Plant responses Flashcards
What are the limitations of plants?
- Many are immobile
- Don’t have rapid nervous systems
Main difference between plant and animal responses?
Plant responses are much slower than in animals
What are the main plant hormones?
- Auxins
- Gibberellins
- Ethene
- ABA (Abscisic Acid)
Sequence of seed germination:
- Seed absorbs water
- Enzymes (proteases, amylase, maltase) are produced
- Products of digestion used to generate ATP and as monomers for new macromolecules
- Gibberellins switch on genes in the embryo that code for these enzymes
- ABA acts as an antagonist to the gibberellins to prevent early germination and is inhibited by changing water content
Experimental evidence of gibberellins: MUTANT SEEDS
- Mutant varieties of seeds that have non working alleles for the genes that allow gibberellin synthesis
- Mutants cannot produce gibberellin
- Mutant seeds do not germinate
- External application of gibberellins allows germination
Experimental evidence of gibberellins: INHIBITORS
- When gibberellin synthesis inhibitors are applied to seeds, they cannot produce gibberellins
- The treated seeds do not germinate
- When either the inhibitors are removed or gibberellins are applied, then the seeds germinate
What are meristems?
The tissue in most plants containing undifferentiated cells
What are the three types of meristematic tissue?
- Apical
- Intercalary
- Lateral
Where is apical meristematic tissue found?
At the tips of roots or shoots
Where is intercalary meristematic tissue found?
At the middle, only in monocot stems at the base of nodes and leaf blades
Where is lateral meristematic tissue found?
At the sides
Role of Auxins: GROWTH OF MAIN APICAL SHOOT
- Auxins bind to receptors in cell membrane
- This causes fall in cellular pH to 5
- Optimum pH for enzymes that keep cell walls flexible
- Cells mature and move away from the meristem, auxins are destroyed
- pH rises and inhibits enzymes that were keeping cell wall flexible
- Cell wall now becomes inflexible and rigid in shape, cell size is fixed
Where are auxins made?
Apical meristems
How are auxins transported?
Translocated in the phloem, mainly from shoot tip to root tip
Role of Auxins: APICAL DOMINANCE
- Auxins produced in the apical meristem
- Translocated down plant in the phloem
- Concentration gradient is established from shoot tip to root tip
- High concentration of auxins inhibit lateral or side shoot growth
- Lower concentrations of auxins allows lateral or side shoot growth
Auxins experimental evidence: APICAL DOMINANCE
-If auxin is applied to the cut surface, from an apical meristem, then apical dominance is restored and lateral shoot growth is repressed
Role of Auxins: ROOT GROWTH
- Auxins produced in root meristems
- Auxins also reaches root by translocation through phloem from shoot tips
- Auxins promote root initiation
- Auxins induce both growth of pre-existing roots and branching of the roots
- High concentrations of auxins inhibit root elongation, but instead enhance branching root formation
Auxins experimental evidence: ROOT GROWTH
- Replacing auxins at a cut apical stem restores root elongation and growth
Role of Gibberellins: INTERNODE LENGTH
- Plants with no gibberellins produce stems with no internodes
- Excessive concentrations of gibberellins produces tall, spindly plants with large internodes
Gibberellins experimental evidence: INTERNODE LENGTH
- Variety of dwarf rice has a short height due to a mutation in the gibberellin synthesis gene, causing it to have short internodes
- Crop averted chronic food shortage feared in 1960s
Why do plants lose their leaves?
If the rate of photosynthesis is too low to keep up with demand of respiration and synthesis of molecules due to lower temperatures and less daylight hours (winter), deciduous trees hibernate and lose all their leaves
What are plant responses to lack of light?
