16 - Plant Responses Flashcards
What is a tropism?
The directional growth of a plant in response to a directional stimulus
What is an abiotic stress?
Stress caused by a non-living factor i.e. lack of water
What is a biotic stress?
Stress caused by a living thing i.e. a pathogen
What is a herbivory stress?
Stress caused to a plant by being eaten
What is phytochrome?
A light-sensitive pigment in plants which exists in two variants, the balance of which can invoke a change
What is photoperiodism?
A plant’s response to seasonal changes in length of night and day due to sensitivity to light availability
What are the two key limitations of plants in terms of responses?
They are rooted and do not have a rapidly responding nervous system
What are 4 important plant hormones?
- ABA 2. Ethene 3. Gibberellins 4. Auxins
What happens to auxin levels in plants as a result of low light levels?
They are also low
What does the plant do in response to low auxin levels caused by low light availability?
Produces the hormone ethene
How does ethene cause abscission?
It switches on plant digestive enzymes. This causes a fatty layer to be laid down in the abscission zone, and also causes the cells on the separation side to swell. This weakens the joint of leaf to stem, causing the leaf to fall off
What is abscission?
When plants shed fruits, leafs, flowers, and seeds.
What is the abscission zone?
Layer of cells at base of petiole that weakens and allows leaf to drop off
Why is it important that plants avoid freezing?
Expanding, frozen water can damage cell walls
What is an adaptation that most plants have to prevent freezing?
The vacuole and cytosol contain certain solvents which lower the freezing temperature of water
What is a specific adaptation which carrots have to prevent freezing?
They produce a protein which binds to ice crystals, stopping them from freezing
What hormone causes closure of guard cells?
ABA
What causes ABA to be released?
When the roots detect low water concentration
How does ABA work?
It causes Ca²⁺ and K⁺ ions to leave the guard cells, increasing the water potential and thus causing water to leave the cells, causing the stomata to close
What are 2 known roles of ethene in plants?
Promotes abscission and fruit ripening
What hormone can stimulate antifreeze production in plants?
ABA
What are 5 known roles of auxins in plants?
- Stimulate ethene production 2. Involved in tropisms 3. Control cell elongation 4. Prevent abscission 5. Maintain apical dominance
What are 3 known roles of gibberellins in plants?
- Cause stem elongation 2. Trigger mobilisation of food stores in seeds at germination 3. Stimulate pollen tube growth in fertilisation
What are 3 known roles of ABA in plants?
- Stimulate stomatal closure 2. Stimulate cold protective response 3. Maintains dormancy of seeds and buds
What is a positive tropism?
Growth towards a stimulus
What is a negative tropism?
Growth away from a stimulus
What is the stimulus for geotropism?
Gravity
What is the stimulus for thigmotropism?
Touch
What is the stimulus for electrotropism?
Electric fields
What are the only 2 places in a plant where growth can occur?
Shoots and meristems
How do gibberellins affect stem length?
More gibberellins mean a longer stem
What stimulates a plant seed to start producing gibberellins?
Absorption of water
What do gibberellins do when produced in seeds?
Produce proteases and amylases
What do the proteases and amylases do in seeds?
Cause a store of protein to be built up, which helps growth and energy release in the seed
What acts as an antagonist to gibberellins in seeds?
ABA
What determines when a seed will germinate?
The relative levels of ABA and gibberellins
What 2 pieces of experimental evidence indicate that gibberellins are involved in seed germination?
- Mutant seeds with no gibberellin production gene do not germinate 2. If gibberellin biosynthesis inhibitors are applied to seeds, they do not germinate
What is an example of an auxin?
Indoleacetic acid (IAA)
Where are auxins produced?
Tips of shoots and roots, meristems
What shoot do auxins stimulate the growth of?
The main, apical shoot
What do auxins affect about the cell wall?
They make it more plastic and stretchy
How do auxins make cell walls more plastic?
They bind to receptors and cause a fall in pH to around 5, the optimal pH for enzymes which keep the cell wall plastic
What happens to auxins as cells mature and what does this cause?
They are destroyed, causing the enzymes maintaining cell wall plasticity to be destroyed and meaning that the cell wall becomes more rigid and fixed in place
High levels of auxins suppress the growth of what?
Lateral shoots
How do auxins cause apical dominance?
At the top of the shoot there are high levels of auxins which inhibit lateral shoot growth, causing the apical shoot to grow better and be dominant. As you move further down the plant auxin levels fall and the lateral shoots become more pronounced
What experimental evidence is there for the role of auxins in maintaining apical dominance?
If the apical shoot is removed, apical dominance stops due to a lack of auxins, and this can be reversed by applying auxins to the cut shoot tip
What do low levels of auxins promote except lateral shoot growth?
Root growth
What are 4 types of plant defence against herbivory?
- Physical defences 2. Chemical defences 3. Pheromones 4. Folding in response to touch
What plant is used folding its leaves in response to touch as a defence strategy?
Mimosa pudica
How does folding its leaves help defend Mimosa pudica against herbivory?
Can knock insects off and startle larger predators
What are 4 examples of plant physical defences against herbivory?
- Barbs/thorns 2. Hairy leaves 3. Stings 4. Fibrous and inedible tissue
What are 3 types of plant chemical defences against herbivory?
- Tannins 2. Alkaloids 3. Terpenoids
How do tannins protect plants against herbivory?
They are very bitter and astringent tasting, and are also toxic to insects
How do alkaloids protect plants against herbivory?
Bitter tasting, affecting metabolism of animals
How do terpenoids protect plants against herbivory?
Can act as toxins to insects and fungi which attack the plant
What is a pheromone?
A chemical made by an organism which affects the social behaviour of other members of the species
Why do plants not use pheromones much?
They do not behave socially
What are 2 examples of plants using pheromones?
- Some plants can release chemicals in their root systems to tell their neighbours that they are suffering from water stress 2. When attacked by insects, maple trees release a pheromone which causes other leaves on the tree to prepare chemical defences such as callose
What are VOCs and how do plants use them?
Chemicals which act as pheromones for other organisms, particularly insects. Plants can use these in a variety of ways- they can attract their attackers’ predator, or create chemicals to repel their attackers
What form at nodes on plant stems containing meristems?
Branches
What type of phototropism do shoots and roots display?
Shoots positive, roots negative
How do auxins cause cell elongation?
The plasticity of cell walls which they create allows the cell to expand as they absorb water
What do increased light levels do to auxins?
Cause the auxins to diffuse away from the light source
What causes plants to grow more quickly in the dark?
Gibberellins
Why does it make sense that plants grow more quickly in the dark?
Would allow them to move towards light more quickly
How does ethene cause fruit ripening?
Increased ethene levels cause increased respiration and increased carbon dioxide levels, causing ripening
Why is it advantageous for fruit to be unripe during transport?
It is harder, as starch is converted to sugar during ripening
What type of plant hormone does rooting powder contain?
Auxins
How is rooting powder used?
You dip a cut stem in it and it increases the chances of roots forming
What type of plant cloning are hormones especially vital in?
Micropropagation
What are 4 advantages of hormonal weedkillers?
- Cheap 2. Can be plant species-specific 3. Non-harmful to animals 4. Can be used in fields of monocot crops to kill dicot weeds
How do hormonal weedkillers often work?
They use synthetic auxins to stimulate unsustainable growth in the weeds
What is Synergism
This is where two hormones work together, complementing each other causing a greater response than they would on their own.
What is Antagonism
This is where two hormones have an opposite affect on each other e.g. one promoting plant growth and one inhibiting it. The balance between the response will determine the plant response.
