Module 5 Section 4: Plant Responses

Question 1

Why have plants evolved responses to their environment

Answer 1

These plant responses have evolved because they provide the plant with a selective advantage (better adapted to survive and reproduce in the environment)

Question 2

What types of stimuli do plants respond to

Answer 2

The environmental stimuli to which plants respond can be abiotic or biotic

Question 3

Examples of plant responses

Answer 3

Tropisms
Responses to touch
Responses to herbivory
Responses to abiotic stress

Question 4

Examples of biotic and abiotic factors

Answer 4

Biotic:
Predators
Human activity
Competition

Abiotic:
Temperature
Soil
Salinity

Question 5

What do plants use hormones to do

Answer 5

Avoid predation – e.g. some plants produce toxic substances

Avoid abiotic stress – e.g. carrots produce “antifreeze” proteins that bind to ice crystals
and lower the freezing point of water, to prevent more ice crystals forming

Survive - to reproduce

Question 6

Why do plants need to respond to their environment

Answer 6

Adapt to changing environments
Avoid abiotic stress
Maximise photosynthesis (obtain more light, water and minerals)
Avoid herbivory
Ensure pollination and seed dispersal

Question 7

Plant physical defences and overall function

Answer 7

Thorns
Hairy leaves
Leaf folding
Bark
Waxy cuticle

Helps protect them from herbivory
( process by which animals eat plants)

Question 8

How do plants chemically avoid herbivory

Answer 8

Produce chemicals such as:
Tannins
Alkaloids
Pheromones

Question 9

What are tannins

Answer 9

Found in upper epidermis of the leaf
Have bitter taste
Toxic to microorganisms and insects
Bind to enzymes produced in saliva and deactivate them or bind to enzymes in the gut which make them hard to digest
Tannins are richly found in tea and red wine

Question 10

What are alkaloids

Answer 10

Chemical with bitter tastes, noxious smells or poisonous characteristics to deter herbivores
Can act as drugs which affect the metabolism of the animal which ingests it which can poison them
Located in growing tips and flowers
Nicotine is an alkaloid which is produced in response to tissue damage, nicotine is poisonous to many insects

Question 11

Examples of alkaloids

Answer 11

Caffeine (toxic to insects and fungi)
Nicotine (released when herbivores graze)
Capsaicinoids (make chillis hot)

Question 12

What are pheromones

Answer 12

Pheromones are chemicals which are released by one individual, which can affect the behaviour and physiology of another member of the same species
These chemicals are capable of acting like hormones outside the body of the secreting individual

Question 13

Examples of pheromones

Answer 13

Alarm pheromones: released by a plant under threat to alert nearby plants of a threat so they can increase chemical defences
Also: food trail pheromones and sex pheromones

Question 14

Forms of abiotic stress for plants

Answer 14

Freezing
Drought
Increased soil water salinity
Presence of heavy metals (e.g. lead, copper, zinc)

Question 15

How can some plants respond to drought and freezing stress

Answer 15

Plants respond to drought by shutting their stomata (reducing water loss through transpiration) or by dropping their leaves

In low temperatures, some plants produce an antifreeze chemical in their cells which decreases the formation of ice crystals, these crystals can destroy plant cells if allowed to form in them

Question 16

What are nastic responses

Answer 16

A non-directional response to stimuli, e.g. thigmonasty
E.g. plants can respond to touch by folding its leaves

Question 17

How do plants carry out the leaf folding response

Answer 17

E.g. can occur in mimosa pudica plant
The leaflets of the leaves fold in rapidly when touched
Caused by rapid water uptake (increase in volume) in cells at the base of each leaflet and rapid loss of water from (and collapse of) adjacent cells
This response is not related to the direction of the touch stimulus and is not considered a tropism so is a nastic movement

Question 18

What is a tropism

Answer 18

A tropism is a directional growth response in which the direction of the response is determined by the direction of the external stimulus
E.g. light or gravity

Question 19

What are positive and negative tropisms

Answer 19

Positive tropism: a growth response towards the stimulus e.g. light
Negative tropism: a growth away from the stimulus e.g. gravity

Question 20

Types of tropisms and what they are stimulated by

Answer 20

Phototropism: response to light (abiotic)
Geotropism: response to gravity (abiotic)
Hydrotropism: response to moisture (abiotic)
Thigmotropism: response to touch (abiotic or biotic)
Chemotropism: response to chemical (abiotic or biotic)

