Plant Physiology

Acid Growth Hypothesis
The cell wall must be strong to withstand the relatively large pressures that can develop. Yet, at the same time, the wall must be capable of growth. This presents an interesting paradox - how is the wall able to be strong, yet allow for cell growth and elongation. One answer is that the wall is "loosened" or undergoes "stress relaxation" to permit wall components to slide past one another. The following experiments are designed to test the hypothesis that wall loosening is moderated by acid (acid growth hypothesis).

Question: Does acid treatment loosen cell walls?

Prediction #1: IF cell walls can be loosened by acid, THEN treating stem or hypocotyl sections with acid buffers should stimulate elongation. The graph below plots the length of etiolated cucumber hypocotyl sections versus time. Initially the pH is 7 and at the arrow is changed to pH 5. Answer the following questions:

Hypothesis #1: Cell walls are loosened by acid.

1. What is a hypocotyl?
2. What does etiolated mean?
3. Suggest a reason why etiolated hypocotyls were used in this experiment.
4. True or False: Hypocotyls elongate faster at pH 7 than at pH 5
5. True or False: Cell elongation is associated with acidic pH
6. What do you conclude about the hypothesis?

Prediction #2: IF cell walls are loosened by acid, THEN regions of cell growth/elongation should be associated with acid production. The diagram below shows maize and sunflower seedlings mounted in a petri dish containing agar. A pH indicator has been incorporated into the agar. Acidic regions are indicated by an "X".

1. Why does the agar become acidified near the tip of a vertically-mounted maize seedling?
2. Why is the acidified region of the vertically-mounted seedling not at the very tip, but a few millimeters behind the tip?
3. Why is the agar acidified only on the side of the sunflower seedling away from the light?
4. Why is the agar acidified only on the upper side of the root of the horizontally-mounted maize seedling?
5. What do you conclude about the hypothesis?

Hypothesis #2: Auxin, a plant growth hormone, is associated with cell elongation and wall acidification.

Prediction: IF protons stimulate elongation and wall acidification, THEN treating seedlings with auxin should stimulate elongation in a manner similar to acids. The graph below shows the length of a stem section in various pH buffers, before and after treatment with auxin. The initial pH in both experiments is pH 7. From these data we can conclude:

1. True or False: Acid stimulates cell elongation
2. True or False: The rate of elongation is slow but constant at pH 7.
3. True or False: The response to auxin is rapid.
4. True or False: Auxin stimulates the rate of elongation at pH 7.
5. True or False: The rate of auxin-stimulated elongation is equal to that of acid-stimulated elongation.
6. What do you conclude about your hypothesis?