5.4 PLANT RESPONSES

Question 1

What are cotyledon?

Answer 1

Cotyledon are plant organs that act as food stores for the developing embryo plants and form the first leaves when the seed germinates
• monocotyledonous plants (monocots)
- a plant that make seeds that contain one cotyledon
- e.g grasses, orchids
• dicotyledonous plant) (dicots)
- a plant that makes seeds that contain two cotyledons
- e.g roses, sunflowers

Question 2

What is the importance of plant hormones?

Answer 2

Plant hormones importance:
• plants are not mobile (they are rooted)
• they lack a rapidly responding nevous system
• however, plants show clear responses to their environment.
- communication between cells and even communication betwen different plants
• plants have evolved a system of hormones which are produced in one region of the plant and transported through transport tissue-to-cell to have an effect in another part or the plant

Question 3

What is auxin?

Answer 3

Auxin:
• controls cell elongation
• prevents leaf fall (abscission)
• maintains apical dominance
• stimulates release of ethene
• involved in fruit ripening

Question 4

What is giberellin?

Answer 4

Gibberellin:
• causes stem elongation
• triggers the mobilisation of food stores in a seed at germination
• stimulates pollen tube growth in fertilisation

Question 5

What is ethene?

Answer 5

Ethene
• causes fruit ripening
• promotes abscission in deciduous
trees

Question 6

What is abscisic acid (ABA) ?

Answer 6

Abscisic acid (ABA)
• maintains dormancy of seeds and buds
• stimulates cold protective responses (e.g antifreeze production)
• stimulates stomatal closing

Question 7

Plant hormones and germination.

Answer 7

Plant hormones and germination:
• the growth of plants from germination of the seed to long-term growth are controlled by plant hormones and environmental factors

Question 8

What is germination?

Answer 8

Germination is the development of a plant from a seed or spore after a period of dormancy.

Question 9

Germination and gibberellins.

Answer 9

Germination and gibberellins:
1. when a seed absobs water, the embryo is activated- this process is imbibition
- this causes
• the cellular constituents to be rehydrated
• the seed coat to rupture as it swells
2. gibberellins are secreted from the embryo as a result of imbibition
•this activates the production of enzymes
•there is evidence that gibberellins switch on genes that code for enzymes
3. enzymes (proteses and amylases) break down the food stores of the seed
• food stores are found in differing locations in monocots and dicots
• dicots: no aleurone layer - starch found mostly in embryo
and not endosperm
• monocots: aleurone layer -food store in endosperm
4. the resulting assimilates can now be translocated and these assimilates cause embryo cell to become metabolically active and grow

Question 10

ABA and gibberellin inhibition.

Answer 10

ABA and gibberellin inhibition:
• ABA acts as an antagonist to giberellins (GA)
• the relative levels of both hormones determine when a seed will guminate
• under times of stress (eg high salinity) , GA concentration falls, and ABA concentration rises, preventing germination

Question 11

What is the evidence of the role of gibberellins?

Answer 11

Evidence for the role or gibberellins:
1. mutant varieties lack the gene to produce gibberellin, so the seeds do not germinate
2. gibberellins then applied to mutant seeds causing the seed to germinate as possible
3. gibberellin biosynthesis inhibitors, applied seeds did not germinate when the inhibitor was removed, they germinated as normal

Question 12

Cell elongation and gibberellins.

Answer 12

Cell elongation and gibberellins:
• gibberellins are the length of the internodes - the regions between
leaves in the stem
• scientists have bred many dwarf varieties of plants where the gibberellin synthesis pathway
• without gibberellin plant stems are much shorter
• makes the plant less tr vulnerable to damage by the weather

Question 13

The discovery of gibberellin.

Answer 13

The discovery of gibberellin:
• gibberellins were discovered because they produced a fungus from the genus fibberella that affects rice
- infected seedlings grew extremely tall and thin
• scientists investigated the rice, isolating the chemical that produced the spindling growth - gibberellins
• it was then discovered that plants also contained these compounds

Question 14

What is herbivory?

