1. Responses in Plants

2. Stimuli Plants respond to:
Phototropism
Geotropism
Hydrotropism
Chemotropism
Thigmotropism
Light
Gravity
Water
Chemicals
Touch

3. Tropisms
A Tropism is a change in the growth of a plant in response to an eternal stimulus
Negative tropism
The growth is away from the stimulus
Positive tropism
The growth is towards the stimulus

4. Phototropism

5. Geotropism

6. Responses in Plants Plants respond by
Changes in their pattern of growth
Flowering
Production of chemicals

7. External Factors Regulating Growth
Light Intensity
Day Length
Gravity
Temperature

8. Internal Factors Regulating Growth
A series of chemicals called growth regulators
These chemicals are produced in meristematic regions
Eg. Root tip and shoot tip

9. Growth Regulators Plant responses are due to growth regulators
A growth regulator is a chemical that controls the growth of a plants
Regulator that promotes growth
Auxin = stimulates cell elongation
Regulator that inhibits growth
Ethene = inhibits cell elongation

10. Growth Regulators They are active in very small amounts
They are produced in the meristems
They are transported in the xylem and phloem
Their effects are dependent on concentration
A small amount of growth regulator can have great effect on growth

11. AUXINS production initiator production site(s), function,
Objectives – What you will need to know from this section
Study auxin as an example of a plant growth regulator under the headings of :
production
initiator
production site(s),
function,
different effects.

12. AUXINS influence includes
AUXINS -- HIGHER LEVEL
Auxins affect virtually every aspect of plant development, and how they respond to environmental stimuli.
AUXINS influence includes
phototropism,
geotropism,
cell enlargement and growth,
apical dominance,
root growth,
fruit development,
vascular development,
and senescence [aging].

13. AUXINS
Promote cell enlargement and growth
Are involved in phototropism—stem bends towards light.
Are responsible for apical dominance, where the main bud inhibits the growth of buds lower down stem.
Regulate the differentiation of the vascular tissue (xylem and phloem).
Indole-3-acetic acid (IAA) is the most abundant naturally-occurring auxin,

14. IAA (Indoleacetic acid) is an auxin that is made in the meristems of shoots, buds and root, and in the tips of coleoptiles [of grasses].

15. Auxin travels backwards from the tip in the vascular tissue and causes cells to elongate (get longer).
This makes the zone of elongation grow and so the tip is pushed further upwards (in shoots) and downwards (in roots).
Movement is by active transport from cell to cell – this requires energy.

16. Auxins promote cell enlargement and growth,
are involved in phototropism and apical dominance
APICAL DOMINANCE
PHOTOTROPISM

17. A tropism is a plant’s response to a stimulus coming from one direction, e.g. sunlight, gravity.
Phototropism is a growth response of a stem towards light, so that it can receive the maximum amount of light for photosynthesis.

18. APICAL DOMINANCE
Auxins are responsible for apical dominance, where the main bud inhibits the growth of buds lower down stem.
This photograph shows side buds sprouting when the main stem is cut off [pruned].

19. Quicker growth here due to more hormones
In shoots, light causes auxin to move down the shaded part of the stem, causing the shoot to grow towards the sunlight. This is called PHOTOTROPISM.
Quicker growth here due to more hormones
Artificial auxins can also be used to kill weeds, stimulate root formation in cuttings, …

22. Uses of plant regulators
Artificial auxins are used in rooting powders to stimulate root formation in stem cuttings.
A synthetic auxin [2,4-D] is used as a selective weedkiller, making the plants grow too fast.
Fruit is transported green and unripe, and can then be quickly ripened by spraying it with ethene.

23. Growth Inhibitors Ethene is a gas. It is produced in: Stem nodes
Ripe fruits
Decaying leaves
It causes:
Fruits to ripen
Leaves to fall in autumn
Plants to age

24. Commercial Growth Regulators
Ripen bananas
Bananas are picked green – ethene is used to stimulate ripening just before distribution
Rooting Powders
Contain synthetic growth regulator NAA. It stimulates rapid root formation on stem cuttings

25. Adaptations for Protection in Plants
Plants need to protect themselves from their environment
They protect themselves against:
Loss of water
Overheating
Infection from micro organisms
Being eaten by herbivores

26. Plant protection
Plants can adapt themselves for protection in two ways:
Structural of anatomical adaptations
Chemical adaptations

27. Structural Adaptations
Bark/epidermis prevents entry of microbes
and reduces loss of water
Thick cuticle on stem or leaves in plants that live in dry places
Thorns prevent plants from being eaten by herbivores
Stinging cell in epidermis prevent plants been eaten
Guard cells change shape when they lose water which causes stomata to close and this reduces water loss

28. Chemical Adaptations
Production of Tannins to make the plant indigestible
Production of toxic chemicals to prevent growth of insect larvae
Production of strychnine and nicotine in legumes that damage nerves and muscles
Production of heat shock proteins to protect enzymes when temperatures are high

29. Most important adaptation
Plants can grow and regenerate throughout their life.
They can produce new roots, stems, leaves.