Plant responses Flashcards
What are tropisms?
Directional growth in responses to environmental cues such as light and gravity
Limitations on plants
They are rooted and not mobile without a rapidly responding nervous system - they are coordinated organisms that show clear responses to their environment, communication between cells and even between different plants
Timescale of plant responses?
Much slower than animal responses
Hormones
Chemicals that are produced in one region of the plant and transported both through the transport tissues and from cell to cell and have an effect in another part of the plant
Important plant hormones?
Auxins, gibberellins, ABA and ethene
Auxins role
Control cell elongation, prevent abscission, maintain apical dominance, stimulate release of ethene, tropisms, involved in fruit ripening
What is abscission?
Leaf fall
Gibberellins
Causes stem elongation, triggers mobilisation of food at germination, stimulate pollen tube growth in fertilisation
Ethene
Causes fruit ripening and promotes abscission in deciduous trees
ABA
Maintains dormancy of seeds and buds, stimulates cold protective responses (antifreeze production), stimulates stomatal closing
How do chemicals interact?
Not in isolation - they work at low concentrations so isolating and measuring them is not easy ; there are multiple interactions
Plant hormones and seed germination
When the seed absorbs water, the embryo is activated and begins to produce gibberellins which stimulate the production of enzymes that break down the food stores found in the seed
Embryo uses these food stores to produce ATP for building materials so it can grow and break out through the seed coat
Giberellins switch on genes which code for amylases and proteases - digestive enzymes required for germination
ABA reacts antagonistically with Gibberellins and that it is the relative levels of both hormones which determine when a seed will germinate
Where is food store in dicot seeds?
Cotyledons
Where is food store in monocot seeds?
Endosperm
Experimental evidence supporting the role of Gibberellins in germination
Mutant varieties of seeds have been bred which lack the gene that enables them to make gibberellins - these seeds do not germinate
If gibberellins are applied to the seeds externally
They then germinate normally
If giberellin biosynthesis inhibitors are applied?
They do not germinate as they cannot make the gibberellins needed for them to break dormancy - once removed, they then germinate
Auxins
Growth stimulants produced in plants - small quantities have powerful effects and they are made in cells at the tip of the roots and shoots and in the meristems ; move down the stem and up the root both in the transport tissue and from cell to cell
Auxins effect on plant growth 1
Stimulate the growth of the main apical shoot by affecting the plasticity of the cell wall ; presence of auxins means the cell wall stretches more easily
How does auxin cause cell wall to stretch?
They bind to receptor sites in the cell membrane, causing a fall in pH to about 5 ; this is the optimum pH for the enzymes needed to keep the walls very flexible and plastic
As the cells mature, auxin is destroyed and as the hormone levels fall, the pH rises so the enzymes maintaining plasticity become inactive (wall then becomes rigid and more fixed in shape and size)
Example of an auxin
IAA (indoleacetic acid)
As levels of auxin increase?
pH decreases
High concentrations of auxins
Suppress growth of lateral shoots - results in apical dominance ; lateral shoots are inhibited by the hormone that moves back down the stem so they do not grow very well (unlike the growth in the main shoot) - further down the stem the auxin concentration is lower so later shoots grow more strongly
If apical shoot is removed, auxin producing cells are removed and so there is no auxin and so as a result the lateral shoots grow faster (freed from apical dominance)
If auxin is applied artificially to the cut apical shoot
Apical dominance is reasserted and lateral shoot growth is suppressed
Low concentrations of auxin
Promotes root growth - up to a given concentration, the more auxin that reaches the roots the more they grow - auxin is produced by the root tips and auxin also reaches the roots in low concentrations from the growing shoots
If the apical shoot is removed, then the amount of auxin reaching the roots is greatly reduced and root growth slows and stops - replacing the auxin artificially at the cut apical shoot restores the growth of the roots
High auxin concentrations
Inhibit root growth
Gibberellins role
Affect the length of the internodes - regions between the leaves on a stem
Variations in plant size and hormones?
