5.1.4 Plant responses

Question 1

What are biotic stresses?

Answer 1

-damage to a plant caused by living organisms i.e: fungi, bacteria, insects, herbivores

Question 2

What are abiotic stresses?

Answer 2

-natural, non-living potential harms to a plant i.e drought, extreme temps, rain
-i.e carrots produce antifreeze proteins at low temps(proteins bind to ice crystals and prevent frost damage by lowering the freezing point of water inside the cells)

Question 3

Herbivory

Answer 3

When plants are eaten by animals

Question 4

Physical defences to herbivory

Answer 4

-thorns
-spikes
-spiny/hairy leaves
-fibrous + inedible tissue
-stings to protect themselves and deter herbivores

Question 5

Chemical defences to herbivory

Answer 5

• can produce toxic chemicals to protect themselves
  -tannins= bitter and hard to digest because they can bind to proteins in the gut
  -alkaloids= bitter, noxious smells and some have poisonous characteristics i.e tobacco produce the alkaloid nicotine in response to tissue damage
  -terpenoids= act as toxins to insects/fungi i.e pyrethin can interfere with the nervous system, others act as repellents

Question 6

Pheromones

Answer 6

• signalling chemicals that produce a response in other organisms i.e alarm pheromones warn nearby plants to secrete callose (makes plant harder to eat)
  -VOCs(like pheromones between plants and other organisms) can diffuse through the air and can elicit gene switching(changing expression between alleles of a gene)
  -i.e corn plants can produce pheromones that attract parasitic wasps–} lay eggs in the caterpillars who eventually die

Question 7

Folding in response to touch

Answer 7

-i.e Mimosa pudica spreads a signal throughout the leaf, causing it to quickly fold
-knock off small insects feeding on the plant, frightens off larger herbivores

Question 8

What is tropism?

Answer 8

• plant growth responses to stimuli from one direction
  -phototropism= growth of plants in response to light from one direction
  -geotropism= growth of the plant in response to gravity
  -hydrotropism= plant growth in response to water(roots are positive)
  -thermotropism= response to temperature
  -thigmotropism= in response to contact with an obect

Question 9

How to tropisms help survival in environmental conditions?

Answer 9

-the direction of the response is related to the direction of the stimulus
-the shoot must grow up towards the light source for photosynthesis and the roots must grow downwards towards the soil for support and nutrients

Question 10

Positive vs Negative

Answer 10

-positive tropisms grow towards the stimulus
-negative tropisms grow away from the stimulus

Question 11

Phototropism

Answer 11

-Shoots are positively phototrophic and grow towards the light
-roots are negatively phototrophic and grow away from it
-helps ensure that shoots receive as much light as possible for max photosynthesis
-the roots will rapidly return back to the soil if they emerge from it i.e due to heavy rain

Question 12

Geotropism

Answer 12

-Shoots are negatively geotropic and grow upwards
-roots are positively geotropic and grow downwards
-adaptation ensures that the roots grow down into the soil and the shoots grow up into the light

Question 13

Plant hormones

Answer 13

-signalling molecules that enable plants to respond to changes in the environment
-help to:
avoid abiotic stress
avoid predators
access resources

Question 14

Auxins

Answer 14

-stimulating cell elongation, inhibits leaf fall(abscission), maintains apical dominance
-also stimulates the release of ethene, inhibit fruit ripening

Question 15

Indoleacetic acid(IAA)

Answer 15

-type of auxin produced in tips of shoots + roots in flowering plants and in the meristem
-moved up and down the plant* to control tropisms by diffusion and active transport(for short distances) and via the phloem(for long distances)
-uneven distribution if IAA causes uneven growth
*both in transport tissue and cell to cell

Question 16

IAA in phototropism

Answer 16

-moves to the shaded side
-stimulates shoot cell elongation on the shaded side(higher conc of IAA)–} results in growth towards the light
-inhibits root cell elongation

Question 17

How do plants still grow in the dark?

