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Plant Growth and Development
Objectives
• Describe phototropism, gravitropism, and
thigmotropism
• Learn about plant hormones and their effects
Understand how varying amounts of
light/dark stimulate/inhibit flowering
Chapter 37
Phototropism
KEY CONCEPTS
• Growth in response to the direction of light
• Plants respond to external stimuli, such as
light, gravity, or touch, with a directional
growth (tropism)
Gravitropism
Thigmotropism
• Growth in response to gravity
• Growth in response to contact with a solid
object
• Tendrils of a vine
Ameoplasts in root tip cells
- contain starch granules
- respond to gravity
- involves hormone auxin
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Plant Hormones
• Organic compounds (chemicals) produced by plants
• Elicit responses that regulate growth and
development
Take Home Message
• Growth and development are influenced by a
plant’s internal conditions and signals from its
external environment
Mechanism for Plant Hormone
How Plant Hormones work!
• Signal Transduction: a receptor converts an
extracellular signal into an intracellular signal
that causes a change in the cell
• Many hormones bind to enzyme-linked
receptors in plasma membrane, triggering an
enzymatic reax.
KEY CONCEPT
Repressor
Protein
Darwins’ Experiments (1870s)
• Although there are many plant hormones, five
have been well-characterized:
• auxins
• gibberellins
• cytokinins
• ethylene,
• abscisic acid
Conclusion: Some substance is transmitted from upper part to lower part
that causes the plant to bend
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Went’s Experiments (1920s)
Plant Hormones: Auxin
•
•
•
•
Cause Tropisms
Apical Dominance
Fruit development
Stimulates root development on stem cuttings
Auxins promote cell elongation
• Cause cells to elongate
• apical meristem
• explains phototropism!
Fig. 36-5
Auxin and apical dominance
- inhibits axillary bud growth
Gibberrelins
1. Bolting in plants (mytosis & hypertrophy)
2. Flowering in plants
Auxins produced by seeds promote fruit
development
2,4-D
3. Seed germination
1. production of Gibberrelins in embryo
2. triggers synthesis of  amylase
3. digests starch in endosperm
4. provides glucose for embryo
2,4,5,-T
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Cytokinins
Auxin-Cytokinin Interactions
• Promote cell division and
differentiation
• Delay senescence
Ethylene
• Ripening fruits
• Apical dominance
• Leaf abscission (as auxin decreases ethylene
increases at abscission layer
• Wound response
• Thigmomorphogenesis
• developmental response to mechanical stressors
such as wind
• Plants produce more ethylene
• Inhibits stem elongation and enhances cell wall
thickening
•
Abscisic Acid – stress hormone
• Stomatal closure caused by
water stress
• Seed dormancy
• temporary state of
reduced physiological
activity
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Photoperiodism
• How do varying amounts of light and darkness
induce flowering?
response of a plant to lengths of daylight
and darkness
• Flowering can be a photoperiodic response
- short-day plant
- long day plant
- day-neutral plant
Short-Day Plants
• Detect lengthening nights of late summer or
fall and flower at that time
Fig. 37-14a, p. 800
Long-Day Plants
• Detect shortening nights of spring and early
summer and flower at that time
Fig. 37-14b, p. 800
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Intermediate-Day Plants
• Flower when exposed to days and nights of
intermediate length
Fig. 37-14c, p. 800
Light
Photoperiodic Responses
Dark
Flash of red light
Light
treatment
24 hours
Short-day
plant
Long-day
plant
Fig. 37-15, p. 801
Phytochrome
• The photoreceptor in photoperiodism
• a family of about five blue-green pigments
• Each phytochrome has 2 forms (Pr and Pfr)
• named for wavelength of light absorbed
Phytochrome
• Pfr (the active form) triggers or inhibits
physiological responses
• flowering
• shade avoidance
• light requirement for germination
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Red light (660 nm)
Circadian Rhythm
Phytochrome
synthesis
Inactive
form
Pr
Phytochrome
degradation
Short-lived
intermediate forms
Active form
Short-lived
intermediate forms
Far-red light
(730 nm)
(Night)
Pfr
• A regular period in growth or activities that
approximates the 24-hour day
• reset by rising and setting of the sun
• Examples:
• opening and closing of stomata
• sleep movements
Physiological
response
(such as
flowering)
Fig. 37-16, p. 801
Sleep Movements
Phytochrome and Cryptochrome
• Both are photoreceptors
• sometimes interact to regulate similar responses
such as resetting biological clock
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