5.1.5 Plant Responses (includes practical skills) Flashcards
what are the 4 main reasons for a plant to respond to its environment?
Limit abiotic stress (drought)
Deter herbivores
Increase photosynthesis rate
Decrease spread of pathogens
What is abiotic stress?
a non-living environmental factor which can cause harm to the plant, i.e drought
why do plants respond to abiotic stress?
increases their chances of suvival
what are the two main response type
response to herbivory and abiotic factors
what does herbivory cause?
loss of leaf surface area and hence decrease in the rate of photosynthesis
what are the reasons to stop herbivory?
Prevent the loss of leaf surface area
prevent the opportunity for vectors to enter the plants and allow pathogens to damage the tissues
What are the 2 types of responses to herbivory?
Chemical defences
Physical defenses
What are the two physical defenses to herbivory?
Permenant structures
Folding,curling or collapsing of the leaves to touch
what are the five chemical responses to herbivory?
Volatile organic compounds
Pheromones
Tannins
Alkaloids
Terpinoids
How to remember the five chemical responses to herbivory?
Violent
people
talk
angrily
too
What is VOC short for?
Volatile organic compounds?
what is a volatile organic compound?
chemicals released into the atmosphere by plants which attract or repel other species
give an example of a VOC
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
what are tannins?
phenols w/ a bitter taste to animals and toxic to insects
where are tannins present?
Grapes and pomegranates
What % of dry mass does tannins take up?
up to 50%
What are alkaloids?
Bitter tasting nitrogenous compounds
bitter taste = deters animals
in high conc = toxic, disrupt metabolism of herbivore = dead
give two example of alkaloids
caffeine and cocaine
what is a terpionid?
large group of compounds which form essential oils
act as toxins against insects and fungi
give an example of a terpinoid
Citronella - produced by lemon grass
Repels insects
What is a pheromone?
chemicals released into the atmosphere
changes behaviour of members of the same species
give an example of a use of a pheromone
maple tree attacked by insects
pheromone released
pheromone absorbed by leaves on other branches
those leaves produce more cellulose
what can VOCs sometime do?
attract or repel predeators to a prest, but also act as a pheremone to warn other plants of the threat
what is a permenant strucutre of a plant to prevent herbivory?
Thorns, spines, hairy leaves, and stingers
what is The folding, curling or collapse of leaves in response to touch?
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)
how do the leaves of mimosa pudica return to normal?
k+ ions into cells, water moves in by osmosis, cells turgid, returns to original
what is tropism?
a directional growth response of the shoots or roots of a plant, towards or away from an environmental stimulus
what are the two movements of tropism?
positive tropism – growth of shoots or roots towards the stimulus;
negative tropism – growth of shoots or roots away from the stimulus
how did tropism come about?
natural selection
what is the stimulus of phototropism?
directional light
what is postive phototropism
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.
what is Negative phototropism
roots - grow into the soil (region of lowest light intensity), anchoring the plant in position and increasing opportunities for water and mineral ion absorption.
what is the stimulus of geotropism/gravitropism
Gravity
what is postive geotropism
roots - grow down into the soil, anchoring the plant in position and increasing opportunities for water and mineral ion absorption
what is Negative geotropism
Shoots - grow upwards, in a direction which
increases the likelihood of greater light
availability for the leaves
what do we use to investigate geotropism?
clinostat
what is the stimulus of thigmotropism?
touch or surface contact
what is postive thigmotropism (no negative)
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.
what is the stimulus for hydrotropism?
water
what is positive hyrdotropism?
roots grow into areas of higher water potential, allowing increased uptake of water into the roots by osmosis.
what is the mechansim for positive phototropism?
- photoreceptors in shoot tip detect direction of light
- cells in shoots produce auxins
- auxin transported downwards from the tip, and accumulates on the shaded side
- 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 - different extent of cell elongation on light and shaded die causes stem to bend as it grows towards the light source
- 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
what are the name of the photoreceptor?
phototropins
what do auxins do on the shaded side of the cell?
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
what is this showing? label it.
the mechanism for positive phototropism
what is the experimental evidence for phototropism?
practical method into phototropism
practal investigation into geotropism
what are plant hormones?
cell signalling molecules in plants that coordinate growth, reproduction, an responses to herbivoes, pathogens and abiotic stress.
why are plant hormones difficult to research?
- effective in low concs ∴ difficult to measure
- different effects depending on concentraion, ie low conc of auxin = growth, but high conc of auxin = inhibits growth
- Different hormone combinations have different effects
- The hormones may have different effects depending on the tissue type or stage of development
- different effects in different species of plant
two types of effects with two different plant hormones
Antagonistic - opposing effect (relative conc = direction of response)
Synergistic = similar effects and together give stronger effect
in plants, where is the highest concentration of hormones
Growing tissues (MERISTEM TISSUE)
i.e shoot and root tips
Do plants have endocrine glands and blood to carry hormones?