Answer 14

Herbivory is the process of animals eating plants.

Question 15

What are plants physical defences against herbivory?

Answer 15

Physical defences against herbivory.
• hairy laves
• spines
• inedible
• tissue

Question 16

What are tannins?

Answer 16

Chemical defences - Tannins:
• part of a group of compounds called phenols produced by many plants
• tannins have a very bitter taste which puts animals off from eating their leaves
- bind to the digestive enzymes produced in the saliva and inactivate them

Question 17

What are alkaloids?

Answer 17

Chemical defences - Alkaloids:
• very bitter basting, nitrogenous compounds found in plant
• many alkaloids act as drugs, affecting the metabolism or animals that take them in and sometimes poisoning them
• eg caffeine- toxic to fungi and insects → caffeine produced by coffee brush seedlings spreads through the soil and prevents the germination of seeds of other plants. caffeine protects against herbivores and competition
• e.g nicotine→ toxin produced in the roots of tobacco plants, transported to the leaves and stored in vacuoles to be released upon leaf consumption

Question 18

What are terpenoids?

Answer 18

Chemical defences - Terpenoids:
• a 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
• pyrethin acts as an insect neurotoxin interfering with insects’ nerous system (produced by chrysanthemums)
• citronella acts as an insect repellent (produed by lemongrass)

Question 19

What are pheromones?

Answer 19

Chemical defences- pheromones:
•a pheromone is a chemical made by an organism which affects the social behaviour of other members of the same species
• plants do not behave socially so do not rely on pheromones very often
• when attacked by pheromones, a maple tree has 2 effects
→ when attacked the maple tree releases pheromones that are absorbed by leaves on other branches- these leaves then make chemicals (e.g callot) to help protect themselves
→ some plants secrete chemicals in their root systems that tell a neighbouring plant that there is water stress
•plants also produce vocs -volatile organic compounds which act like pheromones between the plants and insects. they reach insects by diffusing through the air around the plant, much like a pheromone
• when cabbages are attacked by caterpillars, they produce a chemical that attracts cotesia glometra (parasitic wasp). the wasp lays its eggs in the caterpillars Which are then eaten alive
• apple trees produce voc’s when they have been attacked by aphids, and these repel other aphids from the plant.

Question 20

What is mimosa pudica?

Answer 20

Other responses- mimosa pudica:
• conventional defences against herbivores such as a sharp, prickled stem, being a store of toxic alkaloids
• unconventionally it has the ability for its leaves to fold down and collapse upon contact
• this is thought to frighten off large herbivores and dislodge small insects have landed on the leaves
• the laves recover after around 10 minutes, as a result of the influx of potassium ions, followed by the osmobic movement or water

Question 21

What is antagonism?

Answer 21

Antagonism- if plant homones have opposite effects, the balance between the two determines the response

Question 22

What is synergism?

Answer 22

Synergism- if plant hormones have similar effects, they compliment each other and increase the response

Question 23

Auxin production and transport.

Answer 23

Auxins:
• produced in tips or roots and shoots and meristems
- diffuse in transport tissue from cell to cell
• concentration determines effects

Question 24

Growth of apical shoot.

Answer 24

Auxins- growth of apical shoot:
1. auxin binds to receptors on the cell membrane
2. hydrogen ions are pumped into the cell wall, lowering the pH - this is the optimum pH (5) for the enzyme to make the cell wall more flexible and plastic
3. vacuoles within the cell form and expand as they absorb more water
4. cell wall stretched
5. a large, central vacuole forms as cells mature and enzymes m destroy auxin, causing a rise in pH
6. the cell wall is now rigid, therefore no further elongation occurs

Question 25

Suppression of lateral shoots.