Answer 17

-biological imperative to grow upwards rapidly to reach light for photosynthesis
-seedlings that break through soil first don’t have to compete with other seedlings for light
-gibberellins involved–} can use resources for synthesising leaves + strengthening stems

Question 18

IAA in gravitropism

Answer 18

-moves to the underside of the shoots and roots, causing uneven growth
-promotes shoot cell elongation(grows upwards), inhibits root cell elongation(grows downwards)

Question 19

Investigating phototropism

Answer 19

-take 9 wheat shoots, roughly equal in height and plant them in individual pots in the same soil type
-prepare shoots by covering the tips of 3 shoots with foil, leaving 3 shoots without any foil and wrapping the bases of 3 shoots with foil so only the tip is exposed
-set up in front of light source(same distance, intensity, temp, exposure to moisture etc)
-leave to grow for 2 days and record amount of growth in mm and the direction of growth(quali + quanti)
(Shoots with exposed tips should have had positive phototropism and shoots with tips foiled should have been prevented from growth towards the light, as the tip is where IAA is produced)

Question 20

Investigating gravitropism

Answer 20

-line 3 petri dishes with moist cotton wool(same vol of water and amount of wool in each)
-space out 10 cress seeds on the surface of the cotton wool in each dish and push each seed down into the wool slightly
-tape a lid onto each dish and wrap each one in foil with no gaps(foil prevents light from reaching seeds and being affected by phototropism)
-leave dishes in a relatively warm and constant temp i.e cupboard(not cold otherwise seeds won’t germinate)
-set up your dished at different angles: 90 degrees by propping it upright via attaching it to a wooden block using tape, 45 degrees and attach it the same way, dish C is placed normally(horizontally)
-leave seeds for 4 days then unwrap each dish and note direction of the shoot and root growth of seedlings
(should find that all shoots have grown away from gravity and roots have grown towards gravity)

Question 21

Auxin + apical shoots

Answer 21

-auxin stimulate the growth of the main apical shoot
-can affect the plasticity of the cell wall(can stretch easier)
-auxin molecules bind to specific receptor sites in the plant cell membrane, causing pH to fall to about 5(optimum for enzymes needed to keep walls flexible)
-as the cells mature, auxin is destroyed–} pH rises so the enzymes maintaining plasticity become inactive
-wall becomes rigid and cells can no longer expand and grow

Question 22

Apical dominance

Answer 22

-high concs stimulate the growth of the apical shoot and inhibit the growth of lateral shoots near the top where auxins are produced
-saves energy and prevents side shoots from the same plant competing with the apical shoot tip for light
-allows a plant in an area with lots of plants to grow tall very fast and reach the sunlight

Question 23

Auxin on root growth

Answer 23

-low concs can promote root growth–} produced by the root/lateral tips and can reach the roots at low concs from the growing shoots
-if shoot tip is cut, root growth slows down and eventually stops but lateral shoots near the top grow a lot more
-replacing auxin artificially restores the growth of roots

Question 24

role of auxins in apical dominance

Answer 24

-if the apical shoot tip is removed, the lateral shoots can grow faster as auxin producing cells are removed, so there is no auxin
-if auxin is applied artificially to the cut apical shoot, apical dominance comes back and lateral shoot growth is inhibited
-if tip is removed and replaced with an agar block containing auxin, side shoots don’t grow

Question 25

Investigating the role of auxins on apical dominance

Answer 25

-plant 30 plants of the same type i.e pea plants that are a similar age, height and weight in pots containing the same type of soil
-count + record the number of side shoots growing from the main stem of each plant
-for 10 plants, remove the tip of the shoot + apply a paste containing auxin to the top of the stem
-for another 10 plants, remove the tip of the shoot and apply a paste without auxin to the top of the stem
-leave the final 10 plants as the untreated controls for comparison(know the effect is due to auxin and nothing else)
-leave plants to grow for 6 days(keep all plants in same conditions: light intensity, water etc)
-count number of side shoots growing from the main stem of each plant and create a table showing the average number of side shoots per plant at the start and end

Question 26

Gibberellins

Answer 26

-stimulate seed germination, causes stem elongation and cell division*, stimulates stomatal closure
-increase the length of the internodes= regions between the leaves on a stem, so they can get more light
*helps plants grow very tall i.e dwarf plants catching up to regular plant
-inhibited by Abscisic acid, relative levels of gibberellins and ABA determine whether a seed will germinate

Question 27

What is Abscisic acid?

