Plant Responses

Question 1

Why do plants need chemical coordination?

Answer 1

They are not mobile and do not have rapidly responding nervous systems - so need to be able to respond to changing environment ( e.g. abiotic stress) and to avoid herbivory, so hormones which allow this need to be transported between regions of the plant.
- Must respond to internal and external changes to survive e.g. growing towards light for photosynthesis.

Question 2

How do plants respond to abiotic stress such as shortage of water and what is this coordinated by?

Answer 2

Close their stomata to reduce water loss, this is coordinated by abscisic acid.

Question 3

How do plants respond to other abiotic stresses such as shortage of light?

Answer 3

By growth movements called tropisms.

Question 4

Give four examples of tropism.

Answer 4

• Phototropism
• Geotropism
• Thigmotropism
• Chemotropism

Question 5

What is phototropism? Give example of a region of a plant and what type of phototropism it shows.

Answer 5

The response to light and shoots show positive phototropism as they grow towards light.

Question 6

What is geotropism? Give examples of regions of a plant and what type of geotropism they show.

Answer 6

The response to gravity. Shoots show negative geotropism as they grow away from gravity and towards light. Roots show positive geotropism and grow towards gravity, down into the soil.

Question 7

What is thigmotropism?

Answer 7

The response to touch, some plants grow so that their stem or branch winds around a support.

Question 8

What is chemotropism?

Answer 8

The response to certain chemicals, the pollen tube grows towards the ovum during reproduction.

Question 9

How do plants respond to/avoid herbivory?

Answer 9

• Release tannins which are toxic to microorganisms and herbivores.
• Produce alkaloids which make the tissue taste bitter and deter herbivory.
• Mimosa pudica folds up in response to touch, scares away insect herbivores.
• Produce pheromones to signal to other species and communicate with other plants e.g. if a maple tree is attacked by insects, it releases pheromones which is absorbed by leaves on other branches. These make chemicals such as callose to help protect them.

Question 10

Outline the role of hormones in leaf loss (abscission).

Answer 10

• Occurs during winter.
• Falling light levels lead to a fall in the concentration of auxin in the leaves. Auxin prevents abscission by making cells insensitive to ethylene.
• Cytokinins enter leaves and stop aging.
• When concentration of cytokinins drops, the leaf ages (senescence) and stops production of auxin.
• Abscission zone contains two layers of cells which are sensitive to ethene, triggering production of cellulase.
• Cellulase digests and weakens cell wall in separation layer.
• Vascular bundles sealed off and fatty material (suberin) deposited in cells of stem side of separation layer, forming protective layer to prevent the entry of pathogens.
• Leaf falls off and leaves neat scar.

Question 11

Outline the roles of hormones in seed germination.

Answer 11

• Seed absorbs water and embryo is activated - starts to produce gibberellins.
• Gibberellins then stimulate production of enzymes (turns on genes that code for enzymes such as amylase and protease) which break down the food stores in seed.
• Use food stores to produce ATP for building materials so it can grow and break through the seed coat.

Question 12

Where are the food stores found in dicot and monocot seeds?

Answer 12

• In cotyledons in dicot seeds.

- In endosperm in monocot seeds.

Question 13

What other evidence is there to suggest the role of abscisic acid (ABA) in seed germination?

Answer 13

Acts as an antagonist to gibberellins and it is the relative levels of both hormones which will determine whether a seed will germinate or not.

Question 14

What are examples of experimental evidence supporting the role of gibberellins in seed germination?

Answer 14

• Mutant varieties of seeds have been bred which lack enzymes to make gibberellins - do not germinate. Application of gibberellins externally allows them to germinate.
• If gibberellin biosynthesis inhibitors are applied, the seeds do not germinate as they are unable to make gibberellins needed for them to break dormancy. If inhibition is removed, or gibberellins are applied, germination occurs.

Question 15

Outline the role of hormones in stomatal closure.

