M5 S4 : Plant responses and hormones Flashcards
How do plants increase their chances of survival by responding to changes in the environment?
- They sense the direction of light and grow towards it to maximise light absorption for photosynthesis.
- They sense gravity, so roots and shoots grow the right direction.
- Climbing plants have a sense of touch, so they can find things to climb and reach the sunlight.
What is abiotic stress?
Anything that is potentially harmful to a plant that is natural, but non-living, like a drought (water stress).
How can plants respond to abiotic stress, give example?
Some plants respond to extreme cold by producing their own antifreeze :
- Carrots produce antifreeze proteins at low temperatures. These proteins bind to ice crystals and lower the temperature that water freezes at, stopping more ice from growing.
What is herbivory?
When plants are eaten by animals (including insects).
What ways do some plants respond to herbivory?
- Alkaloids
- Tannins
- Pheromones
What are alkaloids, give example?
These are chemicals that taste bitter, noxious smells or poisonous characteristics that deter or kill herbivores.
Tobacco plants produce the alkaloid nicotine in response to tissue damage. Nicotine is highly poisonous to many insects.
What are Tannins, give example?
These taste bitter, and in some herbivores (e.g. cattle and sheep) they can bind to proteins in the gut, making the plant hard to digest. Both these things deter animals from eating other organisms.
What are pheromones?
Some plants release pheromones in response to herbivory. Pheromones are signalling chemicals that produce a response in other organisms.
What are examples of plants using pheromones in response to herbivory?
- Some plants release alarm pheromones in the air in response to herbivore grazing. Nearby plants detect these chemicals and start making chemical defences, like tannins.
- When corn plants are eaten by caterpillars, they produce pheromones that attract parasitic wasps. These wasps lay their eggs in caterpillars, eventually killing them.
What is an example of how plants fold up when being touched in response to herbivory?
If single leaflet of Mimosa pudica is touched, a signal spreads through the whole leaf causing it to fold up quickly. It may help knock off small insects feeding on the leaf and scare off animals trying to eat it.
What is a tropism?
A response of a plant to a directional stimulus (stimulus coming from a particular direction). Plants respond to stimuli by regulating plant growth.
What is the difference between a positive tropism and a negative tropism?
- A positive tropism is growth towards a stimulus.
- A negative tropism is growth away from stimulus.
What is phototropism?
The growth of a plant in response to light. Shoots are positively phototrophic (grow towards light), roots are negatively phototrophic (grow away from light).
What is geotropism?
The growth of a plant in response to gravity. Shoots are negatively geotropic (grow upwards), roots are positively geotropic (grow downwards).
What are the different tropisms called?
- Phototropism
- Geotropism
- Hydrotropism
- Thermotropism
- Thigmotropism
What is hydrotropism?
Plant growth response to water. Roots are positively hydrotropic.
What is thermotropism?
Plant growth in response to temperature.
What is thigmotropism?
Plant growth in response to contact with an object.
What are growth hormones and which ones do plants use to respond to some stimuli?
Growth hormones are chemicals that speed up or slow down plant growth. They are produced in the growing regions of the plant (e.g. shoot tips and root tips) and they move to where they are needed in other parts of the plant. Examples are :
- Auxin
- Gibberellins
What does auxin do?
The growth hormone that stimulates the growth of shoots by cell elongation. Auxins are produced at the tip of shoots in flowering plants and diffuse backwards to stimulate the cell just behind the tip to elongate. This is where the cell walls become loose and stretchy, so cell gets longer.
If tip of shoot removed, no auxin available so shoot stops growing. Auxin stimulates growth in shoots but high concentrations inhibits growth in roots.
What does IAA stand for?
Indoleacetic acid
What is IAA?
IAA is an auxin that is produced in the tip of shoots and roots in flowering plants. It is moved around the plant to control tropisms, over short distances by diffusion and active transport, and over long distances by phloem. The uneven distribution causes uneven growth.
How do plants use IAA in phototropism?