- Loss of leaves
- Breaking of leaf bud dormancy
- Timing of flowering
- Tuber formation for over wintering
Process of Leaf Abscission:
- Falling light levels leads to lower auxin concentrations
- Leaves respond by increasing the production of ethene
- Ethene causes genes to switch on in the separation layer of the abscission zone
- Enzymes are made that digest the cell wall of the separation layer, causing it to weaken
- Vascular bundles in the petiole are sealed off
- Cells in the protective layer of the abscission zone make suberin and lignin to waterproof and protect the scar when the leaf falls off
- Plant cells at the abscission zone will take in a large amount of water, swell and eventually burst, making the leaf fall off
How can freezing conditions damage plants?
Ice crystals can destroy cell membranes and cause the death of plants
How do plants prevent freezing?
- Produce molecules that act as antifreeze to lower the freezing point of the water so it is less likely to freeze
- E.g. sugars, polysaccharides, amino acids and proteins
- Plant antifreeze molecules produces when genes switch on due to a prolonged fall in temperature and a reduction in day light hours
What conditions are conducive to stomatal opening?
- High light intensity
- High humidity
Normal mechanisms for stomatal opening:
- Proton pump drives H+ from the guard cells
- Guard cells become increasingly negative
- Negative charge opens K+ voltage-gated channels so and uptake of K+ into the cell occurs
- Increase in K+ lowers the water potential inside the cell, resulting in diffusion of water into cell by osmosis
- This increases cell’s volume and turgor pressure
- Rings of cellulose microfibrils that prevent width of guard cells from swelling and the ends of the guard cells are held in place by epidermal cells
- Therefore, extra turgor pressure allows guard cells to lengthen by bowing apart from one another, creating an open pore through which gas can diffuse
What hormone controls stomatal closure?
ABA (Abscisic Acid)
Mechanism for stomatal closure by ABA:
- Root sense water shortage in soil and ABA is released from the roots
- ABA binds to receptor proteins in the guard cell’s membrane
- This causes the concentration of free Ca2+ to increase due to influx from outside the guard cells
- This causes Cl- to exit the guard cells
- This stops any further uptake of K+ into the guard cells and they are then lost from the guard cells
- Loss of these K+ ions causes an increase in water potential, so water leaves the guard cells by osmosis
- Cell becomes plasmolysed, which results in the closing of the stomatal pores
Physical defences to herbivory:
- Thorns, barbs, spikes, spiny leaves
- Fibrous and inedible tissue, hairy leaves, stings
- All discourage herbivores from eating them
3 Chemical defences to herbivory:
- Tannins
- Alkaloids
- Terpenoids
Tannins:
- Bitter tasting
- Phenol based
- Toxic to insects by binding to digestive enzymes in saliva and inactivating them
- Can make up 50% of dry weight of leaves
Alkaloids:
- Bitter tasting, nitrogenous compounds
- Act as drugs, affecting metabolism of herbivores, sometimes poisoning them
- Can prevent germination of other seeds
- E.g. nicotine or caffeine
Terpenoids:
- Form essential oils
- Acts a toxins to insects and fungi
- E.g. pyrethrin acts as a neurotoxin
- Can act as insect repellents e.g. citronella (lemon grass)
What are pheromones?
Chemicals that affect the social behaviour of the same species to protect themselves
Example of pheromones:
- Maple tree is attacked by insects
- Releases pheromones which is absorbed by leaves on other branches
- Leaves make chemicals, like callose, to protect them if they are attacked
- Leaves on branches of nearby trees also prepare for attack in response to original tree’s pheromones
Mimosa Pudica (Folding in response to touch):
- Stimulus transmitted as an electrochemical change in cells from a stimulated leaflet to the leaf’s pulvinus
- Electrochemical change causes K+ ions to flow out of the cells in the extensor side and flow into the cells in the flexor cells
- Water then follows by osmosis
- Flexor cells swell and become turgid, causing the leaflets to fold upwards
- Extensor cells become flaccid
- Then the whole leaf drops
What are tropisms?
Directional growth responses
What are the 3 main types of tropism?
PHOTOTROPISM- growth in response to sunlight
CHEMOTROPISM- growth in response to chemicals
GEOTROPISM- growth in response to gravity
What is phototropism the result of?
