plant responses Flashcards
outline 3 roles of hormones in plants
- cellular communication
- to respond to external stimuli
- to signal to other plants or species nearby e.g. protecting themselves from pests/insects or producing chemical defences against herbivores
what are the 5 main plant hormones
gibberellins
ethene
auxins
cytokinins
abscisic acid - ABA
where are gibberellins found
plant seeds and apical portions of roots and shoots
what do gibberellins do in plants
when seeds absorb large amounts of water, gibberellin is released to start the process of germination - it stimulates enzymes to break down / metabolise food stores
what are gibberellins used for by humans
gibberellins are used to end seed dormancy, so seeds can be planted all year round
they are also used to control fruit ripening
where is ethene found
leaves, stems and young fruits
what does ethene do in plants
it promotes leaf fall in deciduous trees and causes fruit ripening
what is ethene used for by humans
since it affects plant growth, it can be used alongside gibberellins to prevent spoiling / ripening / overripening of fruit during transport, and then to promote fruit ripening when in stores
where are auxins found
found in shoot apical meristems and roots
what do auxins do in plants
auxins are growth factors, produced in tips of roots and shoots, in shoots they cause the stem to grow upwards while also stopping lateral buds from growing, promoting apical dominance
they also prevent leaf fall
why is apical dominance important in a plant
the higher up a plant grows the more light it has access to
how do changing auxin levels affect other hormones in a plant
if auxin levels / production is low, ethene is stimulated
give an example of a specific auxin
indoleacetic acid - IAA
what happens if the growing tip of a plant is removed
this is where the auxins are found / produced, so this means the source of auxins has also been removed, so apical dominance is no longer promoted - with time the lateral shoots will grow and curl towards the light
what are auxins used for by humans
they are used as selective weed killers - since they are growth hormones, they cause weeds to grow very quickly, meaning they absorb nutrients from soil at a faster rate, and so weed killer is also absorbed much faster
they are also used to promote growth of plant cuttings as they allow the roots to develop quickly so plants can self sustain
where are cytokinins found
they are produced in the roots and transported around the plant from there
what do cytokinins do in a plant
they work alongside auxins to stimulate cell division, and play a role in determining the course of differentiation
where is ABA found
in mature leaves, fruits and root caps
what does ABA do in a plant
it slows and inhibits growth, maintains seed dormancy and is involved in stomatal closure - it can start to accumulate when plants experience stresses, such as changes in temperature, water levels or salinity
outline how ABA could help a plant deal with water stress
low water levels are detected by the plant, causing ABA to accumulate and travel to leaves, where high levels of ABA cause the stomata to close, so less water is lost via transpiration
what is ABA used for by humans
it is used to prevent seed development, such as to produce seedless fruit, which is easier to sell
what are the 6 stages of life in a plant
seed dormancy
germination
growth to maturity
flowering
fruit development
leaf abscission
what hormones are involved in seed dormancy
ABA
what hormones are involved in germination
gibberellins
what hormones are involved in growth to maturity
gibberellins, auxins, cytokinins
what hormones are involved in flowering
mostly gibberellins, auxins, cytokinins
also ethene
what hormones are involved in fruit development
mostly cytokinins and ethene
also gibberellins and auxins
what hormones are involved in leaf abscission
ethene and ABA
what is the basic structure of a seed
a seed is a kind of capsule containing the plant embryo
it consists of an aleurone layer, which surrounds the endosperm (like the cytoplasm), which contains starch reserves, as well as the actual embryo
outline how gibberellins control seed germination
1- water is absorbed by the seed
2- this causes gibberellins to be synthesised by the embryo
3- gibberellins stimulate aleurone layer cells to synthesise amylase
4- amylase hydrolyses starch to form maltose
5- maltose is converted to glucose
6- glucose is transported to the embryo providing energy for growth
outline how hormones control stomatal closure
1- ABA binds to receptors on the guard cell membrane
2- Ca2+ ions enter guard cells via channels
3- K+ ions leave guard cells via channels
4- this causes water to leave the guard cells by osmosis to increase water potential inside
5- the guard cells become flaccid, and bend to close the stomata
when + why does leaf loss occur in plants
it can occur during winter when absorption of water is difficult due to frozen soil
