Lab #: 8: Plant Responses to Stimuli (Plant Hormones), Part I Plants & Civilization Spring 2009
Plant Growth & Development
Depends on many factors including:
Light Temperature Gravity Night length Various other external factors Hormone production
Plant Hormones
Organic molecules that coordinate growth and development in plants.
Synthesized in one part of the plant & translocated (moved) to another part.
Active in small amounts
Hormones cause a physiological response in the plant.
5 Groups of Plant Hormones 1. 2. 3. 4. 5.
Auxins Gibberellins Cytokinins Abscisic acid Ethylene
The effects of these hormones can be inhibitory or stimulatory, depending on [conc], tissue affected, & development status of the tissue.
Auxins & Gibberellins Today: you will set-up an experiment in which you treat plants with an auxin or a gibberellin to determine the effects of each on plant growth & development. We will not be studying the effects of other plant hormones during the scope of this lab.
AUXINS
Example: indole-3-acetic acid (IAA)
Occur naturally in several different chemical forms
Synthesized in:
buds of young stems & leaves (e.g. apical bud) embyros seeds fruits
AUXINS
Once IAA is synthesized in a bud, it is translocated to the base of the plant (highest [conc] remaining in the growing tips, less in roots).
http://www.kallus.com/otherplants/coleus/p001957.jpg
IAA causes lengthening of cells in the elongation region of a growing shoot/stem - just behind apical meristem.
AUXINS
However, if [IAA] increases beyond an optimum level, lengthening of the stem is inhibited rather than stimulated!!!
IAA produced in the apical bud of a growing shoot/stem also suppresses the development of lateral/axillary buds.
http://koning.ecsu.ctstateu.edu/apical/decap.gif
Plants Producing Auxin
Result: plant exhibits “apical dominance”
Plant appears to grow upwards rather than outwards (gets taller, but minimal lateral branches are produced).
Plants may appear ‘scrawny’ due to lack of fullness in lateral branches.
What do you think will happen if the apical bud (where IAA is produced) is removed?
Today’s Experiment #1: (work in groups of 4)
Obtain 4 Coleus plants & treat as follows:
1. 2. 3. 4.
Leave intact Remove apical bud Remove apical bud + add control lanolin paste Remove apical bud + add 5000ppm IAA lanolin paste
Be sure to label all plants w/ treatment type, your names, lab section & today’s date.
Don’t forget to make your hypotheses!!!
What you will do…. You will also have a 4th plant w/ apex removed & control lanolin paste added.
http://www.botit.botany.wisc.edu/images/130/Growth_Substances/Auxins/Apical_Dominance/Results_from_lab/Coleus.php?highres=true
Today’s Experiment #1: (continued)
Once you have set-up the experiment & made your initial observations, measurements, & hypotheses, you may continue with the next experiment….
You will record the results of this experiment in lab in 2 weeks (Plant Hormones, Part 2).
Next - testing the effect of Gibberellins on plant growth….
Gibberellins Example: 90+ types, gibberellic acid (GA3) is most prevalent in flowering plants.
Naturally occurring plant steroid hormones.
Synthesized in meristematic regions:
Young leaves Shoot tips Root tips Seeds too!
Gibberellins
Translocated in the xylem & promotes cell elongation (like auxin).
In shoots, stem elongation & rapid growth are controlled directly by gibberellic acid.
In seeds, GA3 causes germination to happen quickly by enhancing cell elongation in the embryonic root (i.e. roots grows quickly to penetrate seed coat).
Gibberellins
Some plants lack the ability to synthesize GA3, but will still react to this hormone if treated with it.
Example: dwarf pea plants
These plants do not produce their own GA3, hence the name “dwarf” peas!
http://oregonstate.edu/dept/hort/faculty/Proebsting/images/isogenic.jpg
Today’s Experiment #2: (work in pairs)
Obtain 2 Dwarf Pea Plants: take initial measurements & make observations.
Then, treat as follows: 1. 2.
Leave untreated – add only control solution. Treat by adding GA3 to the apex.
Be sure to label all plants w/ treatment type, your names, lab section & today’s date.
Don’t forget to make your hypotheses!!!
Today’s Experiment #2: (continued)
Once you have set-up the experiment & made your initial observations, measurements, & hypotheses, you may continue with the next experiment….
You will record the results of this experiment in lab in 2 weeks (Plant Hormones, Part 2).
Next – other ‘plant responses to stimuli’
Tropisms
Directional growth movements that occur in response to stimuli.
Growth TOWARD a stimulus is called a POSITIVE response. Growth AWAY from a stimulus is called a NEGATIVE response.
Gravitropism
Definition: directional plant growth that occurs in response to gravity.
If a plant is placed on its side…
The stem will bend upward toward the sun (against gravity). The root will bend downward (with the gravitational pull) into the soil.
Gravitropism
Movements occur because of UNEQUAL growth.
Unequal growth
Results in bending toward or away from the stimulus. Occurs because of auxin (or other hormone) redistribution.
Gravitropic Response Occurs in 2 steps:
1.
Something in the plant must detect gravity
2.
This “something” is starch grains called statoliths.
Differential growth must occur
One side of the plant grows faster than another, causing the plant to bend.
Gravitropic Response
Plant STEMS vs. ROOTS (both placed horizontally)
Starch grains (statoliths) fall toward lower surface causing an increase in [auxin] in that region.
Statoliths in root cap cause an increase in a growth inhibitor,abscisic acid, on the lower surface.
Lower stem surface grows faster than upper surface, causing upward bending.
Inhibition of growth on lower surface of the root causes a downward bending of root.
http://www.greenlizzy.com/graphics/Tom.jpg
http://pmb.berkeley.edu/faculty/faculty_pages/images/FeldmanImage.jpg
Seed Viability
Seed viability (ability of a seed to grow) is important in the agricultural industry.
If a seed is not capable of growth, a mature plant cannot be produced from it.
Several tests exist to determine whether a seed is viable.
Testing Seed Viability
Tetrazolium Test (TTC) – colorless chemical when oxidized, but becomes pinkish/red when reduced.
Test relies on whether or not the electron transport chain of a seed is functional.
TTC Test (continued)
TTC added to a living cell will interact with the etransport chain by accepting e- that are transferred from the cytochromes. When TTC accepts these e-, it is reduced and turns from colorless to deep pink/red. Abnormal seed
Normal seed
http://www.bioss.ac.uk/smart/unix/mseqacc/images/i10.jpg
Note: Endosperm of normal seed is stained pink/red.
TTC Test (continued)
TTC is a POISON, POISON avoid contact with skin & eyes!! Handle only with gloves & goggles. Wash hands & spills immediately. You will be testing 2 groups of seeds, “A” & “B”, to determine if the seeds are viable. Cut seeds lengthwise & place face down in a petri dish that contains TTC. Put in a dark place for at least 25 minutes before examining.
Today’s Summary:
Set-up experiments testing plant hormones – auxin (IAA) & gibberellin (GA3). Carefully, record ALL initial observations/measurements.
Observe demonstrations, gravitropism of stems.
Set-up seed gravitropism study in corn & bean experiments.
TTC test for seed viability.
Today’s Summary: This lab will be relatively short in length, but you must set-up experiments carefully. The results will be recorded in lab two weeks from today. The data obtained in ‘Part 2’ will be the basis of a short write-up on this lab topic…. start thinking about your hypotheses & possible outcomes!
