Plant responses Flashcards
what is a tropism
response to a stimulus
what is the role of auxins?
control cell elongation, prevent leaf fall, maintain apical dominance, involved in tropisms, stimulate the release of ethene, involved in fruit ripening
-prevent fruit fall by inhibiting ethene
-inhibit root growth at high concentration
-casues shoots to bend towards the light
-causes roots to grow towards gravitational pull
what is the role of gibberellins?
cause stem elongation, trigger the mobilisation of food stores in a seed at germination, stimulate pollen tube growth in fertilisation
what is the role of ethene?
causes fruit ripening, promotes abscission( leaf fall) in deciduous trees
what is the role of ABA (abscisic acid)?
maintains dormancy of seeds and buds, stimulates cold protective responses e.g antifreeze production stimulates stomatal opening
-when water becomes limiting, roots cells synthesise and release ABA. ABA is transported to the leaves and binds to receptors on the plasma membrane of guard cells. The ionic concentration of the guard cells deceases therefore water potential decreases. Water moves out by osmosis.
how does a seed germinate?
-seed absorbs water, embryo is activated and produces gibberellins which stimulate the production of enzymes that break down food stores. The food store is in the cotyledons in the dicot seeds and the endosperm in monocot seeds. Embryo plant uses these food stores to produce ATP for building materials to grow and break out the seed coat.
what evidence is shown about gibberellins and ABA (plant hormone) during seed germination?
-gibberellins switch in genes which code for amylases and proteases- digestive enzymes required for germination
-ABA (plant hormone) acts as an antagonist to gibberellins (interferes with the action of gibberellins) and that it is the relative levels of both hormones which determine when a seed will germinate
what experimental evidence supports the role of gibberellins in the germination of seeds?
-mutant varieties of seeds have been bred which lack the gene that enables them to make gibberellins. These seeds do not germinate unless gibberellins are applied externally
-if gibberellin biosynthesis inhibitors are applied to seeds, they do not germinate as they cannot make the gibberellins needed for them to break dormancy. If inhibitors are removed or gibberellins are applied externally, seeds will germinate
where are auxins found?
tip of roots and shoots and meristems. Meristematic cells produce auxins
the effect of auxins depends on what?
the concentration and any interactions it has with other hormones
what are the different effects auxins have on plant growth?
-stimulate the growth of the main apical shoot
-auxins suppress the growth of lateral shoots (apical dominance)
-low concentrations of auxins promote root growth
how do auxins affect the plasticity of the cell wall (streches more easily)?
auxin molecules bind to specific receptor sites in the plant cell membrane, causing a fall in pH to about 5. This is the optimum pH for enzymes needed to keep the walls very flexible and plastic. As the cells mature, auxins mature, hormone levels fall and pH rises so the enzymes maintaining plasticity become inactive. Cell walls become fixed, inactive and can no longer grow and expand.
Describe the cell elongation process?
-auxins are synthesised in the meristem cells in the zone of cell division.
-auxins diffuse from the tip and bind to receptors in the zone of cell elongation
-binding causes protein channels to open and H+ ions are diffused into the cell, lowering its pH.
-low pH causes cell wall to become more flexible
-cell expand as it absorbs more water. Vacuole gets bigger and cell walls stretch
-auxins become destroyed by enzymes and cell walls become rigid, no further cell elongation is possible
Describe apical dominance?
growth in the main shoot is stimulated by the auxin produced at the tip so it grows quickly. The lateral shoots are inhibited by the hormone that moves back down the stem, so they do not grow very well. Further down the stem, the auxin concentration is lower and so lateral shoots grow more strongly. This allows plant to better compete for light
what is the experimental evidence for apical dominance?
when apical shoot is removed, auxin producing cells are removed so there is no auxin. Lateral shoots are freed from apical dominance and grow faster. If auxin is applied artificially to the apical shoot, apical dominance is reasserted and lateral shoot growth is suppressed.
Describe role of auxins in roots?
up to a given concentration, the more auxin that reaches the roots, the more they grow. Auxin is produced by the root tips and auxin also reaches the roots in low concentrations from the growing shoot. If the apical shoot is removed, the amount of auxin reaching the root is reduced so root growth slows and stops. Replacing auxins at apical shoots can restore root growth. High auxin concentration inhibit root growth.
Describe the role of gibberellins in stem elongation?
higher concentration of gibberellins increase the length of the internodes- the regions between the leaves on a stem. This causes stem elongation making the plant better compete for light.
without gibberellins?
plant stems would be much shorter
what are the advantages to having less gibberellins for shorter stems?
-reduces waste (unedible)
-makes plants less vulnerable to damage by weather and harvesting
what is synergism?
different hormones working together, complementing each other and giving a greater response than they would on their own.
what is antagonism?
if the different hormones working together have opposite effects and (e.g one promoting growth and the other inhibiting it), the balance will determine the response of the plant.
what is photoperiodism?
plants response to length of darkness, lack of light
the sensitivity of plants to day light (or dark length) results from what light-sensitive pigment?
phytochrome which exists in two forms: Pr and Pfr. Each absorbs a different type of light and the ratio of Pr to Pfr changes depending on the levels of light
the lengthening of the dark period triggers?
abscission or leaf fall and a period of dormancy
how does abscission occur?
-falling light levels results in falling concentrations of auxin
-falling auxin concentrations lead to production of ethene hormone
-the abscission zone are made up of two layers of cells sensitive to ethene.
-ethene initiates gene switching in these cells resulting in the production of new enzymes.
-these digest and weaken the cell walls in the outer layer of abscission zone, know as the separation layer
-vascular bundles which carry material in and out of the leaf are sealed off
-fatty material is deposited in the cells of the stem side of the separation layer. This layer forms a protective barrier against pathogens.
-cells deep in the separation zone respond to hormonal cues by retaining water and swelling, putting more strain on the weakened outer layer.
-further abiotic factors such as low temperatures and strong winds finish the process as the strain is too much and leaf separates from the plant. A waterproof scar is left behind.
how can plants prevent freezing?
-in some plants, the cytoplasm and sap in the vacuole contain solutes which lower the freezing point.
-some plants produce sugars, polysaccharides, amino acids and proteins which act as anti-freeze to prevent cytoplasm freezing or protect cells from damage if they do freeze.
how do plants produce their response to prevent freezing?
different genes are suppressed and activated in response. Sustained warm weather and a extended day length reverses these changes.
what are the main abiotic stresses for plants?
water availability and heat
