Plant Growth

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Plant Growth
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Tropisms

• Plants grow in response to cues from their
  environment. These responses are known as
  tropisms.
• Geotropism
• Phototropism
• Thigmotropism

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Geotropism

• Response to the force of gravity
• Roots turn towards the force of gravity
• Stems turn away from the force of gravity

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Phototropism

• Response to light
• Plants grow towards light
• Response to daylight, respond to darkness
• FYI Darwin did some of the first experiments
  investigating this tropism

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Thigmotropism

• Response to touch
• Grow towards solid supports (ivy, beans, vines)
• Adaptation to get more exposure to light, escape
  crowding at ground level

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Plant hormones

• Hormone a substance produced in one part of an
  organism that affects activities in other parts
  of the organism (chemical messengers)
• There are four major plant hormones auxin,
  cytokinin, gibberellin, ethylene
• Three of the four are classes of compounds

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Auxin

• Stimulate cell growth and are produced in the
  apical meristem (rapidly growing region at the
  tip of a root or stem)
• This is the hormone that produces phototropism
• When light hits one side of a plant, auxin builds
  up on the opposite side causing cells on that
  side to grow more which bends the plant towards
  the light.
• In stems auxin stimulates growth to point the
  plant upwards
• In roots auxin inhibits growth causing roots to
  grow downward

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Cytokinin

• Affects the rate of cell growth and cell division
• Ratio between auxin concentration and cytokinin
  concentration determines growth
• Auxins inhibit growth in lateral branches but
  cytokinin stimulates lateral growth

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Gibberellin

• Stimulates cell division and elongation in stems
• Cells in seed produce gibberellin to initiate
  seed sprouting

10
Ethylene

• Stimulates ripening in fruit
• An auxin in fruit tissue stimulates the
  production of ethylene, which in turn stimulate
  ripening
• Commercial growers use ethylne to ripen fruit
  that is harvested before it is ripe

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Life Cycles

• Annuals grow from seed to maturity in one
  season
• Biennials flower and produce seeds in the
  second year
• Perennials live for more than two years

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Timing is everything for a plant

• Reproductive timing must be in sync with all the
  others in the species
• Plants must store food for the winter in
  temperate climates
• Plants must regulate water loss in anticipation
  of the dry season

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Plant Master Clock

• Phytochrome (a red pigment) senses day and night
• Abscisic acid is a hormone regulated by
  phyochrome
• As nights grow longer, chlorophyll production
  stops
• Nutrients are drawn from leaves to the body of
  the plant
• Abscisic acid causes the leaves to drop
• Tips of branches grow thick waxy covers (bud
  scales) over the tips to protect the meristem
  from the winter weather
• Plant adds salts into the sap to keep the water
  in the sap from freezing