Module 5 Section 4: Plant Responses Flashcards
Why have plants evolved responses to their environment
These plant responses have evolved because they provide the plant with a selective advantage (better adapted to survive and reproduce in the environment)
What types of stimuli do plants respond to
The environmental stimuli to which plants respond can be abiotic or biotic
Examples of plant responses
Tropisms
Responses to touch
Responses to herbivory
Responses to abiotic stress
Examples of biotic and abiotic factors
Biotic:
Predators
Human activity
Competition
Abiotic:
Temperature
Soil
Salinity
What do plants use hormones to do
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
Why do plants need to respond to their environment
Adapt to changing environments
Avoid abiotic stress
Maximise photosynthesis (obtain more light, water and minerals)
Avoid herbivory
Ensure pollination and seed dispersal
Plant physical defences and overall function
Thorns
Hairy leaves
Leaf folding
Bark
Waxy cuticle
Helps protect them from herbivory
( process by which animals eat plants)
How do plants chemically avoid herbivory
Produce chemicals such as:
Tannins
Alkaloids
Pheromones
What are tannins
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
What are alkaloids
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
Examples of alkaloids
Caffeine (toxic to insects and fungi)
Nicotine (released when herbivores graze)
Capsaicinoids (make chillis hot)
What are pheromones
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
Examples of pheromones
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
Forms of abiotic stress for plants
Freezing
Drought
Increased soil water salinity
Presence of heavy metals (e.g. lead, copper, zinc)
How can some plants respond to drought and freezing stress
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
What are nastic responses
A non-directional response to stimuli, e.g. thigmonasty
E.g. plants can respond to touch by folding its leaves
How do plants carry out the leaf folding response
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
What is a tropism
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
What are positive and negative tropisms
Positive tropism: a growth response towards the stimulus e.g. light
Negative tropism: a growth away from the stimulus e.g. gravity
Types of tropisms and what they are stimulated by
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)
How can a plant’s movement be both a positive and negative tropism
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
What is phototropism
Where a plant grows towards the light to enable photosynthesis
What is geotropism
When a root grows towards the pull of gravity which provides them with anchorage with the soil and helps take up water for photosynthesis
What is chemotropism and give an example
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
What is thigmotropism
Shoots of climbing plants can wind around other plants and structures to gain support
What are plant hormones
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
What can plant hormones do
Can bring about different effects on the plant
E.g. cell elongation, cell division and cell differentiation
Why do deciduous plants lose leaves in winter
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
Function of auxins in leaf loss
Auxins inhibit leaf loss
They are produced by young leaves
As the leaf gets older, less auxin is produced, leading to leaf loss
Function of ethene
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
How do hormones cause stomata to close
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
Summary of hormones in stomatal closure
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
What do seeds contain
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
Function of gibberellin
Causes stem elongation
Tiggers the mobilisation of food stores at seed germination
Stimulate pollen tube growth in fertilisation
Function of ABA (abscisic acid)
Maintains dormancy of seeds and buds
Stimulates cold protective responses
What are meristems and different types
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
What are auxins
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)
Role of auxins in the shoots
Stimulates cell elongation
High concentration promotes plant growth
Roles of auxins in the roots
Inhibit cell elongation
High concentrations of auxin limit cell growth
Explain how auxin acts in plant shoots (phototropism)
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
Explain how auxin acts in the plant roots (geotropism)
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
Results of Darwin’s experiment:
Removing tip, then placing cap on tip
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
Results of Boysen-Jensen’s experiment:
Mica barrier
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
Results of Boysen-Jensen’s experiment:
Gelatine block
The chemical response could diffuse through the gelatine block from the tip to the rest of the shoot
Results from paál and Went‘s experiment:
Tips and gelatine being off centre
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
What is apical dominance
This is where the main central stem of the plant is dominant and grows more strongly than the side branches
How does apical dominance occur
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)
Function of abscisic acid and cytokinins in apical dominance
ABA: Inhibits all bud growth (not just lateral)
Cytokinins: promote bud growth
Importance of apical dominance
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
How are auxins and gibberellins synergistic
They work together to allow the plant to grow taller
How are auxins and gibberellins antagonistic
They oppose each others actions as gibberellins stimulate the growth of side shoots but auxins inhibit the growth of side shoots
What are gibberellins
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
How do gibberellins stimulate seed germination
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)
Different ways that plant hormones can be used commercially
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
How can ethene be used to control fruit ripening
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
How are auxins used commercially
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
How are auxins used in rooting powder
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
Summary of plant hormones
