1. PLANT RESPONSES TO STIMULI
Response of autumn crocuses (Colchicum purpura) to light [phototropism].
Chapter 21

2. A. Plant Growth Regulated by the action of hormones.
Hormone = a chemical messenger produced in one part of a plant & usually transported to another, where it elicits a response.
Plants have 5 major classes of hormones: auxins, gibberellins, cytokinins, ethylene & abscisic acid.

3. 1. Auxins
First group of plant hormones to be discovered (phototrophic experiments of Charles & Francis Darwin, 1870s).
“Auxin” is derived from the Greek “auxein” which means “to increase”.
Charles & Francis Darwin observed that:
(A) Shoots of oat seedlings always grew toward light source.
(B) Shoots did not grow toward light source if coleoptile tip was covered.
(C) Shoots grew toward light if covering was placed below coleoptile tip.
Thus, something produced in the growing tip enabled the plant to detect & respond to light.

4. Auxins are synthesized in shoot tips, young leaves & seed embryos.
Major Actions
promote elongation of cells (shoots, leaves & embryos)
Auxins stimulate cell elongation by altering the plasticity of cell walls - the walls stretch by taking in more water.
In diffuse light, seedlings grow straight upwards.
When light hits plant from side, auxin builds up on the shaded side of the seedling, causing those cells to elongate more than those on the lighted side.
inhibit growth of lateral buds
inhibit leaf & fruit abscission
stimulate synthesis of ethylene

5. IAA (indolacetic acid) is the most active naturally occurring auxin.
Commercial uses:
stimulate adventitious root growth in cuttings
stimulate some plants to produce fruit (seedless) without being fertilized
2,4-D (synthetic auxin) kills broadleaf weeds, but not grasses
Commonly produced seedless fruits include tomatoes,cucumbers, watermelons & eggplants.

6. 2. Gibberellins
Discovered by Japanese botanists studying “foolish seedling disease” in rice (1926).
Gibberellins are synthesized in young shoots & developing seeds.
Major Actions
promote elongation of cells (shoots, leaves & seeds)
stimulate flowering & fruit development
stimulate seed germination
Foolish seedling disease in rice is caused by the Gibberella fungus. Rice plants infected with the fungus get an overdose of gibberellin, causing them to grow extremely tall, toppling over & dying before they can mature & flower.

7. Commercial uses:
elongate flower stems of cyclamen plants
lengthen stems of grapes

8. 3. Cytokinins
Name derived from fact that cytokinins stimulate cytokinesis.
Cytokinins are synthesized in roots, embryos & fruits.
Major Actions
stimulate cell division (shoots, roots, leaves & seeds)
stimulate growth of lateral buds
delay leaf senescence
Cytokinins were not discovered until 1964.

9. Effects of cytokinins are influenced by auxin concentration.
Apical meristem intact - auxin suppresses lateral bud growth [apical dominance].
In an intact plant, auxin is more concentrated in shoot tips and cytokinin is more concentrated in roots.
As plant grows taller, lateral buds on lower part of plant begin developing. This is due to the higher ratio of cytokinins to auxins in the lower parts of the plant.
Avid gardeners are well aware of apical dominance. They often pinch off the growing tips to create a shorter, bushier plant.
Apical meristem removed - cytokinin concentration increases, stimulating lateral bud growth.

10. Commercial uses:
extend shelf life of leafy vegetables
keep cut flowers fresh
promote branching in Christmas trees

11. 4. Ethylene Only plant hormone that is a gas.
Ethylene is synthesized in all parts of plants, especially ripening fruits, nodes of stems, & dying leaves.
Major Actions
promotes fruit ripening
stimulates leaf & flower senescence
Although the effects of ethylene were known since the early 1900s, it was not known to be produced by plants until 1934.
Ethylene is reason why “one bad apple can spoil the entire bunch”.
All 4 petunia flowers were treated with ethylene. The 2 on the right were not affected by the ethylene treatment because they had been genetically engineered to be insensitive to ethylene.

12. stimulates leaf & fruit abscission
Stem
Leaf stalk
Abscission layer
Ethylene stimulates formation of the abscission layer.
Commercial uses:
ripens fruits that are picked green

13. 5. Abscisic acid (ABA)
Referred to as the “stress hormone” because it helps plants cope with adverse conditions (severe drought, onset of winter).
ABA is synthesized in mature leaves & plants under stress.
ABA was discovered in 1963.

14. induces & maintains seed dormancy Commercial uses:
Major Actions
inhibits growth
closes stomata
induces & maintains seed dormancy
Commercial uses:
inhibits the growth of plants that are to be shipped.
Seeds of many desert plants germinate only after spring rains have washed ABA out of them.

15. B. Plant Movement 1. Tropic Movements
Plant growth directed toward or away from an environmental stimulus.
Phototropism
Growth in response to unidirectional light.
Plant shoots are positively phototropic.
Bending results from auxin accumulation on shaded side of plant.

