tropisms Flashcards
define tropism
when plants respond to stimuli via growth
- triggered by chemical messages produced in response to the stimulus
what 3 factors to 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 toward 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 - 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)
describe gravitropism in the roots
- positive
- IAA moves to lower side of root
- upper side elongates
- lower side bends downwards towards gravity
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 bright 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
what happened in Darwin’s exp 1: intact?
- shoot bends towards unilateral light
- so shoot is positively phototropic
what happened in Darwins exp 2: apex excised
- tip removal prevents response
what happened in Darwin’s exp 3: apex shielded
- lightproof cover is placed over tip of shoot
- no response
what happened in Darwin’s esp 4: light passes through tip
- thin, impermeable layer of mica as barrier
- chemical moves down shaded side
- bends towards light
what happened in Darwin’s esp 5: barrier on shaded side
- movement of chemical down shaded side is prevented by mica
- no response
what happened in Boysen-Jensen’s expt: gelatine inserted & tip replaced?
- tip removed, gelatin block placed and tip replaced
- gelatin allows chemical messages to pass through
- plant bends towards light
