5.1.5 Plant Responses (includes practical skills)

Question 1

what are the 4 main reasons for a plant to respond to its environment?

Answer 1

Limit abiotic stress (drought)
Deter herbivores
Increase photosynthesis rate
Decrease spread of pathogens

Question 2

What is abiotic stress?

Answer 2

a non-living environmental factor which can cause harm to the plant, i.e drought

Question 3

why do plants respond to abiotic stress?

Answer 3

increases their chances of suvival

Question 4

what are the two main response type

Answer 4

response to herbivory and abiotic factors

Question 5

what does herbivory cause?

Answer 5

loss of leaf surface area and hence decrease in the rate of photosynthesis

Question 6

what are the reasons to stop herbivory?

Answer 6

Prevent the loss of leaf surface area
prevent the opportunity for vectors to enter the plants and allow pathogens to damage the tissues

Question 7

What are the 2 types of responses to herbivory?

Answer 7

Chemical defences
Physical defenses

Question 8

What are the two physical defenses to herbivory?

Answer 8

Permenant structures
Folding,curling or collapsing of the leaves to touch

Question 9

what are the five chemical responses to herbivory?

Answer 9

Volatile organic compounds
Pheromones
Tannins
Alkaloids
Terpinoids

Question 10

How to remember the five chemical responses to herbivory?

Answer 10

Violent
people
talk
angrily
too

Question 11

What is VOC short for?

Answer 11

Volatile organic compounds?

Question 12

what is a volatile organic compound?

Answer 12

chemicals released into the atmosphere by plants which attract or repel other species

Question 13

give an example of a VOC

Answer 13

cabbages attacked by white cabbage butterfly caterpillars
cabages produce voc which attracts a parastic wasp ‘Cotesia glomerata’
wasp lays eggs in the caterpillars which hatch and eat+kill caterpillars
The VOC also deter other white cabbages butterflies from laying any eggs

Question 14

what are tannins?

Answer 14

phenols w/ a bitter taste to animals and toxic to insects

Question 15

where are tannins present?

Answer 15

Grapes and pomegranates

Question 16

What % of dry mass does tannins take up?

Answer 16

up to 50%

Question 17

What are alkaloids?

Answer 17

Bitter tasting nitrogenous compounds
bitter taste = deters animals
in high conc = toxic, disrupt metabolism of herbivore = dead

Question 18

give two example of alkaloids

Answer 18

caffeine and cocaine

Question 19

what is a terpionid?

Answer 19

large group of compounds which form essential oils
act as toxins against insects and fungi

Question 20

give an example of a terpinoid

Answer 20

Citronella - produced by lemon grass
Repels insects

Question 21

What is a pheromone?

Answer 21

chemicals released into the atmosphere
changes behaviour of members of the same species

Question 22

give an example of a use of a pheromone

Answer 22

maple tree attacked by insects
pheromone released
pheromone absorbed by leaves on other branches
those leaves produce more cellulose

Question 23

what can VOCs sometime do?

Answer 23

attract or repel predeators to a prest, but also act as a pheremone to warn other plants of the threat

Question 24

what is a permenant strucutre of a plant to prevent herbivory?

Answer 24

Thorns, spines, hairy leaves, and stingers

Question 25

what is The folding, curling or collapse of leaves in response to touch?

Answer 25

in the plant mimosa pudica (Shameplant)
if leaves are touched, they fold down and collapse
Dislodges insects, and scares off herbivores
Returns to normal after 10 mins (k+ ions into cells, water moves in by osmosis, cells turgid, returns to original)

Question 26

how do the leaves of mimosa pudica return to normal?

Answer 26

k+ ions into cells, water moves in by osmosis, cells turgid, returns to original

Question 27

what is tropism?

Answer 27

a directional growth response of the shoots or roots of a plant, towards or away from an environmental stimulus

Question 28

what are the two movements of tropism?

Answer 28

positive tropism – growth of shoots or roots towards the stimulus;
negative tropism – growth of shoots or roots away from the stimulus

Question 29

how did tropism come about?

Answer 29

natural selection

Question 30

what is the stimulus of phototropism?

Answer 30

directional light

Question 31

what is postive phototropism

Answer 31

shoots of a plant gives more success when there is
competition for light. This in turn allows a higher rate of photosynthesis and so increased growth.

Question 32

what is Negative phototropism

Answer 32

roots - grow into the soil (region of lowest light intensity), anchoring the plant in position and increasing opportunities for water and mineral ion absorption.