NO
How do plant hormones travel around the plant
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)
what are the four plant hormones?
auxins
giberelleins
ethene
ABA (abscisic acid)
what are the roles of auxins?
cell elongation
prevents leaf abscision
apical dominance
tropism
stimulates release of ethene
growth of roots
what are the roles of giberellins?
stem elongation
triggers metabolism of food stores during germination
stimulate pollen tube growth in germination
what are the roles of ethene?
causes fruits to ripen
leaf abscision
what are the roles of ABA (Abscisic acid)?
maintains domancy of seeds and buds
stimulates cold protective responses (anti-freeze production)
why does a plant need to be able to close its stomata?
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.
mechanism for stomatal closure
- The hormone ABA is released by leaf cells that suffering a decrease in water potential and a loss of turgor.
- ABA acts as a signal to guard cells, which have complementary receptors in their plasma membranes that ABA binds to.
- 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.
- Once stomata are closed, there is a decrease in loss of water vapour by transpiration,
hence more water retained in the plant
what are the early warning signs for a response to water stress?
- root cells in drier soil produce ABA
- which is carried to the leaves in the transpiration stream
- ABA binds to receptors on guard cells’ plasma membranes
- triggers the pre‐emptive stomatal closure, even before the leaves begin to wilt
where is the apical meristem found?
shoot tip
what does the apical meristem responsible for?
upwards growth of main stem, cuased by repeat mitosis in the apical meristem
what is apical dominance?
where the presence of the apical meristem suppresses growth of side shoots (branches) from the lateral buds that occur at intervals down the stem.
what is the reason for apical dominance?
- 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
- prevents the plant becoming too ‘bushy,’ which could result in overcrowding of leaves that they shade each other (reduces rate of photosynthesis)
what is the apical dominance mechanism?
- The apical meristem in the shoot tip produces very high auxin concentrations.
- The auxin diffuses down the stem.
- 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.
- All the plant’s resources are used to fuel upward growth only.
- Eventually, once the plant becomes very tall, the lowest lateral buds may no longer experience sufficiently high auxin levels to maintain their dormancy.
- Hence the lowest lateral buds are the first to begin side shoot growth.
experimental evidence for the role of auxins in the control of apical dominance.
- 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)
- 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.
what is leaf abscision?
loss of leaves
why do decidouous trees want to lose thier leaves?
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)
when plants lose their leaves, where do the plants get their sugars from?
hydrolysis of stored starch in the roots, translocated in the phloem to where sugars are needed
what must occur for leaf abscission to occur?
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
what main hormones is responsible for leaf abscision?
auxin = pevents
ethene = removes
what is the mechansim for leaf abscision?
- Through the summer, high levels of auxin secretion by (young) leaves inhibit ethene production and therefore prevent abscission. Auxin is antagonistic to ethene.
- In autumn, triggered by the increased length of the dark period (night), auxin levels fall and ethene production therefore increases.
- 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).
- 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!
- 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.
- 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.
- 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).
- This cellulose and pectin digestion creates a line of weakness at the petiole base.
- The final ‘push’ for the leaf to fall will usually be a gust of wind.
what is stem elongation?
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).
what causes cell elongation?
Gibberellins stimulate the elongation of plant stems
how do gibberelins effect stem elongation?
- stimulate cell division by mitosis
- stimulate cell elongation
[Plants with high gibberlein conc have longer internodes, each internode contains more cells and each individual cell is more elongated]
Experimental evidence for the role of gibberellins in the control of stem elongation
- 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
- Genetic analysis of the dwarf plant usually reveals a mutation in a gene for a gibberellin‐synthesis enzyme or recpetors
what hormone is responsible for seed germination?
gibberellins
seed germination mechanism.
- Water is absorbed through the testa (seed coat) into the seed (in a process called imbibition).
- This triggers increased synthesis of the plant hormone, gibberellin, by the embryo within the seed.
- ABA concentration in the seed must be low enough to allow gibberellin to stimulate the events that follow, since ABA is antagonistic to gibberellin.
- Gibberellin activates transcription of the genes for amylase and protease.
- Amylase and protease enzymes are synthesised.
- Amylase hydrolyses stored starch into maltose, for use in respiration.
- Protease hydrolyses stored proteins into amino acids, for use in translation.
- Metabolic processes including respiration increase in rate, allowing increased cell division by mitosis, followed by cell differentiation.
- 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
Experimental evidence for the role of gibberellin in the control of seed germination
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
what are the three commercial uses of plant hormones?
Ethene to promote fruit ripening
auxins in rooting powder
synthetic auxins as weedkillers
describe ethene to promote fruit ripening
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.
describe auxin in rooting powders:
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.
describe synthetic auxins as hormonal weedkillers
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.
