Plant Growth Hormones

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, and the role of another hormone, gibberellic acid, in causing stem growth.

I. Apical Dominance in Bean

(Phaseolus vulgaris cv 'Red Kidney')

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

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

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

D. Prediction: If the hypothesis is correct, then

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

2. Decapitated plants treated on the apex with a lanolin paste containing the hormone should produce as many branches as the intact control and fewer branches than the decapitated (but untreated) plants.

E. Experiment: Earlier in the term you decapitated and treated bean plants in four pots. Retrieve your pots labeled "Intact Control," "Decapitated," "Decapitated + Plain Lanolin," and "Decapitated + 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?____________________________

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!

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. Measure each plant and then calculate the mean.

Individual Plant HeightsMean (cm)
Intact
(Untreated)
      
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. If you have more than one on any of the decapitated treatments, immediately notify your instructor! You are probably in error! Calculate a mean number of internodes for each treatment.

Number of Main-Stem Internodes
for Individual Plants
  Mean  
Intact
(Untreated)
      
Decapitated
(Untreated)
      
Decapitated
(Plain Lanolin)
      
Decapitated
(Lanolin + IBA)
      

 

5. Number of branches. Count the number of side branches on each plant in each pot. Be careful about this: the leaves of bean plants are compound and you should be sure that the branch you count emanates from a leaf axil. The branches are most likely found in the axils of the two simple leaves. If in doubt about whether you have a leaf petiole or a branch, ASK! Do not count branches which are less than 1 cm long. If you find a branch on an intact (untreated) plant, notify your instructor immediately (error likely)! If you find more than four branches on any plant do likewise. Calculate a mean number of branches for each treatment.

Number of 1-cm Branches on
Individual Plants
  Mean  
Intact
(Untreated)
      
Decapitated
(Untreated)
      
Decapitated
(Plain Lanolin)
      
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.

F. Analysis:
Under which treatment does the main stem of bean plants grow taller?

Intact     Decapitated     Decapitated + Plain Lanolin     Decapitated + IBA
Under which treatment do bean plants produce more internodes along the main stem?

Intact     Decapitated     Decapitated + Plain Lanolin     Decapitated + IBA
Under which treatment do bean plants produce more branches?

Intact     Decapitated     Decapitated + Plain Lanolin     Decapitated + IBA
Did the auxin treatment inhibit branch formation?Yes       No

G. 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


II. STEM GROWTH IN PEA PLANTS.

A. Observations: There are two different types of pea plants. In American gardens we plant mostly dwarf varieties that attain a height of less than 1 meter. Gregor Mendel, an Austrian monk, discovered the basis for modern genetics and biotechnology working with European peas. Most of these grow to a height of several meters and are trained up poles or fences.

B. Question: Why is the dwarf plant short?

C. Hypothesis: The dwarf plant is short because it cannot produce an essential hormone.

D. Prediction: If the hypothesis is true, then

1. Dwarf plants should grow taller than controls when treated with the missing hormone

2. Tall plants should grow less than untreated controls when they are treated with an inhibitor of the synthesis of the hormone

E. Experiment: Earlier in the term, you treated young dwarf and tall pea plants with various sprays. Retrieve the pots labeled "Untreated Control," "Water Spray," "GA Spray," and "B9 Drench" for each variety of pea. The untreated control plants were not sprayed with anything. The water spray plants were sprayed with distilled water containing a detergent. The GA spray plants were sprayed with distilled water containing a detergent and 10-4 M gibberellic acid (GA). The B9 Drench plants were drenched with 75 ml of distilled water containing a detergent and 0.5% B9 (2t/l). The detergent is used in each treatment to help wet the waxy surfaces of the leaves so that the chemical solutions can be absorbed. GA is a natural plant hormone and is routinely used to simulate grass growth in the "roughs" of golf courses and to cause seedless table grapes to grow to a full size. B9 is a potent inhibitor of the enzymes that make GA in plants and is routinely used to keep tall plants like geraniums, chrysanthemums and poinsettias dwarf without resorting to pruning. You will now observe the growth of the pea seedlings that have been treated with these sprays. You should have thinned the plants to five uniform individuals per pot. If you need to cull each treatment to five plants per pot.

1. Height of plants. Measure the height of the five plants in each pot. Calculate the mean height of the plants in each pot.
Dwarf VarietyMean
(cm)
Tall VarietyMean
(cm)
Untreated Control              
Water Spray              
GA Spray              
B9 Drench              

2. Number of internodes. Count the number of internodes on each of the plants in each pot. Calculate the mean number of internodes for the plants in each pot.
Dwarf Variety  Mean  Tall Variety  Mean  
Untreated Control              
Water Spray              
GA Spray              
B9 Drench              

3. Size of leaf. Measure the length of the x( ) leaf on each plant and calculate an average leaf length for each treatment. CAUTION: The leaves are compound and may end in a tendril.
Dwarf VarietyMean
(cm)
Tall VarietyMean
(cm)
Untreated Control              
Water Spray              
GA Spray              
B9 Drench              

4. Return the pots to the designated area.

F. Analysis:

1. For the dwarf variety, compared to the water spray treatment, the GA treatment:
increased       had no effect on       decreasedthe plant height
increased       had no effect on       decreasedthe number of internodes
increased       had no effect on       decreasedthe leaf size

2. For the dwarf variety, compared to the water spray treatment, the B9 treatment:
increased       had no effect on       decreasedthe plant height
increased       had no effect on       decreasedthe number of internodes
increased       had no effect on       decreasedthe leaf size

3. For the tall variety, compared to the water spray treatment, the B9 treatment:
increased       had no effect on       decreasedthe plant height
increased       had no effect on       decreasedthe number of internodes
increased       had no effect on       decreasedthe leaf size

For the following two questions, calculate the effect of a treatment using the formula:
treatment height - control (water spray) height
-------------------------------------------------------
control height
x 100

4. Which variety did GA stimulate most?
Dwarf RatioTall RatioAnswer:
Dwarf
Tall

5. Which variety did B9 inhibit most?
Dwarf RatioTall RatioAnswer:
Dwarf
Tall

G. Decision:
The hypothesis: "The dwarf plant is short because it cannot produce an essential hormone" is:rejected       not rejected
What is the primary target of gibberellic acid?# leaves
leaf size
# internodes
internode length


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