1. Plant Responses
Chpt. 33

2. Stimulus and Response
Stimulus: is anything that causes a reaction in an organism, or in any of its parts.
Animal stimuli: - hearing a loud noise
- seeing a pleasant sight
- feeling pain
Plant stimuli: - light
- gravity
- temperature

3. Stimulus and Response
Response: is the activity of a cell or organism as a result of a stimulus.
Animal responses: - movement
- feeding
- production of enzymes
Plant responses: - growth
- flowering

4. Stimulus and Response Structures required for response:
chemical or hormonal system
nerve and sense organ system (animals only)
method of movement, including growth and
muscular and skeletal systems (animals only)
defence or immune system
Note: plants do not have a nervous system instead they
depend on chemical coordination for their
responses.

5. Responses in Flowering Plants
Growth Regulation
External Factors
Internal Factors
External factors that regulate the growth of plants are light intensity, day length, gravity, temperature.
Internal factors involve a number of chemicals that plants produce themselves called growth regulators.
(Note: growth regulators produced in meristematic region of plants)

6. Meristematic Region - Root tips & Shoot tips

7. Responses in Flowering Plants
Tropisms
A tropism is the change in the growth of a plant in response to an external stimulus e.g. sunlight, gravity
Positive tropism – occurs when growth is towards the stimulus
Negative tropism – occurs when growth is away from the stimulus

8. Responses in Flowering Plants
Tropism
Phototropism: is the change in growth of a plant in response to light, usually from one direction.
- stems positively phototropic
- many roots negatively phototropic

9. Responses in Flowering Plants
Tropism
Geotropism (gravitropism): is the change in the growth of a plant in response to gravity.
- stems negatively geotropic
- roots positively geotropic

10. Responses in Flowering Plants
Tropism
Thigmotropism: is the change in growth of a plant in response to touch e.g. vines (tendrils) wrap around objects which help support the plant.

11. Responses in Flowering Plants
Tropism
Hydrotropism: is a change in growth of a plant in response to water.

12. Responses in Flowering Plants
Tropism
Chemotropism: is a change in growth of a plant in response to chemicals.
- roots grow towards minerals e.g. nitrogen,
phosphorous in the soil - positive chemotropism.
- most roots do not grow towards acids or heavy metals
in the soil – negative chemotropism.

13. Growth Regulators
Growth Regulators (hormones): are chemicals that control the growth of a plant.
produced in the meristems.
most are transported in the vascular tissues (xylem
and phloem).

14. Growth Regulators
For the following reasons the exact role of plant growth regulators are difficult to define:
They are active in very small amounts.
Their effects are dependent on their concentration
i.e. the same regulator can have opposite effects at
high or low concentration.
Their effects are dependent on the location in the
plant in which they are acting i.e. the same
concentration of plant regulator can have opposite
effects in the stem and root.
Different regulators interact in different ways.

15. Growth Regulators New roots developing
Growth Promoters: some regulators promote growth e.g.
auxins, gibberellins, cytokinins.
New roots developing

16. Growth shut down for winter
Growth Regulators
Growth Inhibitors: some regulators slow down or inhibit growth e.g. abscisic acid and ethylene (ethene).
Growth shut down for winter

17. Growth Promoters (Higher Level Only) Auxins:
IAA (indoleacetic acid) is the most important auxin.
IAA commonly known as auxin.
Auxin is produced in the meristematic tissue in shoot
tips and also in young leaves and in developing seeds.

18. Auxin as an example of a growth regulator:
Functions of auxin:
stimulates stem elongation
stimulates root growth
causes cells to form into different structures
develops fruit
inhibits side branching in stems
causes phototropism
causes geotropism

19. Auxin as an example of a growth regulator:
Effects of auxins:
1) Tropism: auxins cause cell elongation and growth or
bending e.g. Phototropism.

20. Auxin as an example of a growth regulator:
Effects of auxins:
2) Apical Dominance:
Auxins produced in apex (tip of stem) allow the
apex to grow but inhibit side branches e.g.
cacti – have very few side branches
conifers – inhibition decreases down the stem allowing lower braches to grow more strongly.
If the apex is removed side branches are allowed to
develop and the plant will develop as a low bushy
form.

