plant responses
plants need to respond to stimuli
increases chance of survival
they sense direction of light and grow towards it to maximise light absorption for photosynthesis
they sense gravity so their roots grow in the right direction
climbing plants are sensitive to touch so they find things to climb to reach sunlight
chemical defences against herbivory
alkaloids- chemicals with bitter tastes, noxious smells or poisonous
tannins- tastes bitter and for some organisms are hard to digest as they bind to proteins in gut
pheromones
alarm pheromones- can signal to nearby plant that an organism is grazing so they can produce chemicals such as tannins
response to touch
they are able to fold up
a tropism is a plant's growth response to external stimulus
positive tropism= growth towards stimulus
negative tropism= growth away from stimulus
phototropism
the growth of a plant in response to light
shoots are positively phototropic as they grow towards light
roots are negatively phototropic as they grow away from light
geotropism
growth of plants in response to gravity
shoots are negatively phototropic as they grow up
roots are positively phototropic as they grow down
hydrotropism= growth in response to water
thermotropism= growth in response to temp
thigmotropism= growth in response to contact with an object
responses are bought about by growth hormones
gibberellins= stimulates seed germination, stem elongation, side shoot formation and flowering
auxins= cell elongation
high conc of auxin stimulates growth in shoot but inhibits growth in shoot
indoleacetic acid (IAA)
auxins are produced in tips of shoots of flowering plants
IAA is a type of auxin
IAA is moved around the plant to control tropisms via active transport and diffusion (short distances) or the phloem (long distances)
different parts of plant has varying concs of IAA causing uneven growth
e.g. when IAA moves to shaded part of shoot causing cells to elongate and shoot to bend towards light
e.g. when iAA moves to underside of shoot causing cells to elongate so shoot grows upwards
effects of plant hormones
auxins are involved in apical dominance
auxins stimulate growth of apical bud (shoot) and inhibit growth of side shoots from lateral buds (apical dominance )
preventing side shoots growth saves energy and prevents side shoot competing with apical shoot for light
apical dominance increases speed of growth
if apical bud is improved the plant wont produce auxin so side shoots from lateral buds will grow via cell division and cell elongation
if source of auxin is placed instead of apical bud side shoot growth is inhibited
conc of auxin decreases further away from apical bud
gibberellins
gibberellins stimulate seed germination, stem elongation, side shoot formation and flowering
gibberellins because the stimulate stem elongation allow plants to grow very tall
gibberellins stimulate seed germination by converting start to glucose in seed. the plant embryo in the seed can then use the glucose to begin respiring ad release the energy it needs to grow
gibberellins are inhibited by abscisic acid
gibberellins and auxins work together to affect plant growth
auxins and gibberellins are synergistic- work together
auxin and gibberellins can also be antagonistic - oppose each others action e.g. gibberellins stimulates growth of side shoots but auxin inhibits
hormones are involved in leaf loss in deciduous plants
losing leaves helps them conserve water
leaf loss is triggered by shortening day length
auxin inhibits leaf loss- as leaf ages less auxin is produced
ethene stimulates leaf loss- ethene causes abscission layer to expand, breaking the cell walls and causes leaf to fall off
hormones involve stomatal closure
plants close stomata to reduce water loss through transpiration
guard cells control whether stomata are open- turgid guard cell causes stomata to be open, flaccid guard cell causes stomata to be shut
abscisic acid triggers stomatal closure
ABA binds to receptors on guard cell membranes causing ion channels to open and calcium ions to enter cytosol from vacuole which in turn causes other ion channels to open. ions then leave the guard cell increasing water potential of cells so water leaves by osmosis causing stomata to be flaccid and close
plant hormones have commercial uses
ethene stimulates fruit ripening by breaking down cell walls and converting starch to sugars
auxins are used as selective weedkillers- cause weeds to grow to fast and not have enough leaves causing them to die