The result of movement of auxins across the shoot or root if it is exposed to light stronger on one side than the other
Why are monocots used when investigating phototropism?
The shoot emerges as a single spike with no apparent leaves so are easier to manipulate and observe than a dicot
Why are germinating seeds/young seedlings used when investigating phototropism?
- Easy to work with
- Easy to manipulate
- Able to grow and respond rapidly to any change
- Easy to observe and measure as it affects the whole plant
How can phototropism be tested?
- Remove tip
- Light proof cover over tip
- Thin, impermeable mica on light side
- Thin, impermeable mica on shaded side
- Tip removed, gelatin block inserted and tip replaced
Removing tip:
No response as the tip must either detect the stimulus or produce the messenger (or both) as its removal prevents any response
Lightproof cover:
No response as the light stimulus must be detected by the tip
Thin, impermeable barrier of mica on light side:
Bends towards the light as the movement of the auxins occurs down the shaded side, where it increases growth and causes bending
Mica inserted on shaded side:
No response as movement of chemical down shaded side is blocked by mica
Tip removed, gelatin block inserted and tip replaced:
Bends towards the light as the gelatin allows the auxins to pass through it, but not electrical messages. Therefore, the bending which occurs must be due to a chemical (auxins) passing from the tip
Normal plant shoot:
Shoot bends towards the light as the shoot is positively phototropic and bending occur behind the tip
Effect of unilateral light on phototropism:
Light causes the auxin to move laterally across the shoot so there is a greater concentration on the unilluminated side. This in turn stimulates cell elongation and growth on the dark side, resulting in observed growth towards the light.
How can geotropism be investigated?
- Plants grown on a slowly rotating clinostat (rotating drum) at four revolutions per hour. Roots will grow straight outwards as it cannot detect gravity. Once the clinostat stops, the roots will grow downwards as the plant can detect gravity.
- Seeds can be placed in petri dishes and stuck to the wall of the lab and rotated 90* at intervals as the seedings grow.
Why should geotropism experiments be carried out in the dark or all round light?
So the direction of the growth of roots and shoots isn’t affected by a phototropic response to a unilateral light source
How do plants detect the direction of gravity?
- They contain statoliths, which are dense amyloplasts
- Amyloplasts synthesise and store starch
- Amyloplasts are denser than the cytoplasm and can sediment according to the direction of gravity
- Therefore plants can tell the direction of gravity by how their amyloplasts move
What are some commercial use of plant hormones?
- Control of ripening
- Hormone rooting powders
- Hormonal weed killers
Control of ripening:
- Ethene involved
- Ripening of climacteric fruits
- Ripening linked to peak ethene production
- Triggers series of chemical reactions (greatly increased respiration rate)
- If a bunch of green bananas put in a bag with one ripe banana, the bananas will ripen quicker than the rest of the bunch.
- Ethene from ripe banana stimulates ripening of others
Hormone rooting powders:
- Auxins affect growth of roots and shoots
- Application of auxins to cut shoots stimulates root growth
- Cut stems dipped in hormone rooting powder to increase chance of roots forming and successful propagation
- In both agriculture and horticulture, plant hormones are essential for large scale micropropogation
Hormonal weedkillers:
- If balance of hormones is lost, it can lead to plant death
- Synthetic auxins act as effective weedkillers
- Most weeds are broad leaved dicots
- Synthetic dicot auxins absorbed by weeds and affect their metabolism
- Weeds die as growth rate increases to a point where is become unsustainable
- Narrow leaved monocots (crops) remain unaffected and are freed from competition
- Synthetic auxins used by farmed are:
1. Simple and cheap to produce
2. Low toxicity to mammals
3. Selective
Other commercial uses of plant hormones:
- Auxins used in production of seedless fruit
- Ethene promote fruit dropping in plants, such as cotton, walnuts and cherries
- Cytokinins prevent ageing of ripened fruit and products for transport
- Cytokinins used in micropropagation to control tissue development
- Gibberellins improves size and shape of fruit
- Gibberellins used in beer brewing to speed up malting process
