16- plant responses

Question 1

What are some of the known functions of auxins within plants?

Answer 1

• Controls cell elongation
• prevents leaf fall (abscission)
• maintain apical dominance
• involved in tropisms
• stimulate the release of ethene
• involved in fruit ripening

Question 2

What are some of the functions of gibberellins in plants?

Answer 2

• cause stem elongation
• trigger the mobilisation of food stores in a seed at germination
• stimulate the pollen tube growth in fertilisation.

Question 3

What are some of the roles of ethene in plants?

Answer 3

• Causes fruit ripening
• promotes abscission in deciduous trees

Question 4

What are some of the roles of ABA (abscisic acid) in plants?

Answer 4

• Maintains dormancy of seeds and buds
• stimulates cold protective responses e.g. antifreeze production
• stimulates stomatal closing.

Question 5

Explain the role of plant hormones in seed germination.

Answer 5

• When the seed absorbs water, the embryo is activated and begins to produce gibberellins.
• This in turn stimulates the production of enzymes to break down the food stores found in the seed.
• The food store is in the cotyledons in dicot seeds and in the endosperm in monocot seeds.
• The embryo plant uses these food stores to produce ATP for building materials so it can grow and break through its seed coat.
• Evidence suggests that gibberellins switch on the genes which code for amylases and proteases (the digestive enzymes required for germination).
• These is also evidence suggesting that ABA acts as an antagonist to gibberellins, and it is the relative levels of both hormones that will determine when a seed germinates.

Question 6

Explain 2 pieces of experimental evidence that support the role of gibberellins in the germination of seeds.

Answer 6

• Mutant varieties of seeds have been bred which lack the gene that enables them to make gibberellins. These seeds don’t germinate. If gibberellins are applied to the seeds externally, then then germinate normally.
• If gibberellin biosynthesis inhibitors are applied to seeds, they don’t germinate as they cannot make the gibberellins needed for them to break dormancy. If the inhibitor is removed, or gibberellins are applied, the seed will germinate.

Question 7

What are auxins?

Answer 7

• they are growth stimulants produced in plants e.g. IAA
• They are made in cells in the tip of the roots and shoots, and in the meristems.
• They can move up and down the stem and up the root both in the transport tissue and from cell to cell.
• The effect of the auxin depends on its concentrations and any interactions it has had with other hormones.

Question 8

How do auxins stimulate the growth of the main apical shoot?

Answer 8

• Evidence suggests that auxins effect the plasticity of the cell wall- the presence of auxins means the cell wall stretches more easily. Auxin molecules bind to specific receptor sites in the plant cell membrane, causing a fall in the pH to about 5.
• This is the optimum pH for the enzymes needed to keep the walls very flexible and plastic.
• As the cell matures, auxin is destroyed. As the hormone levels fall, the pH rises so the enzymes maintaining the plasticity become inactive.
• As a result, the wall becomes rigid and more fixed in shape and size and the cells can no longer expand and grow.

Question 9

How does auxin stimulate apical dominance?

Answer 9

• High concs of auxins suppress the growth of lateral shoots. Growth in the main shoot is stimulated by the auxin produced at the tip so it grows more quickly.
• The lateral shoots are inhibited by the hormone that moves back down the stem, so they cannot grow very well.
• Further down the stem, auxin concs are lower, and so the lateral shoots grow more strongly.

Question 10

What is the experimental evidence for the role of auxin in apical dominance?

Answer 10

• If the apical shoot is removed, auxin producing cells are removed and so there is no auxin.
• As a result, the lateral shoots, freed from the dominance of the apical shoot, grow faster.
• If auxin is applied artificially to the cut apical shoot, apical dominance is reasserted and lateral shoot growth is supressed.

Question 11

How do low concentrations of auxin promote root growth?

