Zea mays, corn, maize, mealie

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Taxonomic lineage

cellular organisms - Eukaryota - Viridiplantae - Streptophyta - Streptophytina - Embryophyta - Tracheophyta - Euphyllophyta - Spermatophyta - Magnoliophyta - Liliopsida - commelinids - Poales - Poaceae - PACCAD clade - Panicoideae - Andropogoneae - Zea - Zea mays

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Brief facts

• Maize is widely cultivated cereal grain that was domesticated in Central America. A greater weight of maize is produced each year than any other grain.
• Unlike all other major grain crops, the corn plant has separate male and female flowering parts.
• Corn is the most important and economical source of starch in the United States. Starch is the major carbohydrate storage product in corn kernels comprising 70-72% of the kernel weighton a dry weight basis.
• Together with sorghum and sugarcane, maize belongs to C4 plants (plants that use C4 photosynthesis to fix atmospharic carbon dioxide). C4 photosynthesis is so called because the initial product of photosynthesis is a four carbon compound (oxaloacetate). C4 photosynthesis essentially eliminates the oxygenase activity of Rubisco via anatomical, biochemical and ultrastructural modifications in leaves.
• Archeological and molecular data indicates that modern maize was domesticated from annual Balsas teosinte (Zea mays ssp. parviglumis) in southern Mexico between 6,600 and 9,000 years ago. Selection soon followed: favorable alleles at loci controlling plant morphology and kernel nutritional quality were fixed at least 4,400 years ago, and further selection by Native Americans facilitated maize adaptations to varied environments. Dispersal

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Developmental stages (life cycle)

Life Cycle Stages

The staging system divides corn development into vegetative (V) and reproductive (R) stages. V stages are designated VE (emergence), Vn, where n represents the emerging leaf's order number, and VT (tasseling). Depending on the hybrid each plant develops 20-21 total leaves, silks about 65 days after emergence, and matures about 125 days after emergence.

• seed stage MeSH
  • dormant seed
  • germinating seed embryonic shoot (plumule) and embryonic plant roots (radicle) emerge and grow upwards and downwards respectively; seed should be covered by 1-1 1/2 inches of soil to allow for adequate root system to develop
• vegetative
  • emergence stage VE; coleoptile leaf (1_st) visible; emergence takes from 6-21 days depending on weather conditions
  • seedling MeSH V1 - V4 (1-4 collared leaves are visible); until about 2 weeks after emergence; early whorl stage; functions of seminal roots (roots that grew directly from the seedling) are rapidly taken over by permanent nodal root system, which begins development at each progressively higher node on the stalk (up to 7 to 10 nodes total) and becomes the major supplier of water and nutrients to the plant by the V6
  • first internode
    elongation V5-V6; may lost the coleoptile leaf by this time; plant is approx. 8 inches tall; growing point is just below the ground surface therefore hail and freeze are not that harmful whereas flood, especially in combination with high temperatures, can be deadly; the first internode (~1/2 inch) elongates just below the node of 5_th leaf; this is important reference point for crop growth staging; later, when lower leaves will dry up and fall off, the fifth leaf node can be used as a reference for counting to the top leaf collar
  • ear and tassel
    initiation V6; early whorl stage; 3-4 weeks after emergence; lower leaves (1-4) dry up; tassel formation has been initiated; growing point and tassels are above soil surface (increased vulnerability to freeze and hail); tillers (ear shoots) begin developing
  • stem elongation stages V7-V17; mid-whorl to late whorl; 4-6 weeks after emergence; plant grows rapidly; by V10 new leaf stage occurs every 2-3 days; size of ear and number of potential kernels being established; an ear shoot will develop from every above-ground node, except the last six to eight nodes below the tassel; growth of most lower stalk ears eventually slows, and only the upper one or two shoots develop into a harvestable ear; hybrids that produce more than one harvestable ear are termed prolific
  • tassel emergence VT; 7.5-8.5 weeks after emergence; tassel fully emerged and pollen shed begins 2-3 days prior to silking; tassel is usually fully emerged and stretched out before any pollen is shed
• reproductive
  • silking R1; most critical stage; approx. 8.5-9 weeks after emergence; silks are visible outside the husks; it takes ~24 h for pollen grain to grow down the silk and fertilize the egg; generally 2 -3 days are required for all kernels to be fertilized on a single ear; silks continue to grow 2.5-3.8 cm (1 - 1 1/2 inches each day until fertilized
  • ripening
    • blister stage R2 stage; about 10-14 days after silking; kernels are white, filled with clear fluid and distinct from surrounding cob; kernels are beginning dry matter accumulation; relocation of nutrients from the leaves and stem to the ear begins
    • milk stage R3 stage; about 18-22 days after silking; rapid grain filling period; kernels begin to have yellow colour; inner fluid is milky white
    • dough stage R4 stage; about 24-28 days after silking; top of kernel begins to firm up; usually four embryonic leaves have formed by this time; the reduced fluid and increased solids within the kernel at this time produce a doughy consistency
    • dent stage R5 stage; about 35-42 days after silking; all kernels are dented or denting; starch layer appears shortly after denting as a line across the kernel when it is viewed from the opposite embryo side
    • 1/2 maturity a hard white layer of starch reaches half way down the kernel
  • black layer R6 stage; plant reached physiological maturity; all kernels on the ear have attained their maximum dry weight or maximum dry matter accumulation; hard starch layer has advanced completely to the cob now and a black or brown abscission layer has formed

