tropisms Flashcards
define ‘tropism’
when plants respond to stimuli via growth
- triggered by chemical messages produced in response to the stimulus
what 3 factors do plants respond to?
- gravity
- water
- light
what are tropisms controlled by?
- growth factors
- eg indoleacetic acid (IAA)
describe IAA
- type of auxin
- controls cell elongation in shoots
- inhibits cell growth in roots
- made in tip/root/shoot
- can diffuse to other cells
what is a positive tropism?
growth towards a stimulus
what is a negative tropism?
growth away from a stimulus
describe phototropism in the shoots
- positive
- light is required for light dependent reactions in photosynthesis so plant bends towards light
summarise the process of phototropism in the shoots
- shoot tip cells produce IAA
- causes cell elongation
- IAA diffuses to other cells
- if light is unilateral, IAA diffuses to shade
describe phototropism in the roots
- negative
- roots don’t photosynthesise so don’t need light
- they anchor the plant in soil
- high conc of IAA inhibits cell elongation so cells elongate more on lighter side and bend away from light
describe gravitropism in shoots
- IAA diffuses from upper to lower side
- if plant is vertical, cells elongate and plant grows upwards
- if plant is horizontal, IAA diffuses to lower side, shoot bends upwards (negative tropism)
describe gravitropism in the roots
- IAA moves to lower side of root
- upper side elongates
- lower side bends downwards towards gravity
- positive
define ‘phototropism’
growth of plants in response to light
- towards or away
define ‘geotropism’
plant response to gravity
which hormone is most commonly associated with phototropism?
auxins
IAA
how will plants grow if they are grown in bright, all round light in normal gravity conditions?
straight upwards
- faster
- taller
how will plants grow if they are exposed to light which is brighter on one side than the other?
- shoots grow towards light
- roots grow away
shoots are …. phototrophic
roots are ….. phototrophic
positively
negatively
describe how IAA control tropisms
- IAA is produced in cells at shoot tip
- light causes IAA to gather in shaded side of shoot
- IAA in shoot causes elongation
- one side of shoot elongates faster, uneven growth causes bending
- at root, high conc of IAA inhibits growth and low conc promotes growth
state what happens in each of Darwin’s experiments in discovering the role of IAA
- intact
- apex excised
- apex shielded
what happened in Darwin’s expt 1: intact ?
- auxins gather
- grows towards light
what happened in Darwin’s expt 2: apex excised?
- no auxins if no tip
- grows upwards
what happened in Darwin’s expt 3: apex shielded?
- IAA not produced by auxins as no light
- grows upwards
what happened in Darwin’s expt 4: light passes through tip?
- auxins pass down to shaded side
- grows towards light
what happened in Darwin’s expt 5: barrier on shaded side?
- movement of auxins prevented
- no response
