Basic Concepts

• Seeds form by the combination of male and female gametes
  • Result of fertilization or syngamy
  • Involves stamen and pistil of the flower

• Fertilization in angiosperm plants
  • Self- or cross-fertilization

• Types of asexual reproduction
  • Vegetative (stolons, rhizomes, tubers, bulbs, corms)
  • Apomixis

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Important botanical families

• Poaceae

  • Consists of large-seeded grasses or cereals
    • Comprise approximately 90% of cultivated seed crops
    • Most important source of carbohydrates, protein & other vital food nutrients
    • Crops: wheat, corn, sorghum, oats, barley, rye, millets
• Fabaceae
  • Second most important food family
  • Have a better nutrient balance than cereals
  • Consists of legume food crops or pulses and oilseeds
  • Crops: soybean, chickpea, groundnut, pea, lentil and beans

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Classes of seed producing plants

• Angiosperm
  • Ovule and seed develop within an ovary
• Gymnosperm
  • "Naked seeds" produced without ovaries, flowers or fruits

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Flowering, Fruit and Seed Development

• Sequence:
  1. Flower bud induction and initiation
  2. Flower differentiation and development
  3. Pollination and fertilization
  4. Fruit set and seed formation
  5. Growth and development of fruit
  6. Seed and fruit senescence

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• Flower bud induction and initiation
  • Originates within buds in regions known as meristems
    1. Vegetative meristems give rise to stems, leaves and roots
    2. Reproductive meristems floral organs
  • Primordia are knobby outgrowths within meristem

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• Flower differentiation and development
  • Triggered by certain external stimuli
  • A physiological change that results in development of reproductive primordia
    1. Takes place in advance of flowering
    2. Time required to attain vegetative maturity depends upon the species
       • Flower bud initiation takes place before sprouting in some bulb species

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• Floral induction
  • Induction stimuli:
    1. Temperature
    2. Day-Length
    3. Chemicals
    4. Nutritional

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  • Temperature Stimuli
    • Vernalization - exposure of germinating or moistened seeds to low temperatures
    • Examples:
      1. Winter wheat and rye
      2. Sugar beets and carrots (biennial species)
      3. Peas, lettuce and spinach (induces early flowering)
    • Temperatures range between  1 and 7 C
    • Most commonly found in long-day plants
    • Biennial plants must generally be in a juvenile stage
    • Thermoperiodism - periods of low temperatures separated by high temperatures

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  • Day-Length Stimuli
    • Photoperiodism - floral induction in response to relative lengths of light and darkness
    • Can be influenced by cool night temperatures
    • Categories:
      1. Short-day
         • Examples: cocklebur, strawberry, tobacco, poinsettia
      2. Long-day
         • Examples: sugar beet, coneflower, dill, Hibiscus
      3. Intermediate-day
      4. Day-neutral
         • Examples: artichoke, cucumber, lima bean, tomato
    • Photoperiod requirements
      1. Qualitative - photoperiod requirements are specific
      2. Quantitative - photoperiod hastens the response
  • Phytochrome
    • A reversible photoreceptor formed in the leaves and transported to the meristematic region
    • Pigment exists in two forms: phytochrome
      1. Pr form absorbs red light (660 nm)
      2. Pfr form absorbs far-red light (730 nm) 
    • Conversion of Pr to Pfr in light and how long it remains usually determines the plant's flowering response

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  • Chemical Stimuli
    • Stimuli include natural and synthetic chemicals
    • Useful for commercial applications
      1. Auxin-like compounds
         • Indoleacetic acid
         • Naphthaleneacetic acid
         • 2,4-dichorophenoxyacetic acid (2,4-D)
      2. Gibberellic acid
         • Promotes long-day plants and inhibits short-day plants
      3. Cytokinins
      4. Ethylene
      5. Acetylene

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  • Nutritional Status
    • Carbon-nitrogen ratio is particularly influential
      • Holly: pistillate vs staminate flowers
      • Tomatoes: carbon deficiency causes pollen sterility

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• Floral Initiation
  • Follows a sequence of biochemical events
  • Triggered by changes in relative proportions of various phytohormones
    • Frequently the gibberellin level compared to auxins and abscisic acid (ABA)
  • Can occur only in apices with active DNA and RNA synthesis

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References:

Principles of Seed Science and Technology, 3rd Ed., L.O. Copeland and M.B. McDonald. Chapman and Hall, New York, 1995.

Seeds Handbook, B.B. Desai, P.M. Kotecha and D.K. Salunkhe. Marcel Dekker, Inc., New York, 1997.