Plant responses

Responses to herbivores and abiotic stress

Physical defences
Chemical defences

Example of physical defences

• Thrones

• Strings

• Spikes

• Barbs

• Fibrous inedible leaves

Example of chemical defences

Alkaloids
Pheromones
Folding in response to touch

What are alkaloids?

Nitrogenous, bitter tasking chemicals
They affect the metabolism of the herbivores - can result in death

What are pheromones?

Affect the behaviour of other members of that species through chemical signalling

Examples of abiotic stress

High winds
Excess water
Lack of water
Temperature changes

How to do plants respond to abiotic stress(5)

• Left loss

• Daylight sensitivity

• Abscission

• Preventing freezing

• Stomatal control

How does leaf loss respond to abiotic stress

Loses leaves in winters - as daylight hours decrease - rate of photosynthesis decrease
More energy efficient to lose leaves

How does daylight sensitivity respond to abiotic stress

Photoperiodism (plant being sensitive to lack of light)

How does abscission respond to abiotic stress

When light level decreases ethene switches of genes for enzymes that digest and weaken cell waters at abscission zone, causing leaf to leave the plant

How does preventing freezing respond to abiotic stress

Some plants contain chemicals which act as natural antifreeze to prevent the cytoplasm in cells from freezing

How does stomatal control respond to abiotic stress

Evaporation of water out of open stomata provides a cooling effect to a plant. The opening and closing of the stomata can be controlled by the hormone ABA in response to temperature stress

What is a tropism?

Term given when plants respond ,via growth, to stimuli

Tropisms can be

Negative - growing away from stimulus
Positive - growing towards a stimulus

Plant stimulus (3)

Light
Gravity
Water

How are tropisms controlled ?

By specific growth factors

Key tropism example

indoleacetic acid (IAA)

What is IAA

Type of auxin that controls cell elongation in shoots and inhibits growth of cells in the roots

Where is IAA made

In the tip of the roots and shoots but can diffuse to other cells

What type of phototropism occurs in the shoots?

Positive phototropism

Why does positive phototropism occur in the shoots?

Light is needed for LDR in photosynthesis, so plants grow & bend towards light

How does +ve phototropism occur in the shoots?

1. Shoot tips cells produce auxin, causing cells elongation

2. The IAA diffuses to other cells

3. If there is unilateral light, the auxin will diffuse towards the shaded side of shoot resulting in a higher conc of IAA there

4. Cells on the shaded side elongate more 7 results in the plant bending towards the light source

What type of phototropism occurs in the roots?

Negative phototropism

Why does negative phototropism occurs in the shoots?

Roots don’t photosynthesis s don’t require light

How does -ve phototropism occur in the roots?

High conc of IAA inhibits cell elongation, causing root cells to elongate more on the light side and so root bend away from light

What type of gravitropism occurs in the shoots?

Negative gravitropism

What type of gravitropism occurs in the roots?

Positive gravitropism

How does -ve gravitropism occur in the shoots?

IAA will diffuse from the upper side to the lower side of shoot

If the plant is vertical, this causes the plant cells to elongate & the plant to growth upwards

If the plant is on its side, it will cause the shoot to bend upwards

How does +ve gravitropism occur in the roots?

IAA moves to the lower side of roots so that the upper side elongates and the root bends down towards gravity & anchors the plant in

Plant hormones (examples)

Auxin
Ethene
Gibberellin
Abscisic acid (ARA)

What is auxin?

causes cell elongation in stems and inhibits growth in roots
Prevents leaves form dropping and maintains apical dominance

What is ethene?

A gas that causes fruit to ripen

What is gibberellin?

Stimulates seed germination, stem elongation and pollen tube growth in fertiliasation

What is abscisic acid (ABA)?

Stimulates stomatal closing and maintains dormancy of seeds

What is apical shoot?

What is apical dominance?

Auxin results in one main shoot growing which inhibits the growth of lateral shoots due to the high conc of auxin in apical shoot

A high of conc of auxin

the stronger the apical dominance & less growth there will be of lateral shoots

Evidence for apical dominance

If apical shoot is removed (which contains auxin-producing cells in the tips) than lateral shoots shart to grow again

When artificall auxin appilled deccrease soemthing

Gibberellins role:

Seed germination
Stem elongation

How does gibberellins control stem elongation?

Collection of hormones that help plants grow by stimulating elongation in the stem

The high the conc of gibberellins

the more elongated the stem will become

Evidence for stem elongation

• Dwarf variety of plants have low level of gibberellins

• adding gibberellins result in them growing to the same heigh as non-dwarf varieties

• Horticulturalists and farms apply G to shorter plants to stimulate growth

Commercial use of ethene

Used to control ripening of fruits
Unripe fruits can be picked and transport whilst firm, then sprayed with E before it is sold

Commercial use of auxin

Rooting power - encourage the growth of new roots from plant cuttings
Weedkiller - sprayed over weeds which then grow too quickly, their stems give way and die