Question 33

what is the stimulus of geotropism/gravitropism

Answer 33

Gravity

Question 34

what is postive geotropism

Answer 34

roots - grow down into the soil, anchoring the plant in position and increasing opportunities for water and mineral ion absorption

Question 35

what is Negative geotropism

Answer 35

Shoots - grow upwards, in a direction which
increases the likelihood of greater light
availability for the leaves

Question 36

what do we use to investigate geotropism?

Answer 36

clinostat

Question 37

what is the stimulus of thigmotropism?

Answer 37

touch or surface contact

Question 38

what is postive thigmotropism (no negative)

Answer 38

stems/tendrils of soft‐stemmed climbing plants allows them to coil around solid structures, supporting the plant as it grows taller and thus allowing its leaves more access to light.

Question 39

what is the stimulus for hydrotropism?

Answer 39

water

Question 40

what is positive hyrdotropism?

Answer 40

roots grow into areas of higher water potential, allowing increased uptake of water into the roots by osmosis.

Question 41

what is the mechansim for positive phototropism?

Answer 41

1. photoreceptors in shoot tip detect direction of light
2. cells in shoots produce auxins
3. auxin transported downwards from the tip, and accumulates on the shaded side
4. in zone of elongation, the higher conc of auxins on shaded side cause increase in cell elongation:
   a) auxins bind to complementary receptors
   b) enzyme activates enzyme which break cross-links between cellulose microfibrils
   c) cell wall becomes more stretchy
   d) stretchy cells elongate more when they take water in by osmosis
5. different extent of cell elongation on light and shaded die causes stem to bend as it grows towards the light source
6. The growth of the shoots to the light source makes it more competitive for light, and therefore a higher rate of photosynthesis and glucose production

Question 42

what are the name of the photoreceptor?

Answer 42

phototropins

Question 43

what do auxins do on the shaded side of the cell?

Answer 43

in zone of elongation, the higher conc of auxins on shaded side cause increase in cell elgation:

a) auxins bind to complementary receptors
b) enzyme activates enzyme which break cross-links between cellulose microfibrils
c) cell wall becomes more stretchy
d) stretchy cells elongate more when they take water in by osmosis

Question 44

what is this showing? label it.

Answer 44

the mechanism for positive phototropism

Question 45

what is the experimental evidence for phototropism?

Question 46

practical method into phototropism

Question 47

practal investigation into geotropism

Question 48

what are plant hormones?

Answer 48

cell signalling molecules in plants that coordinate growth, reproduction, an responses to herbivoes, pathogens and abiotic stress.

Question 50

why are plant hormones difficult to research?

Answer 50

1. effective in low concs ∴ difficult to measure
2. different effects depending on concentraion, ie low conc of auxin = growth, but high conc of auxin = inhibits growth
3. Different hormone combinations have different effects
4. The hormones may have different effects depending on the tissue type or stage of development
5. different effects in different species of plant

Question 51

two types of effects with two different plant hormones

Answer 51

Antagonistic - opposing effect (relative conc = direction of response)

Synergistic = similar effects and together give stronger effect

Question 52

in plants, where is the highest concentration of hormones

Answer 52

Growing tissues (MERISTEM TISSUE)

i.e shoot and root tips

Question 53

Do plants have endocrine glands and blood to carry hormones?

Answer 53

NO

Question 54

How do plant hormones travel around the plant

Answer 54

Directly from cell-to-cell via simple diffusion via plasmodesmata

carrier or channel proteins to cross plasma membrane by facilitated diffusion or active transport

transpiration steam and/or translocation (hormones detected in cell sap)

Question 55

what are the four plant hormones?

Answer 55

auxins

giberelleins

ethene

ABA (abscisic acid)

Question 56

what are the roles of auxins?

Answer 56

cell elongation

prevents leaf abscision

apical dominance

tropism

stimulates release of ethene

growth of roots

Question 57

what are the roles of giberellins?

Answer 57

stem elongation

triggers metabolism of food stores during germination

stimulate pollen tube growth in germination

Question 58

what are the roles of ethene?

Answer 58

causes fruits to ripen

leaf abscision

Question 59

what are the roles of ABA (Abscisic acid)?

Answer 59

maintains domancy of seeds and buds

stimulates cold protective responses (anti-freeze production)

Question 61

why does a plant need to be able to close its stomata?

Answer 61

Water stress is a form of abiotic stress in which the rate of water vapour loss in transpiration is higher than the rate of water uptake by root hair cells from the soil.

This imbalance leads to loss of cell turgor and hence the wilting of stems and leaves.

Wilting causes decreased photosynthesis rate, as leaves are no longer held up to the light optimally.

