9- chemical control in plants Flashcards
auxins origin
Produced by cells at shoot apical meristems. Travel cell to cell via auxin transporters.
function of auxins
• Cell elongation.
• Apical dominance (suppresses lateral buds)
Apical dominance (suppresses lateral buds)
• Auxin inhibits lateral bud growth, causing most growth to be upwards.
• The shoot grows longer and does not branch sideways as much.
• Removing the tip means that there is less auxin, so there is less suppression.
auxins on root growth
• Auxin stimulates root growth - roots grow faster and longer in its presence.
• It is used by gardeners on cuttings to promote root growth.
auxins mechanism for cell elongation
- Auxins are synthesised in the cells at cells at shoot apical meristems.
- Auxins diffuse from the tip to the zone of elongation.
- Auxins bind to specific receptor sites on the cell membrane of newly formed cells.
- This activates proteins in the cell membrane, which pump H+ ions into the cell walls.
- The pH is lowered, which provides the optimum conditions for expansins to act.
- Expansins break bonds between cellulose microfibrils, increasing cell wall flexibility.
- The cells absorb water by osmosis and the flexible cell walls allow the cells to expand.
- As the cells mature, they become situated further from the tip due to new cells forming.
- Eventually, auxins in the receptor sites are destroyed by enzymes and the pH rises.
- Bonds form more readily between cellulose microfibrils.
- The cell walls become more rigid and cannot expand anymore.
cytokinins origin
Generated at shoot base to encourage lateral growth.
cytokinins function
• Lateral bud growth.
• Work with ethene for leaf abscission.
• Stimulate cell division in meristems.
gibberellins function
• Cell elongation, especially in stem internodes.
• Stem lengthening.
• Key in seed germination process.
gibberellins mechanism
- The seed absorbs water. This stimulates the embryo to secrete gibberellin, which diffuses to the outer aleurone layer.
- Gibberellin is a transcription factor, and it stimulates the cells of the aleurone layer to express the amylase gene and synthesise amylase.
- The amylase diffuses into the endosperm, where it digests the starch reserves into the sugars maltose and glucose.
- The sugars are absorbed by the embryo and used for aerobic respiration and as raw materials for biosynthesis of protein,
carbohydrates and lipids, to form new cells.
synergism
When the effect of 2 substances acting together is greater than the sum of the effects of the substances acting alone - they enhance the effect of each other.
antagonism
When the effect of 2 substances acting together is less than the sum of the effects of the substances acting alone - they inhibit / decrease the effect.
stem growth
Auxin and gibberellin work synergistically to promote cell elongation.
root growth
Auxin and gibberellin work antagonistically. Auxin promotes it and gibberellin inhibits it.
apical dominance
• Auxin and gibberellin work synergistically.
• Auxin and cytokinins work antagonistically.
bud dormancy
Auxin and cytokinins appear to work synergistically.
phytochrome
• A blue-green plant pigment that absorbs light.
• Found in very small quantities in leaves and seed.
• Exists in 2 interconvertible forms.
Pr
• Physiologically inactive form, absorbs red light.
• Has a wavelength of ~660 nm.
Pfr
• Physiologically active form, absorbs far red light.
• Has a wavelength of ~730 nm.
• The relative level of Pfr causes the plant response.
the importance of light in plants
• Photosynthesis.
• To influence the direction of growth (phototropism).
• To influence development
light affect on the formation of chlorophyll
• When seedlings are grown in the dark, they become etiolated.
• The stem becomes thinner and taller to try and reach light.
• The plant turns yellow as chlorophyll is broken down (not needed).
light affect on germination in some seeds
High levels of Pfr are needed for germination in lettuce seeds.
light affect on the time of flowering
• The photoperiod is the day length.
• Long day (short night) plants (e.g. clover) → flower when the photoperiod exceeds a critical value, therefore they flower in the summer.
• Short day (long night) plants (e.g. chrysanthemum) → flower when the photoperiod is lower than a critical value, therefore they flower in the autumn.
• Flowering of day-neutral plants (e.g. tomatoes) → are not affected by photoperiod.
how phytochrome controls flowering- long day and short night plants
• Need high levels of Pfr to promote flowering.
Flowering is prevented by:
• Short days or long nights.
• Shining a far red light during the short night, so there is less Pfr.
how phytochrome controls flowering- short day and long night plants
• Needs low levels of Pfr to promote flowering.
Flowering is prevented by:
• Long days or short nights
• Shining a red light during the short night, so there is more Pfr.
phytochrome mechanism
- Photoperiod is detected by the phytochrome system in leaves below the apex of plants.
- A signal is transmitted to the apex to initiate flowering.
A. The signal can pass through grafts to stimulate flowering in a different plant, so it must be a chemical message that can diffuse through cells.
B. The signal must therefore be a hormone. It is called florigen.
C. Florigen has yet to be identified and could be a combination of other hormones. - Gibberellin appears to act synergistically with florigen to promote flowering.
- Other aspects of plant growth and development are also controlled by light, including seed germination, the size and shape and number of leaves and chlorophyll synthesis.
- In all cases, phytochrome is the light receptor and it switches on gene expression.
- The control of growth and development in plants by specific wavelengths of light is called photomorphogenesis (e.g. etiolation).