Plants that have short stems produce few/no gibberellins; dwarf plants - without gibberellins, shorter stems, reduces waste and makes the plant less vulnerable to damage by weather and harvesting
What should be done at the end of an experiment on plant hormones?
Important to make serial dilutions to observe the effects of different concentrations of the hormones as they can have different effects on growth - usually involve large numbers of plants, spread of data from each experimental group should be measured using standard deviation
Synergism
Different hormones working together, complementing each other and giving a greater response than they would on their own
Antagonism
Substances have opposite effects - one promoting growth and one inhibiting it, balance will determine the response of the plant
Abiotic stresses
Changes in day length, cold and heat, lack of water, excess water, high winds and changes in salinity - plant responses involve both physical and physiological adaptations
Lack of water adaptations
Thick cuticles, hairy leaves, sunken stomata or a wilting response in hot, dry or extremely windy conditions - develop aerenchyma if they grow in an aquatic environment
What causes plants to change in physical appearance?
Range of daylight hours and temperatures vary as well - thus they affect the rate of photosynthesis so seasonal changes have a big impact on the amount of photosynthesis possible
What point then comes after physical changes?
Amount of glucose required for respiration to maintain the leaves and to produce chemicals from chlorophyll that might protect them against freezing is greater than the amount of glucose produced by photosynthesis - a tree that is in leaf is more likely to be damaged or blown away by winter gales which means deciduous trees in temperature climates lose all of their leaves in winter and remain dormant until the days lengthen and temperatures rise again in spring
Daylight sensitive
Lack of light that is the trigger for change - this is known as photoperiodism ; breaking of the dormancy of lead buds so they open up and time of flowering in a plant
Sensitivity of plants to day length?
Results from a light-sensitive pigment called phytochrome - exists in two forms Pr and Pfr ; each absorb a different type of light and the ratio of Pr to Pfr changes depending on the levels of light
Abscission?
Lengthening it the dark triggers a period of dormancy during winter months and leaf fall
Falling light = falling concentrations of auxin ; leaves respond to the falling auxin concentrations by producing the gaseous hormone ethene - at the base of the leaf stalk is a region called the abscission zone, made up of two layers of cells sensitive to ethene
Ethene turns on genes - producing new enzymes which digest and weaken the cell walls in the outer layer of the abscission zone (SEPARATION LAYER)
What else happens building up to leaf fall?
Vascular bundles which carry materials into leaf are sealed off and fatty material is deposited in the cells on the stem side to form a protective scar when the leaf falls (preventing entry of pathogens) ; cells deep in separation zone respond to hormonal cues by retaining water and swelling, thus more strain on outer layer
ABIOTIC STRESSES like low temperatures or strong winds finish the process and the strain is too much, leaf separates from the plant leaving a neat scar behind
Preventing freezing
If cells freeze, membranes disrupted and thus they will die - but plants have evolved mechanisms that protect their cells in freezing conditions ; cytoplasm of plant cells and sap in vacuoles has solutes which lowers the freezing point and some plants produce sugars/polysaccharides/amino acids/proteins which act as antifreeze to prevent cytoplasm from freezing or protect the cells from damage even if they do freeze
When do plants become frost resistant?
During the winter months - different genes are suppressed and activated in response to a sustained fall in temperatures along with a reduction in day length, preparing the plants to withstand frosty conditions
Sustained spell of warm weather along with extended day length reverses these changes in the spring
Major abiotic stresses?
Heat and water availability
Water availability response?
Open and closing stomata when to cool and conserve water respectively
Opening and closing of stomata?
In response to abiotic stresses is largely under the control of the hormone ABA - leaf cells appear to release ABA under abiotic stress causing stomatal closure
When level of soil water fall and transpiration is under threat?
