Coordination In Plants Flashcards
Define stimulus
A change in the environment that causes an organism to change its behavior
What is a tropism?
A growth movement of part of a plant in response to a unidirectional stimulus
Give an example of phototropism in plants
- shoots and coleoptiles are positively phototropic
- Some roots are negatively phototrophic for example some roots of climbers like ivy
Give an example of geotropism in plants
- Shoots are negatively geotropic
- Roots are positively geotropic
Give an example of chemotropism in plants
- Pollen tubes are positively chemotropic in response to chemical produced at micropyle of ovule
- Hyphae of some fungi are positively chemotropic eg mucor
Give an example of hydrotropism in plants
- All plant roots are positively hydrotropic
Give an example of thigmotropism in plants
- Tendrils are positively thigmotropic
- Central tentacles of sundew (an insectivorous plant) are positively thigmotropic
Give an example of aerotropism in plants
- Pollen tubes are negatively aerotropic
- Breathing roots of mangroves are positively aerotropic
What is a taxis?
A movement of an entire organism in response to a unidirectional stimulus
Give an example of phototaxis
- Euglena swim towards light
- Chloroplasts move towards light
- Earthworms and cockroaches move away from light
Give an example of chemotaxis
- Sperms of mosses, ferns and liverworts swim towards substance’s released by the ovum
- Motile bacteria move towards various food substances
- Mosquitoes avoid insect repellents
Give an example of aerotaxis
- Motile bacteria move towards oxygen
Give an example of geotaxis
- Larvae of some cnidarians swim towards the sea bed
What is a nasty?
A non-directional movement of part of a plant in response to an external stimulus
Give an example of photonasty
- Leguminous plants lower their leaves in the dark and raise them in the light
- Sunflowers close at night
Give an example of thermonasty
- Petals of flowers like crocus and tulip open at temperatures around 16 degrees celsius and close below this
Give an example of thigmonasty
- Venus fly trap leaves close rapidly when touched
What are plant hormones/ growth substances?
These are chemical substances produced in plants which accelerate, inhibit or otherwise modify growth
What are the five major plant growth substances?
- Auxins (Indoleacetic acid) [growth promoter]
- Gibberellins [growth promoter]
- Cytokinins [growth promoter]
- Abscissic acid [growth inhibitor]
- Ethene [growth inhibitor]
What do auxins do and where do they mostly occur?
- They are growth promoters
- Found at the apices of shoots and roots
What is the mode of action of auxins in plants?
- When present in a plant cell, they promote movement of hydrogen ions from the cytoplasm to the cell wall
- This makes the cell wall more acidic in nature as it gains a lower pH
- The cell wall becomes loosened due to breaking of hydrogen bonds catalyzed by enzymes called expansins
- There is osmotic uptake of water into the cell
- There is increased turgor and the cell expands, leading to elongation
What are the effects of auxins?
- Promotes growth in shoots
- Inhibits growth in roots
- Enforces dormancy of lateral buds
- Promotes growth of adventitious roots and lateral roots
- Inhibits leaf loss (abscission)
- Promotes cell division (with cytokinins)
- Increases ethene production
- Promote parthenocarpy (fruit formation without fertilization)
- Selective weed killers
- Promote tropic responses which enable the plants to respond to gravity and light
How do extremely high amounts of auxins inhibit growth in plants?
Stimulate the production of ethene which inhibits growth
Where are cytokinins mainly found?
Actively growing tissues eg roots, embryos and fruits
What are the effects of cytokinins?
- Stimulate cell division (with auxins)
- Delay leaf aging
- Promote formation of buds
How do cytokinins show anti-aging effects to the plants?
Retard the aging of certain plant organs by inhibiting protein breakdown
(Dipping leaves in cytokinin solution stay green much longer)
Define apical dominance
The ability of the apical bud to suppress the development of axillary buds
What is the effect of cutting off the apical bud of a plant?
Development of axillary buds and the plant becomes bushy
Where are gibberellins synthesized?
Germinating seedlings
What are the effects of gibberellins in plants?
- Break seed dormancy
- Fruit growth
- Seed germination
- Promote lateral branching
- Stem elongation
- Stimulates enzyme production during seed germination
- Plays a role in cell division and tissue differentiation
How do gibberellins support the growth of cereal seedlings?
- Stimulate the synthesis of digestive enzymes such as alpha amylase that mobilize stored nutrients
What is the effect of ethene on plants?
- Promotes fruit ripening
- Controls abscission (shedding)
What is the effect of abscissic acid in plants?
- Induces seed dormancy
- Induces abscission
- Stress responses especially due to water deficiency
- Inhibits growth
- Controls stomatal closings
What is water stress?
Situation where a plant losses more water than it absorbs
Define abscission
The organized shedding of part of the plant.
