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Seed GerminationName________________________

In this exercise you will learn about propagating plants sexually (from seeds). It is expected that you have read the appropriate sections in the textbook prior to coming to class.

The effect of planting depth on seed germination.

Observation. Seed companies recommend different planting depths for seeds. Lettuce seeds are small and need light to germinate. Pea seeds are quite a bit larger.

Question. Can lettuce seeds or pea seeds germinate better if planted deeply in soil?

Hypothesis. Lettuce seeds germinate poorly when planted deeply compared to peas.

Prediction. If the hypothesis is true, then pea seeds planted deeply will germinate much better than lettuce seeds planted at the same depth. Moreover, pea seeds may be light-inhibited and germinate poorly when planted shallowly.

Experiment. Earlier in the semester, you planted four pots with lettuce and pea seeds at various depths in the soil. You should locate your pots and make your observations. Record the number of seedlings appearing above the soil and calculate the percent germination:

 Lettuce SeedsPea Seeds
Planting
Depth
Number
Germinating
Germination
(% of 20)
Total
Germinated
Germination
(% of 10)
0 cm % %
1.5 cm % %
3 cm % %
6 cm % %
Other observations: Peas planted at 0 cm were unusually: 
Analysis.
The optimum depth for lettuce seeds is:   0     1.5     3     6   cm.
The optimum depth for pea seeds is:   0     1.5     3     6   cm.
The plants growing taller when planted at 6 cm than at 0 cm:   lettuce     pea  

Decision.The hypothesis: "Lettuce seeds germinate poorly when planted deeply compared to peas." is:

  rejected       not rejected  
-   /20

The effect of fruits on seed germination.

Observation. Seeds do not germinate inside their fruits even though it is moist and warm inside the fruit.

Question. Do the juices surrounding seeds inhibit their germination?

Hypothesis. The juices surrounding seeds inhibit their germination.

Prediction. If the hypothesis is true, then seeds placed in fruit juice will not germinate as well as seeds placed in water.

Experiment. Obtain three Petri Dishes from the supply and put one disc of filter paper in the bottom of each dish. The bottom is the smaller of the two dishes and fits inside the cover. Label the dish covers (and bottoms!) with the marking pen. The labels should be:

Water in light       Tomato Juice in light       Abscisic Acid in light

Put 5 mL of distilled water in the first dish, 5 mL of tomato juice in the second dish, and 5 mL of 1 mM Abscisic Acid in the third dish.

With your partner, carefully count out three groups of 50 'Salad Bowl' lettuce seeds. Of course the counts must be accurate, but WORK QUICKLY! Place one group of 50 seeds into each dish on top of the moist paper.

Place the three dishes under the lights to allow the seeds to germinate.

After a few days and after a week, count out the number of seeds germinating and express as percent germination.

 After ______ DaysAfter Eight Days
TreatmentTotal
Germinated
Germination
(% of 50)
Newly
Germinated
Total
Germinated
Germination
(% of 50)
Water + light  %  %
Tomato Juice + light  %  %
Abscisic Acid + light  %  %

 
Other observations: 
Analysis.
More seeds germinated in:   water     tomato juice     abscisic acid  
Is there good evidence that the active
hormone in tomato juice is abscisic acid?
  yes     no  
Decision. The hypothesis: "The juices surrounding seeds inhibit their germination" is:
  rejected       not rejected  
-   /20

The effect of light on seed germination.

Observation. Seeds of some species germinate in the light but do not do so well in the dark. The pigment phytochrome has two forms, Pr and Pfr which absorb red and far-red light, respectively. Perhaps this pigment helps seeds detect the light.

Question. Do species requiring light to germinate use the phytochrome pigment to determine if they are in the light or darkness?

Hypothesis. Light-sensitive seeds do use the phytochrome pigment.

Prediction. If the hypothesis is true, then lettuce seeds placed in red light will germinate much better than seeds placed in darkness or far-red light. Moreover, seeds in far-red light should be particularly inhibited.

Experiment. Obtain three Petri Dishes from the supply and put one disc of filter paper in the bottom of each dish. Label the dish covers (and bottoms!) with the marking pen. The labels should be:

Water in red light       Water in far-red light       Water in dark

Put 5 mL of distilled water in each dish, and obtain a square of aluminum foil so that you are ready to wrap the "dark dish immediately. BE READY!

With your partner, carefully count out three groups of 50 lettuce seeds. Of course the counts must be accurate, but WORK QUICKLY! Place one group of 50 seeds into each dish on top of the moist paper.

IMMEDIATELY wrap the "dark" dish in aluminum foil so that it is completely enshrouded. Place the "red light" dish under red cellophane with fluorescent lighting. Place the "far-red light" and the "dark" dishes under red and blue cellophane with incandescent lighting.

After a few days and one week, count out the number of seeds germinating and express as percent germination. After 2-5 days, move all the seeds into white light.

 After ______ DaysAfter Eight Days
TreatmentTotal
Germinated
Germination
(% of 50)
Newly
Germinated
Total
Germinated
Germination
(% of 50)
Water in red light  %  %
Water in far-red light  %  %
Water in darkness  %  %
Analysis.
The most seeds germinated in ______ days of:   red light       far-red light       darkness  
The fewest seeds germinated in ______ days of:   red light       far-red light       darkness  
The germination-stimulating form of phytochrome is:   Pr       Pfr  
Decision.
The hypothesis: "Light-sensitive seeds do use the phytochrome pigment" is:   rejected       not rejected  
-   /21

Questions to Answer at Home

Defend the statement: "Lettuce seeds need light to germinate" using the data you obtained in laboratory.

Lettuce seeds do need light to germinate, because germination of seeds was   greater     less     in the dark than in white light. Moreover, after ___ days of darkness, the ungerminated seeds were exposed to white light and   more     no more     germinated.

Condemn the statement: "Lettuce seeds need light to germinate" using the data you obtained in the laboratory.

Lettuce seeds do not need light to germinate, because germination of seeds in the darkness was more than   0     5%     in white light control  .

Explain how you can tell whether far-red light had any effect.

Seed germination after ___ days in far-red light was   greater     equal to     less    than seed germination in the control seeds. These control seeds were in __________________________.

The failure of a seed to sprout could be due to the fact that its germination is indeed inhibited. But there is also a second possibility: perhaps the treatment does not inhibit seed germination directly, but simply kills the embryo. Such a situation would certainly not produce a germinating seed! In this exercise you exposed the seeds treated with far-red light to white light for an additional time and observed germination again. Present arguments for both possibilities!

Far-red light inhibits germination--My evidence is that germination after the white light exposure was:

 

Far-red light kills the embryo--My evidence is that germination after the white light exposure was:

 

 
-   /9

 

This page © Ross E. Koning 1994.

 

 

Go to the Course Schedule Page.

 

Go to the Plant Physiology Information Homepage.

 

Send comments and bug reports to Ross Koning at koningre∂gmail⋅com.