Answer 11

• Up to a given conc, the more auxin that reaches the roots, the more they grow. Auxin is produced by the root tips and auxin also reaches the roots in low concs from the growing shoots.
• If the apical shoot is removed, then the amount of auxin reaching roots is greatly reduced and root growth slows and stops.
• Replacing the auxins artificially at the cut apical shoot restores the growth of the roots. High auxin concs inhibit root growth.

Question 12

What are the effects of gibberellins?

Answer 12

They are involved in the germination of seeds and are important in the elongation of plant stems during growth.
- They affect the length of the internodes, the regions between the leaves on the stem.

Question 13

How can gibberellins be used in farming?

Answer 13

• scientists have bred many different dwarf varieties of plants where the gibberellin synthesis pathway is interrupted.
• Without gibberellins, the plant stems are much shorter.
• This reduces waste and also makes the plants less vulnerable to damage by weather and harvesting.

Question 14

What is synergism and antagonism?

Answer 14

• If different hormones work together, complementing each other and giving a greater response than they would on their own, the interaction is known as synergism.
• If the substances have opposing effects e.g. one promoting growth and one inhibiting it, the balance between them is known as antagonism.

Question 15

Explain the process of abscission.

Answer 15

• Falling light levels results in falling concs of auxin. The leaves respond to the falling auxin concentrations by producing ethene.
• At the base of the leaf stalk is a region called the abscission zone. This is made of 2 layers of cells sensitive to ethene.
• Ethene seems to indicate 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.
• The vascular bundles which carry materials into and out of the leaf are sealed off. At the same tie, 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.
• Cells deep in the separation zone respond to hormonal cues by retaining water and swelling , putting more strain on the already weakened outer layer.
• Then further abiotic factors such as low temps or strong autumn winds finish the process.
• The strain is too much and the leaf separates from the pant. A neat, waterproof scar is left behind.

Question 16

Why do plants have to prevent themselves from freezing?

Answer 16

• If cells freeze, their membranes are disrupted and they will die.

Question 17

How do plants prevent freezing?

Answer 17

• The cytoplasm of the plant cells and the sap in the vacuoles contain solutes which lower the freezing point.
• Some plants produce sugars, polysaccharides, amino acids, and even proteins, which act as antifreeze to prevent the cytoplasm from freezing, or to protect the cells from damage even if they do freeze.
• Most species only produce the chemicals that make them hardy and frost resistant during the winter months.
• Different genes are suppressed and activated in response to a sustained fall in temps along with a reduction in day length.

Question 18

How does ABA control the opening and closing of stomata?

Answer 18

• The leaf cells appear to produce ABA under abiotic stress, causing stomatal closure. However, scientists think that the roots also provide an early warning of water stress through ABA.
• ABA is transported to the leaves where it binds to receptors on the plasma membrane of the stomatal guard cells.
• ABA activates changes in the ionic conc of the guard cells, reducing the water potential and therefore turgor of the cells. As a result of reduced turgor, the guard cells close the stomata and water loss by transpiration is greatly reduced.

Question 19

What are the common physical defences that plants have against herbivory?

Answer 19

Thorns, barbs, spikes, spiny leaves, fibrous and inedible tissues

Question 20

What are tannins?

Answer 20

• They’re a part of a group of compounds called phenols produced by many plants.
• They have a very bitter taste which puts animals off eating the leaves.
• They’re toxic to insects, they bind to the digestive enzymes produced in the saliva and inactivate them.
• Tea and red wine are both rich in plant tannins.

Question 21

What are alkaloids?

Answer 21

• A large group of very bitter tasting, nitrogenous compounds found in many plants.
• Many of them act as drugs, affecting the metabolism of animals that take them in and sometimes poisoning them.
• They include caffeine, nicotine, morphine and cocaine.
• Caffeine is toxic to fungi and insects and the caffeine produced by coffee bush seedlings spreads through the soil and prevents the germination of the seeds of other plants- so caffeine protects the plant both against herbivores and plant rivals.

Question 22

What are terpenoids?

