9.3 Flashcards
what plant growth factors (plant name for hormone) bring about chemical control in plants?
auxins
cytokinins
gibberellins
what type of auxin are we looking at and what is its function? 3
- IAA = indoleacetic acid
- involved in cell elongation (IAA controls phototropism)
- suppression of lateral buds to maintain apical dominance
- in low conc promote root growth (controlling phototropism)
how do auxins cause cell elongation?
- zone of cell division: auxins synthesised in meristem cells (in shoot)
- zone of cell elongation: auxins diffuse from tip and auxins bind to receptor on CSM and activate AT of H+ ions into cell walls
- this causes pH of walls to lower (~5) - enzymes break bonds bw adj cellulose microfibrils - slide past each other easily - walls become flexible
- low pH makes cell walls flexible - so they can stretch to accommodate more water by osmosis - bc of turgor pressure - flexible cell walls stretch - cells elongate + expand
- then after auxin is destroyed by enzymes - cell becomes rigid no more elongation possible
- this enables expansion + growth of cells
where are auxins produced
- young shoots and move from shoots down to roots
how do auxins distribute
- shoot tip acts as photoreceptor
- if shoot illuminated from all sides - auxin distributed evenly and move DOWN shoot to shaded regions
- illuminated only one side - auxins move to shaded part of shoot - elongation of shaded side only - bending of shoot towards light
functions of gibberellin?
- stimulate elongation at cell internodes
- stimulate growth of fruit
- stimulate end of dormancy in seeds + germination
- stimulate ‘bolting’ = rapid growth and/or flowering
how does gibberellin stimulate germination?
- seed absorbs water - activating embryo
- activated embryo secretes gibberellin - which diffuse to aleurone layer
- gibberellin stimulates this layer to produce amylase which diffuses to endosperm layer + amylase breaks down starch to glucose - can be used for rest + growth
- enzymes produced in response to G digest endosperm - products released = useful to make NEW CELLS + GERMINATE
functions of cytokinins?
- promote cell division in apical meristems + cambium through interactions w auxins
- promote lateral bud development
- work synergistically w ethene to promote abscission of leaves, flowers + fruits
what is synergy? give an example
- working tg + complementing each other to give greater response
- auxins + gibberellins in growth of stems
what is antagonism? give an example
- substances have opp effects
- auxins + cytokinins on maintenance of apical dominance
what is apical dominance and how do auxin and cytokinins affect it?
= one lead shoot in young plant grows bigger and faster than all others - growth of all other lateral buds = inhibited bc of HIGH AUXIN levels produced by first emerging shoot
- cytokinin stimulated development of lateral buds
what is phytochrome
a plant pigment that exists in 2 interconvertible forms: Pr (biologically inactive form + absorbs RED light eg sunlight) & Pfr (biologically active)
how do the two phytochrome forms convert ?
- pr converts to pfr rapidly in red light (daylight)
- pfr converts to pr in rapidly far red light or slowly in darkness
how does phytochrome control flowering?
- regulates flowering based on day length (photoperiodism)
1. LONG DAY PLANTS: flower when days = long, nights = short; in these plants Pfr promotes flowering as during long day pr converted to pfr and during nights enough pfr remains to stimulate flowering
2. SHORT DAY PLANTS: pfr inhibits flowering in these plants; during long nights - pfr converts back to pr - removing inhibition + allowing flowering
how do phytochrome control phortomorphogenesis - and what os it?
= plant growth + development in response to light
- phytochrome synthesised as Pr so when seedling emerges from seed underground it only has pr as not exposed to light - this means seedling = etiolated (tall + thin, fragile stems w long internodes, small yellowed leaves, little root growth)
- this is reversed when steam breaks thru soil as pfr is made and pfr stimulates leaf development + production of chlorophyll
- pfr acts as transcription factor (switching genes on and off in nuclei of plant cells) - to control Dif aspects of growth + development in plants
