REGULATION OF PLANT GROWTH

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REGULATION OF PLANT GROWTH
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Plant growth is influenced by a number of
external and internal factors.

• The external factors affecting plant growth are
• ? light
• ? temperature
• ? humidity
• ? oxygen
• ? carbon dioxide
• ? soil water and soil nutrients
• ? pressure (altitude)
• ? gravity

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• Plant Hormones

• Most plant hormones are made in minute quantities
  by actively dividing tissues at the tips of roots
  and stems. Once produced, they are transported to
  various parts of the plant.
• The most important plant hormones are
• Auxins
• Giberrelins
• Cytokinins
• Ethylene

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AUXINS

• Hormones that affect the plant growth are called
  auxins. They may stimulate or slow growth,
  depending on the type of the tissue and the
  amount of hormone.
• Auxins are synthesized mainly in shoot meristem.
• These hormones increase plant growth by
  stimulating cells to lengthen.
• In addition they cause cell to differentiate.
• Auxins also affect the process of abscission
  the dropping off of leaves, flowers, or fruits
  from a plant.

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AUXINS
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GIBBERELLINS

• Hormones that affect the plant growth and
  development of fruits and seeds are called
  gibberellins.
• Unlike auxins, they are distributed evenly
  throughout the plants tissues.
• They have important effects on stem growth.
• Commercially they are used to stimulate flowering
  and to increase fruit size.

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CYTOKININS

• Cytokinins stimulate cell division and growth
  during seed germination.
• They are thought to work together with auxins in
  stimulating cell differentiation.

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ETHYLENE

• Ethylene, is a gas that plays role, along with
  auxins, in abscission.
• It also stimulates the ripening of many fruits
  and controls aging of the plant.

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AUXINS and TROPISM

• The growth of a plant in a specific direction in
  response to a stimulus is called a tropism.
• Plant growth or movement toward a stimulus is
  called positive tropism, while movement away from
  a stimulus is called negative tropism.

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AUXINS and TROPISM
The stem of a plant that is growing toward the
light is an example of positive phototropism.
Roots, show negative phototropism.
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Phototropism
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AUXINS and TROPISM
Roots generally show positive geotropism- they
grown down into the ground in the direction of
the force of gravity. Stems show negative
geotropism.
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Geotropism
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Corn geotropism
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Tropism, negative response to gravity Onion
(Allium cepa)
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AUXINS and TROPISM
When the tendrils of a grapevine wind themselves
around the stem of another plant, they are
showing thigmotropism- growth in response to
touch.
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AUXINS and TROPISM
Hydrotropism is observed in plants whose roots
grow toward water.
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AUXINS and TROPISM
The growth responses seen in tropism are thought
to be caused by uneven distribution of auxins in
the affected plant parts.
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In the positive photoperiodism of stems, for
example, the concentration of auxins becomes
higher on the shaded side of the stem than on the
lighted side. Thus, the cells on the shaded side
grow faster than the cells on the lighted side.
The uneven rates of growth on opposite sides of
the stem result in bending toward the side less
rapid growth. In this case the stem bends toward
the light.
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NASTIC MOVEMENTS

• A plant movement that is in response to a
  stimulus but independent of the direction of the
  stimulus is called nastic movement.
• Most nastic movements involve changes in the
  internal pressure or turgor pressure of specific
  cells.

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Rapid movements such as closing leaves involve
changes in the turgor pressure in cells
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Venus flytrap
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PHOTOPERIODISM

• The flowering of many plants is in response to
  changes in the length of day over the course of
  the year.
• The response of a plant to changes in the length
  of day or night is called photoperiodism.
• (Photoperiodism is the non-directional
  developmental responses to non-directional but
  periodic light stimuli.)
• In many types of plants, flowering and other
  processes, such as leaf abscission, are
  controlled photoperiodically.

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Long-day plants
Short-day plants
? Plants that flower during short days were
called (or short-night plants). ? They flower
when there are short periods of darkness. ?
These plants usually bloom in the summer. ? ex
clover, potato, beet, poppy and gladiolus.

• ?Plants that flower during short days (or
  long-night plants).
• They require long periods of darkness in order to
  flower.
• These plants flower in the early spring.
• ex Morning glory, tulip, chrysanthemum and aster

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Short-day plants
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Long-day plants
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Day-neutral plants

• ?Plants whose flowering is unaffected by the
  lengths of light and dark.
• These plants have long flowering season.
• ex tomato, cucumber, dandelion, strawberry,
  string bean and corn.