Plant responses to the Environment

1
Life Processes in Plants

• Plant responses to the Environment

2
Plant hormones

• ? General functions of the following plant
  hormones
• Auxins
• Gibberellins
• Abscisic acid
• Investigate geotropism and phototropism by
  controlling variables
• ? Role of auxins in geotropism through the
  regulation
• of differential growth
• ? Role of auxins in phototropism through the
  regulation
• of differential growth
• ? Uses of plant hormones in agriculture
• Auxins as herbicides
• Auxins to control apical dominance
• Auxins and gibberellins to increase/stimulate
  fruit development

3
Plant defense mechanisms

• ? The role of the following as plant defense
  mechanisms
• Chemicals (protection) against infection from
  pathogenic micro-organisms and from being eaten
  by herbivores)
• Thorns (protection from being eaten by animals/to
  reduce water loss

4
Plant hormones

• Performs a specific function, which controls
  further growth and development of the plant.
• Stimulation a process is turned on or
  activated.
• Inhibition a process is turned off or stopped.

5
Auxins

• Group of plant hormones that are mainly produced
  in the apical meristem of the stems and roots.
• Indole acetic acid is the most common natural
  auxin.
• Auxins move down in the phloem to cells lower
  down in the plant.
• Small amounts of auxins are produced in the
  growing points of roots.

6
Functions of auxins

• Stimulate cell division.
• Stimulate cell elongation.
• Promotes apical dominance (agriculture).
• Develops adventitious roots.
• Cause tropic movement in stems and roots.
• Stimulates the development of fruit.

7
Functions of gibberellins

• Stimulate stem elongation.
• Stimulate root growth.
• Promote flowering.
• Promote the growth of lateral buds.
• Stimulate the germination of seeds.

8
Functions of abscisic acid

• Inhibits cell division and growth in stems and
  roots.
• Prevents bud and seed growth in winter.
• Promotes the abscission of leaves and frui.t
• Controls the stomata opening and closing.
• Speeds up fruit ripening.

9
Role of auxins in phototropism

• Phototropism is the growth movement of a plant or
  part of a plant in response to unilateral light
  stimulus.
• A high auxin concentration builds up on the side
  of the stem furthest of the light source.
• Cell division and elongation are stimulated and
  grow faster.
• The stem curves in the direction of the light and
  positively phototropic.
• Negative phototropism occurs in roots grow
  away from a light source.

10
Role of auxins in geotropism

• Young seedling is placed horizontally roots
  grow downwards and stem upwards.
• Auxins accumulate on the lower side of the root.
• Higher auxin concentration on the lower side of
  the root inhibits cell division and elongation.
• Cells on the upper side of the root grow faster
  lower auxin concentration.
• Root curves downwards positively geotropism.
• Stem curves upwards negatively geotropism.

11
Uses of plant hormones in agriculture

• Root formation in cuttings.
• Production of seedless fruit.
• Ripening of fruit.
• Increase in fruit size.
• Stimulation of germination of seeds.
• Weed control.
• Apical dominance.

12
Auxins as herbicides

• Synthetic herbicides (weed killers) contain high
  concentrations of auxins.
• Accelerate the metabolism of broad-leaved
  dicotyledonous weeds and stimulate growth.
• Weeds grow so fast that water absorption is
  insufficient.
• Plants weaken and die.

13
Apical dominance

• Apical dominance occurs when the apical bud of
  the main shoot grows more strongly than lateral
  bud below.
• Removing the apical bud from the top of the
  plant, auxin is also removed.
• More lateral grow below the apical bud.
• Pruning promotes the grow of lateral branches.
• The value of pruning plant grows bushier and
  fruit formation can be controlled.

14
Plant defense mechanisms- thorns

• Sharp thorns protect the plants against
  herbivores.
• Less damage to plants by herbivores and plants
  continue to grow.
• Reduce water loss.

15
Plant defense mechanisms- chemicals

• Many plants produce chemicals and play a role in
  the defence against herbivores and
  pathogenic-organisms.
• Chemicals are poisonous or influence the
  digestibility of plant material.