Question 21

How can a plant’s movement be both a positive and negative tropism

Answer 21

A plant’s growth towards light is positive phototropism but negative geotropism
A plant root’s growth into the soil is positive geotropism but negative phototropism

Question 22

What is phototropism

Answer 22

Where a plant grows towards the light to enable photosynthesis

Question 23

What is geotropism

Answer 23

When a root grows towards the pull of gravity which provides them with anchorage with the soil and helps take up water for photosynthesis

Question 24

What is chemotropism and give an example

Answer 24

The growth of an organism controlled by an external chemical stimulus
Occurs in flowers where pollen tubes are attracted by chemicals to grow down the style towards the ovary

Question 25

What is thigmotropism

Answer 25

Shoots of climbing plants can wind around other plants and structures to gain support

Question 26

What are plant hormones

Answer 26

Chemical messengers which coordinate responses in plants These hormones are produced in one region of the plant and travel (through the transport tissues or from cell to cell) to different parts of the plant in order to target them

Question 27

What can plant hormones do

Answer 27

Can bring about different effects on the plant E.g. cell elongation, cell division and cell differentiation

Question 28

Why do deciduous plants lose leaves in winter

Answer 28

Deciduous plant lose leaves in winter to help conserve water in times when it may be difficult to absorb water from the soil and when there is less light for photosynthesis This is triggered by the shortening day length in autumn Controlled by auxins and ethene

Question 29

Function of auxins in leaf loss

Answer 29

Auxins inhibit leaf loss They are produced by young leaves As the leaf gets older, less auxin is produced, leading to leaf loss

Question 30

Function of ethene

Answer 30

Ethene stimulates leaf loss Ethene is produced by ageing leaves As the leaves get older, more ethene is produced A layer of cells (abscission layer) develops at the bottom of the leaf stalk where leaf joins the stem Abscission layer separates the leaf from the rest of the plant Ethene stimulates the cells in the abscission layer to expand, breaking the cell walls and causing the leaf to fall off

Question 31

How do hormones cause stomata to close

Answer 31

The plant hormone abscisic (ABA) is able to trigger stomatal closure ABA binds to receptors on guard cell membranes Causes specific ion channels to open which allows calcium ions to enter the cytosol from the vacuole The increased concentration of calcium ions in the cytosol causes other ion channels to open These ion channels allow ions (such as K+) to leave the guard cells This raises the water potential of the cells Water then leaves the guard cells by osmosis Guard cells become flaccid and the stomata close

Question 32

Summary of hormones in stomatal closure

Answer 32

ABA binds to specific receptors on guard cell membranes ABA causes calcium ions (Ca2+) to move into the cytoplasm of the guard cells The increased concentration of Ca2+ causes other ion channels (e.g. K+) to open Ions leave guard cell which increases water potential inside the guard cell Water leaves the guard cells by osmosis Guard cells become flaccid causing the stoma to close

Question 33

What do seeds contain

Answer 33

Embryo: will grow into the new plant when the seed germinates Endosperm: starch-containing energy store surrounding the embryo Aleurone layer: a protein rich layer on the outer edge of the endosperm

Question 34

Function of gibberellin

Answer 34

Causes stem elongation Tiggers the mobilisation of food stores at seed germination Stimulate pollen tube growth in fertilisation

Question 35

Function of ABA (abscisic acid)

Answer 35

Maintains dormancy of seeds and buds Stimulates cold protective responses

Question 36

What are meristems and different types

Answer 36

These are areas where plant growth occurs Apical meristems: tips of roots and shoots Lateral bud meristems: give rise to side shoots Lateral meristems: help roots and shoots get wider

Question 37

What are auxins

Answer 37

Auxins are hormones produced at the tip of a shoot or root Regulate plant growth Moved by diffusion and active transport over short distances and through the phloem for longer distances Also called Indole-3-acetic acid (IAA)

Question 38

Role of auxins in the shoots

Answer 38

Stimulates cell elongation High concentration promotes plant growth

Question 39

Roles of auxins in the roots

Answer 39

Inhibit cell elongation High concentrations of auxin limit cell growth

Question 40

Explain how auxin acts in plant shoots (phototropism)

Answer 40

Auxin activates a proton pump in the plasma membrane. This causes the secretion of H+ ions into the cell wall. The resultant decrease in pH of the cellulose cell wall is the optimum pH for the enzyme ‘Expanase’. Expanase breaks the bonds in the cellulose fibres. Cell wall becomes more flexible. The cell can now take in more water and become turgid. This stretches/elongates the cell