IAA moves to the more shaded parts of the shoots and roots, so uneven growth. IAA inhibits root growth and promotes shoot growth.
What is PAG 11 : Practical investigations into phototropism? What would you expect the results to be?
Investigating how plant shoots respond to light :
- Take 9 wheat shoots (roughly equal height) and plant them in individual pots (with same soil type).
- Cover tips of 3 shoots with a foil cap (A), leave 3 without foil (B) and the last 3 cover the bases (only exposing the tip)(C).
- Set up the shoots in front of light source. Make all shoots same distance from light source and they experience the same light intensity. Control other variables like temperature and exposure to moisture (by growing the shoots in a propagator and watering them all equally).
- Leave the shoots for 2 days.
- Then record the amount of growth (mm) and direction of growth of your shoots.
Results :
Shoots with exposed tips (B and C) grow towards light source (positive phototropism). Covering foil cap (A) prevents growth towards light as the tip is most sensitive to light and since covered, it will continue to grow straight up. Covering the base (C), the shoot should still grow towards light.
What is PAG 11 : the practical investigation into geotropism (one of the ways)? What are the results?
- Line 3 petri dishes with moist (not soaking wat) cotton wool. Use same amount of water and cotton wool in each dish.
- Space 10 cress seeds on cotton wool on each dish and slightly push each seed down.
- Tape lid onto each dish and wrap each one in foil (no gaps). This prevents light from reaching the shoots from germinating seeds and stops phototropism.
- Leave the petri dishes somewhere where the temperature is warmish and pretty constant ( a cupboard). Not anywhere too cold (less than room temperature) as seeds may not germinate.
- Set up dishes so placed at different angles ( 90, 45 and 0 degrees). Label dishes.
- Leave the dishes for 4 days.
- Then unwrap each dish and see the direction of shoot and root growth in cress seedlings.
Results :
Whatever angle the dishes are placed at, the shoots have grown away from gravity (negative geotropism) and roots grow towards gravity (positive geotropism).
What is an apical bud?
The shoot tip at the top of a flowering plant.
What is apical dominance?
Auxin stimulates the growth of the apical bud and inhibits the growth of side shoots from lateral buds. The apical bud is dominant over the lateral bud.
How does apical dominance benefit a flowering plant?
As apical dominance prevents side shoots from growing, it saves energy (prevents side shoots from same plant competing with the shoot tip for light). As energy isn’t being used to grow side shoots, apical dominance allows a plant in an area of lots of other plants, to grow tall very fast, past smaller plants to reach sunlight.
What happens when you remove the apical bud from a plant?
The plant will not produce auxins, so side shoots will start growing by cell division and cell elongation.
What happens when you remove the apical bud and replace it with a source of auxin (agar block containing auxin)?
Side shoot development is inhibited.
Why do very tall plants have have side shoots starting to grow near the bottom?
Auxin becomes less concentrated as they move away from the apical bud to the rest of the plant. If a plant grows very tall, the bottom of the plant will have low auxin concentration, so side shoots will start to grow near the bottom.
What is PAG 11 : Investigating the role of auxins in apical dominance? What are the results?
- Plant 30 plants of same type (pea plants) with similar age, height and weight in pots containing the same soil type.
- Count the number of side shoots from main stem of each plant.
- For 10 plants, remove the tip of shoot and apply paste containing auxin on top. For next 10 remove tip of shoot and apply paste without auxin on top. Leave the final 10 as they are. These are the controls.
- Leave plants for 6 days. Keep the plants in the same conditions (light intensity, water, etc).
- Then count number of side shoots growing from main stem of each plant.
Results :
Removing the tips of shoots caused more side shoots to grow. Removing the tips and applying auxin prevents more side shoots growing.
What are Gibberellins?
A growth hormone that is produced in young leaves and in seeds. They stimulate seed germination, stem elongation, side shoot formation and flowering.
How do Gibberellins stimulate growth of plants?
