5.1.5 - plant responses Flashcards
tropisms
response to environmental cues
limitations on plants
they are not mobile, and do not have a rapidly responding nervous system
roles of auxin
control cell elongation, prevent leaf fall, maintain apical dominance
roles of gibberellin
causes stem elongation, trigger the mobilisation of food stores in germination
ethene
causes fruit ripening, and promotes abcission
ABA
maintains dormancy of seeds and buds, stimulates cold protective responses
seed germination process
- seed absorbs water, embryo is activated and produces gibberellins
- stimulates production of amylase and protease to break down food stores in seed
- uses the glucose from breaking down, to produce ATP for building materials so it can grow and break out through the seed coat
experimental evidence supporting the role of gibberellins in the germination of seeds
- seeds that have been bred which lack the gene to make gibberellins, do not germinate.
- if gibberellin biosynthesis inhibitors are applied to seeds they do not germinate as they cannot make the gibberellins needed
how do auxins stimulate elongation?
- binds to receptors on cell wall
- releases H+ ions into the cell wall
- this lowers the pH
- this makes the cellulose more flexible
- activates expansin enzymes
- allows the cells to elongate
apical dominance
growth in the main shoot is stimulated by the auxin produced at the tip so it grows quickly -lateral shoots are inhibited by the hormone that moves down the stem sp do not grow very well.
what happens to the lateral shoots if the apical shoot is removed
they are freed from the dominance of the apical shoot, so they grow faster
gibberellins role in elongation
affect the length of the internodes - the region between the leaves on a stem.
synergism
different hormones working together, complementing eachother and giving a greater response than they would on their own
antagonism
different hormones having opposite effets, and the balance of these will determine the response of tge plant
examples of abiotic stresses
changes in day length, cold and heat, lack of water, high winds, and changes in salinity
why do some plants lose leaves in winter?
the conditions are not right for photosynthesis
waste of essential substances
may be blown by strong winds or heavy rainfall
photoperiodism
plants are sensitive to a lack of light in their environment
abcission
leaf fall
what happens to auxin levels, when light intensity falls
lowers the conc of auxin
process of abcission
- lower conc of auxin causes production of ethene
- the ethene switches on genes in the abcission zone, producing new enzymes
- these digest and weaken cell walls in the outer layer, becoming the separation layer
- vascular bundles are sealed off
- protective scar when leaf falls off, preventing entry of pathogens
why is it bad if cells freeze?
membranes will be disrupted
adaptations for preventing freezing
cytoplasm of the plant cells and the sap in the vacuoles contain solutes whuch lower the freezing point.
sugars or proteins that act as antifreeze
what hormone controls stomatal opening and closure?
ABA
physical defences of plants
thorns, barbs, spikes, spiny leaves, fibrous and inedible tissue, hairy leaves etc
chemical defences - tannins
bitter taste, which puts off animals eating the leaves, toxic to insects
chemical defences - alkaloids
bitter tatsing, nitrogeneous compounds found in many plants, include caffeine or nicotine
chemical defences - terpenoids
large group of compounds which act as toxins to insects and fungi that may attack the plant
pheromone
chemical made by an organism which affects the social behaviour of other members of the same species.
volatile organic compounds (VOCs)
act like pheromones between themselves and other organisms, diffusing through the air in and ariund the plant.
examples of plants folding in response to touch
venus fly trap - when the leaves are touched, they fold down and collapse.
phototropism
growth in plants in response to light
geotropism
response to gravity
chemotropism
response to chemicals
thigmotropism
response to touch
growth when there is all round light
the plant will grow straight and upwards, the auxin is distributed evenly
growth when the light is on one side
the auxin will gather on the shady side, elongate, and the plant will slant towards the light
roots and phototropism
roots grow away from the light
example of forcing growth in plants
rhubarb
etiolation
upward growth which takes place in a plant grown in the dark
why are etiolated plants thin and pale?
no chloropyll because no light
geotropism and shoots
negatively geotropic (they grow away frim gravity)
roots and geotropism
positively geotropic (grow towards the gravitational pull)
why do roots grow towards gravity?
to ensure they grow into the soil.
which plant hormone is involved in ripening fruits?
ethene
climareric
fruits that continue to ripen after they have been harvested
e.g. bananas, tomatoes and mangoes
what happens to the ethene to cause ripening?
linke to a peak of ethene production triggering a series of chemical reactions including a greatly increased respiration rate.
why is it important to control the ripening of fruit?
prevents wastage of fruit during transport and increases the time available for them to be sold.
role of auxin in micropropagation
dipping the cut stem of a cutting into hormone rooting powder (contains auxin) to improve the chances of roots forming and of successful propogation taking place.
what happens if the balance of the hormones in a plant is lost?
interrupt the metabolism of the whole plant and it may lead to plant death - this is sometimes useful for weeds etc
why are weeds not good for crop yield?
interefere with crop plants, competing for light, space, water and minerals
how do auxins deal with the problem of weeds?
scientists have developed synthetic auxins which act as very effective weedkillers - the plants absorb the auxins and affect the metabolism, becoming unbalanced and die. these do not affect the desired plants
what hormone can be used in the production of seedless fruit?
auxins
commercial use of cytokinins
prevent ageing of ripened fruit and products such as lettuces and in micropropagation to control tissue development