Define synergism
Interaction of plant growth substances in which two or more substances supplement each other’s activities and their combined effect is much greater than the sum of their separate effects
Give examples of synergy in plant growth substances
- Gibberellins & auxins > Shoot growth
- Cytokinins & auxins > Cell elongation
Define antagonism
Interaction of plant growth substances in which two substances have opposite effects on the same process, one promoting and the other inhibiting
Give examples of antagonism in plant growth substances
- Cytokinins & auxins > Lateral/ axillary bud growth
- Gibberellins & auxins > Lateral branching
- Gibberellins & abscissic acid> Seed dormancy
What are meristems?
Regions of growth of a plant
What is vernalisation?
A process by which flowers are stimulated to flower after exposure to cold treatment
(It is a requirement for some plants to hasten flowering)
Define photomorphogenesis
The effect of light upon the development of organisms
What is an etiolated plant?
A plant grown in the dark or little light thus it lacks chlorophyll and appears white or pale yellow rather than green.
What is the significance of etiolation?
Allow maximum growth in length with minimum use of carbon reserves
True or false
Once an etiolated plant is exposed to light, it remains etiolated
False
(Normal growth ensues)
What is a photoperiod?
The relative length of day and night
Define photoperiodism
The response of an organism to changes in length of day or night
Briefly explain the concept of flowering of short day plants and give examples of such
Short day plants generally flower when light periods are short
Examples; strawberries, chrysanthemums
Briefly explain the concept of flowering of long day plants and give examples of such
Long day plants generally flower when days are longer and nights are shorter
Examples; lettuce, clover
What is a long day plant?
A plant that will flower when light periods exceed the critical day length
What is a short day plant?
A plant that flowers only if the period of darkness exceeds the critical length
What are day-neutral plants?
These are plants whose flowering appears to be unaffected by the photoperiod
True or false
If short day plants have their long night interrupted by flashes of light they do not flower
True
(Method used by some florists)
What other name can be given to short day plants and long day plants respectively?
Short day plant= long night plant
Long day plant = short night plant
What is phytochrome?
A pale blue light sensitive protein used for detecting the length of darkness in plants
What are the two interconvertible forms of phytochrome?
- Phytochrome red/ P660 (absorbs red light like in the day)
- Phytochrome far red/ P730 (absorbs far red light like at night)
What is the active form of phytochrome?
Phytochrome Far Red
(It determines the release of florigen)
Describe the mechanism by which phytochrome is used
- Synthesized in Pr form
- Pr is converted to Pfr when exposed to red light; the conversion is rapid
- Pfr is converted back to Pr when exposed to far red light or in darkness; the conversion is gradual
- A build up of Pfr stimulates flowering in long day plants but inhibits flowering in short day plants
- High Pfr in long day plants stimulates florigen secretion while high Pr in short day plants stimulates florigen secretion
True or false
Pfr inhibits flowering in short day plants
True
What are the differences between short day and long day plants?
LONG DAY PLANTS
- Flower when the photoperiod exceeds the critical day length
- Interruption during light period with darkness inhibits flowering
- Flowering occurs if the long dark period is interrupted by a flash of light
- Dark period is not crucial for flowering
- Flowering occurs under alternating cycles of short day followed by shorter dark periods
SHORT DAY PLANTS
- Flower when the photoperiod is less than the critical day length
- Interruption during light period with darkness does not inhibit flowering
- Flowering is inhibited if the long dark period is interrupted by a flash of light
- Long continuous and uninterrupted dark period is crucial for flowering
- Flowering does not occur under alternating cycles of short day followed by short light periods
Define parthenocarpy
This refers to fruit formation in plants without fertilization taking place.
Results in seedless fruits
Occurs in plants like pineapples
In what ways does growth in plants take place?
- Cell division
- Cell elongation (due to osmotic intake of water)
- Cell differentiation (specialization of cells)
What are the commercial roles of auxins / uses of auxins to a farmer?
- Adventitious root formation for farmers who plant stem cuttings
- Inhibiting abscission of plant parts such as fruits
- Increases ethene production which stimulates ripening of fruits
- Promote parthenocarpy (fruit formation without fertilization taking place)
- Selective weed killers of broad-leaved weeds
Describe the mode of action of gibberellins in breaking seed dormancy and in seed germination
- There is entry of water into the seed by imbibition.
- Gibberellins are made by the embryo
- They diffuse from the embryo to the Aleurone layer
- This results in production of enzymes such as alpha amylase
- The enzymes travel from the Aleurone layer to the endosperm where they catalyze the hydrolysis of complex food substances into simple ones. Eg starch to glucose, proteins to amino acids, lipids to glycerol and fatty acids
- The products are taken to the embryo
- Amino acids make up plant structures
- Glucose is respired to form energy for growth of the plant
What is seed dormancy?
This is the inability of a seed to germinate despite the necessary conditions being present.
What are the commercial roles of gibberellins?
- Break seed dormancy allowing plants to germinate
- Fruit growth useful to fruit farmers
- Seed germination increasing yields
- Promote lateral branching increasing yields
Which hormone does the same work as vernalization?
Gibberellins
(Both activate the seedling to germinate)