Answer 25

Auxins- suppression of lateral shoots:
apical dominance - the growth and dominance of the main shoots as a result of the supression or the lateral shoots by auxin
• growth in the main shoot is stimulated by the auxin produced at the tip so it grows quickly
• lateral shoots are inhibited by the auxin that moves down the stem, so they do not grow very well
• further down the stem the auxin concentration is lower, so lateral shoots grow man strongly

Question 26

Low concentration and root growth.

Answer 26

Auxins - low conc and root growth.
• up to a certain concentration, the more auxin that reaches the roots, the more they will grow
• auxin is produced in the root tips and some auxin reaches the roots from growing shoots
• however, high concentrations of auxin inhibit root growth.

Question 27

How is auxin similar to animal hormones?

Answer 27

Auxin similar to animal hormone:
• both produced in one part of the organism and transported to another
• affect a target area
• affect cells by binding with receptors and bringing about a response

Question 28

How does auxin differ to animal hormones?

Answer 28

Auxin direrent to anima hormone:
• auxin is not a protein or steroid
• it is not made in a gland
• not transported through blood stream

Question 29

Ethene and control of ripening.

Answer 29

Ethene - control of ripening
• gaseous hormone
• climacteric fruits produce high concentrations or ethene e.g bananas, tomatoes, mangoes and avocados
• high ethene concentration = increase in chemical e.g increase in respiratory rate= ripening
•changes that occur as a result of ripening.
• changes in cell wall structure
• pigment accumulation
• flavour and aromatic volatile production
• conversion of starches in sugar

Question 30

Auxin and micropropagation.

Answer 30

Auxin-micropropagation:
• a cutting is a small piece of the stem of a plant, usually with leaves on
• micropropagation is used to multiply plants such as those that have been genetically modified or bred through conventional plant breeding methods

Question 31

Auxin and hormonal weed killer.

Answer 31

Auxin- hormonal weedkiller:
• synthetic auxins act as very effective weedkillers
• monocot plants with narrow leaves such as rice, maize, and wheat are staples of diets
• most weeds are dicot plants with broad leaves which absorb the chemicals
• affecs their metabolism by increasing growth rate so much that it is unsustainable and they die

Question 32

Auxins and seedless fruit.

Answer 32

Auxins - sedless fruit:
• natural and artificial auxins supplied externally to unpollinated flowers can induce fruit growth in tomato and in other horticultural plants
• suggests that these hormones can replace the signals provided by pollination and fertilisation
• a fruit that develops as a result of this is called a parthenocarpic fruit.

Question 33

What is photoperiodism?

Answer 33

Photoperiodism is the response of an organism to seasonal changes in the period of daylight hours
→ many plant responses are affected by photoperiodism including:
• the breaking or dormancy of leaf buds so that they open up
• timing or flowering in a plant
• when tubers (storage stem of certain plants e.g Potato) are formed for overwintering

Question 34

What is daylight sensitivity?

Answer 34

Daylight sensitivity:
•plants are responsive to day length due to the presence of two pigments of phytochrome
• the ratio or each alters in response to differing levels of light
• red right - Pr 7 stimulates seed germination (produced when more daylight)
• far red light - Pfr inhibits seed germination (produced when less daylight)

Question 35

Light intensity and abscission.

Answer 35

Light intensity and abcission:
abscission-the fall of leaves from a plant
• light and temperature affect the rate of photosynthesis, therefore seasonal changes affect plants significantly
• eventualy there comes a point when-
- demand for glucose (for respiration to maintain leaves and to produce the chemicals required to protect plants against freezing is greater than the amount of glucose produced by photosynthesis

Question 36

How does abscission work?