it can also occur in hot dry climates to minimise water loss
how does leaf abscission occur
a layer of parenchyma cells - called the abscission layer - develops at the base of the leaf stalk, which have thin walls that are easily broken
ethene then stimulates the break down of cell walls in the abscission layer
this causes the leaves to drop off
- in young plants auxins also play a role in leaf loss as they make leaf stalks insensitive to ethene
what happens to auxin concentration in plants as they grow
as plants age the conc of auxins decreases
how do auxins regulate growth
by controlling cell division and elongation
how is growth regulated differently in the shoots and roots
in the shoots growth is stimulated
in the roots growth is inhibited
how to tropisms work
movement and changes in concentrations of auxins allows plants to respond to environmental factors
tropism definition
a directional response
phototropism definition
a plants directional response to light
what kind of phototropism is displayed in the shoots and roots
positive phototropism in the shoots, negative phototropism in the roots
outline how shoots show positive phototropism
when a plant received light on on side, the auxins will move to the other side of the plant
this causes the shaded side to grow and elongate more
as a result the plant bends towards the light
why is positive phototropism in shoots useful
this allows plants to face the sun, which increases rates of photosynthesis
outline how roots show negative phototropism
when roots grow, the auxins move to the shaded bottom side, but here is inhibits growth
this means the top side nearer to the light elongates more
as a result roots grow down
why is negative phototropism in roots useful
it increases the probability of roots growing into soil towards water
gravitropism / geotropism definition
a plants directional response to the earths gravitational field
outline how roots show positive geotropism
auxins in the root tips is pulled down towards the bottom side of the roots
this inhibits growth causing the top to bend downwards
hydrotropism definition
a plants directional response to water - tendency to grow towards water
chemotropism definition + example
a plants directional response to chemicals
e.g. pollen tubes grow down the style towards the ovary where fertilisation happens
thigmotropism definition
the tendency of a plant to wind around other plants or solid structures to allow it to grow + support itself
nastic movement definition
a non directional response
what are 4 examples of abiotic stresses plants can response to
changing daylight hours
freezing
heat
water availability
how have plants adapted to cope with daylight hours
seasonal changes in hours of daylight impacts the amount of photosynthesis possible
if the amount of glucose needed for respiration to maintain leaves is greater than the amount of glucose being produced, leaf fall will occur
how have plants adapted to cope with freezing temperatures
decreasing temperatures can cause plant cells to freeze, which can disrupt cell surface membranes and kill cells
to prevent this, plant cells have lots of solutes in cytoplasm and cell sap which decreases the freezing point of the cells, and cells can also produce proteins that act antifreeze during winter
how have plants adapted to cope with heat
stomatal opening/closing helps plants to deal with heat - if it is very hot stomata will open to allow water to cool down the plant as it evaporates out of leaves
how have plants adapted to cope with water availability
stomatal opening/closing helps plants to deal with water - if water is scarce, stomata will close to conserve water, as a result if ABA production in roots which then travels to guard cells
what are 2 ways that plants have adapted to biotic stresses
physical defences
chemical defences
give 6 examples of a physical plant defence + its function
thorns, barbs, spikes, spines, inedible tissue, stings, etc
- these discourage herbivory, protecting plants
give 4 examples of chemical defences in plants
tannins
alkaloids
terpenoids
pheromones
what are tannins + how do they act as a plant defence
they are part of a group of compounds called phenols
they make up 50% of dry weight in the leaves, and are bitter tasting to animals, and toxic to microbes as they bind to and inhibit digestive enzymes in saliva - discourages herbivory
what are alkaloids + how do they act as a plant defence
they are part of a large group of bitter tasting compounds
they can act as drugs, affecting the metabolism of animals or poisoning them
e.g. caffeine - toxic to fungi + insects, nicotine, morphine, cocaine
what are terpenoids + how do they act as a plant defence
these are compounds which form essential oils, and they can often act as toxins to insects + fungi e.g. pyrethin in crysanthemums
they also repel insects
what are pheromones
these are chemicals made by an organism which affect the social behaviours of other members of the same species
