Labmanual Sexual Propagation and Seed Dormancy

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					                      Sexual Propagation and
                      Seed Dormancy


       Students will propagate by seed and look at some of the factors important
in seed propagation.

       The propagation of plants is achieved by a variety of means. The two
major classes of propagation are sexual and asexual. Sexual propagation
involves propagation by seed.

       Sexual propagation (or seed propagation) generally results in some
seedling variation. Sexual reproduction involves the union of male and female
gametes (fertilization). The progeny contain genetic information from both
parents. Consequently, progeny may resemble either, neither or both parents
depending on the genetic make-up of the parents.

Seed Germination

     Seed Germination Sequence of Events

             Water Imbibition Influenced by seed coat permeability.

             Enzyme Activation Enzymes break down stored food, aid in
             translocating materials and aid in synthesis of cellular structures.

             Embryo Growth Initiation Embryo is stimulated to grow.

             Seedling Establishment This occurs when roots are taking up
             H20 and nutrients and the shoot is undergoing photosynthesis.
 Environmental Requirements for Germination

       Water Water is needed for imbibition and enzyme activation.

       Air (CO2 and O2) When germination begins, respiration increases
       very rapidly, which demands increased amounts of O2.

       Temperature Each plant species has an optimal temperature
       range for seed germination.

       Light Some seed require light; i.e. lettuce.

 Types of Germination

       Epigeal Germination The cotyledons rise above the soil surface.
       Most dicots undergo epigeal germination.

       Hypogeal Germination The cotyledons remain below the soil
       surface. All monocots have this type of germination and few dicots.

Figure 26. Types of germination (Peffley, 1998).
Part I Seed Viability and Viability Testing
       Whether or not a seed contains a viable embryo is important in
understanding germination. If the seed does not contain a viable embryo it will
not germinate.

        Viability is defined as the capability of a seed to germinate and produce a
normal seedling. Testing seed viability generally follows a standard procedure
and is run under optimum conditions. If a germination test produces low
results = low quality seed. A germination test that produces high results may not
= field conditions.


       Students will be divided into groups. Each group will test seeds of two
types of plants. Each group should count out 50 seeds for each plant. Each
group will also need 4 paper towels and plastic bags.


       Wet a paper towel. Make a row of 5 seeds on the edge of the paper
towel, fold over, and place a second row of 5 seeds on the paper towel, fold
again, and repeat until 25 seeds have been placed in rows on the paper towel.
Place the folded paper towel into a plastic bag, and label the bag with the group
number, seed type and date. Repeat until all 100 seeds are placed in a paper
towel in a bag.
       In 7 days count the numbers of seeds that have germinated and perform
the below calculation to determine percent germination. Technical definition of
germination is radicle visible.

Germination Percentage Calculation

                        # of Seed Germinated
                                                  x    100
                          # of Seed Planted
 Figure 27. Viability Test.


                                  Corn 2001
          # Seed Planted      # Seed Germinated   % Germination
          Batch 1

          Batch 2

                                  Okra 1997
          # Seed Planted      # Seed Germinated   % Germination
          Batch 1

          Batch 2
Part II Seed Dormancy
       Seed dormancy is the inability of a seed to germinate even though
optimal environmental conditions are present for germination. The purpose of
dormancy is a biological mechanism to prevent premature germination especially
when conditions for survival are not good.
       Dormancy can result from physical or physiological conditions in seed.
The three most common causes of seed dormancy are: 1) dormant embryo, 2)
impermeable seed coat (testa), and 3) chemical inhibitors within the seed.
       We will look at dormancy resulting from an impermeable seed coat.
Scarification involves making a notch in the seed coat with a file or sharp object,
boiling water, acid treatment or other method to break dormancy caused by an
impermeable seed coat.


      Each group will need:
          a flat filled with potting media
          3 plastic labels
          30 Mesquite (Prosopis glandulosa) seed
          Pliers
          Razor blade or sharp object


       H2O Soaked 1 liter of water needs to be heated to boiling; 10 seeds will
be placed in the boiling water for one minute. This will scarify the seed coat to
some degree, and then seed can be removed from the heat and allowed to soak
in water for 12 hours.
       Physical Scarification Physically scarify 10 seed by scratching the surface
of the seed coat with a razor blade or sharp object.
       Control No treatment applied to remaining 10 seed.

      Plant seeds in a flat filled with potting media as follows:
          10 untreated seed in a row labeled Control
          10 water-soaked seed in a row labeled H2O Soaked
          10 scarified seed in a row labeled Physical Scarification

      Labels should also contain group names and date.

Make observations for two weeks during the lab period on the germination
percent, time of germination, and effects treatments had on these factors.

 Week      Treatment      Germination                 Observations

          Boiled and
          H2O Soaked

Week 1    Physical


          Boiled and
          H2O Soaked

Week 2    Physical


            Sexual Propagation and Seed Dormancy
            Worksheet and Questions

Answer the following questions.

   1. How many seed germinated and what was the % germination?

      Corn 2001

      Okra 1997

   2. What can germination percent tell you about seed quality?

   3. If you were purchasing seed, would you look at germination percent
      before buying? Why?

   4. Which seed treatment gave the fastest germination?

   5. Which seed treatment had the highest germination percent?
6. Discuss which was the best treatment and why. Discuss in terms of
   germination rate, germination percent, ease of treatment, and potential for
   use in large-scale production).

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