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Corn seed anatomy

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Maize flowers

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Corn plant terminology

• tassel inflorescence of male flowers; the tassel consists of several long, indeterminate branches bearing short determinate branches (spikelet pairs) that bear two spikelets (compact auxiliary btanches of grass inflorescence that in maize consists of two bracts subtending one reduced male flower)
  • stamen pollen-producing reproductive organs which are collectively referred as androecium
    • stalk also filament; the part of the stamen
      on which anther develops
    • anther the terminal part of a stamen in which
      the pollen grains are produced
      • microspore smaller of the two types of
        spore produced in heterosporous
        plants; develops in the pollen sac
        into a male gametophyte
        • male
          gametophyte microspores divide twice
          to produce 3 celled
          pollen grain/tube
          (a male gametophyte);
          two of the cells are sperm;
          other is called vegetative
          cell , or tube cell
          • sperm cell two sperm cells
            are produced;
            one sperm cell
            fuses with the
            egg resulting
            in zygote;
            other sperm cells
            fuses with central
            cell giving start
            to development of
            triploid tissue
            called endosperm
            which surrounds
            the embryo and serves
            an absorbtive/nutritive
            function in seed
          • tube cell haploid cell that
            comprise two sperm
            cells and facilitates
            delivery of the
            sperm into ovary

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• ear inflorescence of female flowers; it consists of a single spike with short branches each producing two single-flower spikelets
  • pistil
    • style slender part of a pistil, situated
      between the ovary and the stigma (main part of silk)
    • stigma the receptive apex of the pistil of a flower,
      on which pollen is deposited at pollination
    • ovary
      • ovule
        • megaspore also called macrospore;
          gives rise to female
          gametophyte
          • female
            gametophyte also called
            megagametophyte
            or embryo sac;
            the female gametophyte
            consists of 7 cells
            • egg one of small 6
              cells, which fuses
              with the sperm cell
              giving rise to the
              plant embryo
            • central cell large cell, which
              has 2 haploid, or
              polar nuclei; it
              fuses with the
              second sperm cell
              giving rise to
              triploid tissue
              called endosperm

• leaf MeSH
  • leaf blade
  • sheath part of the leaf originating from the node and running parallel to the culm or stem itself
  • ligule a membrane that is located between the culm and the leaf blade

• seed MeSH corn seed is also called kernel and known botanically as a caryopsis
  • pericarp the tissue surrounding a seed that develops from the ovary wall of the flower; in corn is also called husk, hull, or bran
    • epidermis
    • mesocarp
    • cross cells
    • tube cells
    • seed coat a testa; outer coat of a seed
  • endosperm the triploid nutritive tissue formed within the embryo sack of seed plants; the endosperm provides about 83 percent of the kernel weight
    • soft endosperm
    • horny endosperm
    • aleurone a granular protein found in the endosperm of many seeds or forming the outermost layer in cereal grains
  • embryo also called a germ
    • plumule the rudimentary terminal bud of a plant embryo situated at the end of the hypocotyl, consisting of the epicotyl and of immature leaves (3rd and 4th leaves)
      • epicotyl the stem of a seedling or embryo located
        between the cotyledons and the first true
        leaves
      • coleoptile a protective sheath enclosing the shoot
        tip and embryonic leaves of grasses
    • cotyledon seed leaf (as opposing to the true leaf); also called scutellum
    • hypocotyl the part of the axis of a plant embryo or seedling plant that is below the cotyledons
    • radicle embryonic root
    • coleorhiza a protective sheath enclosing the embryonic root of grasses
  • tip cap dead tissue found where the kernel joins the cob

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References

• Bortiri E, Hake S. Flowering and determinacy in maize. J Exp Bot. 2007;58(5):909-16. Epub 2007 Mar 3.
• Buckler ES, Gaut BS, McMullen MD. Molecular and functional diversity of maize. Curr Opin Plant Biol. 2006 Apr;9(2):172-6.
• Brown NJ, Parsley K, Hibberd JM. The future of C4 research--maize, Flaveria or Cleome? Trends Plant Sci. 2005 May;10(5):215-21.
• Ritchie SW, Hanway JJ and Benson GO. How a Corn Plant Develops. Special Report No. 48. Iowa State University of Science and Technology Cooperative Extension Service Ames, Iowa, June 1993
• Major topic Zea mays - free full text articles in PubMed

Websites

• Corn Production
• Corn Development and Key Growth Stages

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