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Plant Responses Chpt. 33. 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.

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Plant Responses Chpt. 33

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Plant Responses

Chpt. 33

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

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

- production of enzymes

  • 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.

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)

Meristematic Region - Root tips & Shoot tips

  • Responses in Flowering Plants

  • Tropisms

  • A tropismis 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

Responses in Flowering Plants


Phototropism:is the change in growth of a plant in response to light, usually from one direction.

- stems positively phototropic

- many roots negatively phototropic

Responses in Flowering Plants


Geotropism (gravitropism):is the change in the growth of a plant in response to gravity.

- stems negatively geotropic

- roots positively geotropic

Responses in Flowering Plants


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.

Responses in Flowering Plants


Hydrotropism:is a change in growth of a plant in response to water.

Responses in Flowering Plants


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.

  • 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).

  • 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.

New roots developing

Growth Regulators

Growth Promoters:some regulators promote growth e.g.

auxins, gibberellins, cytokinins.

Growth shut down for winter

Growth Regulators

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

  • 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.

  • 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

  • Auxin as an example of a growth regulator:

  • Effects of auxins:

    • 1) Tropism: auxins cause cell elongation and growth or

    • bending e.g. Phototropism.

  • 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.

Apical Dominance

  • 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.

  • 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.

  • 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

Role of auxin (IAA) in phototropism:

Quicker growth here due to more hormones

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

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, flowersand fruits and the ageing of plants.

  • Ethene stimulates the production of more ethene.

  • 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.

  • 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

  • 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.

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

Plant Adaptations for Protection

Anatomical (structural) adaptations:

Bark/epidermis prevents entry of microbes and reduces loss of water

Thorns on the epidermisprevent 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.

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


Heat shock proteins: excessive heat may cause plant enzymes to lose shape and become denatured. Heat shock proteins are producedto protect enzymes when temperatures are high i.e. above 40oC

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