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Plant Responses to Stimuli

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					Plant Responses to Stimuli
         Plants versus Animals
► Plants as living organisms have the ability to:
► 1) use energy to obtain materials from the
  external environment, and use energy to
  rearrange those materials into new plant
  substances.
► 2) With very few exceptions, plants acquire all the
  matter and energy they need: without changing
  location (as must most animals, protists, and
  bacteria), without preying on other living
  organisms (as animals do), and without relying on
  matter assembled by other organisms (as fungi
  do).
    How are new individuals formed?
►   Some plants, such as the
    Bryophyllum grow new
    individuals from a leaf.
►   Some trees, such as sumac
    and poplar, grow new
    individuals, called suckers,
    from the roots.
►   Spider plants and
    strawberries grow stems
    from which new individuals
    can become established at
    some distance from the
    original plant.
►   Many grasses grow from
    nodes in their roots.
               Attainment of Nutrients
►   Most plants obtain their nutrients
    from the air and the soil.           ► P.Plant
                                         ► (Not
►   Some plant species however, are
    able to “prey”, or eat and gain      Newfoundland)
    nutrients from animals.
►   The pitcher plant (the provincial
    flower of Newfoundland and
    Labrador) and the Venus flytrap
    are consumers.
►   They trap and digest insects in
    order to obtain nutrients that are
    not available in the nutrient-poor
    soil they grow in.
►   These plants are also producers
    because they photosynthesize, as
    well.
Development in the Meristem
►   What gives plants their amazing
    ability to grow throughout their
    lives?
►   Although mitosis and cell division
    occurs throughout a plant as it
    grows, eventually new growth is
    restricted to small regions of
    unspecialized tissue collectively
    called the meristems
►   Growth there results from the
    accumulation of rapidly dividing
    cells.
►   When a cell in the meristem
    divides, one of the two resulting
    cells remains in the meristem.
    The other cell becomes part of
    the plant body.
►   Initially, all meristem cells are
    identical in structure, and they
    have no specialized function.
Development in the Meristem
►   As they divide repeatedly,
    they begin to differ in:
     Shape
     relative proportions of their
      various organelles
     functions they can perform.
►   Changes result in the cells
    becoming specialized for
    particular functions, such
    as photosynthesis,
    storage, and support.
       Types of Meristem Tissue
►   Two main types of meristem
    tissue.
►   1) apical meristem tissue:
    located in the root and shoot tips
    of plants. Division of apical
    meristem cells results in growth
    of roots, leaves, and flowers.
►   Protected by a root cap in the
    root.
►   Protected by a terminal bud in
    the stem.
►   In colder climates, the terminal
    buds stop growing in the winter
    and resume growing in the
    spring.
►   These buds are protected by bud
    scales, which fall off when
    growth begins in the spring.
       Types of Meristem Tissue
► 2)  Lateral meristem
  results in the growth of
  tissue beneath the bark
  of tree stems
► division of cells results
  in the thickening of
  cylinders of tissue.
► Most woody plants
  have two kinds of
  lateral meristems: a
  vascular cambium
  and a cork cambium.
                      Meristems
►   Meristem tissue enables
    plants to grow from
    cuttings.
►   Growing plants from
    cuttings is the basis of
    plant cloning: growing
    genetically identical
    copies of an organism
    from a single cell or part
    of an organism.
►   For some species,
    growing plants from
    cuttings can be much
    faster than growing them
    from seed.
         Internal Regulation of Plant
          Growth and Development
►   Plants can grow to their maximum height
    when environmental conditions are
    optimal.                                  ► Fertilizer
►   Optimal conditions include adequate
    moisture, warmth, light, and
    nutrients.
►   Fertilizers promote plant growth and
    development by providing additional
    nutrition.
►   Pesticides and fungicides promote                         Alice…R.I.P
    plant growth by controlling numbers of
    insects and fungi that feed on plants.
►   Plant growth and development are also
    controlled by the plant’s own hormones.
►   Hormone: a chemical compound
    manufactured by specialized tissue in one
    body part of an organism but that
    governs or regulates the activity of     ►Pesticide
    another body part or parts.
