Plant responses (Module 5)

Question 1

State the range of plant responses

Answer 1

1. Responses to abiotic stress (water levels, daylight, temperature)
2. Herbivory (chemical defences, folding in response to touch)
3. Tropisms (specific growth in response to): Light (phototropism), Gravity (geotropism)

Question 2

Explain how hormones allow plants to make responses to environmental factors

Answer 2

1. Plants do not have a nervous system that allows fast responses
2. But plants can respond to the environment with hormones (not to be mistaken for an endocrine system)
3. Where a chemical produced by cells in one part of the plant
4. Moves to a different part of the plant from cell to cell, or via plant transport mechanisms (xylem, phloem)
5. And cause changes in behaviour in other cells and tissues (via cell signalling)
6. Often affecting cell growth
7. As a result these responses are usually relatively slow compared to that of animals

Question 3

Give examples of plant hormones

Answer 3

1. Auxins
2. Gibberellins
3. Abscisic acid (ABA)
4. Ethene

Question 4

Describe the effects of auxins on plants

Answer 4

1. Cell elongation
2. Prevent leaf fall (abscission)
3. Promote apical dominance
4. Affects root and shoot growth
5. Stimulates release of ethene (fruit ripening)

Question 5

Describe the effects of gibberellins on plants

Answer 5

1. Stem elongation
2. Mobilisation of energy storage carbohydrate during germination
3. Stimulate pollen tube growth during fertilisation

Question 6

Describe the effects of ethene on plants

Answer 6

1. Fruit ripening

2. Promotes abscission in deciduous trees

Question 7

Describe the effects of Abscisic acid (ABA) on plants

Answer 7

1. Maintains dormancy of seeds and buds
2. Stimulates cold protective responses (antifreeze production)
3. Stimulates stomatal closing

Question 8

Describe the role of gibberellins in seed germination

Answer 8

1. Dormant seed absorbs water which activates the embryo
2. Activated embryo cells produce the hormone gibberellin
3. Gibberellin results in cell signalling that causes genes for amylases and proteases to be switched on
4. Starch and protein are hydrolysed to glucose/maltose and amino acids respectively
5. Providing the cells of embryo with respiratory fuel and material for biosynthesis of new cells
6. ABA acts antagonistically to this process, which is initiated when gibberellin levels are higher than ABA

Question 9

Describe the role abscisic acid (ABA) in stomatal closure

Answer 9

1. Stomatal opening causes plant cooling due to water transpiration/evaporation
2. Stomatal closure results in less water loss (greater water conservation) due to reduced transpiration/evaporation
3. Leaf cells release ABA during times of water stress
4. Plant root cells also produce ABA when soil water levels are low
5. ABA is transported to the leaves
6. ABA causes cell signalling that changes the activity of ion carriers in the cell membrane, causing ions to be removed from the guard cell.
7. The raised water potential in the guard cell causes water to leave by osmosis
8. Reduced turgor of the guard cells causes the stomata to close

Question 10

Describe the role of ethene in leaf fall (abscission)

Answer 10

1. Deciduous trees lose their leaves in winter to conserve energy (less light for photosynthesis in the winter) and reduce the risk of being blown over by wind
2. In response to reduced light levels, auxin levels fall
3. Auxin levels falling cause an increase in the production of ethene
4. Due to the process of differentiation and cell specialisation cells in the abscission zone are responsive to ethene.
5. Cell signaling is triggered that causes genes for enzymes that weaken the cell wall to be switched on.
6. Vascular bundles (xylem and phloem) transporting materials to the leaf are sealed off
7. Without a supply of water and nutrients the leaf dies, and falls away
8. Fatty material is deposited on the stem side of the abscission zone, producing a waterproof scar that also prevents the entry of pathogens.

Question 11

Explain how auxins such as (indoleacetic acid) IAA affect the growth of apical shoots

Answer 11

1. Auxins are produced and released by meristem cells at the shoot tip in the zone of cell division
2. Auxins diffuse down the shoot tip and bind to cells in the zone of elongation
3. Cell signalling is triggered that results in hydrogen ions to be pumped out of cells, lowering the pH in the cell walls
4. This lower pH (pH 5) activates enzymes in the cell wall that maintain its plasticity
5. When water enters these cells due to osmosis, the cells elongate (as the cell wall is somewhat stretchable)
6. Mature cells further down the shoot break down auxin, in these areas the pH is higher, and cell walls are more rigid, and less able to elongate.

Question 12

Describe the evidence for the role of auxin in apical dominance

Answer 12

1. Shoot tips produce high levels of auxin and are the fastest growing parts
2. While the apical bud is attached, the high levels of auxin suppress the growth of the lateral buds
3. When the apical bud is removed, the lateral buds grow faster….
4. ….As the there are no longer high levels of auxin from the apical buds and the apical dominance has been removed
5. When the apical bud is removed, and auxin is applied the apical dominance returns
6. When the apical bud is removed, auxin is applied but not allowed to move down the stem, the apical dominance is removed.

Question 13

Describe the experimental evidence for the role of gibberellins in stem elongation

Answer 13

1. Seedlings infected with the fungus Gibberella grow very tall and thin, because the fungus produces gibberellins
2. Plants that have short stems produce no, or very little gibberellins
3. Plants modified to disrupt gibberellin synthesis are dwarf varieties

Question 14

Describe the experimental evidence for the role of gibberellins in seed germination

Answer 14

1. Mutant varieties of seeds, bred to have alleles which cause a lack of production of gibberellins do not germinate. 2. When gibberellins are externally applied, germination occurs normally
2. Gibberellin synthesis can be disrupted with inhibitor compounds. Inhibitor-treated seeds do not germinate.
3. When gibberellins are applied, they germinate normally.

