External factors and plant growth
– Chapter 28
Tropism
 A tropism is a growth response involving bending of a plant toward or away from an external stimulus
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Positive tropism – move towards stimulus
Negative tropism
– move away from stimulus
Phototropism
 Bending in response to light

Shoots exhibit positive phototropism
 Caused by elongation of cells on dark side of shoot
Went’s experiment

Cut off coleoptile tips and placed in agar
 Put agar blocks on tip of shoot tip
 Saw elongation where auxin diffused
Phototropism and auxin
 Light can decrease sensitivity of cells to auxin on one side
 Light can destroy auxin
 Light moves auxin to shaded side of tip
Briggs et al.
experiments

Same amount of auxin in coleoptile tip found in light and dark tip
Briggs et al.
experiments

Same amount of auxin when coleoptile tip is split by glass
Briggs et al.
experiments

Same amount of auxin when coleoptile tip and agar block split with glass
Briggs et al.
experiments
 Differential amount of auxin when coleoptile tip and agar block split with glass, but tip intact
Gravitropism

Response to gravity
 Shoots are negatively gravitropic
 Roots are positively gravitropic

Gravitropism and auxin
 Auxin redistributes from upper side to lower side
 In shoots, higher concentration on lower side promotes elongation and shoot turns up
 In roots, higher concentration on lower side restricts elongation and shoot turns down
Gravitropism and calcium
 Ca/Calmodulin complex
 Calcium diffuses to upper surface of shoots and towards the lower surface of roots
Gravitropism and cytokinin
 Cytokinin accumulates in lower side of gravistimulated roots
Gravity perception
 Movement of amyloplasts
 Starch sheath cells in shoot
 Columella in rootcap
What next?
 Starch-statolith hypothesis

 Amyloplasts act as gravity sensors
Protoplast pressure hypothesis

 Entire protoplast is gravity sensor
Tensitegrity model
 Tensional integrity is structural integrity created by interactive tension
 between structure components
Disruption of integrity increases calcium levels
Thigmotropism
 Response to touch
 Tendrils
Circadian rhythms
 Regular 24-hour cycles in plant function
Biological clocks
 Rhythms continue even when environment is constant
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
Rhythms are internal
Maintains rhythm with temperature changes
Sychronization
 Biological clocks are 21 to 27 hours with constant environment
 Biological clocks are sychronized or entrained with environment to maintain 24 hour cycle

Entrainment
Photoperiodism
 Flowering under certain daylength conditions
 Important for other organisms too
Photoperiodism

Three general patterns
 Short-day plants flower in early spring or fall and must have a light period shorter than a critical length
 Long-day plants flower in summer and must have a light period longer than a critical length
 Day-neutral plants flower without respect to daylength
 Flash of light during dark period long-day plant will flower even if daily light period is shorter than time required
 Flash of light period during dark period for short-day plant will inhibit flowering
Photoperiodism
 Can be very precise

Varies with species
 1 day or many days
 Type of light during dark period for long-day plants is important
 Only germinate if last flash is red light
Phytochrome

Phytochrome is a photoreceptor in plants
 Two interconvertable forms
 Pr absorbs red light and is biologically inactive
 Pfr absorbs far-red light and is biologically active
Phytochrome
 During daytime…..

 Since daylight contains both types of light an equilibrium of Pr and Pfr will exist in daytime (60% Pfr at noon)
At night….


Levels of Pfr decline due to dark conversion and destruction
Flash of FR light during night causes conversion of Pfr to Pr. Flash of R light reverses this.
Etoliation
Phytochrome and shade response

Greater FR light under canopy
 Greater FR:R ratio causes increased internodal elongation

Hormonal flowering control
 Substance from leaf called florigen stimulates flowering
 Florigen is probably gibberellin and anthesin
 Another unidentified hormone inhibits flowering
Dormancy
 Time of arrested growth

Dormant bud or embryo has specific cues for reactivation
Seeds
 Stratification
 Cold requirement simulates winter
 Drying
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

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Prevents germination in moist fruit
Abrasion
 Movement
Gastrointestinal tract
Fruit inhibitory substances
Seed banks
Bud dormancy
 Acclimation precedes dormancy
 Cold period may be necessary
 Drying

Addition of gibberellin
Vernalization

Cold temperatures can stimulate flowering
Nastic movement

Plant movements that occur in response to a stimulus but whose direction is independent of the position of the origin of the stimulus
 Thigmonastic movements
Thigmomorphogenesis
 Alteration of growth patterns in response to mechanical stimuli
Heliotropism
 Solar tracking
 Leaves move with moving sun