Answer 36

Abscission- how it works:
1. there is a fall in daylight hours
2. as a result, there is a fall in the concentration of auxin
3. leaves respond to this, by producing the gaseous hormone ethene
4. between the stem and the leaf petiole, there is a region called the abcission zone that is made up of two layers of layers that are sensitive to ethene
5. ethene initiates gene switching (gene regulators in these cells, resulting in the production of new enzymes
6. the enzymes digest the cell wall in the outer layer or the abscission zone known as the separation layer
7. vascular bundles are then scared off between the leaf and the stem, and fatty material is deposited in the stem side of the separation layer. this foms a protective layer to prevent the entry or pathogens
8. cells deep within the separation zone respond to further hormonal cues by retaining water and swelling, adding strain to the outer layer
9. further abiotic fav such as low temperature and high winds finish the process - the leaf separates from the plant and a waterproof scar forms

Question 37

Temperature and preventing freezing.

Answer 37

Temperature and preventing freezing:
•a decrease in temperature affects many plants
• if cells freze, membranes become disrupted, and they will die
• plants have developed many adaptations to prevent this

Question 38

Preventing freezing adaptations (plants).

Answer 38

Preventing freezing adaptations:
• cytoplasm and vacuoles of plant cells contain solutes that lower the freezing point or the cell
• some plants produce sugars, amino acids, polysaccharides and proteins that act as antifreeze to prevent the cytoplasm from freezing/cell damage
• frost resistant chemicals activated in winter and suppressed in warmer temperatures

Question 39

Water availability and stomatal control.

Answer 39

Water availability and stomatal control:
• stomata play an important role in response to change in temperature and water availability
• the role or ABA is fundamental for stomatal control
• initially it was thought that leaf cells release ABA under abiotic stress causing stomatal closure.

Stomatal conbrol :
• scientists now believe that roots provides an early warning of water stress, releasing ABA from roots which is transported to the leaves
1. ABA binds to receptors on stomatal guard cell
2. ABA activates in ionic concentration of guard cells, raising the water potential and therefore, turgor of cells as water leaves by osmosis
3. reduced turgor causes the guard cells to close and water loss is reduced.

Question 40

What are mastic movements?

Answer 40

Nastic movements- relatively rapid responses of a plant to a stimulus, non-directional usually from changes in turgor

Question 41

What is a tropism?

Answer 41

A tropism is a growth response in a plant to a unidirectional stimulus
positive tropism = towards stimulus
negative tropism = away from stimulus

phototropism- light
geotropism- gravity
thigmotropism- touch stimulus
chemotropism- chemical stimulus

Question 42

Phototropism introduction.

Answer 42

Photo tropism - introduction:
*light can be unilateral (from one direction) or multilateral (from all directions)
• stems show positive phototropism
• roots show negative phototropism

Question 43

Multilateral light.

Answer 43

Multilateral light:
plants will grow straight upwards in both low level and high level multilateral light.

Question 44

Unilateral light.

Answer 44

Unilateral light:
plants in partially shaded regions will grow towards the light to maximise light available for photosynthesis) - this is a result or the way auxin moves within the plant under the influence of light
• blue light activates phototropism’s
• it is believed that light causes auxin to diffuse laterally across the shoot, so there is a greater concentration on the shaded side
- cell elongation on the shaded side then results in observed growth towards the light

Question 45

What is geotropism?

Answer 45

Geotropism :
• in normal conditions, plants always recieve a unilateral gravitational stimulus. gravity always acts downwards
• shoot are usually negatively geotropic (growing away from the gravitational pull)
•roots are positively geotropic (growing towards the gravitational pull
• this ensures roots grow into the soil and shoots grow towards the light

Question 46

What is the process of cell elongation?

Answer 46

Process or cell elongation :
1. auxin binds to receptors on the cell membrane
2. H+ ions are pumped into the cell wall, lowering the pH -this is the optimum pH (5) for the enzyme to make the cell wall more flexible and plastic
3. racols within the cal form and expand sis thy form mor water
4. cell wall stretches
5. a large, central vacuole forms as cells mature and enzymes destroy auxin causing a rise in pH
6. the cell wall is now rigid therefore no further elongation occurs