Plant Responses Flashcards
what are some plant responses to abiotic stress
leaf loss in deciduous plants
daylength sensitivity
leaf fall
preventing freezing
stomatal control
describe how leaf loss in deciduous plants is beneficial for the plant
Trees in temperate climates – great environmental changes in year
Glucose required for respiration to maintain leaves + to produce chemicals from chlorophyll to protect against freezing > glucose produced from respiration
Leaf tree more likely to be damaged + blown over
how do plants prevent freezing + why is this necessary
Cytoplasm + sap in vacuoles – have solutes = lower freezing point
Produce sugars / polysaccharides / amino acids – act as antifreeze
how do plants control stomata + why is this necessary
Shut stomata for drought – reduce transpiration
Controlled by ABA
Open stomata – cool plant
what are the physical defences to herbivory
thorns
barbs
spiny leaves
fibrous + inedible tissue
hairy leaves
folding in response to touch
what is an example of a plant that folds in response to touch
Mimosa pudica plant
how does the Mimosa pudica plant fold in response to touch
leaflets of touch-sensitive leaves fold in rapidly when touched
caused by rapid water uptake (+ therefore increase in volume) in cells at the base of each leaflet and rapid loss of water from (+ therefore collapse of) adjacent cells
maybe local bioelectrical signal
what are the chemical defences in response to herbivory
tannins
alkaloids
terpenoids
pheromones
what are tannins and how are they a chemical defence
part of phenols – water soluble carbon compound
stored in vacuoles
bitter taste
toxic to insects – bind to digestive enzymes produced in saliva + inactivate them
what are alkaloids and how are they a chemical defence
nitrogenous compound
bitter taste
caffeine – toxic to fungi / insects
nicotine – toxin produced in tobacco plants
what are terpenoids and how are they a chemical defence
toxin to insect + fungi
neurotoxin = disrupts nervous system
what are pheromones and how are they a chemical defence
chemical made by organism that affects social behaviour of other members of same species
if tree attacked – produce pheromones
absorbed by leaves on other branches + other trees
leaves make callose to protect
what is tropism
growth movement of a part of a plant in response to a directional stimulus
what type of tropism occurs when the plant grows towards stimulus
positive tropisms
what type of tropism occurs when the plant grows away from stimulus
negative tropism
what is phototropism in response to + why is it beneficial
light
Ensures plants get max light – max photosynthesis
what is geotropism in response to + why is it beneficial
gravity
Ensures shoots/roots from germinating seeds grow in the right direction regardless of the orientation of the seed – positive + negative
what is hydrotropism in response to + why is it beneficial
moisture
Root tips normally grow towards damper areas in soil – increase access to water
what is thigmotropism in response to + why is it beneficial
touch
Important in climbing plants – adapted to climb as it provides greater access to light etc. allows these plants to detect living/non-living support + curl around it
what is chemotropism in response to + why is it beneficial
chemicals
Pollen tubes grow down the flower’s stigma towards the ovules
are shoots positively or negatively phototrophic
positively phototrophic
describe the experimental evidence for phototropism
are roots positively or negatively phototrophic
negatively phototrophic
describe how phototropism occurs
high conc of auxins supress growth of lateral shoots
diffuses to shaded side of shoot tip
causes active transport of H+ into cell wall
disruption of H bonds between cellulose molecules
make cell more permeable
cells on shaded side elongate faster due to higher turgor pressure
Once growing directly towards light = transport of auxin stops
grows straight towards light
describe the experimental evidence for movement of auxins
how does rate of growth change in response to light
in dark p reach surface fast to get light
gibberellins – responsible for extreme elongation of internodes (region between leaves on cell)
when exposed to light – growing slower
are shoots positively or negatively geotropic
negatively geotropic
are roots positively or negatively geotropic
positively geotropic
main roles of auxins
cell elongation
prevent lead fall - abscission
maintain apical dominance
involved in tropism
involved in fruit ripening
describe the experimental evidence for geotropism
main roles of gibberellins
stem elongation
trigger mobilisation of food stores in seeds at germination
stimulate pollen tube growth
main roles of ethene
fruit ripening
abscission in deciduous trees
main roles of ABA / abscisic acid
maintain dormancy of seeds + buds
stimulates cold protective responses - antifreeze productions
stimulates stomatal closing
state the hormones involved in seed germination
gibberellins
ABA
what are gibberellins role in seed germination
When seed is shed from the parent plant = dormant – little water + metabolically inactive
(allows survival in harsh conditions)
when conditions are right - seed absorbs water
