentation gene is turned off long enough by a transposon, the grain will be completely unpigmented. The reddish- purple patterns caused by transposons may be blotches, dots, irregular lines and streaks

F. Many endosperm types developed by selection 1. DENT (USA, N. Mexico): large kernels with central core of floury endosperm which shrinks upon drying 2. FLINT (Argentina, Africa): large smooth kernels with mainly hard endosperm but with small floury center A. POP (IO,NE, Central Mexico): small smooth kernels with hard endosperm

Endosperm types cont inued 3. SWEET (MN, WI): Large kernels with carbohydrates stored as sugars 4. FLOURY (Andean countries): large smooth kernels with floury endosperm 5. WAXY (Asia) Normally maize is 27% amylose (straight chain) and 63% amylopectin. Waxy corn has 100% amylopectin. Used to substitute for cassava starch.