The closure of stomata is a response to water stress. This response can be highly effective since a very high proportion of transpirational water loss occurs as diffusion of water vapour through open stomata.

Question 62

mechanism for stomatal closure

Answer 62

1. The hormone ABA is released by leaf cells that suffering a decrease in water potential and a loss of turgor.
2. ABA acts as a signal to guard cells, which have complementary receptors in their plasma membranes that ABA binds to.
3. Guard cells respond to ABA by losing their cell turgor via water loss by osmosis to surrounding cells, such that both guard cells in a pair collapse and seal the stoma between them.
4. Once stomata are closed, there is a decrease in loss of water vapour by transpiration,
   hence more water retained in the plant

Question 63

what are the early warning signs for a response to water stress?

Answer 63

1. root cells in drier soil produce ABA
2. which is carried to the leaves in the transpiration stream
3. ABA binds to receptors on guard cells’ plasma membranes
4. triggers the pre‐emptive stomatal closure, even before the leaves begin to wilt

Question 64

where is the apical meristem found?

Answer 64

shoot tip

Question 65

what does the apical meristem responsible for?

Answer 65

upwards growth of main stem, cuased by repeat mitosis in the apical meristem

Question 66

what is apical dominance?

Answer 66

where the presence of the apical meristem suppresses growth of side shoots (branches) from the lateral buds that occur at intervals down the stem.

Question 67

what is the reason for apical dominance?

Answer 67

1. all the plant’s resources are used to fuel upward growth at the highest possible rate, rather than outwards growth - Allows leaves of the plant above those of other plants, giving a competitive advantage in light absorption
2. prevents the plant becoming too ‘bushy,’ which could result in overcrowding of leaves that they shade each other (reduces rate of photosynthesis)

Question 68

what is the apical dominance mechanism?

Answer 68

1. The apical meristem in the shoot tip produces very high auxin concentrations.
2. The auxin diffuses down the stem.
3. If the lateral buds experience high enough auxin concentrations, their growth is suppressed, i.e. they are kept in a dormant state, and no side shoots grow.
4. All the plant’s resources are used to fuel upward growth only.
5. Eventually, once the plant becomes very tall, the lowest lateral buds may no longer experience sufficiently high auxin levels to maintain their dormancy.
6. Hence the lowest lateral buds are the first to begin side shoot growth.

Question 69

experimental evidence for the role of auxins in the control of apical dominance.

Answer 69

1. Removal of the shoot tip results in the growth of side branches from the lateral buds - auxin level in lateral buds is low once site of auxin production is removed.

If paste containing auxin is immediatly place in the area where the shoot tip was removed, side shoots do not grow

(apical dominance maintained - auxin diffusing down the stem - high conc at lateral buds - growth inhibited)

2. If a plant that shows apical dominance is turned and kept upside down, side branches start to grow

apical dominance lost

auxin not transported to the lateral buds in high concs to supress growth if it moves against gravity.

Question 70

what is leaf abscision?

Answer 70

loss of leaves

Question 71

why do decidouous trees want to lose thier leaves?

Answer 71

winter = the days are short = low rate of photosynthesis

if leaves were present, no benefit. Glucose used in respiration and water lost via transpiration (Leaves = disadvantage)

Question 72

when plants lose their leaves, where do the plants get their sugars from?

Answer 72

hydrolysis of stored starch in the roots, translocated in the phloem to where sugars are needed

Question 73

what must occur for leaf abscission to occur?

Answer 73

there must be a weakening in a layer of cells (the abscission or separation layer) at the base of the leaf stalk (petiole), where it joins to the main stem

Question 74

what main hormones is responsible for leaf abscision?

Answer 74

auxin = pevents

ethene = removes

Question 75

what is the mechansim for leaf abscision?

Answer 75

1. Through the summer, high levels of auxin secretion by (young) leaves inhibit ethene production and therefore prevent abscission. Auxin is antagonistic to ethene.
2. In autumn, triggered by the increased length of the dark period (night), auxin levels fall and ethene production therefore increases.
3. As a consequence of higher ethene levels in the leaf, chlorophyll is broken down (resulting in the loss of green colour from the leaf; the red/orange/yellow colours result from the increase visibility of accessory pigments called carotenoids).
4. The magnesium ion (Mg2+) from the centre of each chlorophyll molecule is transported in the phloem to cells elsewhere in the plant for storage, so it can be reused in the spring for synthesis of new chlorophyll. These ions are too valuable to be lost in the leaves that are about to fall!
5. Once withdrawal of valuable ions from the leaf is complete, a protective layer of waterproof, impermeable suberin (a waxy material) is produced in abscission zone, in the part of stem adjoining the base of the petiole.
6. This protective layer will seal the wound left when the petiole breaks away, and in particular blocks the vascular tissue (xylem and phloem). This removes the opportunity for pathogens to enter the wound, and prevents leakage of sugars and evaporation of water from exposed vascular tissue.
7. Ethene also stimulates synthesis of cellulase and pectinase enzymes, resulting in digestion of the cellulose and pectin in cell walls and middle lamellae (sticky material holding cells together) specifically in the abscission layer (also called the separation layer: the band of cells at the base of the petiole).
8. This cellulose and pectin digestion creates a line of weakness at the petiole base.
9. The final ‘push’ for the leaf to fall will usually be a gust of wind.

