Plant responses

Question 1

Explain why plants need to respond to their environment

Answer 1

1. Need to respond to abiotic stresses
2. Animals wanting to eat them
3. Pathogens

Question 2

List the different types of response they have with examples of each.

Answer 2

1. Plant defences against herbivory (chemical or physical)
2. The control of growth (phototropism, geotropism, apical dominance and stem elongation
3. The triggering of specific events (leaf loss, seed germination, stomatal closure, preventing freezing)

Question 3

Define the term tropism

Answer 3

A growth response by a plant in response to a unidirectional stimulus

Question 4

Name, and give an example of, 4 different types of tropism.

Answer 4

1. Phototropism- response to light
2. Geotropism- response to gravity
3. Chemotropism- response to chemicals
4. Thigmotropism- response to touch

Question 5

Define plant hormone

Answer 5

1. Chemical signals to coordinate growth and responses

2. They do not always act in the same way as animal hormones

Question 6

Define target tissue

Answer 6

Specific cells which hormones act on. Have receptors specific for the hormone

Question 7

Outline how plant hormones can be transported around plants and have their effect

Answer 7

1. They are released by cells
2. Travel through the plant by diffusion or active transport across and between cells and by mass flow in xylem or phloem.
3. They then trigger a response in target cells or tissues.

Question 8

List 5 plant hormones and describe the effects of each of them.

Answer 8

1. Auxins- control cell elongation, prevent leaf fall (abscission), maintain apical dominance, involved in tropisms, stimulate the release of ethene, involved in fruit ripening.
2. Gibberellin- cause stem elongation, trigger the mobilisation of food stores in a seed at germination, stimulate pollen tube growth in fertilisation
3. Ethene- causes fruit ripening, promotes abscission in deciduous trees
4. ABA- Maintains dormancy of seeds and buds, stimulates cold protective responses, stimulates stomatal closing.
5. Cytokinins- promote cell division

Question 9

Define meristems

Answer 9

Tissue found at regions of growth in plants. Contains stem cells.

Question 10

Name the 4 types of meristem in plants and describe where they are and their effect on the growth of the plant.

Answer 10

1. Apical meristems at the tips of roots and shoots
2. Lateral bud meristems giving rise to side shoots
3. Lateral meristems (in cambium) are responsible for roots and shoots getting wider
4. Intercalary meristems, located between the nodes where the leaves and buds branch off the stem, cause shoots to get longer

Question 11

Describe what auxins are and what their effect depends on

Answer 11

1. Growth stimulants produced in plants e.g. indoleacetic acid IAA
2. Small quantities can have powerful effects
3. They are made in cells at the tip of the roots and shoots and in the meristems.
4. Auxins can move down the stem and up the roots both in the transport tissue and from cell to cell.
5. The effect of auxin depends on its concentration and any interactions it has with other hormones.

Question 12

Describe how auxins, such as IAA, lead to growth in the apical shoot

Answer 12

1. Auxins affect the plasticity of the cell wall- presence of auxins means the cell wall stretches more easily
2. Auxin molecules bind to specific receptor sites in the plant cell membrane, which triggers a second messenger system and cells pump H+ ions out, causing a fall in pH to about 5 which breaks hydrogen bonds within cellulose
3. This is the optimum pH for the enzymes needed to keep the walls very flexible and plastic.
4. As the cells mature, auxin is destroyed.
5. As the hormone levels fall, the pH rises so the enzymes maintaining plasticity become inactive
6. As a result the wall becomes rigid and more fixed in shape and size and the cells can no longer expand and grow

Question 13

Describe how auxins, such as IAA, lead to growth in the lateral shoots

Answer 13

1. High concentration of auxins suppress the growth of lateral shoots- results in apical dominance
2. Growth in the main shoot is stimulated by the auxin produced at the tip so it grows quickly .
3. The lateral shoots are inhibited by the hormone that moves back down the stem, so they do not grow very well.
4. Further down the stem, the auxin concentration is lower so the lateral shoots grow more strongly,
5. If the apical shoot is removed, the auxin-producing cells are removed and so there is no auxin and the lateral shoots grow faster.
6. If auxin is applied artificially to the cut apical shoot, apical dominance is reasserted and lateral shoot growth is supressed

Question 14

Describe how auxins, such as IAA, lead to growth in the root growth

Answer 14

1. Low concentrations of auxins promote root growth
2. Up to a given concentration, the more auxin that reaches the roots, the more they grow.
3. Auxin is produced by the root tips and auxin also reaches the roots in low concentrations from the growing shoots.
4. If the apical shoot is removed, then the amount of auxin reaching the roots is greatly reduced and root growth slows and stops.
5. Replacing the auxin artificially at the cut apical shoot restores the growth of the roots.
6. High auxins concentrations inhibit root growth.