Question 15

Describe the ways in which plants can defend against herbivory with chemical responses

Answer 15

1. Tannins: phenolic compounds which taste bitter to animals and are toxic to insects
2. Alkaloids: bitter tasting nitrogenous compounds, some of which act as poisons
3. Terpenoids: oily substances acting as toxins to insects that may feed on the plants (inhibiting the functioning of their nervous system)
4. Pheromones are chemicals that affect the behaviour of other members of the same species. Examples of this are:
   4(a) Pheromones that cause the leaves of nearby trees to make callose
   4(b) Volatile organic compounds (VOCs) produced when a plant is attacked by a pest that alter gene expression via cell signalling, altering behaviour in another species which may deter the pest. For example, production of VOCs may attract a predator of the pest attacking the plant.

Question 16

Explain how plants can defend against herbivory by folding in response to touch

Answer 16

1. Some plants such as Mimosa pudica will fold down leaves in response to touch
2. This may reduce accessibility by the herbivore, or scare them away, or dislodge any attacking insects
3. Touching the leaves causes movement of potassium ions into specialised cells on the top and out of cells at the bottom parts of the base of the leaf
4. So water enters the cells at the top of the ‘joint’, and water leaves the cells at the bottom
5. Flexible cell walls allows the upper cells tissues to expand, and lower cells to contract
6. Causing the leaf to fold downward at the joint
7. In the absence of further stimulus, ion concentrations return to normal and the leaf rises back

Question 17

Explain why tropisms are important responses for the survival of a plant

Answer 17

1. Tropisms are growth-related movements of plants in response to directional environmental stimuli such as light and gravity
2. This is best exemplified by the emerging shoot and root during germination
3. Where the shoot grows upward in the direction of light (and opposite to the direction of gravitational pull) to maximise the energy it has to photosynthesise (positive phototropism, negative geotropism)
4. And the root(s) grow downward into the soil to maximise water and mineral uptake, and provide better support for the rest of the plant (negative phototropism, positive geotropism)

Question 18

Explain how shoots are able to make positive phototropic responses

Answer 18

1. Unilateral light (light of a particular direction) causes the plant hormone auxin to redistribute/move laterally in the shoot
2. It accumulates on the opposite side to that facing the light source (auxin moves away from the light)
3. Auxin causes cell elongation (in shoots) on the side not facing the light
4. causing the shoot to bend towards the light.

Question 19

Explain how shoots are able to make negative geotropic responses

Answer 19

1. Gravity causes auxin to accumulate on the lower side of a growing shoot
2. In shoots auxin causes cell elongation (on the lower side more than the upper)
3. Resulting in the shoot bending away from the direction of gravity

Question 20

Explain how roots are able to make a negative phototropic response

Answer 20

Negative phototropic
1. light causes auxins to accumulate on the lower side of the root
2. Recall that varying auxin concentrations affect different parts of the plant in different ways
3. At the concentrations present in the root, auxin inhibits cell elongation
4. Therefore, roots bend away from the light source
Positive geotropism

Question 21

Explain how roots are able to make a positive geotropic response

Answer 21

1. Gravity causes auxins to accumulate on the lower side of the root
2. Recall that varying auxin concentrations affect different parts of the plant in different ways
3. At the concentrations present in the root, auxin inhibits cell elongation
4. Therefore, roots bend towards the direction of the gravitational stimulus

Question 22

Describe practical investigations into phototropism

Answer 22

1. plants bend towards a unidirectional light source
2. when the shoot tip is removed, it loses the ability to bend towards the light (shoot tip needed for detection or production of chemical messenger)
3. when the shoot tip is protected from the light, it prevents a phototropic response (shoot tip is needed to detect directional light stimulus)
4. artificially causing auxin accumulation on one side of the shoot (with a mica plate) causes bending (differential chemical messenger is required for response)
5. preventing auxin accumulation in the presence of a unidirectional light source (with a mica plate) prevents bending (movement of chemical messenger is needed for response)
6. using an auxin-infused gelatine block reproduces the effects of a removed shoot tip (the chemical messenger involved is auxin)

Question 23

Describe the practical investigations into geotropism

Answer 23

1. Gravitational stimulus can be controlled/varied by the use a clinostat
2. The rotating drum can be controlled to observe the effect of direction of gravitational stimulus on the growth of shoots and roots.

Question 24

Describe the use of plant hormones to control ripening of fruits

Answer 24

1. Ethene promotes climacteric fruit ripening
2. This allows less ripe fruit to be picked and transported (less likely to be damaged)
3. Ethene is then used to ripen the fruit
4. There is a more uniformly ripe product available to the public

Question 25

Describe the use of plant hormones in rooting powders

Answer 25

1. Auxin can be applied to cut stems (cuttings) to promote the growth of roots.
2. This can increase the success of propagation of plants through cuttings.
3. Plant hormones are also used in micropropagation, ensuring that the growth media contain the right hormones and hormones concentrations increases the differentiation and growth of calluses into plants.

Question 26

Describe the use of plant hormones as weedkillers

Answer 26

1. The presence, concentration and balance of hormones affects the growth of plants
2. High concentrations of hormones can kill plants, as this balance is lost.
3. Synthetic auxins (designed to affect weeds but not plants of importance, understanding this is subjective) can increase the growth of weeds until it is unsustainable and leads to the weed’s death
4. This reduces competition for water, light, nutrients and increases growth of the ‘important’ plants