16. Growth in response to gravity.
Gravitropism
Growth in response to gravity.
Plant roots are positively gravitropic.
Plant shoots are negatively gravitropic.
Believed that amyloplasts in root cells function as statoliths (gravity detectors).
Gravitropism causes shoots to grow up & roots to grow down despite seed orientation at planting.
Amyloplasts - plastids that store starch.
Shift in statoliths signals redistribution of auxin.

17. Growth in response to touch.
Thigmotropism
Growth in response to touch.
Passion vine tendrils exhibit positive thigmotropism.
Coiling is controlled by auxin & ethylene.
Cells in contact with object grow at a slower rate, making the tendril coil around the object.

18. 2. Nastic Movements
Plant movements that are not oriented with respect to a stimulus.
Thigmonasty
A nastic response to touch.
Ex. leaflet folding of “sensitive plant”
Closing of Venus flytrap is also a thigmonastic response.

19. Photonasty (“sleep movement”)
A nastic response to daily rhythms of light & dark.
Ex. movement of prayer plant leaves
During the day, leaves of prayer plant (Maranta) lie horizontally - maximizes their interception of sunlight.
At night, the leaves fold vertically.

20. C. Response to Seasonal Changes
Many plant processes (flowering, seed germination, senescence, dormancy) occur at specific times of the year.
Plants track seasons by measuring photoperiod (relative lengths of daylight & darkness).

21. 1. Flowering
Long-day plants bloom when light periods are longer than some critical length.
Flower in spring or early summer
Short-day plants bloom when light periods are shorter than some critical length.
Long-day (short-night) plants include daffodil, lettuce, corn, iris, clover.
Short-day (long-night) plants include chrysanthemum, ragweed, poinsettias.
Flower in late summer or fall

22. Day-neutral plants do not rely on photoperiod to stimulate flowering.
Flower spring, summer & fall
Long-day & short-day plants actually respond to length of night rather than length of day.
Thus, short-day plants require a specific period of uninterrupted darkness to flower.
Day-neutral plants include violets, roses, snapdragons, dandelions, sunflowers, tomatoes.

23. Experiments of Karl Hamner & James Bonner demonstrated the importance of night length in flowering of clover & cocklebur.
Cocklebur is a short-day (long-night) plant. It requires 15 or fewer hours of light (9 or more hours of darkness) to flower.
Hamner & Bonner discovered that cocklebur would not flower if they interrupted the 9 hour (or longer) period of darkness with a 1-minute flash of light. This 1-minute flash of light appeared to “reset” the internal clock of the cocklebur, treating the light interrupted night as 2 short nights.
Clover plants also treated the light interrupted night as 2 short nights, and responded by producing flowers.
1-minute flash of light

24. Plants measure photoperiod with help of phytochrome, a blue pigment molecule that exists in 2 forms:
Pr = red-absorbing (660nm)
Pfr = far-red-absorbing (730nm)

25. Red light** Far-red light   Pr  Pfr Pr  Pfr
In daylight:
Red light** Far-red light
 
Pr  Pfr Pr  Pfr
(rapid conversion) (rapid conversion)
In darkness:
Pr < Pfr
(slow spontaneous conversion)
**Sunlight consists of much more red light than far-red light. Thus, most of the phytochrome is converted to the Pfr form during daylight hours.

26. Effects of alternate flashes of red & far-red light
A) Long-day plant flowers when daylight exceeds a critical length (night is short).
B) Short-day plant flowers when daylight is less than a critical length (night is long).
C) If interrupt night with flash of red light, long-day plant will flower, but short-day plant will not (period of uninterrupted darkness was too short).
D) If the flash of red light is followed closely by a flash of far-red light, the effect of the red light is cancelled, so plants are unaffected.
E) If 3 flashes occur in rapid succession R-FR-R, plants will respond the same as if receiving a single flash of red light.
F) If 4 flashes occur in rapid succession R-FR-R-FR, plants respond as if flashes did not occur.
Conclusion: It is the final light flash that affects the plant’s measurement of night length.

27. 2. Seed Germination Phytochrome affects seed germination.
In many weeds:
red light stimulates germination
Pr  Pfr
far-red light inhibits germination
Pfr  Pr
If seeds are buried too deeply in soil, Pfr is lacking & germination does not occur.

28. D. Circadian Rhythms
Plant responses that occur daily.
Circadian rhythms are regulated by biological clocks, which are controlled:
internally by genes
externally by environmental factors
Genetic control of circadian rhythms is demonstrated by the fact that the rhythms will continue even if the plant is exposed to continuous light or continuous dark.
A change in photoperiod can reset an plant’s biological clock.

29. Examples of circadian rhythms include:
flowering of the evening primrose
photonastic movements of prayer plant
opening of stomata
secretion of nectar
solar tracking (heliotropism) of sunflowers
Evening primrose blooms only in the late afternoon.
Stomata open in the morning and close at night.
Nectar secretion occurs during daylight hours.
Solar tracking - the ability of leaves or flowers to follow the sun’s movement across the sky. Ex. Sunflowers & buttercups.
Experiments with buttercups revealed that blue light (detected in the upper stem) controls solar tracking through auxin.