Question 15

Describe the basic model of phototropism

Answer 15

1. The basic model of the way plants respond to light as they grown was based on experiments where shoots were entirely in the dark or in full illumination- rarely case in real life
2. Phototropisms are the results of the movement of auxins across the shoot or root if it is exposed to light that is stronger on one side than the other
3. If plants are grown in bright all-round light in normal conditions of gravity they grow more or less straight upwards.
4. In even but low light they grow straight upwards and faster and taller than in bright light
5. If plants are exposed to light that is brighter on one side or to unilateral light that only shines from one side, then the shoots of the plant will grow towards the light and the roots, if exposed, will grown away
6. Shoots are positively phototropic and roots are negatively phototropic.

Question 16

Describe how a plant can grow towards light

Answer 16

1. Exposure to light that is more strong on one side than the other causes auxin to move laterally across the shoots, so there is a greater concentration on the unilluminated
2. This in turn stimulates cell elongation and growth on the dark side, resulting in the observed growth towards the light.
3. Once the shoot is growing directly towards the light, the unilateral stimulus is removed. The transport of auxin stops and the shoot then grows straight towards the light.

Question 17

What happens when a normal plant shoots is exposed to unilateral light

Answer 17

1. Shoot bends towards the light

2. The shoot is positively phototropic, bending occurs behind the tip

Question 18

What happens when a plant shoot with the top removed is exposed to unilateral light

Answer 18

1. No response

2. The tip must either detect the stimulus or produce the messenger (or both) as its removal prevents any response

Question 19

What happens when a light proof cover is placed over intact tip of shoot and it is exposed to unilateral light

Answer 19

1. No response

2. The light stimulus must be detected by the tip

Question 20

What happens when thin, impermeable barrier of mica is put in the illuminated side of the shoot and it is exposed to unilateral light

Answer 20

1. Movement of chemical down shaded side and bends towards the light
2. Mica on the illuminated side of the shoot allows the hormone to pass only down the shaded side where it increases growth and causes bending

Question 21

What happens when mica is inserted on the shaded side of the shoot and it is exposed to unilateral light

Answer 21

1. Movement of chemical down shaded side is prevented by mica so no response

Question 22

What happens when the tip is removed and a gelatin block inserted and tip replaced in the shoot and it is exposed to unilateral light

Answer 22

1. Movement of chemical down shaded side and it bends towards the light
2. As gelatin allows chemicals to pass through it, but not electrical messages, the bending which occurs must be due to a chemical passing from the tip

Question 23

What experiment can be used to show the movement of auxin across a shoot

Answer 23

1. Have two shoots one intact and one split
2. First experiment- keep shoots in the dark- total auxin produced approximately the same whether shoot is split or not
3. Second experiment- Shoot in unilateral light total auxin produced approximately the same whether shoot split or not ?
4. Third experiment- Shoot in unilateral light but undivided- auxin accumulates on the dark side and lower on the lit side when the soot is intact but when divided the auxin concentration is approximately the same both sides. This suggests that normally auxin is transported across the shoot in unilateral light from the lit side to the dark side

Question 24

Design an experiment to investigate one factor affecting phototropism in seedlings/coleoptiles.

Answer 24

Many different ways:

1. Germinate and grow seedlings in different conditions of dark, all-round light, and unilateral light. Observe, measure and record the patterns of growth. Time-lapse photography can give a good record of the changes as they take place
2. Germinate and grow seedlings in unilateral light with different colour filters to see which wavelengths of light trigger the phototropic response.
3. Repeat some of the classic experiments- cover the tips of coleoptiles, place auxin-impregnated agar jelly blocks or lanolin on decapitated coleoptiles, place auxin-impregnated agar blocks on one side only of decapitated coleoptiles.

