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Pruning Name________________________

You have already examined the effect of auxin on causing the growth of roots on treated leaf and stem cuttings. Small amounts of auxin hormone mixed with talcum powder stimulated roots to form on the leaf petioles of your cuttings. Today you will investigate the role of auxin in formation of branches on a plant, in this case a tall cultivar of Pisum sativum.

Observation: When the apical bud of a plant is removed, the stem f orms lateral branches. The apex of a plant seems to produce hormones that signal the lateral buds of a plant to remain dormant.

Question: What causes plants to form branches when a plant is pruned?

Hypothesis: The decapitation of the apical bud removes the source of a hormone that inhibits branching.

Prediction: If the hypothesis is correct, then

1. Plants will produce more or longer branches when decapitated than when left intact and

2. Decapitated plants treated on the apex with a lanolin paste containing the hormone should produce branches of length and number similar to the intact control and fewer or shorter branches than the decapitated (but untreated) plants.

Experiment: Earlier in the term you decapitated and treated tall pea plants in four pots. Retrieve your pots labeled: "Intact," "Decapitated," "Lanolin," and "IBA."

1. Observe the plants carefully. The intact control plants were untreated and the apical bud is intact. The decapitated plants were treated by removing the apical bud and any attached immature leaves and a terminal stub of the stem should still be evident. The decapitated + IBA plants were treated similarly, but a dollop of lanolin containing 5000 ppm auxin (IBA) was applied to the stub. This dollop has probably been absorbed, depending upon the amount you used and the temperature of the greenhouse.

What has formed at the treated stem tip?_______________spelling counts!

2. If you did not do so before, now eliminate plants in each pot until five remain in each one. As you do this, be sure to eliminate the unusual plants. For example, if any new plants have sprouted in a pot (you can tell in three of the pots because they will not be decapitated!), eliminate them. Eliminate any unusual plants. Hopefully the remaining plants will look very uniform!


 
-   /1

3. Height of plants. Measure the height of the plants in each pot; measure from the soil surface to the apical bud of the main stem for undecapitated plants. For decapitated plants, measure from the soil surface to the end of the decapitated stump. Measure each plant to the nearest whole centimeter and then calculate the mean. As usual, round the mean to one decimal place.

  Individual Plant HeightsMean
(cm)
measure from soil to apical bud Intact
Untreated
             .
measure from soil to stump Decapitated
Untreated
             .
Decapitated
Plain Lanolin
             .
Decapitated
Lanolin + IBA
             .

4. Number of internodes. Count the number of internodes along the main stem of each of the plants in each pot. Calculate a mean number of internodes for each treatment.

  Number of Main-Stem Internodes for Individual PlantsMean
All intact should have
more than two!
Intact
Untreated
             .
If any of the decapitated
have more than two,
show the instructor!
Decapitated
Untreated
             .
Decapitated
Plain Lanolin
             .
Decapitated
Lanolin + IBA
             .

5. Number of branches. Count the total number of side branches arising from the axil of leaf #1, #2, or #3 in each pot as directed. Be careful about this: the leaves of pea plants are compound and you should be sure that the branch you count emanates from a leaf axil. If in doubt about whether you have a leaf petiole or a branch, ASK! Calculate a mean number of branches for each treatment.

  Number Branches in Axil of Leaves #1-#3 on Individual PlantsMean
If you find a branch on any
intact plant, show instructor!
Intact
Untreated
             .
If you find more than two branches
on any plant, show your instructor!
Decapitated
Untreated
             .
Decapitated
Plain Lanolin
             .
If you find branches in these
plants, show instructor!
Decapitated
Lanolin + IBA
             .

6. Dispose of the plants and potting soil as directed, then rinse out the pots and place them in the designated area.


-   /24

Analysis:

Under which treatment does the main stem of pea plants grow taller?

  Intact       Decapitated      Decapitated + Plain Lanolin       Decapitated + IBA  

Under which treatment do pea plants produce more internodes along the main stem?

  Intact       Decapitated      Decapitated + Plain Lanolin       Decapitated + IBA  

Under which treatment do pea plants produce more branches?

  Intact       Decapitated      Decapitated + Plain Lanolin       Decapitated + IBA  

Did the auxin treatment inhibit branch formation or growth?   yes       no  

Decision: The hypothesis: "The decapitation of the apical bud removes the source of a hormone that inhibits branching:

is:   rejected       not rejected  .

When one of the buds grows out from a decapitated plant, are the cells of the bud:

    being genetically re-programmed into a new pathway of development, or

    continuing in their original genetic pathway?

The purpose of the Decapitated + Plain Lanolin treatment was:

 

What evidence do you have that the hormone involved in branch inhibition is auxin?

 

Have you eliminated the possibility that some other hormone could be involved?   yes       no  

Do you have evidence that, if a shrub is repeatedly trimmed (all
stems decapitated), the plant will remain shorter but become bushier?   yes       no  


-   /10

 

This page © Ross E. Koning 1994.

 

 

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