Plant Responses Flashcards
Define tropism
A directional growth response in which the direction of the response is determined by the direction of the external stimulus.
Give an example of phototropism
Shoots grow towards light (they are positively phototropic), which enables them to photosynthesise.
Give an example of geotropism
Roots grow towards the pull of gravity. This anchors them in the soil and helps them to take up water, which is needed for support (to keep cells turgid)
Give an example of chemotropism
Pollen tubes grow towards chemicals given off by the ovary in a flower
Give an example of thigmotrophisms
Shoots respond to touch by growing around other plants.
Define a nastic response
An instant response
What is meristematic tissue
Growing points in a plant where immature, undifferentiated cells are still capable of dividing
What are plant hormones/growth factors
Co-ordinate plant growth, produced in one part of the plant and transported to the target cells by active transport and diffusion in the xylem and phloem vessels
What are cytokinins
Growth factors which promotes cell division(mitosis) in the meristem
What are auxins
Growth factors stimulating shoot growth
How do auxins work
H+ is actively pumped into the cell by an ATPase enzyme.
The resulting low pH allows wall loosening enzymes to break bonds within the cellulose so that the wall becomes less rigid and can expand as water moves into the cell by osmosis
Inhibits cell growth of lateral buds
Inhibits leaf fall
What are gibberellins
Growth factors which Promotes elongation of stems - mitosis cell elongation
What is abscisic acid
A growth factor which Causes stomotal closure when plants are stressed by low water availability
What is ethene
Growth factor which Stimulates leaf fall by increasing cellulase production in the abscission zone
Promotes fruit ripening
What happens to a shoot when light is shone from above
The auxins will be spread equally along the side of the plant, spreading from the tip, causing cell elongation across the zones of elongation.
What happens to a shoot when light is shone only on one side
Auxins will move down from the shoot tip, towards the shaded side of the shoot. Only those cells on the shaded side elongate and the shoot will bend towards the light.
Why does a shoot bend towards light
Auxins produced in the tips of the plant and are transported down the shoot to bring about cell elongation
Photosensitive enzymes in the tip absorb blue wavelengths of light and become more active causing the auxins to redistribute towards the shaded area
Because there are more auxins on the shaded side the cells elongate more and the shoot bends towards the light.
What are 3 roles of auxin in apical dominance
high concentrations of auxins inhibit lateral bud growth and low levels of auxins encourage lateral bud growth
Auxins are produced by the apical bud and move down the stem
At the top of the stem the auxin concentration is high and constantly produced, this inhibits the growth of lateral buds which remain dormant.
Further down the stem the concentration of auxins is lower, the lateral buds are stimulated to grow, this means that plants grow into a cone shape, so higher leaves don’t shade lower ones.
If the apical bud is removed, the dormant lateral buds grow due to the lack of auxin
If the apical bud is removed and auxins are placed on the cut tip, the lateral buds will remain dormant
What is the role of Gibberellins
Gibberlellin brings about stem elongation by loosening cell walls which causes cell elongation.
Why do Gibberellins cause stem elongation
Gibberlellin works by affecting gene expression. The hormone moves through the plasma membrane and into the cell nucleus where it binds to a series of receptor proteins eventually resulting in the breakdown of DELLA proteins allowing transcription of the gene.
(Della proteins bind to transcription factors inhibit cell division and expansion)
Define etiolated
When plants are grown without a light source so continuously grow upwards in the search for light.
Define deciduous plants
Plants which lose their leaves in autumn and replace them again in the spring/summer
What is the role of ethene in senescence
As leafs age the production of auxin falls and the production of ethene increases.
Layer of cells called the abscission layer develops at the bottom of the leaf petiole.
Ethene stimulates the production of the enzyme cellulase which hydrolyses the bonds in cell walls in the abscission layer and the cells expand
A protective layer of cells with wax in their cell walls forms and seals off the xylem and phloem from the leaf, preventing against fungal infections.
Leaf is broken off by the wind and under goes decomposition where the nitrates from the leaf are eventually returned to the leaf via the nitrogen cycle
Define senescence
Leaf loss
Give 3 commercial uses of Auxins
Taking cuttings - dipping the end of the cutting in auxins promotes the growth of roots
Producing seedless fruit - treating unpollinated flowers with auxins stimulates fruit growth without fertilisation; consequently the fruit has no seeds.
Herbicides - Auxins promote shoot growth to an extent that the stem falls over and dies
Give 2 commercial uses of Gibberellin
Extending shelf life of fruit - Gibberlellin delay the ripening of citrus fruit and stimulate seed germination (barley malting)
Sugar production - Gibberlellin elongate the stems of sugar canes meaning more sugar
Give 2 commercial uses of Cytokinins
Keeping lettuce green - cytokinins delay leaf senesence so leafy vegetables and cut flowers stay green for longer
Micro propagation - cytokinins promote bud and shoot growth in small tissue samples taken from the parent plant
Give 2 commercial uses of ethene
Ripening of the plant - Fruit is picked and shipped before it is ripe, then it is hung in rooms with ethene gas in the atmosphere to promote ripening
Encourage fruit drop - ethene is released in orchards to make all fruits ready for picking at the same time
Define abscission
Shedding of a leaf
Define apical dominance
The inhibition of the growth of lateral buds by the presence of an active apical meristem
What are the stages of apical dominance
1- ATPase enzyme on the plasma membrane
2- H+ ions are atcively transported into the cell wall
3- The pH is then lowered
4- This stimulates expansins (wall loosening enzyme) now has optimum conditions
5- The bands between cellulose are broken
6- Walls become less rigid and can expand so cells can divide
Define petiole
The leaf ‘stem’ that breaks away from main stem
What are the stages of leaf abscission
1- Auxins inhibit abscission
2- Cytokinins stop leaves senescing
3- Cytokinin level drops so senescing begins
4- Leaf senescence causes auxin production to drop
5- Cells in abscission zone become sensitive to ethene
6- Ethene production increases due to decrease in auxin concentration
7- Enzyme celluclase is increased which digests cell walls in abscission zone
8- The petiole separates from the stem
9- Leaf lost
Where does the most cell division occur
Near the tip
Where does the maximum cell elongation occur
Just below the tip
Define coleoptile
A sheath and enclosed leaf
What is an intact coleoptile’s response to light
Positive phototropism
What materials prevent the shoot growing towards the light
Materials not permeable to water
Why do plants respond to stimuli
Helps the plants to avoid stress, to avoid being eaten and to survive long enough to reproduce
How do plant hormones move around the plant
Active transport
Diffusion
Mass flow in phloem or in xylem vessels