►   Even though a hormone may circulate
    throughout an organism, it will act only
    on specific “target” tissues or
    organs.
            Hormonal Control of Plant
                Growth: Auxin
►   In the early 1800s, experiments undertaken by
    Charles Darwin and his son Francis described
    the effects of a mysterious “influence” that
    affected the growth of grass seedlings.
►   The seedlings normally grew toward a light
    source, however this behaviour was not seen if
    the tips of the grass seedlings were covered
    with an opaque capsule that did not let light
    through.
►   The remainder of the plant, where the growth
    actually occurs, was still exposed to light.
►   If the tip of the seedling were covered with a
    gelatin capsule, which allowed light to pass
    through, then the seedling would grow towards
    the light as expected.
►   For many years, researchers conducted
    experiments to try to explain the nature of this http://www.tutorvista.com/content/biolo
    observation.                                     gy/biology-iv/plant-growth-
►   In 1926, Frits Went performed a series of        movements/growth-regulators.php
    experiments that showed there was a chemical
    messenger in the grass seedlings. This
    chemical could enhance plant growth. He
    named the substance auxin, from the Greek
    work auxein, which means “to increase”.
   Hormonal Control of Plant
Growth PROMOTOR HORMONES
► Other discoveries about plant hormones came as a result
  of people noticing unusual growth in plants.
► For example, observers noticed that a rice plant infected
  with the fungus Gibberella fujikoroi grew abnormally tall.
  In 1935, researchers were finally able to isolate the
  chemical compound that caused the accelerated growth,
  and named it gibberellic acid. They discovered that
  applying artificially-manufactured gibberellic acid to a plant
  not infected by the fungus caused the plant to grow
  abnormally tall.
► Scientists and researchers continued to search for other
  plant hormones that might affect growth and development.
► Two types of plant growth hormones: promoter
  hormones, which are hormones that cause growth, and
  inhibitor hormones, which are hormones that block
  growth.
The Classes of
Hormones
►   Auxins are a class of hormone that
    is produced in the apical meristem
    of shoots.
►   There are both natural and
    synthetic auxins that promote
    cell elongation, the development of
    vascular tissue, and trigger the
    development of above-ground
    stems, which help support the
    plant.
►   These hormones also cause leaves
    to drop after they are no longer
    needed.
►   The stimulation of cell elongation
    occurs because auxin increases the
    plasticity of the plant cell wall.
►   The more plastic the cell wall is, the
    more it can stretch during active
    cell growth.
The Classes of
Hormones
►   Gibberellins are also produced in the apical meristem and act
    to increase stem length.
►   Increase the uptake of starch in the embryo of germinating
    seeds and stimulate the development of vascular tissue.
►   The effects of gibberellins include taller and stronger plants,
    plants that flower early, or genetically dwarf plants that grow to
    normal heights.
►   Used in commercial crops to increase fruit size, and cluster size
    in grapes.
►   They can delay the ripening of citrus fruits and speed up the
    flowering of strawberries.
     The Classes of Hormones
►   Cytokinins promote cell division and cell differentiation.
►   Cell differentiation occurs when specialized cells are
    needed to perform certain functions.
►   They also delay the aging of leaves and fruit.
►   Work by influencing the synthesis and activation of
    proteins that are required for mitosis.
►   Oligosaccharins are a recently discovered class of
    growth promoters. They stimulate plants to manufacture
    antibiotics in response to attack by fungi or bacteria.
►   This allows the plant to grow to its full potential because
    the negative influences of pests are diminished.
     The Classes of Hormones:
             Inhibitors
► There  are two classes of hormone growth
  inhibitors.
► 1) abscisic acid (ABA): synthesized mainly in
  mature green leaves, fruits, and root caps.
► Inhibits the germination of seeds, inhibits the
  growth of buds in plant stems, and blocks the
  intake of carbon dioxide by controlling the opening
  and closing of leaf stomata.
► Abscisic acid also blocks the action of growth
  promoting hormones.
      The Classes of Hormones:
              Inhibitors
► 2) Ethylene: a gaseous hydrocarbon. It occurs as a
  natural plant hormone.
► Stimulates the aging of plant tissues, the ripening and
  sweetening of fruit, and can also speed up the dropping of
  leaves from trees.
