Flowers Floral Induction

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Flowers / Floral Induction
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Objectives

• Review flower form function
• Consider the diversity of flowers floral
  structures
• Discuss how flowering in controlled in plants
• Consider how horticulturists manipulate flowering

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Functions of Flowers

• For the plant
• Reproduction
• Allows for maintenance of genetic diversity (
  adaptability to changes / evolution)
• For the horticulturist
• Ornamental value (pleasure, sales profit)
• Fruit and seed production (1st step)

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Pistil
Ovary
Stigma
Ideal Flower
Style
Stamen
Anther
Filament
Petal (coll. corolla)
Sepal (coll. calyx)
Receptacle
Peduncle
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Whorls of Floral Parts

• Pistil
• Anther
• Petals
• Sepals

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Functions of Floral Parts

• Peduncle/Receptacle
• Calyx (sepals)
• Corolla (petals)
• Stamens
• Pistil

• Support
• Protection in the bud
• Attract pollinators
• Produce distribute male gametes
• Produce contain ovules, screen male gametes,
  place for seed development

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Variations on the Ideal

• A flower lacking any one or more parts (sepals,
  petals, stamens, pistils)
• A flower lacking one sex or the other

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A flower with all parts

• COMPLETE

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A flower lacking some part

• INCOMPLETE

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A Flower with both sexes

• PERFECT
• The flower may or may not have sepals and/or
  petals, but it does have both sexes

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A flower lacking one sex

• IMPERFECT
• The flower may or may not have sepals and/or
  petals, but it has only one sex

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• All imperfect flowers are incomplete
• Think about it if a flower lacks one sex it
  must lack one part
• Not all incomplete flowers are imperfect!
• Think about it if a flower lacks one part it
  may still have both sexes present

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Plants with imperfect flowers

• May have both sexes on the same plant
• Monoecious
• (literally monoone, oikoshouse Gr.)
• One house, one plant
• Or only one sex on any single plant
• Dioecious
• (literally ditwo, oikoshouse Gr.)
• Two houses, two plants

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Monoecious (1 house)
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Dioecious (2 houses)
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Example species

• Monoecious
• Corn (maize)
• Birch
• Begonia
• Squash

• Dioecious
• Holly
• Bittersweet
• Honeylocust
• Ash
• Ginkgo

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Monoecious (1 house)
Squash
Sweet Corn
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Dioecious (2 houses)
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Inflorescences

• Structures that bear flowers
• May be
• determinate - elongation growth stops and then
  flowering occurs
• Indeterminate - elongation growth continues so
  flowers continue to develop

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Inflorescence Types
Raceme
Solitary
Umbel
Corymb
Panicle
Spike
Head
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Induction of Flowering

• Flowering requires the change of the meristem
  (growing point) of a stem from vegetative (stem
  and leaves) to floral (flower or flowers)
• This change does not occur by accident!
• It is triggered by internal or external cues

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Why control flowering?

• Allow all members of a species to flower at about
  the same time (synchrony)
• Time seed maturity before unfavorable
  environmental conditions occur (ie.winter)
• To insure the plant is large enough (vegetative
  phase) to support the development of flowers,
  fruits, and seeds (reproductive phase)

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Important Factors

• Photoperiod
• (literally photolight, periodduration during a
  24 hour period)
• Light Intensity
• Temperature
• Juvenility
• (sort of, but not exactly, how old a plant is)

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Photoperiodism

• Plants have evolved mechanisms to make use of the
  fluctuation in daylength in the temperate regions
  of earth as an environmental cue for flowering.
• Those plants that are not photoperiodic are
  termed day neutral

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Fluctuation in Daylength
16 hours
12 hr.
Hours of Light
9 hours
Dec
Jan
March
Sept
June
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Photoperiodism - Original View

• A photoperiodic plant will only flower when the
  length of the day is more or less than a critical
  length
• Short day (SD) plants flower when the day
  length is less than a critical length
• Long day (LD) plants flower when the day length
  is more than a critical length

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Photoperiodism - Actual Mechaism

• A photoperiodic plant will only flower when the
  length of the night (or dark period) is more or
  less than a critical length

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Photoperiodism - Actual Mechaism

• Short day (SD) plants flower when the night
  length is more than a critical length - thus,
  they are really long night plants
• Long day (LD) plants flower when the night
  length is less than a critical length - thus,
  they are really short night plants

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Photoperiodism - Actual Mechaism

• We know it is the length of the night or dark
  period that is important because you can use
  night interruption (a short period of light in
  the middle of the night) to change a short day
  (long night) into a long day (short night)
  (really two short nights).

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Photoperiod Effects
Short
Long
day plant
day plant
Light Period
Light Period
Light Period
Light Period
Light Period
Critical Length of Dark
Light Period
24 hour period
Dark Period
Dark Period
Light Flash
Dark Period
Dark Period
Dark Period
Dark Period
Dark Period
Dark Period
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How do plants sense the length of the dark
period?

• Phytochrome P
• Protein chromophore (changeable by light)
• Occurs in two forms
• PR red light form
• PFR far red light form
• Phytochrome is changed from one form to another
  by light or by degradation during darkness PR
  RFR

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Important Factors

• Photoperiod
• (literally photolight, periodduration during a
  24 hour period)
• Light Intensity
• Temperature
• Juvenility
• (sort of, but not exactly, how old a plant is)

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Light Intensity

• Flowering (reproduction) is an energy intensive
  process
• Light intensity affects energy availability
  because it affects carbohydrate production
  through photosynthesis
• Low lightlow carboslow energy accumulation
• Low lightno flowering

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Light Intensity

• Horticultural application
• Young fruit trees are sometimes girdled just
  above the soil (bark removed from a ring around
  the trunk) to interrupt the phloem
• This prevents carbohydrates from moving to the
  roots from the top of the tree, resulting in
  carbohydrate accumulation in the tree and more
  (or earlier) flowering!

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Trunk Girdling - Phloem interruption
CHO
CHO
CHO
Carbohydrates cannot move beyond girdle - they
collect in top of tree and provide energy for
flowering
CHO
CHO
Trunk girdle
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Important Factors

• Photoperiod
• (literally photolight, periodduration during a
  24 hour period)
• Light Intensity
• Temperature
• Juvenility
• (sort of, but not exactly, how old a plant is)

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Temperature - Low

• Some plants require exposure to cold temperatures
  (1-7ºC, 34-45ºF) to flower, generally several
  weeks duration
• Exposure to these cold temperatures called
  Vernalization
• Examples Easter lily, foxglove, apple

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Temperature - High

• Some plants are induced to flower through
  exposure to high temperatures
• Examples cool season vegetables
• spinach
• cabbage
• called bolting

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Important Factors

• Photoperiod
• (literally photolight, periodduration during a
  24 hour period)
• Light Intensity
• Temperature
• Juvenility
• (sort of, but not exactly, how old a plant is)

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Juvenility

• Two stages of plant development
• Juvenility - cannot flower even with appropriate
  environment
• Mature - capable of flowering with proper
  conditions

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Juvenility

• May last weeks or years
• Mechanism unknown, but involves
• Plant size
• Hormones
• Carbohydrates
• Other metabolic factors
• Examples oak, pine trees, English ivy

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Summary

• Flower form/function, terminology
• Control of flowering
• Environmental triggers
• Night length
• Temperature
• Light intensity
• Juvenility Maturity