Chapter 29 External Factors and Plant Growth

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Chapter 29External Factors and Plant Growth
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Outline

• Intro
• Tropisms
• Circadian rhythm
• Seed dormancy
• Bud dormancy
• Nastic movements
• Solar tracking

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Intro

• Mechanisms to sense/respond to
  external factors
• time/light/season/etc.
• dormancy during winter
• flowering at appropriate times
• Ability to adapt/withstand large changes
  to environment by
  altering development
• red maple

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s

• Megs Talk

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Tropisms

• Tropism bending/curving toward/away
  from external stimulus
• growth toward a stimulus
• growth away from stimulus -
• Phototropism bending of shoot toward light
• due to elongation of cells on shady side
• controlled by auxin
• auxin migrates to shady side

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Tropisms

• Gravitropism bending of plant to orient shoot
  upward/root downward
• gravitropism downward growth of root
• - gravitropism upward growth of shoot
• controlled by auxin
• shoot
• auxin migrates to bottom side of stem
• promotes elongation on bottom side
• root
• auxin migrates to bottom side of
  root and inhibits normal cell
  expansion on bottom side

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Tropisms

• Starch-statolith hypothesis
  gravity perception due to sedimentation of
  amyloplasts which act as gravity sensors
• statoliths
• Protoplast/hydrostatic pressure hypothesis
  gravity perception due to hydrostatic pressure
  changes by protoplast on cell wall
• Plasmalemma central control hypothesis gravity
  perception involves disruption of actin meshwork
  and Ca2 channels
• Asymetrical distribution of cytokinins
• accumulation on underside of curved
  root inhibits growth

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Tropisms

• Hydrotropism growth of roots toward moisture
• ? water potential
• involves rootcap and Ca2
• Thigmotropism growth response when
  influenced by contact w/ solid
  object
• tendrils
• cells touched shorten
• other side elongates
• memory
• sensitive plant

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Circadian Rhythms

• Circadian rhythm rhythmic changes in activity
  w/ 24 hr cycle
• flowers/leaves open at night
• close in dark
• diurnal (daily) events
• internally controlled by biological clock
• kept accurate/synchronized (entrained) by
  environmental cues
• light and temperature cycles

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Circadian Rhythms

• Sleep mvmts in leaves (nyctinasty)
• horizontal/open to light in day
• vertical/closed at night
• occurs in total darkness
• blue light opening
• red light darkness
  closing

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Circadian Rhythms

• turgor changes in ventral and dorsal motor cells
  in pulvinus at base of petiole
• due to K/Cl- fluxes across PM
• leaflets close
• swelling when dorsal motor cells accumulate K
  and Cl-
• shrinking when ventral motor cells release K and
  Cl-
• leaflets open
• ?????

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Effects of Light on Growth

• In addition to being necessary for Ps, light
  affects plant form and growth
• etiolation form and growth of seedlings grown
  w/o light
• pale, tall, spindly looking vs dark green,
  shorter and
  stockier
• pale b/c chloroplasts do not mature
• light is cue for change in form from etiolated
  (w/o light) to
  normal (w/ light) growth

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Photoperiodism

• Photomorphogenesis influence of light on
  development
• light growth (above ground) vs dark growth (below
  ground)
• blue light
• phototropism and guard cells
• red light
• vegetative and reproductive development
• Photoperiodism biological response to change in
  proportion in light/dark in 24-hr cycle
• phytochrome pigment absorbs red and far-red
  light

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Photoperiodism

• Phytochrome is photoreversible
• effects of red light (650-680 nm) reversed by
  exposure to far-red light (710-740 nm)
• 2 forms of phytochrome
• absorbs red (Pr) (looks blue)
• absorbs far-red (Pfr) (blue-green)
• Pr Pfr (physiologically active form)
• in darkness phytochrome is Pr

red light
far-red light
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Photoperiodism

• Phytochrome allows plant to sense/respond to
  shading
• Redfar-red light varies
• more far-red light during sunset, underground,
  under canopy
• leaves absorb red but transparent to far-red
• RFR and shading
• shade avoidance response plants lengthen as
  response to shading (? RFR)
• more FR converts Pfr to Pr ? PfrPtotal ?
  stem extension
• allows plants to
  
  reach for light

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Photoperiodism

• RFR and seed germination
• light requirement for germination of many
  small-seeded herbaceous/grassland sp.
• FR inhibits germination
• ? R converts Pr to Pfr
  ?
  PfrPtotal seed germination
• allows plants to wait to
  germinate until
  little
  competition for light

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Photoperiodism

• 3 flowering responses to light
• short-day plants
• flower early spring or fall
• must have light period shorter
  than
  critical period
• long-day plants
• flower summer
• must have light period longer
  than critical
  period
• day-neutral plants
• flowering unaffected
  by
  daylength

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Dormancy

• Dormancy condition of arrested growth
• occurs during unfavorable condititions
• water/temperature
• resumes when conditions are favorable
• survival mechanism
• endogenous inhibitors maintain dormancy when
  conditions are favorable

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Seed Dormancy

• Seed dormancy mechanism to protect seed from
  germinating during unfavorable conditions
• temperate environments
• accumulation of cold temperatures
• many need to dry out prior to germination
• some seeds require exposure to
• cold (stratification)
• light
• fire
• mechanical/chemical abrasion
• break hard seed coat
• allows entry of air and water
• water
• leach away of inhibitors
• seed storage/banks

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Bud Dormancy

• Embryonic shoot in bud
• apical meristem, nodes, internodes, leaves,
  primordia
• Covered by bud scales (or not)
• protective
• dessication, heat loss, O2
• contain growth inhibitors
• Survive adverse conditions
• Do not elongate
• metabolically slowed
• Initiated in summer

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Bud Dormancy

• Summer
• physical/physiological changes prepare for winter
  (acclimation)
• induced by shorter days
• Cold hardiness ability of plant to survive
  winter weather
• Cold required to break dormancy
• Specific hours required
• forcing
• Cold required for many sp.
• Period of dryness or light
  required for other sp.
• Needs match environment

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Nastic Movements

• Nastic movements movements
  that occur in response to stimulus
• Nyctinastic movements night closure, up/down
  movement due to light cycles
• droop in night, spread in day
• due to turgor changes of parenchyma cells at leaf
  base
• Thigmonastic (seismonastic) movements closure
  due to mechanical stimulation (touch/shaking/elect
  ical/thermal)
• sensitive plant
• due to changes in turgor
  in pulvinus
  (thickening
  at leaf base)

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Thigmomorphogenesis

• Thigmomorphogenesis alteration of growth
  pattern due to mechanical stimulation
• greenhouse grown plants are tall, spindly
• rubbing/bending inhibits elongation, promotes
  thickening/shortening
• due to wind, rain, animals walking by
• poinsettas

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Solar Tracking

• Solar tracking (heliotropism) diurnal movement
  of lvs/flowers
• pulvini at leaf/flower bases
• diheliotropism diurnal movement of lvs/flowers
  keeping plant organs perpendicular to sun rays,
  maximize Ps
• paraheliotropism diurnal movement of
  lvs/flowers keeping plant organs parallel to sun
  rays, minimize sun rays

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