Question 76

what is stem elongation?

Answer 76

increase in the length of internodes, which are the regions of stem between nodes (which are the location of meristem tissue and hence lateral buds and/or leaves).

Question 77

what causes cell elongation?

Answer 77

Gibberellins stimulate the elongation of plant stems

Question 78

how do gibberelins effect stem elongation?

Answer 78

1. stimulate cell division by mitosis
2. stimulate cell elongation

[Plants with high gibberlein conc have longer internodes, each internode contains more cells and each individual cell is more elongated]

Question 79

Experimental evidence for the role of gibberellins in the control of stem elongation

Answer 79

1. dwarf plants = lower gibberellin levels, fewer cells inbetween internodes + less elongated

adding gibberellins (via water or stem graft from regular plant) causes increased stem elongation

2. Genetic analysis of the dwarf plant usually reveals a mutation in a gene for a gibberellin‐synthesis enzyme or recpetors

Question 80

what hormone is responsible for seed germination?

Answer 80

gibberellins

Question 81

seed germination mechanism.

Answer 81

1. Water is absorbed through the testa (seed coat) into the seed (in a process called imbibition).
2. This triggers increased synthesis of the plant hormone, gibberellin, by the embryo within the seed.
3. ABA concentration in the seed must be low enough to allow gibberellin to stimulate the events that follow, since ABA is antagonistic to gibberellin.
4. Gibberellin activates transcription of the genes for amylase and protease.
5. Amylase and protease enzymes are synthesised.
6. Amylase hydrolyses stored starch into maltose, for use in respiration.
7. Protease hydrolyses stored proteins into amino acids, for use in translation.
8. Metabolic processes including respiration increase in rate, allowing increased cell division by mitosis, followed by cell differentiation.
9. A first shoot (radicle) and first root (plumule) emerge from the seed, breaking through the testa. This is possible due to the high hydrostatic pressure of elongating cells

Question 82

Experimental evidence for the role of gibberellin in the control of seed germination

Answer 82

Seeds from plants with a mutation in the gene coding for a gibberellin synthesis enzyme cannot make gibberellin upon imbibition of water; such seeds will fail to germinate, suggestion gibberellin synthesis is essential for germination.

If gibberellin‐synthesis inhibitors are applied to (normal) seeds, they fail to germinate

If gibberellin is applied externally to the mutant or inhibited seeds, they will germinate normally

Question 83

what are the three commercial uses of plant hormones?

Answer 83

Ethene to promote fruit ripening

auxins in rooting powder

synthetic auxins as weedkillers

Question 84

describe ethene to promote fruit ripening

Answer 84

Fruits such as bananas are picked when unripe, transported in this unripe state (as hard fruit are less easily bruised and damaged) and then exposed to ethene near/at point of sale.

The ethene causes the fruit to rapidly ripen.

Ensuring this happens only once the fruit is about to be sold reduces food waste due to damage in transit or the oversupply of ripe fruits in shops.

Question 85

describe auxin in rooting powders:

Answer 85

To stimulate root growth in a stem cutting, the base of the cutting is dipped in a ‘hormone rooting powder,’ containing auxin.

This increases the chance of successful vegetative propagation as the auxin stimulates growth of roots at the base of the cutting.

The cutting will now grow into a complete plant, a genetically identical clone of the plant the cutting was taken from.

Question 86

describe synthetic auxins as hormonal weedkillers

Answer 86

Synthetic auxins act as selective weedkillers that kill broad‐leaved weeds but not narrow‐leaved cereal crops (or grasses)

These chemicals are relatively cheap to obtain and are generally not toxic to humans and other animals;

This means that farmers can spray their crops (and gardeners their lawns) to selectively kill the weeds that compete with the crop (or grass) for light, space, water and mineral ions, increasing yield

The effect of the weedkiller is to cause rapid, unsustainable growth of the stems of the weeds, which become too weak to support themselves, collapse and die.