Question 25

Describe what happens when plants grow in the dark

Answer 25

1. Plants grow more rapidly in the dark.
2. If a plant is in the dark the biological imperative is to grow upwards rapidly to reach the light ot be able to photosynthesise.
3. The seedlings that break through the soil first will not have to compete with other seedlings for light.
4. Evidence suggests that it is gibberellins that are responsible for the extreme elongation of the internodes when a plant is grown in the dark.
5. Once a plant is exposed to the light, a slowing of upwards growth is valuable.
6. Resources can be used for synthesising leaves, strengthening stems, and overall growth. Scientists have demonstrated that levels of gibberellin fall once the stem is exposed to light.
7. The rapid upward growth which takes place in a plant grown in the dark is etiolation
8. Etiolated plants are thin and pale- little chlorophyll develops.

Question 26

Describe the mechanism for geotropism

Answer 26

1. In normal conditions, plants always receive a unilateral gravitational stimulus- gravity always act downwards
2. The response of plants to gravity can be seen in the laboratory using seedlings placed on their sides either in all-around light or in the dark.
3. Shoots are usually negatively geotropic and roots are positively geotropic.
4. This adaptation ensure that the roots grow down into the soil and the shoots grow up to the light.
5. Geotropism are also known as gravitropisms.

Question 27

Design an experiment to investigate one factor affecting geotropism in shoots or roots.

Answer 27

1. The geotropic response can be investigated in shoots and roots using a rotating drum known as a clinostat.
2. The plants can be grown on a slowly rotating clinostat (4 rotations per hour) so the gravitational stimulus is applied evenly to all sides of the plant- and the root and in the dark shoots grow straight
3. Alternatively, the seeds can be placed in petri dishes stuck to the wall of the lab, and the dishes rotated 90 degrees at intervals as the seedlings grow. A geotropic response in the roots can be seen within about two hours.

Question 28

List 7 physical defences plants have against herbivory.

Answer 28

1. Thorns
2. Barbs
3. Spikes
4. Spiny leaves
5. Fibrous and inedible tissue
6. Hairy leaves
7. Stings

Question 29

State 3 chemical defences plants have against herbivory

Answer 29

1. Tannins
2. Alkaloids
3. Terpenoids

Question 30

Describe how tannins help protect plants from herbivory

Answer 30

1. Part of a group of compounds called phenols produced by many plants
2. Make up 50% of dry weight of leaves
3. Have very bitter taste which deters herbivores
4. Toxic to insects- bind to digestive enzymes produced in saliva and inactivate them.

Question 31

Describe how alkaloids help protect plants from herbivory

Answer 31

1. Very bitter tasting nitrogenous compounds found in many plants.
2. Many affect the metabolism of animals that take them in and sometimes poisoning them.
3. Include caffeine, nicotine, morphine and cocaine
4. Caffeine is toxic to fungi and insects, the caffeine produced by coffee bush seedlings spreads through the soil and prevents germination of the seeds of other plants.

Question 32

Describe how terpenoids help protect plants from herbivory

Answer 32

1. Large group of compounds produced by plants which often form essential oils but also often act as toxins to insects and fungi that might attack the plant
2. Pyrethrin acts as an insect neurotoxin interfering with the nervous system.
3. Citronella is an insect repellent produced by lemon grass

Question 33

Define the term pheromone

Answer 33

1. Pheromone is a chemical made by an organism which affects the social behaviour of other members of the same species.
2. Because plants do not behave socially they do not rely a lot on pheromones, but they do use them to defend themselves sometimes

Question 34

Describe two examples of plants using pheromones to defend themselves

Answer 34

1. If a maple tree is attacked by insects, it releases pheromones which is absorbed by leaves on other branches. These leaves then make chemicals such as callose to help protect them if they are attacked. Leaves on nearby trrs also prepare for attack in response to these chemical signals
2. Some evidence that plants communicate by chemicals produced in the root systems and one plant can ‘tell’ a neighbour if it is under water stress

Question 35

Define VOCs

Answer 35

1. Volatile organic compounds act like pheromones between themselves and other organisms particularly insects
2. They diffuse through the air in and around the plant.
3. They are usually only produced when the plant detects attack by an insect pest through chemicals in the saliva of the insect.