► The production of ethylene gas by plants can stimulate
  other plants to ripen.
► Initially noted when bananas ripened quickly if they were
  left near oranges.
► The ripening action of ethylene has led to its use in
  agriculture. For example, tomatoes may be picked while
  green and then ripened artificially by the application of
  ethylene.
                        Plant Tropisms
►   Plants exhibit the ability to orient
    themselves in response to external
    stimuli such as light.
►   A directional growth response to
    unequal stimulation from the external
    environment is called a tropism, and
    it controls the growth pattern of the
    plant.
►   Various external stimuli affect the
    production of plant hormones - results
    in the directional growth of a plant.
►   In tropism, the plant may grow either
    toward or away from the stimulus.
►   Growth toward the stimulus is a
    positive tropism. Growth away from
    the stimulus is a negative tropism.
    There are three major kinds of
    plant tropisms that are affected by
    light, gravity, and touch.
                        Plant Tropisms
►   1) Phototropism occurs when the
    growth of a plant is affected by light.
►   In general, plants are positively
    phototropic, that is, they grow toward
    light.
►   Roots are negatively phototropic and
    grow away from light.
►   The growth is caused by differing
    amounts of auxin produced on the
    light and dark sides of the stem.
►   Auxin accumulates on the shaded side
    of the stem, which causes the cells
    there to elongate.
►   This causes the stem bend toward the
    light.
►   Turning the plant around will cause the
    stem to bend in the other direction,
    however, it will not change the original
    curve in the stem because it is the
    result of growth.
                Plant Tropisms
►   2) Gravitropism is a plant’s response to gravity.
►   Causes roots to grow downwards (positive gravitropism)
    and shoots and stems to grow upward (negative
    gravitropism).
►   Benefits the plant, because shoots that grow upward will
    receive light and roots that grow downward will receive
    nutrients from the soil.
►   3) Thigmotropism is the response of plants to touch.
    This behaviour is a caused by specialized cells in the
    epidermis of the plant. Vining plants demonstrate a strong
    positive thigmatropism
►   The vines grow toward the object touching them causing
    them to coil around the object. Other plants demonstrate a
    negative thigmotropism.
        Nastic Responses in Plants
►   Another type of response, called nastic
    movements, are caused by a stimulus that
    is not directional.
►   For example, the leaves on a mimosa plant
    fold up when the plant is touched
►   This response might seem to illustrate a
    negative thigmotropism, however, it is
    neither directional nor permanent.
►   The leaflets fold downward in the same way
    regardless of the direction of the stimulus.
►   These movements are not a result of
    growth, but rather a change in turgor
    pressure in the cells at the base of each
    leaflet.
►   A sudden drop in pressure causes the cells
    to become limp and the leaflets fold down.
►   Once the stimulus has ceased, the turgor
    pressure in the cells rises once again and
    the leaflets open.
►   Another example of a nastic response is the
    hinged leaf of a Venus’s flytrap.
►   The movement of an insect on the leaf
    triggers the hinged leaf to close, trapping
    the insect between the leaves.
                     Commercial Use of
                     Growth Regulators
►   Over the past century, scientists have
    learned much about plant growth
    hormones.
►   Horticulturists and other agricultural
    scientists use this knowledge of plant
    growth regulators to influence the
    growth and development of crops and
    ornamental plants.
►   Most growth regulating hormones used
    for commercial purposes are
    synthetically produced rather than
    extracted from plants.
►   For example, there appears to be only
    one naturally occuring auxin, but many
    more synthetic auxin-like growth
    regulators exist.
►   Although these synthetically produced
    hormones are not identical to natural
    auxins, their chemical action is similar,
    and the plants respond as they would
    to naturally occurring auxin.
                   Commercial Use of
                   Growth Regulators
►   A large industry is based on the
    manufacture of artificial plant
    growth regulating hormones.
►   Some of the growth regulating
    hormones that are produced can
    be very specialized
►   For example, a chemical treatment
    can be applied to ornamental trees
    to prevent them from growing too
    tall and interfering with utility
    lines. From the ground, the trees
    look normal, but the tops look as if
    they have been pruned flat.
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