Plant responses

Question 1

E.g. of plant responses?

Answer 1

1. responses to abiotic stress
2. responses to herbivory
3. groth responses - tropisms
4. role of plant hormones

Question 2

responses to herbivory?

Answer 2

chemical & physical defences

Question 3

growth responses?

Answer 3

tropisms - directional growth responses

Question 4

e.g. of plant hormones?

Answer 4

• Auxins
• Gibberellin
• Ethene
• Abscisic acid (ABA)

Question 5

3 main types of plant response?

Answer 5

• Nastic movements - rapid changes caused by reversible changes in the turgidity of cells
• production of chemicals
• tropism

Question 6

Abiotic stress?

Answer 6

• changes in the abiotic (non living) factors in a plant’s envir

Question 7

What is a plants response to changing daylight and why?

Answer 7

• Many different plant responses are affected by the photoperiod including the breaking of dormancy of the leaf buds so that they open up, the timing of flowering in a plant and when tubers are formed in preparation of overwintering.
• The sensitivity of plants to day length/dark length results from a light-sensitive pigment called phytochrome.
• This exists in 2 forms- Pr & Pfr.
• Each absorbs a different type of light and the ratio of pr to Pfr changes depending on the levels of light.

Question 8

Photoperiodism ?

Answer 8

plants are sensitive to a lack of light in their environment.

Question 9

plants response to freezing T?

Answer 9

• The cytoplasm & sap in vacuoles contain solutes which lower the freezing point.
• Some plants produce sugars, polysaccharides, AAs and proteins which act as antifreeze to prevent the cytoplasm from freezing or to protect the cells from damage if they do freeze
• Most species only produce chemicals that make them frost resistant during winter

Question 10

plants response to high T?

Answer 10

• Stomata opened
• To cool the plant as water evaporates from the cells in the leaves in transpiration.
• The opening and closing of stomata in response to abiotic stresses is largely under the control of the hormone ABA`

Question 11

plants response to lack of water?

Answer 11

• Stomata closed
• The opening and closing of stomata in response to abiotic stresses is largely under the control of the hormone ABA.
• Leaf cells release ABA under abiotic stress, causing stomatal closure

Question 12

herbivory?

Answer 12

The consumption of plant material by a herbivore.

Question 13

Physical defences against a herbivore?

Answer 13

Thorns, barbs, spines, stings, fibrous inedible tissues etc

Folding in response to touch

Question 14

chemical defences against a herbivore?

Answer 14

Eg tannins, alkaloids, terpenoids, pheromones

Question 15

Thorns detail?

Answer 15

• thorns, fibrous indigestible tissue, etc

* hairy leaves & stings to protect themselves and discourage herbivores from eating them

Question 16

leaves folding up detail?

Answer 16

• Mimosa pudica defences such as toxic alkaloid and the stem has sharp prickles.
• If leaves are touched, they fold down and collapse, this frightens off larger herbivores and dislodges small insects.
• The leaf falls in a few s and recovers over 10-12 mins due to the K+ movement into specific cells, followed by osmotic water movement.
• When the leaf is touched, there is an electrochemical change in the cells which causes the active movement of K+ into cells on the upper, flexor side of the pulvinus, while K+ ions are similarly moved out of cells on the lower side.
• Water follows the K+ ions by osmosis, so turgor increases in the top cells and decreases in the lower.
• There are elastic tissues in the cells that increase this effect.
• As a result, the leaflet/ whole leaf bends down

Question 17

Chemical defences - tannins?

Answer 17

• phenols produced by many plants. Very bitter taste which deters animals from eating the leaves.
• Toxic to insects - they bind to digestive enzymes in saliva and inactivate them.

Question 18

Chemical defences - alkoids?

Answer 18

• bitter tasting compounds.
  • Many act as drugs affecting metabolism of animals and sometimes poisoning them.
  • Alkaloids include caffeine (toxic to fungi & insects, and caffeine produced by coffee bush seedlings spreads thru the soil preventing germination of seeds of other plants), nicotine

Question 19

Chemical defences - pheromones?

Answer 19

• chemical made by an organism which affects the social behaviour of other members of the same species.
• Plants produce volatile organic compounds which act like pheromones between themselves and other organisms.

Question 20

Chemical defences - terpenoids

Answer 20

Terpenoids - often form essential oils but also act as toxins to insects and fungi that may attack the plant. E.g. pyrethrin acts as an insect neurotoxin & citronella repels insects

Question 21

Positive photropism?