Question 36

Describe 3 examples of when VOCs are used by plants to defend themselves

Answer 36

1. When cabbages are attacked by caterpillars, they produce a chemical signal which attracts the parasitic wasp. This insect lays its eggs in the caterpillars which are then eaten alive, protecting the plant. The signal from the plant also deters any other butterflies from laying their eggs.
2. When apple trees are attacked by spider mites they produce VOCs which attract predatory mites that come and destroy the apple tree pests.
3. Some types of wheat seedling produce VOCs when they have been attacked by aphids and these repel other aphids from the plant.
4. Sometimes a VOC produced by a plant will also act as a pheromone so that neighbouring plants begin to produce the VOC before they are actually attacked

Question 37

Describe how Mimosa pudica folds in response to touch.

Answer 37

1. Sensitive plant Mimosa pudica is one of a small number of plants which move at a speed you can see.
2. If the leaves are touched, they fold down and collapse.
3. Scientists think this frightens off larger herbivores, and dislodges small insects which have landed on the leaves.
4. The leaf falls in a few seconds, and recovers over 10-12 minutes as a result of potassium ion movement into specific cells, followed by osmotic water movement.

Question 38

Define senescence

Answer 38

The process by which cells irreversibly stop dividing and enter a state of permanent growth arrest without undergoing cell death. Occurs before abscission and is inhibited by cytokinins

Question 39

Define abscission

Answer 39

The fall of leaves

Question 40

Describe the role of plant hormones in leaf loss in deciduous plants.

Answer 40

1. The lengthening of the dark period in winter triggers abscission.
2. The falling light levels results in falling concentrations of auxin.
3. The leaves respond to the falling auxin concentration by producing the gaseous plant hormone ethene.
4. At the base of the leaf stalk is a region called the abscission zone, made up of two layers of cells sensitive to ethene.
5. Ethene seems to initiate gene switching in these cells resulting in the production of new enzymes. These digest and weaken the cell walls in the outer layer of the abscission zone, known as the separation layer
6. The vascular bundles which carry materials into and out of the leaf are sealed off.
7. At the same time fatty material is deposited in the cells on the stem side of the separation layer. This layer forms a protective scar when the leaf falls, preventing the entry of pathogens
8. Cells deep in the separation zone respond to hormonal cues by retaining water and swelling, putting more strain on the already weakened outer layer.
9. Then further abiotic factors such as low temperatures or strong winds finish the process- the strain is too much and the leaf separates from the plant leaving a neat waterproof scar is left behind.

Question 41

Describe how plants respond to prevent freezing

Answer 41

1. If cells freeze, their membranes are disrupted and they will die.
2. The cytoplasm of the plant cells and the sap of in the vacuoles contain solutes which lower the freezing point.
3. Some plants produce sugars, polysaccharides, amino acids and even proteins which act as antifreeze to prevent the cytoplasm from freezing or protect the cells from damage even if they do freeze
4. Most species only produce chemicals which make them hardy and frost resistant during the winter months.
5. It appears that different genes are suppressed and activated in response to a sustained fall in temperatures along with the reduction in day length, effectively preparing the plants to withstand frosty conditions.

Question 42

Explain why a plant might close its stomata and describe this process including the role of plant hormones.

Answer 42

1. Plants can respond to stress of heat and water availability, opening the stomata can cool the plant as water evaporates from the cells in the leaves in transpiration or to close the stomata to conserve water
2. Opening and closing od stomata in response to abiotic stresses is largely under control of the hormone ABA
3. The leaf cells appear to release ABA under abiotic stress, causing stomatal closure.
4. Scientists think that the roots also provide an early warning of water stresses through ABA
5. e.g when the levels of soil water fall and transpiration is under threat, plant roots produce ABA which is transported to the leaves here it bind to receptors on the plasma membrane of the stomatal guard cells.
6. ABA activates changes in ionic concentration of the guard cells, reducing the water potential and therefore the turgor of the cells.
7. As a result of reduced turgor, the guard cells close the stomata and water loss by transpiration is greatly reduced.

