Plant reponses Flashcards
What is a tropism
a plant that shows directional growth in response to environmental cues
Key roles of auxins
- cell elongation
- prevent leaf fall
- fruit ripening
- involved in tropisms
- ethene release
Key roles of gibberelins
- stem elongation
- trigger mobilisation in of food stores in seed germination
- stimulate pollen tube growth in fertilisation
Key roles of ethene
- fruit ripening
- abscission in deciduous trees
Key roles of abscisic acid (ABA)
- stimulate stomatal closing
- stimulate cold protective responses
- maintain seed and bud dormancy
What is a cotyledon
a food store in a dicot seed
How do gibberellins stimulate seed germination
- gibberellins begin to be produced when a seed absorbs water and the embryo is activated
- gibberellins stimulate the production of enzymes that break down food stored
- embryo uses food stores to produce ATP for building materials to break through seed coat
Role of auxins in growth
Apical dominance- growth of the main branch only
How do auxins affect cell wall plasticity
- auxin present means the wall stretches more easily
- auxin molecules bind to receptors in the plant cell membrane, causing the pH to fall to 5
- pH 5 is the optimum for enzymes to keep the walls stretchy
- as the cells mature, auxin is destroyed, causing pH to rise so enzymes maintaining plasticity are inactive
- walls become rigid so cells can no longer grow and expand
What do high concentrations of auxins do
- suppress the growth of lateral shoots (branches)
- results in apical dominance
- shoots lower down aren’t affected so that lower lateral shoots grow better
What is an internode
- regions between leaves on a stem
What is synergism
- different hormones work together, complimenting each other
- give a response greater than they would individually
What is antagonism
- substances have opposite effects, the balance between them determines the plants response
- give an effect smaller than they would individually
Why do plants lose their leaves as a result of abiotic stress
- glucose required for respiration and to maintain leaves > glucose produced by photosynthesis
What is photoperiodism
- the idea that plants are sensitive to lack of light in their environment
What is phytochrome
-a light sensitive pigment, existing in 2 forms, Pr and Pfr
- each of the forms absorbs a different type of light
What is the abscission zone
- made up of 2 layers of cells, sensitive to ethene
- at the base of the leaf stalk
Process of leaf abscission
- falling light levels results in lower auxin concentrations
- leaves respond to low auxin by producing gaseous hormone ethene
- ethene initiates gene switching on in the abscission zone
-gene switching on results in production of new enzymes which digest and weaken cell walls in the separation layer - vascular bundles that carry materials in and out the leaf are sealed off
- fatty material is deposited into the cells on the stem side of the separation layer (forms protective scar later)
- cells in separation zone respond to hormonal cues by retaining water and swelling, straining the outer layer
- more abiotic stressors put to much strain on the leaf, it falls off leaving a scar
What is the separation zone
cells in the outer layer of the abscission zone
How do cells prevent freezing
- cytoplasm and sap in the vacuole contain solutes that lower the freezing point
- some solutes act as antifreeze- stop cytoplasm freezing
- different genes are suppressed and activated in response to change in daylength
Role of ABA in stomatal control
- open and close stomata in response to abiotic stress
- leaf cells now appear to release ABA when under abiotic stress
What is herbivory
- the process by which herbivores eat plants
Physical defences against herbivory
thorns, spikes, hairy leaves, stings, inedible tissue
Chemical defences against herbivory
- Tannins
- Alkaloids
- Terpenoids
- Pheromones
- VOC’s
-Fold in response to touch
Tannins
- have a bitter taste to animals
- toxic to insects- bind and deactivate digestive enzymes in saliva
Alkaloids
- bitter tasting, nitrogenous compounds
- e.g. caffeine, nicotine, cocaine, morphine
- Caffeine toxic to fungi and insects
Terpenoids
- act as toxins to insects and fungi
- e.g. phyrethin interferes with insect nervous system
- some act as insect repellent
Pheromones
- chemicals made by an organism that affect the social behaviours of other members of the same species
- mainly animals only, e.g. maple trees
VOC’s
- volatile organic compounds
- act as pheromones between plants and other organisms
- e.g. attract herbivore predators
Plants that respond in response to touch
- Venus fly trap
- Mimosa Pudica= folds and collapses in seconds, removes herbivore, scares others, plants has prickles and toxic alkaloids
Phototropism
- plant growth in response to light from 1 direction
Geotropism
- plant growth in response to gravity
Chemotropism
- plant growth in response to chemicals
Thigmotropism
- plant growth in repsonse to touch
How are plants phototropic
- results of auxin movement
- if exposed to light brighter on 1 side (unilateral light), shoots grow towards light, roots grow away
- shoots= phototropic, roots= negatively phototropic
- enables maximum photosynthesis
Effects of unilateral light on a plant
- causes auxin to move laterally, with a greater concentration on the unilluminated side
- this stimulates cell elongation and growth towards the light
- results in bend plant
Growth of plants in the dark
- grow more rapidly in the dark- grow upwards to reach light ASAP
- gibberellins responsible for extreme internode elongation in the dark
Shoot and root geotropism
- shoots= negatively geotropic, grow against gravity
- roots= positively geotropic, grow towards gravitational pull
Hormones controlled in plant ripening
- ethene
- fruit is harvested a long time before it is ripe so it can be transported easier
- when time for sale, exposed to monitored levels of ethene to ripen
Hormones affecting rooting powders and micropropagation
- auxin application to cut shoots stimulates production of new roots
- cuttings can cause roots to appear
- dipping the cut stem into hormone rooting powder increases chances of successful propagation occurring
- easier to develop cuttings for people to sell
Hormone weedkillers
- synthetic auxins used as weed killers= plants affected by weeds are often monocot, weeds are dicot
- if synthetic dicot auxins are used, destroys weeds without harming plant
Uses on plant hormones commercially
- fruit ripening
- rooting powder/ micropropagation
- weed killers
- auxin involved in seedless food production
- ethene promotes fruit dropping
- cytokinin prevent aging of ripened fruit
- gibberellins also delay ripening and aging, improve fruit shape and size
