A. Observation. You have already observed some of the effects of auxin and gibberellin when used at optimal concentrations. Some plant hormones can be used in very high amounts with no negative effects; others are toxic to plants when applied at high concentration. You probably observed the thickening of the stem tip of peas that had a 1% IAA paste applied to the stump. Perhaps auxin then is a hormone that could be used to kill pea plants if applied in large amounts. 2,4-D is a synthetic form of auxin that is particularly potent. Clover is a relative of peas that is a common lawn pest (?); some people like clover in their lawn for reasons that your instructor will discuss. Here we will use rapid growing radish to simulate clover in a lawn of grass. We will use large popcorn to simulate small grass plants in our"lawns."
B. QUESTION: Would large amounts of auxin be toxic to some plants but not others?
C. HYPOTHESIS: Strong auxin will kill radish but will not kill a"lawn" of popcorn.
D. PREDICTION: If the hypothesis is correct, then among a mixture of radish and popcorn plants only the radish will die when sprayed with a solution containing 1 mM 2,4-D. The popcorn will remain healthy.
E. EXPERIMENT: Some time ago you counted out some popcorn and radish seeds and planted them together in three pots. A few weeks ago you marked one pot"Untreated Control" and returned it to the greenhouse. A second pot was marked"Detergent Control," the plants were sprayed with detergent water, and the pot was returned to the greenhouse. A third pot was marked"2,4-D plus Detergent," the plants were sprayed with 1 mM 2,4-Dichlorophenoxyacetic acid in detergent water, and the pot was returned to the greenhouse. 2,4-D works best when plants are exposed to bright light. You also counted the number of radish and popcorn seedlings in each pot and observed the leaf color for both plant species in each pot. You should transfer those numbers to the spaces below.
| Why is it important to have these starting numbers and observations available? |
1. Today you will observe the pots of plants once again. Retrieve the three pots from the greenhouse. Make the same observations you did before.2. Clean up your bench area.
a. Put all used soil and counted plants in the designated trash container.b. Brush the bench top clean and wash it down.
c. Remove all labels from the pots, rinse out the empty pots, and put them in the designated area.
d. If you have spilled soil on the floor, sweep it up in the dustpan and dispose of it in the designated trash container.
| Untreated Control | Data before spraying | Data after spraying | ||
|---|---|---|---|---|
| Popcorn | Radish | Popcorn | Radish | |
Number of Living Seedlings | ||||
| Proportion of Living Total (See explanation in Analysis) | ||||
Percentage of Living Total | ||||
Leaf Color | ||||
| Detergent Control | Data before spraying | Data after spraying | ||
|---|---|---|---|---|
| Popcorn | Radish | Popcorn | Radish | |
Number of Living Seedlings | ||||
| Proportion of Living Total (See explanation in Analysis) | ||||
Percentage of Living Total | ||||
Leaf Color | ||||
| 2,4-D + Detergent | Data before spraying | Data after spraying | ||
|---|---|---|---|---|
| Popcorn | Radish | Popcorn | Radish | |
Number of Living Seedlings | ||||
| Proportion of Living Total (See explanation in Analysis) | ||||
Percentage of Living Total | ||||
Leaf Color | ||||
F. ANALYSIS:
1. Calculate the total number of plants in each pot by adding the number of living radish and popcorn plants you counted. Then divide the actual number of living radish plants by the calculated total to get a proportion then multiply by 100 to get a percentage. Repeat this calculation for the popcorn plants. Enter the results in the tables above. Remember that you should not show any fractions in the tables...divide out to a real proportion (number between 0 and 1). If you have rounded carefully, the sum of %corn and %radish should total 100%, right?
2. Compared to the percentage before spraying, the final percentage indicates that the proportion of popcorn plants in the...
| untreated control pot: | increased stayed the same decreased |
| detergent control pot: | increased stayed the same decreased |
| 2,4-D treated pot: | increased stayed the same decreased |
3. Compared to the percentage before spraying, the final percentage indicates that the proportion of radish plants in the...
| untreated control pot: | increased stayed the same decreased |
| detergent control pot: | increased stayed the same decreased |
| 2,4-D treated pot: | increased stayed the same decreased |
4. Compared to the number before spraying, the final number of popcorn seedlings in the:
| untreated control pot: | increased stayed the same decreased |
| detergent control pot: | increased stayed the same decreased |
| 2,4-D treated pot: | increased stayed the same decreased |
5. Compared to the number before spraying, the final number of radish seedlings in the:
| untreated control pot: | increased stayed the same decreased |
| detergent control pot: | increased stayed the same decreased |
| 2,4-D treated pot: | increased stayed the same decreased |
6. In terms of color, the popcorn plants treated with 2,4-D were:
| greener the same yellower | than untreated controls |
| greener the same yellower | than detergent-treated controls |
7. In terms of color, the radish plants treated with 2,4-D were: (Note: brown=dead=yellower!)
| greener the same yellower | than untreated controls |
| greener the same yellower | than detergent-treated controls |
G. DECISION:
| The hypothesis,"Strong auxin will kill radish but will not kill a"lawn" of popcorn" is: | rejected not rejected |
H. OTHER QUESTIONS: Answer these questions at home after you have analyzed the results of your experiment.
1. Why did you use an untreated control and another detergent-treated control?
Untreated control tells us:
Detergent control tells us:
2. Why is estimating an overall color for many plants a weak approach for science?
a:
b:
3. Is 2,4-D an effective lawn weed killer?
Yes No
4. In what class of crop plants could 2,4-D be successfully used to control weeds?
Hint:"Corn" or"Radish" would be too specific!!
______________________
5. Would 2,4-D be an effective weed control spray for all crops?
Yes No
6. What precautions should you take in using products containing 2,4-D around your home landscape? (Do not consider human and pet safety in this answer, consider only precautions for plant safety)
This page © Ross E. Koning 1994.
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Koning, Ross E."Weed Control". Plant Physiology Website. 1994. http://koning.ecsu.ctstateu.edu/plants_human/weedlab.html (your visit date).
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