Plant And Animal Responses Flashcards
What are the key limitations on plants?
There’re rooted - not mobile
They do nay have rapidly responding nervous system.
What are auxins responsible for?
Control of cell elongation Prevent leaf fall (abscission) Maintain Apical dominance Involved in Tropisms Stimulate release of ethene Involved in fruit ripening.
What do gibberellins control?
Cause stem elongation
Trigger mobilisation of food stores in a seed at germination
Stimulate pollen tube growth in fertilisation.
What does ethene control?
Causes fruit ripening
Promoted abscission in deciduous trees
What does ABA (Abscisic Acid) do?
Maintains dormancy of seeds and buds
Stimulates cold protective responses
Stimulates antifreeze production
Stimulates stomata closing
Explain seed germination in relation to the presence of plant hormones?
Seed absorbs water and the embryo is activated and produced gibberellins. The gibberellins stimulate production of enzymes that break down food stores found in the seed. Embryo plant used food stores to produce ATP for building materials so it can grow and break through seed coat.
Evidence suggests gibberellins switch on genes that code for enzymes like amylases and proteases - the digestive enzymes requires for germination. And also that ABA acts as an antagonist to gibberellins.
What experimental evidence supports the role of gibberellins in the germination of seeds?
Mutant varieties of seeds have been bred which lack the gene which enables them to make gibberellins, these seeds do not germinate. If gibberellins are externally applied, then the seeds germinate.
If gibberellin biosynthesis inhibitors are applied to seeds, they do not germinate as they cannot make the gibberellins required to break dormancy. If inhibitor removed or gibberellins applied. The seeds germinate normally.
Explain how auxins co tell growth of main apical shoot?
Stimulate growth of main apical shoot. Evidence suggests auxins impact the plasticity of cell walls, more auxin = stretchier cell wall. Auxins bind to receptors in plant cell membrane and cause a fall in pH to around 5. 5 is the optimal pH for enzymes needed to keep walls flexible and plastic. As cells mature, the auxin levels fall and the pH rises again = less plasticity, as enzymes aren’t active and walls become rigid and more fixed in shape.
How do auxins suppress the growth of lateral shoots?
High concentrations of auxin suppress the growth of lateral shoots. Results in apical dominance.
Growth in main shoot is stimulated by the auxin produced at the tip so it grows quickly. The lateral shoots are inhibited by the hormone that runs down the stem so they don’t grow very well. Further down the stem auxin concentration is lower and therefore lateral shoots grow better.
Is apical shoot removed, auxin producing cells also removed so there’s no auxin, lateral shoots therefore grow very quickly.
How do low concentration of auxins promote root growth?
Up to a given conc, the more auxin that reaches the roots, the more they grow. Auxin is produced by the root tips and auxins also reaches the roots in low concentrations from the growing shoots. If the apical shoot is removed then the amount of auxin reaching the roots is drastically reduced and root growth stops. Replacing auxin artificially at cut apical shoot restores the growth of roots.
High auxin concentrations inhibit root growth.
What are gibberellins involved in?
Germination of seeds
Elongation of internodes between the leaves on a stem.
Short stems = little to no gibberellins
How can you investigate the effect of plant hormones on the growth of shoots, roots and the germination of seeds?
Growing seedlings hydroponically (in nutrient solution rather than soil) in serial solutions of different hormones or applying different concentrations to cut ends of stems or roots and observing the effects
Serial dilutions are important as you can see the impact on growth at different concentrations of the hormones.
What is synergism?
If different hormones can work together and complement eachother and give a greater response than they would on their own. Beneficial.
What is antagonism?
If the substances used have opposite effects, one promoting growth and the other inhibiting it.
Name some examples of abiotic stresses in plants?
Change in day length
Cold and heat
Lack of water
Excess water
High winds
Changes in salinity
Explain Leaf loss in deciduous plants as a response to abiotic stress?
Plants that grow in temperate climates experience great environmental changes during the year. As both light and temperature affect the rate of photosynthesis seasonal changes can have a big impact on amount of photosynthesis possible.
The point comes where the glucose required for respiration to maintain the leaves and to produce chemicals from chlorophyll might protect against freezing is greater than the glucose supplied by photosynthesis.
As a result deciduous trees lose all of their leaves in winter in order for them stay alive, they remain dormant until days lengthen and temperature rise again.
What is daylight sensitivity?
Plants are sensitive to a lack of light in their environment = photoperiodism.
Lack of light can act as a trigger for change.
Lots of light may cause the breaking of dormancy of the leaf buds so they open up, the timing of flowering in a plant when tubers are formed in prep for overwintering.
The sensitivity to light results from a pigment called phytochrome. Exists in two forms Pr and Pfr, each absorbs a different type of light and the ratio of Pr to Pfr depend on levels of light.
Describe how abscission or leaf fall occurs?
Falling light levels result in a falling concentration of auxins. Leaves respond to this by producing ethene. Ethene switches on genes in the abscission zone and triggers the production of new enzymes. The enzymes digest and weaken the cell walls in outer layer of abscission zone.
Vascular bundles are sealed off, and at the same time fatty material is deposited in cells in stem side of abscission layer. This layer of fat forms a protective layer when the leaf falls
Cells in separation zone retain water and begin to swell as a response to stress putting additional strain on the outer layer. Other abiotic factors such as high winds contribute and then the strain becomes too much and the leaf falls off.
How do plants prevent freezing?
The cytoplasm of plant cells and sap in the vacuoles contain solutes which lower the freezing point. Some plant produces sugars, polysaccharides, amino acids and even proteins which act as antifreeze to the cytoplasm, and protecting cells from damage even if they do freeze.
Most species can produce chemicals which makes them hardy against frost during winter. Genes can be suppressed and activated in response to sustained fall in temperature along with reduction in day length, effectively the plants prepare to withstand frosty conditions. Sustained warm weather will reverse these changes in spring.