Answer 27

-ABA is released by leaf cells and by roots as an early warning sign that water levels in soil have fallen
-maintains dormancy of seeds and buds
-stimulates cold protective responses i.e antifreeze production
-stimulates stomatal closing

Question 28

Synergistic

Answer 28

-auxin and gibberellins sometimes work together to affect plant growth + have a big affect
-i.e work together to help plants grow very tall

Question 29

Antagonistic

Answer 29

-auxin and gibberellins can have opposite effects i.e Gibberellins stimulate the growth of side shoots but auxins inhibit it
-ethene and auxin oppose each other in abscission and fruit ripening

Question 30

Why do deciduous plants lose their leaves in winter?

Answer 30

-triggered by the shortening day length in autumn
-helps to conserve water when it is hard to absorb water from the soil
-less light for photosynthesis
-amount of glucose required for respiration and to produce antifreeze chemicals is more than glucose produced from photosynthesis

Question 31

Effect of auxin on leaf loss

Answer 31

-inhibits leaf loss
-produced by younger leaves–} as leaf gets older, less auxin is produced leading to leaf loss
-allows for photosynthesis and glucose storage to occur

Question 32

Effect of ethene on leaf loss

Answer 32

-stimulates leaf loss
-leaves respond to falling auxin conc by producing ethene–} initiates gene switching in the abscission zone(2 layers of sensitive cells) producing new enzymes
-enzymes digest and weaken cell walls of abscission zone(separation layer)
-fatty deposits form on the stem side of separation layer, forming a protective scar t prevent pathogens entering
-also, as leaves get older, more ethene is produced

Question 33

Preventing freezing

Answer 33

-cytoplasm of the plant cells and sap in the vacuoles contain solutes that lower the freezing point
-antifreeze proteins to prevent cytoplasm from freezing/protect the cells from damage if they do freeze

Question 34

Stomatal closure

Answer 34

-happens under abiotic stress to reduce water loss via transpiration
-stomatal closure is due to guard cell becoming flaccid and the pores closing
- ABA can trigger stomatal closure because it binds to the receptors on the guard cell membranes= -cause specific ion channels to open, which allow calcium ions to enter the cytosol from the vacuole
-increases conc of calcium ions in the cytosol causes other ion channels to open which allow ions i.e potassium to leave the guard cell–} raises water potential
-water leaves via osmosis and guard cells become flaccid and stomata close

Question 35

Commercial use of ethene

Answer 35

-stimulates enzymes to break down cell walls, break down chlorophyll and convert starch into sugars
-makes fruit soft, ripe and ready to eat
i.e bananas are harvested and transported before ripe and exposed to ethene on arrival so they all ripen at the same time on the shelves or at customers’ homes

Question 36

Commercial use of auxins

Answer 36

• Selective weed killers: -make weeds produce long stems instead of lots of leaves, makes weeds grow too fast so they can’t get enough water/nutrients and die
• rooting hormones(in rooting powder): -make a cutting and plant it so it can be grown into a new plant. Lots of the same plant can be grown quickly and cheaper from just 1 plant

Question 37

Other commercial uses of plant hormones

Answer 37

-auxins can be used in the production of seedless fruit
-ethene is used to promote fruit dropping in plants i.e cotton, walnuts
-gibberellins can be used to delay ripening and ageing in fruits, to improve the size and shape of fruits and in beer to speed up the malting process

Question 38

Role of ethene

Answer 38

-promote abscission
-promote fruit ripening

Question 39

Evidence for hormonal balance

Answer 39

-mutate genes that make a hormone
-disrupt the production pathway(normally as the plant is growing)
-cut the shoot tip(removing meristems and therefore auxins)—} see changes to plant, apply hormones externally, observe further changes

Question 40

Seed germination

Answer 40

-water is absorbed by seed embryo which produces gibberellins
-activates genes for amylase and protease
-breaks down food stores into glucose which provides energy for respiration and to release energy needed for growth and to break through seed coat