Answer 22

• A large group of compounds produced by plants which often form essential oils but often act as toxins to insects and fungi that might attack the plant.
• Pyrethrin, produced by chrysanthemums, acts as an insect neurotoxin, interfering with the nervous system.
• Some terpenoids act as insect repellents e.g. citronella.

Question 23

What are pheromones?

Answer 23

• It is a chemical made by an organism which affects the social behaviour of other members of the same species.
• As plants don’t behave socially, they don’t rely a lot on pheromones

Question 24

Give an example of where pheromones have been used by plants to defend themselves.

Answer 24

• If a maple tree is attacked by insects, it releases a pheromone which is absorbed by leaves on other branches. These leaves then make chemicals such as callose to help protect them if they are attacked.
• It has also been observed that leaves on the branches of nearby trees also prepare for attack in response to these chemical signals.

Question 25

What are VOCs?

Answer 25

Volatile organic compounds, act like pheromones between themselves and other organisms, particularly insects.
- They diffuse through the air in and around the plant. Plants use these chemical signals to defend themselves.
- They are usually only made when the plant detects attack by an insect pest through chemicals in the saliva of the insect. This may elicit gene switching.

Question 26

Give an example of when plants use VOCs

Answer 26

• When cabbages are attacked by the caterpillars of the cabbage white butterfly, they produce chemical signals that attract a parasitic wasp.
• The insect lays its eggs in the caterpillars which are eaten alive, protecting the plant.
• The signal also deters any other female cabbage white butterflies from laying their eggs.

Question 27

How can ethene be used to control the ripening of fruit?

Answer 27

• Ethene is widely used commercially in the production of perfectly ripe climacteric fruit for greengrocers and supermarkets. These fruit are harvested when they are fully formed but long before they are ripe, and then cooled, stored and transported.
• The unripe fruit is hard and much less easily damaged during transportation around the world than the ripe versions.
• When the fruit are needed for sale, they are exposed to ethene gas under controlled conditions. 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.
• This careful control of ripening prevents a lot of wastage of fruit during transport, and increases the time available for them to be sold.

Question 28

How can hormone rooting powders be useful in micropropagation?

Answer 28

• Application of auxin to cut shoots stimulates the production of roots. This makes it much easier to propagate new plants from plant cuttings.
• A cutting is placed in compost or soil, or even water. Roots may eventually appear and a new plant forms.
• Dipping the cut stem in hormone rooting powder increases the chances of roots forming and successful propagation taking place.

Question 29

How do hormonal weedkillers work? How can they be used in agriculture?

Answer 29

• If the balance between hormones within a plant is lost, it can interrupt the metabolism of the whole plant and may lead to the death of the plant.
• Many of the staple foods around the world are narrow-leaved monocot plants e.g. rice. Most weeds are broad leaved dicots. If synthetic dicot auxins are applied as weedkillers, they are absorbed by the broad leafed plants and affect their metabolisms.
• The growth rate increases and becomes unsustainable so they die.
• The narrow leaved crop plants are not affected and continue to grow normally.

Question 30

What are other ways that plant hormones can be used commercially?

Answer 30

• Auxins in the production of seedless fruit
• Ethene to promote fruit dropping in plants e.g. cotton and cherries
• Cytokinins are used to prevent ageing of ripened fruit and products such as lettuces and in micropropagation to control tissue development.
• Gibberellins can be used to delay ripening and ageing fruit, to improve size and shape of fruits and in beer brewing to speed up the malting process.

Question 31

What is nastic movement?

Answer 31

• Relatively rapid response of a plant to a stimulus, non-directional, usually from changes in turgor e.g. a Venus fly trap. It is reversible.

Question 32

How does phototropism occur?

Answer 32

• Phytotropins in plasma membrane are activated by blue light
• Causes auxin to diffuse to shaded side of plant
• Auxin stimulates cell elongation
• once growing in that direction, unilateral stimulus is removed so auxin distribution becomes more even.