Plant roots produce ABA which is transported to the leaves where it binds to receptors on the plasma membrane of the stomatal guard cells - ABA activates changes in the ionic concentration of the guard cells, reducing g water potential and therefore turgor of the cells ; guard cells thus close the stomata and water loss via transpiration is greatly reduced
Physical defences to Herbivory
Thorns, spikes, barbs, fibrous/inedible tissue, hairy leaves and even stings to protect themselves
Chemical defences - stinging nettle
Counts as a physical and chemical defence in its vicious trichomes (stinging hairs)
Chemical defences - tannins
Type of phenol ; can make up to 50% of the dry weight of the leaves, have a very bitter taste which puts animals off from eating the leaves
TOXIC to insects by binding to the digestive enzymes in their saliva and inactivating them
Tea and red wine have many tannins
Chemical defences alkaloids
Bitter tasting ; most of them act as drugs by affecting metabolism of herbivores and poisoning them - CAFFEINE NICOTINE COCAINE ETC
Caffeine produced by coffee bush seedlings spreads through the soil and prevents the germination of the seeds of other plants - caffeine protects against herbivores and the plant’s rivals
Nicotine
Toxin produced in the roots of tobacco plants, transported to the leaves and stored in vacuoles to be released when the leaf is eaten
Terpenoids
Often form essential oils but also act as toxins to insects and fungi ; pyrethron acts as an insect neurotoxin for example, interfering with the nervous system - some terpenoids act as insect repellents (citronella produced by lemon grass for example)
Pheromones?
Chemical which affects the social behaviour of other members of the same species - plants do not rely a lot on pheromones BUT in a few instances they could be regarded as using pheromones
Examples of plants using pheromones
Maple tree attached by insects - releases pheromones - absorbed by leaves on other branches - make chemicals such as callose to help protect them if they are attacked
Plants communicate by pheromones via root systems and one plant can tell a neighbour if it is under water stress
What do plants use to defend themselves?
Volatile organic compounds - VOCs
When cabbages are attacked by the caterpillars
Produce a chemical signal which attracts the parasitic wasp - insect lays its eggs in the caterpillars which are then eaten alive, protecting the plant. Signal from the plant also deters any other female cabbage white butterflies from laying their eggs - if cabbage is attacked by a different insect it’ll send out another signal specific to that insect
When Apple trees are attacked by spider mites?
Produce VOCs which attract predatory mites that come and destroy the Apple tree pests
VOC extension?
Sometimes may also act as a “pheromone” in causing neighbouring plants to produce VOC before they too are attacked
Responding to touch
If leaves are touched, they fold down and collapse - dislodging small insects and frightening large herbivores ; leaf falls in a few seconds and recovers some time layer after K+ movement followed by osmotic water movement
Tropisms
Plant growth responses to stimuli from one direction
Response to light
Phototropism
Response to gravity
Geotropism
Response to chemicals
Chemotropisms
Response to touch
Thigmotropism
Why must a plant respond to tropisms?
To maximise their own benefit - shoot and root must keep growing in the right direction, shoot must grow up for photosynthesis to take place and the roots must grow downwards into the soil to provide support, minerals and water
Why does research use germinating seeds/Young seedlings?
Easy to work with and manipulate as they are growing and responding rapidly - changes also tend to affect the whole organism rather than a small part, making tropisms much easier to observe and measure
What seedlings are usually used?
Those of monocots as it is a single spike with no apparent leaves - easier to manipulate and observe than a ditto
Must remember that coleoptiles are relatively simple plant systems - important to remember that the control of the responses to light in an intact adult plant may be more complex
If plants are grown in bright, all-round light in normal conditions/even but low light
They will also grow straight upwards
Shoots
Positively phototropic
Roots
Negatively phototropic
Survival value of phototropism
Ensures that shoots maximise amount of light for photosynthesis
Ensures roots turn back into the soil (towards water)
If shoot tip is removed/barrier put on it?
No response - tip must detect the stimulus and produce the auxin
Mica on the illuminated side of the shoot?
Allows shoot to bend towards light as the auxin passes down the shaded side where the increased growth occurs and it causes bending
Mica on shaded side
No response - impermeable to chemicals thus auxin cannot travel down
Gelatin block inserted
Permeable to chemicals like auxin - DOES NOT ALLOW ELECTRICAL CHEMICAL THROUGH THOUGH - bending still occurs in shoots as can be passed through
Effect of light?