Answer 15

• Stomata close and open through the action of the guard cells.
• Guard cells possess receptors for abscisic acid (ABA).
• Binding of ABA activates number of chemical pathways inside cells, causing an increase in pH and transfer of Ca2+ ions from vacuole to the cytoplasm.
• Ca2+ stimulates loss of charged ions such as K+, NO3- and Cl-.
• This increases water potential so water moves out of cell by osmosis.
• Reduces turgor pressure which changes shape of the cell and the stoma closes.

Question 16

Outline experimental evidence for the role of auxins in apical dominance.

Answer 16

• Auxins are produced at tip of the shot and inhibits the growth of lateral buds.
• When plant tip (apex) is removed, lateral buds start to grow. When ring of auxin transport inhibitor is applied below the apex of the shoot, the lateral buds grow.
• New evidence suggests ABA inhibits bud growth instead, and high auxin levels keep ABA levels high, so when tip is removed, ABA levels drop so bud starts to grow.
• Cytokinins promote bud growth, directly applying this can override apical dominance effect. High levels of auxin make the shoot apex a sink for cytokinins so most of this goes to apex, when apex is removed, cytokinins are spread evenly around the plant.

Question 17

Outline the experimental evidence for the role of gibberellins in stem elongation.

Answer 17

• Elongation of stems stimulated by gibberellins.
• Scientists have bred many dwarf varieties of plants where gibberellin pathway is interrupted, without gibberellins the plant stems are much shorter.
• Growing seedlings with certain fungi has been shown to increase seedling growth by causing stem elongations. Gibberellins can be isolated from these fungi.
• Applying a cream containing gibberellins extracted from fungi will cause stem elongation.

Question 18

What are commercial uses of auxin?

Answer 18

• Promoting root growth in cuttings.
• Producing seedless fruit.
• Synthetic auxins used as selective weedkillers.
• Hormone rooting powder to promote root growth in cuttings.

Question 19

Commercial uses of gibberellins?

Answer 19

• Delaying ripening and ageing in fruit.
• Improve size and shape of fruits.
• Used in beer brewing to speed up malting process by activating stored barley to produce malt.

Question 20

Commercial uses of ethene?

Answer 20

• Speeding up fruit ripening and promoting fruit drop.

- Promoting growth of lateral branches.

Question 21

Commercial uses of cytokinins?

Answer 21

• Delaying leaf senescence to avoid discolouring of vegetables - increases shelf life.
• Promoting bud and shoot growth during tissue culture.
• Promoting growth of lateral buds.

Question 22

How can the effect of plant hormones on growth be investigated?

Answer 22

• Make serial dilutions to observe the effect of different concentrations of a hormone, as they can have different effects on growth at different concentrations.
• Use large number of plants.
• When measurements have been completed, the spread of data from each experimental group should be measured using standard deviation.

Question 23

Why must plants grow and respond to variations in their environment?

Answer 23

To be able to make maximum use of their environmental conditions.

Question 24

Why does most research on tropisms use young seedlings and germinating seeds?

Answer 24

• Easy to work with and manipulate.
• Growing and responding rapidly, so any changes are easy to observe.
• Any changes affect whole organism rather than a small part (as with a mature plant), making tropisms easier to observe and measure.

Question 25

What are methods in which phototropism can be investigated?

Answer 25

• Germinate and grow seedlings in different conditions of dark, all-round and unilateral light. Observe, measure and record patterns of growth. Time-lapse photography can give good record of changes as they take place.
• Germinate and grow seedlings with unilateral light, with different colour filters to see which light wavelength triggers a phototropic response.
• Remove tips.
• Place auxin impregnated agar block jelly on only one side of decapitated coleoptile.

Question 26

Outline plants responses to growing in the dark and the hormone responsible for this response.

Answer 26

• Grows rapidly so that it can reach light to be able to photosynthesise.
• Gibberellins responsible for extreme elongation of internodes in the dark.
• Once plant reaches light, upward growth slows down to use resources to strengthen stems, synthesise leaves and for overall growth.
• Levels of gibberellins fall once plant is exposed to light.