Question 41

Explain how auxin acts in the plant roots (geotropism)

Answer 41

Cells in the tip of the root produce IAA, which is then transported along the root. The IAA is initially transported to all sides of the root. Gravity influences the movement of IAA from the upper side to the lower side of the root. A greater concentration of IAA builds up on the lower side of the root than on the upper side. More auxin in a root means LESS elongation. As IAA inhibits the elongation of root cells, and there is a greater concentration of IAA on the lower side, the cells on this side elongate less than those on the upper side. The relatively greater elongation of cells on the upper side compared to the lower side causes the root to bend downwards towards the force of gravity

Question 42

Results of Darwin’s experiment: Removing tip, then placing cap on tip

Answer 42

Tip removal: The tip of the coleoptile was responsible for detecting light and producing chemicals to react to the light Lightproof cap: The tip detects light and if covered, the plant will not be able to react to directional light

Question 43

Results of Boysen-Jensen’s experiment: Mica barrier

Answer 43

Mica on light facing side: Response is chemical as its inhibited by mica (not electrical) Mica on shaded side: Chemical response needs to be produced down the side opposite to the stimulus to cause cell elongation on shaded side Could not pass to this side so plant didn’t grow

Question 44

Results of Boysen-Jensen’s experiment: Gelatine block

Answer 44

The chemical response could diffuse through the gelatine block from the tip to the rest of the shoot

Question 45

Results from paál and Went‘s experiment: Tips and gelatine being off centre

Answer 45

Cutting off tip and placing it off centre in the dark: Side with tip grew more than the other side so shoot curved Allowing hormones to diffuse into gelatine block then placing it off centre on shoot in the dark: Side with gelatine block grew more so shoot curved Greater the concentration of hormone in block, the more coleoptile curved

Question 46

What is apical dominance

Answer 46

This is where the main central stem of the plant is dominant and grows more strongly than the side branches

Question 47

How does apical dominance occur

Answer 47

This is due to the apex of the stem which has an actively growing apical bud. The apical bud contains the plant hormone auxin. Auxin inhibits the growth of lateral buds. Removing the apical bud stops auxin being produced Results in the growth of lateral buds (that were previously dormant) Other hormones are also involved (e.g. abscisic acid and cytokinins)

Question 48

Function of abscisic acid and cytokinins in apical dominance

Answer 48

ABA: Inhibits all bud growth (not just lateral) Cytokinins: promote bud growth

Question 49

Importance of apical dominance

Answer 49

Ensures that a plant will use its energy to grow up towards the light in order to outcompete other plants As distance between apical bud and side buds increases, auxin inhibition diminishes so side buds grow further down

Question 50

How are auxins and gibberellins synergistic

Answer 50

They work together to allow the plant to grow taller

Question 51

How are auxins and gibberellins antagonistic

Answer 51

They oppose each others actions as gibberellins stimulate the growth of side shoots but auxins inhibit the growth of side shoots

Question 52

What are gibberellins

Answer 52

Produced in young leaves and in seeds Stimulate seed germination, stem elongation, side shoot formation and flowering Do not inhibit the plants growth in any way

Question 53

How do gibberellins stimulate seed germination

Answer 53

Trigger the breakdown of starch into glucose in seed Plant embryo in the seed can use the glucose to start respiring and release the energy it needs to grow Gibberellins are inhibited by abscisic acid (seed germination prevented)

Question 54

Different ways that plant hormones can be used commercially

Answer 54

As the growth and development of plants is controlled by hormones, these hormones can be used as: Selective weed killers Rooting powders Controllers of ripening

Question 55

How can ethene be used to control fruit ripening

Answer 55

Ethene stimulates enzymes that break down cell walls, they break down chlorophyll and convert starch into sugars This makes the fruit soft, ripe and ready to eat E.g. bananas are harvested and transported before they’re ripe because they’re less likely to be damaged this way, then exposed to ethene on arrival so they ripen in the shops

Question 56

How are auxins used commercially

Answer 56

Used in selective weed killers (herbicides) Make weeds produced long stems instead of lots of leaves Makes weeds grow too fast so they can’t get enough nutrients or water so they die

Question 57

How are auxins used in rooting powder

Answer 57

Auxins make a plant cutting grow roots Cutting can then be planted and grown into a new plant Many cuttings can be taken from one original plant and treated with rooting hormones Lots of plants can be grown quick and cheap from one plant

Question 58

Summary of plant hormones

Answer 58