Plants & Human Affairs - Seed Germination
Cherries.wmf (7140 bytes) Plants & Human Affairs (BIOL106)  -  Stephen G. Saupe, Ph.D.; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321; ssaupe@csbsju.edu; http://www.employees.csbsju.edu/ssaupe

Introduction to Seed Germination

 Objectives:  The purpose of this lab experience is to provide an opportunity to:

  1. measure seed germination percentage and rate
  2. learn the requirements for seed germination
  3. study the effect of light/dark on germination
  4. grow a plant to maturity
  5. learn about a particular species of plant

 Introduction:
   
A seed is essentially a baby in a suitcase carrying its lunch.  "Baby" refers to the embryo, or immature plant, that will grow and develop into the seedling and ultimately the mature plant.  The "suitcase" is the seed coat (or testa) that surrounds the seeds and "lunch" refers to the nutritive source for the germinating seedling.  The food for the germinating seedling may be stored in part of the embryo itself, such as the fleshy cotyledons of a bean seed, or it may take other forms including endosperm, which is a special starch-rich storage tissue that surrounds the embryo. 

     A seed is officially considered to have germinated when the young root, called the radicle, emerges from the seed coat.  To germinate, a seed requires three things – water, oxygen, and a suitable temperature.  Water uptake, also called imbibition, is the first stage of seed germination.  During this process the dry seed, which typically has a water content of less than 10%, absorbs water and swells.  This process serves to hydrate the dry components of the seed and active the metabolic machinery necessary for germination.  Among the early metabolic activities occurring in the seed is the breakdown of starches stored in the seed into simple sugars that can be used for energy and building blocks for necessary cellular structures.  

   
Except for the first half-hour or so of germination when little oxygen is present, seed germination and subsequent seedling growth requires oxygen.  It is required, in large part, for use in the cellular structures, called mitochondria, to produce ATP (energy).  A suitable temperature is necessary to optimize the metabolic reactions required for germination.  The seeds of every species have an optimal temperature for germination; some species, such as the gourds and squashes prefer warm temperatures while other species such as radish can tolerate cooler temperatures for germination.

    A seed that has not germinated because it is lacking one or more of the necessary requirements for germination is termed quiescent.  These seeds are simply "resting", waiting for the appropriate conditions for germination.  Given water, oxygen and/or a suitable temperature, a quiescent seed will germinate.  However, even if given the proper conditions, a seed may not germinate.  These seeds may fail to germinate because the seed is either dormant or "dead".  

    Dormant seeds have the potential to germinate but are prevented from doing so by some mechanism.  Thus, even though all the proper growth conditions are present, they don't germinate unless they have been "primed" and there dormancy mechanism has been overcome.  There are many dormancy mechanisms in seeds.  For example, when some seeds, like hemp, are shed they have immature embryos that will not germinate until they undergo a period of development (called after-ripening).  Other seeds, like apple, require a cold treatment, called stratification, for germination.  Many of our native plant seeds must be stratified.  Some seeds have a hard seed coat that needs to be nicked (called scarification) for germination.  This usually occurs as the result of natural freeze-thaw cycles.  Still other seeds require a period of heat in order to germinate.  Many of these species are winter annuals that germinate in the late summer/early fall.  

    Ultimately, the function of these varied dormancy mechanisms is to enable the seed time to disperse from the parent plant and to avoid germinating during unfavorable weather.  Humans have attempted to breed dormancy mechanisms from our crop plants.  Although an advantage for a wild plant, dormancy is a problem if a farmer who want the crop to germinate uniformly and immediately upon planting.

   
It’s not easy to tell if a seed is “dead”.  Only if it fails to germinate when provided the proper conditions and any dormancy mechanisms are broken can we consider a seed “dead”.

    Seed companies typically test the germination of seeds before sale. The results of these tests, the germination percentage, are typically provided on a seed packet.  Most crop seeds lose viability rapidly after a few years.  However, a few long-lived seeds are known.  For example, mustard seeds show good germination after even 50 years.  

1.  Species Selection/Research.
    There will be a variety of seeds available in the lab.  Record as much data about the seeds as possible.