Causes auxin to move laterally across the shoot so there is a greater concentration on the unilluminated side - stimulates cell elongation and growth on the dark side, resulting in growth towards the light
Once shoot is growing towards the light
Unilateral stimulus is removed - transport of auxin stops and shoot grows straight towards the light ; LIGHT DOES NOT DESTROY THE AUXIN
Experiments for photosynthesis
Germinate and grow seedlings in unilateral light with different colour filters to see which wavelengths of light trigger the phototropic response
When a plant is grown in the dark?
Grows more rapidly - biological imperative is to grow upwards rapidly to reach the light to be able to photosynthesise ; seedlings that break through the soil first will not have to compete with the other seedlings for light
Gibberellins are responsible for extreme elongation of internodes when a plant is grown in the dark - once a plant is exposed to light, a slowing of upwards growth is valuable as resources can be used for synthesising leaves and strengthening stems
Once the stem is exposed to light
Levels of gibberellins fall
Rapid upward growth which takes place in a plant in the dark…
Known as etiolation
Etiolated plants are thin and pale because the plant is deprived of light, little chlorophyll develops in the leaves
Normal conditions - geotropism
Always receive a unilateral gravitational stimulus - gravity always acts downwards, response of plants to gravity can be seen in the laboratory using seedlings placed on their sides either in all round light or in the dark
Shoots
Negatively geotropic
Roots
Positively geotropic
Geotropism
Ensures that roots grow down into soil and shoots grow up to the light
Clinostat?
Used to investigate shoots and roots - rotating the plants, the shoot and root both grow straight
Climacteric fruits
Fruits that continue to ripen after they have been harvested
Ethene production does what?
Triggers a series of chemical reactions which also greatly increases the respiration rate
Climacteric fruits
Bananas, tomatoes, mangoes and avocados
Non-climacteric fruit
Oranges, strawberries and watermelons - do not produce large amounts of ethene
Ethene
Widely used commercially as a stimulant in fruit ripening - fruits are harvested LONG before they are ripe and then cooled, stored and transported… unripe fruit is hard and much less easily damaged during transport around the world
When the fruit are needed for sale?
Exposed to ethene gas under controlled conditions - ensures that each batch of fruit ripens at the same rate and are all at the same stage to be put on the shelves for sale to the public (PREVENTS A LOT OF WASTAGE OF FRUIT DURING TRANSPORT - increases time available for them to be sold)
What does auxin do to shoots?
Application of auxin stimulates the production of roots which makes it much easier to propagate new plants from plant cuttings
What is a cutting?
A small piece of the stem of a plant - usually with some leaves on ; if this is placed in compost or soil or even water, roots may eventually appear and a new plant forms - dipping the cut stem into hormone rooting powder increases the chances of roots forming and of successful propagation
Micropropagation
Thousands of new plants are grown from a few cells of the original plant - control the production of the mass of new cells and then the differentiation of the clones into new plants
Hormonal weedkillers
Interactions between hormones are finely balanced to enable the plant to grow - if this is lost, it can interrupt the metabolism of the whole plant and may lead to plant death ; sometimes this is what we want to achieve (weeds are plants that grow where they are not wanted - commercial food crops are vital globally for producing the food people need to eat
Why are weds not wanted?
They interfere with crop plants, competing for light, space, water and minerals
Synthetic auxins?
Most weeds are broad leaved cults (crop plants are narrow leaved monocots) - these synthetic dicot auxins are only absorbed by broad leaved plants and affect their metabolism ; growth rate increases and becomes unsustainable ; narrow leaved crop plants are unaffected and are frees from competition ; SIMPLE AND CHEAP TO PRODUCE - SELECTIVE
Advantage of synthetic dicot auxins?
Low toxicity to mammals
Auxin usage?
In seedless fruit
Ethene use
Promote fruit dropping in plants such as cotton, walnuts and cherries
Cytokinins use?
Prevent ageing of ripened fruit and products like lettuce ; in micropropagation to control tissue development
Gibberellins
Delay ripening and ageing in fruit - improve the size and shape of fruits - beer brewing to speed up malting process (allowing grain to germinate)