Question 43

Describe the process of germination in seeds including the role of plant hormones.

Answer 43

1. When the seeds absorb water, the embryo is activated and begins to produce gibberellins.
2. They in turn stimulate the production of enzymes that break down the food stores found in the seeds.
3. The food store is in the cotyledons in dicot seeds and endosperm in monocot seeds
4. The embryo plant uses these food stores to produce ATP for building materials so it can grow and break out through the seed coat
5. Evidence suggests that gibberellins switch on genes which code for amylases and proteases- the digestive enzymes required for germination
6. There is also evidence suggesting ABA acts as an antagonist to gibberellins and that it is the relative levels of both hormones which determine when a seed will germinate

Question 44

Describe the evidence supporting a link between gibberellins and seed germination.

Answer 44

1. Mutant varieties of seeds have been bred which lack the gene that enables them to make gibberellins. These seeds do not germinate. If gibberellins are applied to the seeds externally, they then germinate normally
2. If gibberellin biosynthesis inhibitors are applied to seeds, they do not germinate as they cannot make the gibberellins needed for them to break dormancy. If the inhibition is removed, or gibberellins are applied, the seeds germinate.

Question 45

Define apical dominance

Answer 45

The growth and dominance of the main shoot as a result of the suppression of lateral shoots by auxin.

Question 46

Describe the evidence supporting a link between auxin and apical dominance, and describe the evidence that suggests that this link is not directly causal.

Answer 46

1. When the shoot of the apex is removed, one or more lateral buds grow
2. When IAA is put on cut end of shoot after decapitation apical dominance is maintained-IAA can artificially maintain apical dominance
3. Auxin transport inhibitor placed below the shoot apex, lateral buds grow- IAA plays a role in maintaining apical dominance naturally
4. After decapitation auxin concentration of lateral buds increased suggesting it is not directly linked and other hormones must be at play
5. Evidence shows cytokinins stimulate lateral bud growth and auxins control the accumulation of cytokinins

Question 47

Describe how gibberellins lead to growth.

Answer 47

1. Gibberellins affect the length of internodes- regions between the leaves on a stem. Stimulate elongation (by loosening cell walls) and cell division (by stimulating production of protein that controls cell cycle)
2. Discovered as were produced by fungus which affects rice. The affected seedlings grew extremely tall and thin. Scientists investigated rice and isolated chemicals- gibberellins which produce the same spindly growth in the plants.
3. It was then discovered that plants themselves produce the same compounds.
4. Plants that have short stems produce few or no gibberellins.
5. Scientists have bred many dwarf varieties of plants where the gibberellin synthesis pathway is interrupted.
6. Without gibberellins plant stems are much shorted.
7. This reduces waste and makes the plants less vulnerable to damage by weather and harvesting

Question 48

Draw a table to compare the action of gibberellins and auxins in the growth of stems.

Answer 48

1. Example molecules: A- IAA G- Gibberellic acid GA 1
2. Effect on stem length: A- increase G- increases
3. pH of molecule: A- acidic G- acidic
4. Effect on cell length: A- increase G- increase
5. Effect on cell division: A- none G- increase
6. Effect on cell wall: A- loosens G- loosens
7. Location where they stimulate growth: A- lateral buds, apical meristems G- stems at internodes.

Question 49

Describe how to investigate the effect of plant hormones on growth

Answer 49

Many different ways including:

1. Growing seeds hydroponically (in nutrient solution rather than soil), in serial dilutions of different hormones or applying different concentrations of hormones to the cut ends of stems or roots and observing the effects.
2. In most experiments it is important to make serial dilutions to observe the effects of different concentrations of the hormones as they can have different effects on growth at different concentrations
3. Experiments investigating the effect of hormones on plant growth usually involve large numbers of plants.
4. When you have completed your measurements, the spread of data from each experimental group should be measured using standard deviation

Question 50

Describe the idea of synergism and antagonism

Answer 50

1. Most plant hormones do not work on their own but by interacting with other substances- very fine control can be achieved
2. If different hormones work together, complementing each other and giving a greater response then they would on their own, the interaction is known as synergism
3. If the substances have opposite effects- one promoting growth and one inhibiting it, the balance between them will determine the response of the plant- antagonism.