Common name:  _____________________________

Variety/Cultivar: _____________________________

Scientific name:  _____________________________

Family name:  _______________________________

Source of seeds (CSB/SJU Herbarium Collection, commercial seed packet, etc):

If a commercial seed packet, indicate the year packaged, germination percentage, directions provided by the company to grow the plants, and any other pertinent information from the seed packet.  If from the CSB/SJU herbarium collection, record any pertinent information.

Learn more about your chosen species and complete a "Plant Portrait" form (available in class or via the web)

2.  Germination Percentage and Rate.
                Germination percentage is an estimate of the viability of a population of seeds.  The equation to calculate germination percentage is:  

GP = seeds germinated/total seeds x 100

    To provide an idea of the germination rate, time course of seed germination, and uniformity in seed germination we will determine the GP at different time intervals after planting and then plot these data.

  1. Obtain a petri dish and filter paper.
  2. Count out 50 seeds (fewer if large seeds) and place a layer of filter paper in the dish
  3. Moisten the paper with enough water so that the seeds just barely begin to float (about 10 ml).
  4. Put on the lid and place the dish in the light.
  5. Record the number of seeds that germinate daily.  Record your data in Table 1.  Remove the seeds as they germinate.
  6. Complete Table 1.
  7. Plot Percent Germination during Time Interval vs. days. 
  8. Plot Cumulative Percent Germination vs. days.

Table 1.  Germination data for seeds in the light

Date Started

Days since start

Total Seeds in Treatment

# Seeds Germinated During Time Interval

Total Seeds Germinated Since To

Percent Germination during time interval (per day)

Cumulative Percent Germination

 

0

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Questions:

  1. What was the final germination percent of your seeds?  Was it what you expected?  Explain.
  2. Explain each of your two graphs.  What do these tell you about your seeds?


3.  Light vs Dark Germination
   
Some seeds require light for germination.  In this exercise we will test to see if your seeds have a light requirement.  We will set this up in the form of a typical experiment.  

Question:        Do our seeds have a light requirement for germination?

Hypothesis:    My seeds do not require light.

Predictions:    If my seeds do not require light, then the germination percentage for the seeds planted in the light and dark should be the same.

Protocol:

  1. Set up another set of seeds identical to the first set you prepared (in #2 above).  Record the number of seeds that you use in Table 2.
  2. Wrap this dish in aluminum foil and label it with your name and date.
  3. When the seeds in the light have finished germinating, open the foil and record the number of seeds that germinated in Table 2.
  4. Transfer your final data from Exercise 2 into Table 2.

Results:
   
Calculate the germination percent in the dark.  How does this compare to the seeds germinated in the light? 

Statistical Test:
    It is likely that we will need to perform a statistical test to determine if our germination rates are significantly different.  We will use a Chi square 2 x 2 contingency table test.  This is available through the Concepts of Biology web site.  We will discuss it in class.

Null hypothesis (Ho):

Results:  x2 = _____________________   p = ___________________


Conclusion: (does light affect seed germination?  did you expect it to?  explain)

 

Table 2.  Comparison of light and dark germination

Treatment

# Seeds Germinated

# Seeds not germinated

Percent germination

Dark

 

 

 

Light (from exercise 2)

 

 

 


4.  Plant Growth.
   
Finally, we will grow our plants to maturity in the greenhouse.  Plant your seeds in potting mix according to the directions provided in class.  Monitor your plants during the next few weeks.  Prepare a growth curve by measuring the height of your plants at weekly intervals and describe the growth.

Assignment:  At the completion of this exercise, turn in:

  1. Plant Portrait for your species

  2. Tables 1 & 2

  3. Statistical treatment for light vs. dark germination (i.e., null hypothesis, chi square value and the associated probability, conclusion)

  4. graph percent germination per time interval vs. day

  5. graph of cumulative percent germination vs. day

  6. a one-page (maximum) summary of the experiment addressing the questions posed in the lab.


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Last updated:  08/25/2008 / © Copyright  by SG Saupe / URL:http://www.employees.csbsju.edu/ssaupe/index.html
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