produces gibberellins
Gibberellin molecules diffuse into the aleurone layer + stimulate cells to make amylase
amylase hydrolyses starch molecules in the endosperm for monocot seeds
(or cotyledons in dicot seeds)
produces soluble maltose molecules
maltose is converted to glucose and transported to the embryo
respired by the embryo
breaking dormancy + providing energy for growth
how does the gibberellin molecules stimulate cells to make amylase
been shown that gibberellin does this by regulating genes = increase in transcription of mRNA coding for amylase
describe ABA’s role in seed germination
has opposite effect to gibberellin – antagonist
maintaining dormancy by inhibiting amylase production
how is seed germination controlled overall
determined by the balance of ABA + gibberellins
describe briefly how seed germination occurs
what is an endosperm
a starch-containing energy store surrounding the embryo
what is an aleurone layer
a protein-rich layer on the outer edge of the endosperm
state the hormone involved in leaf loss of deciduous plants
auxin
ethene
describe how ethene is involved leaf loss occurs in deciduous plants
Falling light levels – falling levels of auxin
Produce gas hormone ethene
Base of leaf stalk – abscission zone
(Has 2 layers of sensitive cells to ethene)
Ethene causes new enzyme production in there
Digest + weaken cell wall in outer layer of abscission zone // separation layer
Vascular bundles sealed off
Fatty material deposited on stem side of separation layer = form protective scar
Leaf separates from plant
describe how auxin is involved leaf loss occurs in deciduous plants
inhibit leaf loss + produced in young leaves = leaf stalks insensitive to ethene
The concentration of auxins in leaves decreases as they age until leaf loss can once again occur in response to ethene
state the hormones involved in stomatal closure
ABA
describe how stomatal closure occurs
water stress = abscisic acid (ABA) produced
Guard cells have ABA receptors on cell surface membranes
ABA binds receptors
inhibits proton pumps
(stops active transport of H+ out of guard cells)
+ causes Ca2+ to move into cytoplasm of the guard cells via cell surface membranes
Ca2+ = second messengers:
- cause channel proteins to open
- negatively charged ions leave guard cells
- stimulates the opening of further channel proteins
- allow K+ ions to leave guard cells
loss of ions increases the water potential of the guard cells
Water leaves guard cells by osmosis
become flaccid = stomata close
how does auxins cause apical shoot growth
auxins stimulated in meristem cells
diffuse away from tip
bind to receptor sites
vacuoles form + low pH develops
low pH of cells walls - keep flexible
allows cells to explain as they absorb water
vacuoles get bigger + cell walls stretch
large central vacuole forms
auxin destroyed by enzymes
cell walls rigid
no further cell elongation
what is apical dominance
auxins produced at the growing tip at the apex of a plant stem cause the stem to grow upwards + stop lateral buds from growing
describe how auxins cause apical dominance
high conc of auxins supress growth of lateral shoots
growth in apical shoot stimulated by auxin at tip
lateral shoots inhibited by hormone that moves back down – don’t grow well
further down stem – auxin conc is lower
lateral roots grow more strongly
describe the experimental evidence for apical dominance
apical bud of the first test plant is removed (decapitated)
allows the lateral buds to grow
second (genetically identical) test plant is decapitated but this time the cut tip immediately replaced with an agar block containing auxin
restores the inhibition of lateral bud growth and no lateral buds grow
what is the experimental evidence for the role of gibberellins in stem elongation
when given gibberellic acid – grow taller
what is the experimental evidence for the role of gibberellins in germination
mutant varieties of seeds – lack gene that allows them to make gibberellins
seeds do not germinate
if applied externally – germinate normally
if gibberellin biosynthesis inhibitors applied – no germinate
commercial use of ethene
stimulate fruit to ripen by increasing respiration rate
used for fruits that are delicate and soft when they are ripe (e.g. bananas) + that can be easily damaged during transport
commercial use of auxins
rooting powders + micropropagation
hormonal weed killers
unpollinated flower fruit development
produce seedless fruit
how is auxin involved in rooting powders
auxins = stimulate cuttings to grow new roots
cutting – small piece of stem
if cutting placed in soil + rooting powder – increases chance of roots forming + successful propagation
sold commercially in the form of rooting powders
how is auxin involved in hormonal weed killers
weeds interfere with crop plant – compete for light / space / water / minerals
weeds – wide leaves
High conc auxin - rapid growth
become distorted and damage + allow pathogens to enter
normal plants fine – narrow leaved + do not absorb a lot of auxin
what is the commercial use of cytokinin
prevents ageing of ripened fruit
what is the commercial use of gibberellins
delay ripening
make unpollinated flowers develop fruit