Question 51

Describe why deciduous plants lose their leaves as a respnse to abiotic stress

Answer 51

1. As light and temperature affect the rate of photosynthesis, seasonal changes have a big impact on the amount of photosynthesis possible.
2. There is a point where the amount of glucose required for respiration to maintain the leaves, and to protect them against freezing is greater than the amount of glucose produced by photosynthesis.
3. Alos, a tree that is in leaf is more likely to be damaged or blown over by winter gales.

Question 52

How are plants sensitive to daylight

Answer 52

1. Photoperiodism- sensitive to lack of light in their environment
2. Many different plants responses are affected by the photoperiod including the breaking of dormancy of the leaf buds so they open up, the timing of flowering in a plant and when tubers are formed in preparation for overwintering.
3. The sensitivity to day length results from a light-sensitive pigment called phytochrome
4. This exists in two forms Pr and Pfr.
5. Each absorbs a different type of light and the ration of Pr to Pfr changes depending on the levels of light.

Question 53

Describe the role of plant hormones in controlling the ripening of fruits.

Answer 53

1. Ethene is incolved in the ripening of climacteric fruits- fruits that continue to ripen after they have been harvested. E.g bananas, tomatoes, mangoes and avocados
2. Their ripening is linked to a peak of ethene production triggering a series of chemical reactions including a greatly increased respiration rate.
3. Non climacteric fruits ( oranges, strawberries and watermelon) do not produce large amounts of ehten and do not ripen after picking
4. The effect of ethene can easily be seen if part of a bunch of green bananas is put into a bag with a single ripe banana.
5. The bunch with the ripe banana will ripen faster than the rest of the bunch, even if the temp is exactly the same in both cases. Ethene from ripe banana stimulates rapid ripening of green ones.

Question 54

Explain the benefits of using plant hormones to control ripening of fruit for sale in shops

Answer 54

1. Ethene is widely used commercially in the production of perfectly ripe climacteric fruit for greengrocers and supermarkets
2. These fruit are harvested when they are fully formed but long before they are ripe, and then cooled, stored and transported.
3. Then unripe fruit is hard and much less easily damaged during transport than ripe versions
4. When the fruit are needed for sale, they are exposed to ethene gas under controlled conditions.
5. This ensures that each batch of fruit ripens at the same rate and are all at the same stage to be put on the shelves for sale to the public.
6. This prevents a lot of wastage of fruit during transport and increases the time available for them to be sold.

Question 55

State the plant hormone in rooting powders and explain why rooting powders are useful.

Answer 55

1. Auxin affects the growth of both shoots and roots.
2. Scientists have discovered that the application of auxin to cut shoots stimulates the production of new roots
3. This makes it easier to propagate new plants from plant cuttings.
4. THis has made it much easier for horticulturists to develop cuttings to sell and for individuals taking their own cuttings.
5. Many plants are now propagated on a large scale by micropropagation- plant hormones are essential in this process- they control the production of the mass of new cells and then the differentiation of the clones into tiny new plants.

Question 56

Describe an example of how a hormonal weed killer works and explain why it is useful in agriculture

Answer 56

1. Weeds interfere with crop plants, competing for light, space, water and minerals
2. Scientists have developed synthetic auxins which act as very effective weedkillers.
3. Many of the main staple foods around the world are narrow-leaved monocot plants- rice, maize, wheat.
4. Most of the weeds are broad-leaved dicots.
5. If synthetic dicot auxins are applied as weedkiller, they are absorbed by the broad leaved plants and affect their metabolism.
6. The growth rate increases and becomes unsustainable so they die.
7. The narrow-leaved crop plants are not affected and continue to grow normally, freed from competition.
8. Auxins are cheap and simple to produce, have a very low toxicity to mammals and are selective

Question 57

List 7 other commercial uses for named plant hormones.

Answer 57

1. Auxins- can be used to produce seedless fruit
2. Ethene- is used to promote fruit dropping in plants such as cotton, walnuts and cherries
3. Cytokinins- are used to prevent ageing of ripened fruit and products such as lettuces and in micropropagation control tissue development.
4. Gibberellins- can be used to delay ripening and ageing in fruit, to improve the size and shape of fruits and in beer brewing to speed up the malting process.