Answer 21

Plant shoots grow toward the light

Question 22

Positive geotropism?

Answer 22

roots grow towards gravity

Question 23

The mechanism of phototropism?

Answer 23

• Phototropin receptors become phosphorylated when they absorb blue light.
• This phosphorylation causes the sideways movement of auxin
• which is actively transported to the shady side of the plant, either due to a redistribution of transporter proteins, or a higher rate of transporter activity on the shaded side
• Auxin binds to receptor proteins, which causes H+ ions to be transported into the cell walls.
• This drop in pH activates proteins which break cross links in the cell walls, allowing them to elongate when water moves into the cell by osmosis

Question 24

auxin accumulates on the ?

Answer 24

shaded side of the shoot

Question 25

Positive and negative geotropism?

Answer 25

\+ = roots grow in the same direction as the force of gravity
- = - it grows in the opposite direction to the force of gravity.

Question 26

Plant hormones?

Answer 26

• cell signalling

* bind to receptors

Question 27

plant hormones are often present in?

Answer 27

small quantities

Question 28

plant hormones often act ?

Answer 28

together producing a different affect than them on their own. They Often have different effects in different species, in different parts of the same plant, or at different stages of its life cycle

Question 29

Role of ethene & auxin in leaf fall?

Answer 29

• Falling light levels result in falling auxin concentrations.
• The leaves respond to falling auxin conc by producing the gaseous plant hormone ethene.
• The abcission zone, found at the vase of the leaf stalk, is made up of 2 layers of cells sensitive to ethene.
• Ethene initiates gene switching in these cells resulting in the synthesis of new enzymes which digest and weaker the cell walls in the outer layer of the abcission zone.
• Vascular bundles are sealed off, fatty acids are deposited in the cells on the stem side of the separation layer.
• This layer forms a protective scar when the leaf falls, preventing pathogen entry.
• Cells in the separation zone respond to hormonal cues by retaining water and swelling, putting more strain on the already weakened outer layer.
• Further abiotic factors (e.f. wind) cause he leaf to separate from the plant.

Question 30

Seed germination: hormones?

Answer 30

• Gibberellin is produced by the seed embryo as it absorbs water.
• Gibberellin switches on genes that code for amylase and protease enzymes.
• These enzymes release respiratory substrate molecules which are used to make ATP
• The ATP is used for biosynthesis – growth – so the embryo can break out through the seed coat and produce leaves so photosynthesis can start

Question 31

experimental evidence for the role of gibberellins in seed germination?

Answer 31

• Mutant varities of seeds which lack the gene needed to make gibberellins don’t germinate
• if gibberellins are applied externally, they germinate
• if gibberellin biosynthesis inhibitors are applied to seeds they can’t make the gibberellins needed to break dormancy
• if inhibitor removed or gb applied, seeds germinate

Question 32

Stem elongation is caused by?

Answer 32

gb - causes growth between the nodes of a stem

Question 33

Apical dominance?

Answer 33

• Auxins, generally, stimulate growth
• Auxin molecules bind to receptors in cell membranes causing H+ ions to be transported by carrier proteins out of cells into the cell wall.
• This fall in pH causes the cell walls to remain flexible and elongate easily.
• Auxins are produced in the apical meristem, and cause apical dominance by stimulating growth of the apical shoot, and inhibiting growth of lateral shoots.
• If the apical bud is removed, apical dominance disappears, and lateral shoots grow making the plant bushier.

Question 34

the effect of plant hormones is diff depending on?

Answer 34

their conc

Question 35

Commercial uses of auxin?

Answer 35

• Auxins used to prevent fruit abscission. Fruits ripen more fully.
• Auxins used to promote fruit formation, even if flowers have not been pollinated. Seedless fruit.
• Hormone rooting powder. Auxins stimulate root growth from cuttings.
• Selective herbicides. Synthetic auxins kill weeds, but not grass.

Question 36

commercial uses of ethene?

Answer 36

Ethene causes fruits to ripen. Used to promote ripening after transportation.

Question 37

commercial uses of gibberellin?

Answer 37

• Promotes growth and aids production of eg. seedless grapes.
• Prevents fruit abscission.
• Increases yield of sugar cane.
• Stimulates germination.

Question 38

in general, what are the commercial uses of plant hormones?

Answer 38

1, control of fruit ripening - ethene
2. hormone rooting powder
3. micropropagation - cloning
4 selective weed killer - synthetic dicot auxin increase growth rate of weeds unsustainably causing them to die