Download Exercise 8 – Plant Hormones

Exercise 8 – Plant Hormones
Biol 1012, S2008, Lee and Little
Goals
•
Develop a better understanding of some of the various roles of plant hormones during growth
and development.
Introduction
Hormones are organic molecules that play critical roles in plant development. Often these effects are
rather specific and only very small amounts of hormone are necessary to produce a response. Some
plant physiologists prefer the term “plant growth substances” rather than “hormones.”
A number of different types of hormones are found in plants. For the most part, they can be placed
into one of the following five well-accepted categories: abscisic acid, auxins, cytokinins, ethylene,
and gibberellins. Although their effects may be quite specific and a given response is often
associated with one particular hormone, all five groups of hormones may affect a single process. The
exact concentration range of maximum biological activity differs for different processes and species,
and the total hormone balance is as important as the level of hormone.
In this exercise, you will observe the effects of 3 different hormones in a series of demonstrations.
The effects of gibberellic acid (GA) on stem elongation
Observe the demonstration of pea plants with the following treatments:
A. Genetically dwarf pea control
B. Genetically dwarf pea with GA sprayed on leaves
C. Genetically tall pea control
1. What effect did GA have on the genetically dwarf plant? Is this consistent with what you learned
in lecture?
2. State a hypothesis that is supported by the plants in this demonstration:
Floral senescence
Observe the demonstration of the snapdragon inflorescences with the following treatments:
A. In bell jar with bruised apple
B. In free air
3. What effect did “sharing air” with a bruised apple have upon the snapdragon?
4. Using information from lecture, what plant growth substance do you think is responsible for this
effect?
5. Describe a better control for our demonstration:
The effects of auxin (IAA) on leaf senescence
Senesced leaves (that have lived out their useful lifespan) typically do not remain on a plant. Instead,
the plant “drops” them through a process called abscission. View the demonstration slide of a leaf
abscission layer.
In the abscission process, the plant must receive a signal that the parts of the leaf (petiole and blade)
are no longer functional and are no longer needed. What is that signal?
Several sets of leaves on each Coleus plant in this demonstration have been “de-bladed.” That is, the
blades have been removed from the petioles that are still attached to the plant. Each set of opposing
petioles has received a different treatment at its tip, as indicated by the white binder ring:
C:
No substance control
L:
Lanolin control
IAA: Lanolin with IAA (a type of auxin)
Lanolin is a greasy fat that comes sheep’s wool. Botanists often use it as a medium to apply auxin, a
non-polar, fat-soluble hormone.
6. The two different types of controls are often called “positive” and “negative.” Which is which?
7. Are the data shown in the demonstration consistent with what you learned about auxin in lecture?
Why or why not?
References
Davies, P.J. 1995. Plant Hormones: Physiology, Biochemistry and Molecular Biology. 2nd Edition. Kluwer
Academic. Boston.
Moore T.C. 1989. Biochemistry and Physiology of Plant Growth Hormones. 2nd Ed. Springer Verlag. New
York.
Salisbury, F.B. and C.W. Ross. 1992. Plant Physiology, Fourth Edition. Wadsworth Publishing
Company, Inc., Belmont California, 6