RipeningSenescence AbscissionDormancy

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Ripening/Senescence/Abscission/Dormancy
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Objectives

• Understand the process of fruit ripening in
  fleshy fruits
• Learn about ethylene gas and its role in the
  process
• Understand what senescence and abscission are,
  and the parallels to ripening
• Introduce dormancy

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Ripening Of Fleshy Fruits

• Last part of the fruit development process

Ripening Period
Cell Expansion Period
Fruit Fresh Weight
Maturity (Full sized fruit)
Cell Division Period
Pollination/ Fertilization
Time
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Ecological Function of Fruit Ripening

• To aid in seed dispersal, by making the seed
  container attractive to animals that will spread
  the seeds
• Ripening must happen when the seeds are mature
• In order for ripening and seed maturity to happen
  together, the plant must control the process

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So What Is Ripening?

• Increase in sweetness due to conversion of
  starch sugars
• Softening due to a breakdown of cell walls
• Less tartness due to decrease in acidity
• Increase in flavor compounds
• Color change
• Changes in respiration due to all

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Starch to Sugar Conversion
Banana
Sugar

Starch
Days
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Softening of Fruit
Soluble pectin
content of soluble pectin
Fruit Firmness, kg pressure
Firmness
Days in storage
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What is Control Mechanism?

• That insures
• ripening and seed maturity
• happen together

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Controlling Ripening

• The mechanism plants use is a gas
• Ethylene C2H4 produced in response to seed
  maturation (less auxin???)
• How does ethylene control ripening?
• Stimulates production of enzymes that
• Convert starch to sugar
• Break down cell walls
• Break down acids

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Ripening Of Fleshy Fruits
Cell Expansion Period
Ripening Period
Ethylene
Increasing Fruit Fresh Weight, Respiration Level,
Gas Production Etc.
Respiration
Cell Division Period
Flavor components
Pollination/ Fertilization
Optimum eating stage
Optimum storage stage
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Ripening Stimulated by C2H4

• Rise in respiration cellular activity is called
  the
• Note Not all fruits are climacteric fruits!

CLIMACTERIC !
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Climacteric goes on . . .

• Once the climacteric is started by a rise in
  ethylene production, it promotes itself and
  becomes
• That is, it becomes self sustaining (feedback
  promotion).

Autocatalytic ! ! !
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Climacteric goes on . . .
Signal from seed
Ethylene stimulates Enzyme production
Fruit Softens, Sweetens, Etc.
Ethylene is produced
Autocatalysis
Ethylene itself stimulates more ethylene
production
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Are all fruits picked ripe?

• Yes, many are picked ripe . . .
• Examples blueberries, strawberries, grapes,
  local production of many kinds

• No . . .
• Not when they are hort. vegetables!
• Examples green beans, peas, summer squash

• No . . .
• Not when they are to be shipped a long distance
• Examples bananas, peaches

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A Horticultural Manipulation

• What does the autocatalytic climacteric mean for
  fruit handling?
• It means that some fruits that are picked unripe
  and shipped long distances can be gassed with
  C2H4 and caused to ripen, literally by
  artificially beginning the climacteric.

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Example

• Bananas
• but the ripening often doesnt include as much
  sweetness increase or flavor increase as might
  happen on the plant (vine-ripened or
  tree-ripened)

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Other Ethylene roles!

• Fruit ripening is a terminal or End Point
  process
• Other, related Terminal or End Point processes
• Senescence
• Abscission

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Senescence

• Death (degradation) of a tissue or organ
• Senescence occurs once the function of the tissue
  or organ is done
• Example flower petals, once pollination and
  fertilization is accomplished, there is no need
  to further attract pollinator insects

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Senescence

• Senescence is stimulated by ethylene!
• Senescence is an active growth process, in which
  some of the cellular contents (minerals and
  simple molecules) are actively relocated via
  phloem to other parts of the plant for re-use

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A Horticultural Manipulation

• Horticulturally, in cut flowers, it is desirable
  to retard the falling off of petals, resulting in
  what is called longer vase life.
• Silver thiosulfate (Ag ions) is used to
  chemically block ethylene production.
• Biotechnology may produce flowers without C2H4
  production (last forever)!

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Abscission

• Separation of a plant organ or tissue from the
  plant
• Abscission may occur after senescence, at
  ripeness (fruit), or in response to an
  environmental trigger or stress
• Example deciduous tree leaves, in response to
  lengthening nights in late summer/fall (of
  course, phytochrome is involved too!)

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Abscission Layer
Vascular system
Bud
Leaf petiole
Stem
Layer of cells that actively degrades
between-cell layers, leading to tissue separation
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Abscission

• This too, involves ethylene as an initiator and
  promoter
• Abscission is an active growth process, in which
  the abscission layer cells develop in such a way
  (softening of the layers that hold cells
  together) which leads to separation!

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A Horticultural Manipulation

• Horticulturally, in cherry production, trees with
  nearly ripe fruit are sprayed with Ethephon, a
  material that produces ethylene, to stimulate
  abscission of the fruit.
• Harvest is then done by shaking the tree and,
  like magic, the cherries fall off!

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Cherry Tree Shaker
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Cherry Tree Shaker
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Dormancy (another end point)

• Plant development is halted, but metabolism (or
  life processes) continue
• Dormancy often involves other plant hormones, but
  not ethylene
• Examples
• Seeds
• Buds

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Seed Dormancy

• Physical
• Hard seed coat, so air/water cant get in, even
  if temperature is correct
• Physiological
• Hormones present in the seed prevent the seed
  (embryo) from beginning development

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Bud Dormancy

• Physical
• Environmental factors prevent stem development
  (especially temperature)
• Physiological
• Internal plant hormones prevent bud from
  beginning development

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Summary

• Many changes occur during fleshy fruit ripening
• In some fruits ripening becomes autocatalytic,
  called the climacteric
• Ripening, senescence, and abscission, all involve
  ethylene gas in initiation and regulation of the
  process