They stimulate the stems of plants to grow by stem elongation (helps plants grow very tall). If a dwarf variety of plant treated with gibberellins then they will all grow same height as the tall variety. Gibberellins do not inhibit plant growth in any way (unlike auxins).
How do gibberellins stimulate seed germination? What inhibits this?
They trigger the breakdown of starch into glucose in the seed. The plant embryo in the seed can use this glucose to begin respiring, releasing energy that it needs to grow. Gibberellins are inhibited (and therefore seed germination is inhibited) by the hormone abscisic acid.
Why are auxins and gibberellins said to be synergistic?
They work together to have a really big effect, exa :
Auxins and gibberellins work together to help plants grow very tall.
Why are auxins and gibberellins said to be antagonistic?
They oppose each other’s actions, exa :
Gibberellins stimulate the growth of side shoots, but auxins inhibit the growth of side shoots.
What is PAG 11 : Investigating the role of gibberellins in stem elongation? What are the results?
- Plant 40 plants (like dwarf pea plants) that are a similar age, height and mass in pots containing the same type of soil.
- Water 20 of them in same way and keep conditions the same. These are my negative control.
- The other 20 will grow in same conditions but water them with a dilute solution of gibberellins (100 mg dm^-3).
- Let plants grow for 28 days.
- Every 7 days record the length of each plant and calculate the mean stem length for plants watered normally and watered with dilute solution of gibberellins.
Results :
The stems grow more when they are watered with a dilute solution of gibberellins.
What is a negative control?
Used to check that only the independent variable is affecting the dependent variable.
What are deciduous plants?
Plants that loose their leaves in winter.
What is abscission?
Leaf loss.
Why do plants loose leaves in winter?
Loosing leaves helps plants to conserve water (lost from leaves), during cold parts of year. This is where it may be difficult to absorb water from the soil (soil water may be frozen) and there is less light for photosynthesis.
How do auxins effect leaf loss?
Auxins inhibit leaf loss. Auxins are produced by young leaves, as the leaf gets older, less auxin produced leading to leaf loss.
How does ethene effect leaf loss?
Ethene stimulates leaf loss. Ethene is produced in aging leaves, as leaf gets older, it produced more ethene. A layer of cells called the abscission layer develops at bottom of leaf stalk (where leaf joins to the stem). The 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.
Why do plants need stomata closure?
In order to reduce water loss through transpiration. This happens because of the guard cells, found either side of the stomatal pore. When guard cells full of water, they are plump and turgid so pore is open. When the guard cells lose water, they are flaccid, making the pore close.
How does ABA (abscisic acid) trigger stomatal closure?
- ABA binds to receptors on the guard cell membrane. This causes specific ion channels to open causing calcium ions to enter cytosol from the vacuole.
- Increased calcium ion concentration in cytosol causes other ion channels to open. These ion channels allow other ions (like potassium ions) to leave the guard cell, raising the water potential of the cells.
- Water leaves the guard cells by osmosis.
- Guard cells become flaccid and the stomata close.
What is the cytosol?
Liquid part of the cytoplasm in a cell.
How can ethene be used to ripen fruit? Give example.
Ethene stimulates enzymes that break down cell walls, break down chlorophyll and convert starch to sugars. This makes the fruit soft, ripe and ready to eat. Exa :
Bananas are harvested and transported before ripe so less likely to be damaged. They are exposed to ethene on arrival so all ripen at same time (on shelves and in people’s homes).
How can auxin be used by farmers and gardeners?
- Auxins are used in selective weedkillers (herbicides). They make weeds produce long stems and lots of leaves. This makes the weeds grow very fast, so the plant can’t get enough nutrients or water, so they die.
- Auxin are used as rooting hormones (in rooting powder). They make a cutting (part of plant like a stem cutting) which can be planted and grown into a new plant. Many cuttings can be taken from just one original plant and treated with rooting hormones. This means lots of plants can be grown quickly and cheaply from just one plant.