21. Apical Dominance

22. Auxin as an example of a growth regulator:
Effects of auxins:
3) Fruit Formation:
IAA is made in developing seeds and stimulates food to form in the fruit around the developing seeds.
If artificially applied to flowers before pollination and fertilisation occur, IAA will cause the ovary to enlarge and form seedless fruit (parthenocarpic fruit) e.g. seedless grapes, oranges etc.
4) Root Growth:
- At low concentrations IAA causes roots to grow.
- IAA can be applied artificially to stimulate rooting.

23. The mechanism of a plant response to light
Phototropism
(Higher Level)
Auxin and cell elongation:
Auxin stimulates the activation of enzymes in the cell
wall which break the bonds between cellulose strands.
This loosens cell walls allowing them to expand.
Cell elongation is essential for normal growth and
tropisms.

24. Role of auxin (IAA) in phototropism:
IAA is produced in the growth tips of the stem.
If the stem is exposed to light from one side IAA will move down the shaded part of the stem.
The higher concentration of IAA in the shaded cells will cause them to elongate more than the cells on the bright side of the stem.
As a result of this uneven elongation the stem bends towards the light i.e. phototropism

25. Quicker growth here due to more hormones
Role of auxin (IAA) in phototropism:
Quicker growth here due to more hormones

26. Expt. 20: To investigate the effect of IAA growth regulator on plant tissue.

27. Growth Inhibitors Ethylene (ethene):
Only growth regulator that is a gas.
Made by plants in stem nodes, ripe fruits and decaying leaves.
It promotes ripening of fruit, the fall of leaves, flowers and fruits and the ageing of plants.
Ethene stimulates the production of more ethene.

28. Growth Inhibitors Abscisic Acid:
produced in leaves, stems and root caps.
known as stress regulator of plants as it causes
plants to respond to harmful conditions e.g.
- in dry conditions causes stomata to close.
- in winter causes production of bud scales.
- inhibits germination in seeds allowing them to remain dormant during the winter.

29. Commercially prepared Growth Regulators
*You must know two uses of commercially prepared plant growth regulators*
Plant growth regulators can be produced outside of plants by artificial or synthetic methods:
1. Rooting Powders: artificial auxins are used in rooting powders to stimulate root formation in stem cuttings e.g. NAA

30. 2. Tissue Culturing:
this process allows pieces of plant material to be
grown to form entire new plants.
if a piece of plant tissue is grown in high auxin
concentration it will develop into a mass of similar
cells called a callus.
3. Ethene:
Fruit is transported green and unripe, and can then be quickly ripened by spraying it with ethene e.g. Bananas.

31. Plant Adaptations for Protection
*You must know four methods of plant protection*
Plants can’t move and so must defend themselves from their environment.
They protect themselves against:
Loss of water
Overheating
Infection from micro organisms
Being eaten by herbivores
Plants can adapt themselves for protection in two ways:
Structural or anatomical adaptations
Chemical adaptations

32. Plant Adaptations for Protection
Anatomical (structural) adaptations:
Bark/epidermis prevents entry of microbes and reduces loss of water
Thorns on the epidermis prevent plants from being eaten by herbivores e.g. blackberry bushes
Stinging cell in epidermis prevent plants been eaten e.g. nettle leaves.
Guard cells change shape (shrivel) when they lose water which causes stomata to close and this reduces water loss.

33. Plant Adaptations for Protection
Chemical adaptations:
Stress Proteins: when infected by a micro-organism the plant can sometimes produce stress proteins e.g. phytoalexins. Stress proteins act in different ways e.g.
- damaging micro-organisms by attacking their cell walls.
- promoting the formation of specialised plant cell walls that stop the spread of the micro-organism.
- stimulating nearby plant cells to respond to the
micro-organism.
Heat shock proteins: excessive heat may cause plant enzymes to lose shape and become denatured. Heat shock proteins are produced to protect enzymes when temperatures are high i.e. above 40oC