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 |
Seed Germination
Objectives: The purpose of this lab
experience is to provide an opportunity to:
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.
Select
a seed by: (1) examining the seeds available in the lab; (2)
recovering your seeds from a commercially-available fruit; or (3) from a wild
plant.
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 you are using seed from a commercial grower, 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. If you obtained
your seed from a fruit, describe the procedures you used.
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. Click here to visit a Gink & Go dialog about this topic.
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:
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:
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): |
|
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:
Plant Portrait for your species
Tables 1 & 2
Statistical treatment for light vs. dark germination (i.e., null hypothesis, chi square value and the associated probability, conclusion)
graph percent germination per time interval vs. day
graph of cumulative percent germination vs. day
a
one-page (maximum) summary of the experiment addressing the questions posed
in the lab.
| Top | PHA Home | PHA Course Materials| SGS Home | |
Last updated: 08/25/2008 / � Copyright by SG Saupe / URL:http://www.employees.csbsju.edu/ssaupe/index.html