1. Biology, 9th ed, Sylvia Mader
Chapter 27
Chapter 27
Control of Plant Growth/Response
Control of Growth and Responses in Plants

2. Outline Plant Responses Plant Hormones Photoperiodism Tropisms
Nastic Movements
Plant Hormones
Auxins
Gibberellins
Cytokinins
Abscisic Acid
Ethylene
Photoperiodism

3. Plant Responses Tropisms
Plant growth toward or away from a unidirectional stimulus is called a tropism
Positive is towards stimulus
Negative is away from stimulus
Phototropism - Light
Gravitropism - Gravity
Thigmotropism - Touch

4. Phototropism

5. Positive phototropism:
Occurs because cells on the shady side of the stem elongate
A pigment related to riboflavin thought to act as a photoreceptor when phototropism occurs
Auxin migrates to shady side of stem
Shady sides elongate faster than bright side

6. Stems with root caps grow downward
Gravitropism
When a plant is placed on its side, the stem grows upward, opposite of the pull of gravity
Stems with root caps grow downward
Root cells contain statoliths
Auxin is responsible for:
Positive gravitropism of roots, and
Negative gravitropism of shoots

7. Gravitropism

8. Thigmotropism
Unusual growth due to contact with solid objects is called thigmotropism
Coiling of tendrils
Thigmomorphogenesis occurs when the entire plant responds to the presence of environmental stimuli
Wind
Rain

9. Coiling Response

10. Nastic Movements Nastic movements: Seismonastic movements result from:
Do not involve growth and
Are not dependent on the stimulus direction
Seismonastic movements result from:
Touch
Shaking, or
Thermal stimulation
Sleep movements:
Occur daily in response to light and dark changes
Prayer Plant

11. Seismonastic Movement

12. Sleep Movement

13. Circadian Rhythms A circadian rhythms:
Biological rhythms with a 24-hour cycle
Tend to be persistent
Rhythm is maintained in the absence of environmental stimuli
Caused by a biological clock

14. Almost all communication on a plant is done by hormones
Plant Hormones
Almost all communication on a plant is done by hormones
Synthesized in one part of the plant
Travels within phloem in response to the appropriate stimulus

15. Auxins
Auxin is produced in shoot and apical meristem and is found in young leaves and in flowers and fruits
Apically produced auxin prevents the growth of axillary buds
Apical dominance
Weak solution of auxin applied to woody cutting causes rapid growth of adventitious roots
Promotes fruit growth

16. Apical Dominance

17. Demonstrating Phototropism

18. Auxins bind to plasma membrane receptors
How Auxins Work
Auxins bind to plasma membrane receptors
Activated proton pumps H+ out of cell
Cell wall loosens
Turgor pressure increases due to the entry of water
Cell enlarges

19. Auxin Mode of Action

20. Gibberellins Gibberellins: Growth promoting hormones
Bring about elongation of the resulting cells
Gibberellic acid
Stem elongation
Breaking of dormancy

21. Effect of Gibberellins

22. Gibberellic Acid: Structure and Mode of Action

23. Cytokinins Cytokinins
A class of plant hormones that promote cell division
Derivatives of adenine
Prevent senescence
Initiate growth

24. Interaction of Hormones

25. Abscisic Acid Abscisic acid (ABA):
Initiates and maintains seed and bud dormancy, and
Brings about closure of stomata
Produced by:
Any “green tissue” with chloroplasts
Monocot endosperm, and
Roots

26. Abscisic Acid: Control of Stoma Opening

27. Ethylene is involved in abscission
Once abscission has begun:
Ethylene stimulates certain enzymes
Cause leaf, fruit, or flower to drop
Also ripens fruit by increasing activity of enzymes that soften fruit

28. Functions of Ethylene

29. Photoperiodism Photoperiodism:
Any physiological response prompted by changes in day or night length
Short-day plants
Long-day plants
Day-neutral plants
Some plants may require a specific sequence of day lengths in order to flower

30. Photoperiodism and Flowering

31. Conversion of forms allows a plant to detect photoperiod changes
Phytochrome
Phytochrome is a blue-green leaf pigment that alternately exists in two forms
Phytochrome red (Pr)
Phytochrome far-red (Pfr)
Conversion of forms allows a plant to detect photoperiod changes
Also promotes seed germination and inhibits stem elongation

32. Phytochrome Conversion Cycle

33. Phytochrome Control of Growth Pattern

34. Review Plant Responses Plant Hormones Photoperiodism Tropisms
Nastic Movements
Plant Hormones
Auxins
Gibberellins
Cytokinins
Abscisic Acid
Ethylene
Photoperiodism

35. Biology, 9th ed, Sylvia Mader
Chapter 27
Ending Slide Chapter 27
Control of Plant Growth/Response
Control of Growth and Responses in Plants