Plant Responses

Question 1

What are the main reasons plants respond to their environment

Answer 1

Avoid abiotic stress
Maximise photosynthesis
Obtain more light, water, minerals
Avoid herbivory/grazing/disease
Ensure germination in suitable conditions, or pollination, or seed dispersal

Question 2

What are the three chemical plant defences against herbivory

Answer 2

• Tannins - toxic to microbes and taste bitter to animals
• Alkaloids - taste bitter and are sometimes poisonous e.g. nicotine
• Pheromones - some plants produce these signalling molecules in response to grazing to communicate with other plants

Question 3

Define tropism

Answer 3

A directional growth plant response
Determined by the direction of external stimuli

Question 4

What are the types of tropism

Answer 4

Phototropism - light
Geotropism - pull of gravity
Chemotropism - attracted by chemicals
Thigmotropism - shoots of climbing plants wind around solid structures

Question 5

Define nastic responses (nasties)

Answer 5

Non-directional response to external stimuli

Question 6

Define photoperiodism

Answer 6

Responses to day length or photoperiod

Question 7

What controls plant responses

Answer 7

Hormones / plant growth regulators
- chemical signals
- target cells/ tissues
- receptors

NOT produced in endocrine glands
Produced in a variety of tissues
Very small quantities

Question 8

What are the main plant hormones/ growth regulators (5) and what do they do

Answer 8

• auxins - inhibit leaf abscission, cell elongation and inhibit lateral growth
• gibberellins - seed germination, stem growth
• abscisic acid - stomatal closure, inhibits seed germination and growth
• ethene - promote fruit ripening
• cytokinins - promote cell division

Question 9

Where are auxins produced then transported to

Answer 9

At the apices (apex)
Transported by diffusion or active transport
To the Zone of Elongation

Question 10

Describe phototropism (positive)

Answer 10

Illumination causes auxins to move down the shoot tip and causes elongation of cells
If the illumination is only on one side, auxins will move to the shaded side causing elongation and the shoot bends towards the light

Question 11

What occurs in the zone of elongation

Answer 11

Build up of hydrogen ions accumulates in cell wall (due to hydrogen pump)
Causing a low pH which is optimum for expansins (wall loosening enzymes) - breaking bonds within cellulose
And increased H+ conc disrupts hydrogen bonds
Cell walls become less rigid and cell expands as it takes on water

Question 12

What are the differences between negative and positive phototropism

Answer 12

Negative - in the roots, auxins inhibit elongation on the shaded side root bends away from the light
Positive - in the apex of shoots, auxins promote elongation on the shaded side, shoot bends towards light

Question 13

Define abscission

Answer 13

A leaf or fruit fall from a Plant

Question 14

What is the cause of abscission

Answer 14

Auxin production decreases,
ETHENE production increases
Causes production of CELLULASE - digests cell walls in the abscission zone
Petiole separates from stem

Question 15

Define apical dominance

Answer 15

Auxins from the apex (apical bud) inhibit lateral growth

Question 16

Describe how gibberellins promote seed germination

Answer 16

• seed absorbs water and embryo releases gibberellin
• gibberellin causes amylase to be produced
• amylase breaks down starch into glucose
• glucose used for respiration so embryo can grow

Question 17

How does abscisic acid promote stomatal closure

Answer 17

ABA triggers stomatal closure when there is a lack of water
ABA binds to receptors on the guard cell membranes
Causes K+ ions to leave the cell
Raises water potential of cells
Water leaves cell via osmosis and stomata closes

Question 18

How is leaf loss (abscission) in deciduous plants controlled?

Answer 18

1. As leaf ages, cytokinin and auxin levels lower, ethene levels increase
   2.triggers production of cellulase enzymes which weaken leaves by breaking down cell walls in abscission layer
2. Leaves break from the branch. Below abscission layer, Suberin layer forms to prevent entry of pathogens

Question 19

List the functions of gibberellins

Answer 19

Stimulate:
Germination
Elongation at cell internodes
Fruit growth
Rapid growth/ flowering

Question 20

List the functions of auxins

Answer 20

Involved in trophies responses e.g. IAA
Control cell elongation
Suppress lateral buds to maintain apical dominance
Promote root growth e.g. in rooting powders

Question 21

How are auxins and cytokinins used commercially?

Answer 21

Auxins: rooting powder, growing seedless fruit, herbicides, low concentrations prevent leaf and fruit growth, high concentrations prevent leaf and fruit growth, high concentrations promote fruit drop

Cytokinins: prevent yellowing of lettuce leaves, promotes shoot growth

Question 22

How are gibberellins and ethene used commercially?

Answer 22

Gibberellins: delay senescence in citrus, elongation of apples and grape stalks, brewing beer for malt production, increase sugar cane yield, speed up formation in conifers, prevent lodging

Ethene: speeds up ripening, promotes lateral growth, promotes fruit drop Cytokinins

Question 23

What statistical test is most appropriate for testing whether there was a significant different between the stem lengths of the seedlings in group a and group b
And why

Answer 23

(Unpaired) T-test

Idea of comparing two means