Plant Growth and Development

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Plant Growth and Development
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Types of Growth

• Apical meristem plant tissue made of actively
  dividing cells. Primary growth and located at
  the tip of roots and shoots.
• All growth at the apical meristem is called
  primary growth which increases the height, not
  the width.
• Secondary growth arises from lateral meristems
  and that increases the width of a plant.

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Shoot Primary Growth

• Begin with mitosis and once cell division occurs,
  each cell grows longer.
• Then elongated cells specialize (differentiate)
  into different cell types parenchyma, epidermal,
  vascular.
• Tissues form stems, leaves, and organs for sexual
  reproduction.

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Root Primary Growth

• Root apical meristems produce the cells of the
  root cap and all other cell types in the root.
• There are several zones in the root
• Root cap protects the meristem as it pushes
  through the soil.
• Zone of elongation most cells stop dividing but
  increase in length. Phloem matures and xylem
  starts to form.
• Zone of cell division cells formed by meristem
  continue to divide.
• Zone of maturation (differentiation) tissues of
  the root begin to take on specialized roles.
  Root hairs begin to form.

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Secondary Growth

• Happens in woody plants after the first season.
• Wood is a product of secondary growth from
  lateral meristem
• The vascular cambium is an example of a lateral
  meristem as it gives rise to secondary phloem and
  secondary xylem
• Cork cambium is another example as it makes the
  cells that form the bark

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Tropisms and Growth Regulators

• Tropisms are growth responses to external
  stimuli.
• Positive tropisms are growths towards the
  stimulus
• Negative tropisms are growths away from the
  stimulus
• Phototropism stems show a positive tropism
  response to sunlight, where roots show a negative
  tropism response.
• Gravitropism stems show negative to gravity,
  where roots show positive.
• Thigmotropism is a change in the direction of
  growth in response to contact. Climbing vines
  are an example.

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Hormones

• There are several groups of plant hormones
  auxins, gibberellins, cytokinins, and ethylene.
• Auxins
• Plants like grasses have a sheath that covers
  young leaves as the seedling emerges from the
  ground. This sheath responds to sunlight by
  bending towards it.
• Auxin is released from the apical bud (tip) when
  exposed to light and travels away from the light
  and downwards, causing the cells to elongate.
• This results in an uneven elongation of cells,
  and the stem bends towards the light.
• Produced in the roots by the apical meristems.
• Helps the roots grow with gravity, and works the
  opposite to stem cells, high amounts of auxin,
  inhibit elongation.
• Help roots grow around rocks.

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• Some herbicides contain auxins that cause plants
  to undergo elongation at an unsustainably rapid
  rate and die.
• Synthetic auxins are used to induce cell
  elongation in fruits.
• If you cut off the apical bud, you can stunt the
  growth of the plant and possibly cause the plant
  to grow more flowers, fruit or leaves.

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• Gibberellins
• Produced in apical meristems AND in leaf
  chloroplasts
• Work with auxins to elongate stems and roots
• Promote leaf growth and flowering
• Seed germination
• Controls fruit development
• We spray fruit with gibberellins, to get larger
  fruit.

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Cytokinins

• Promote cell division
• Found in tissues that are actively dividing such
  as meristems, young leaves, seeds.
• Can also slow cell aging and used on lettuce and
  mushrooms to keep them from going bad.

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Ethylene

• Plant growth regulator
• Induces changes that protect a plant against
  environmental stress (leaf drop in the fall and
  in drought conditions)
• Released when a plant has a wound
• Stimulates developmental stages like fruit
  ripening, shoot and root growth, flower opening.
• Because they are involved in fruit ripening,
  farmers and grocers try to control the amount of
  ethylene produced by adding more when they want
  them to be ripe and using filters to remove the
  ethylene when they want to keep them from
  ripening too quickly.