Módulo 4 de 4 · Texto
Plant responses
- Auxins
- cell elongation, prevent leaf abscission, apical dominance, stimulate release of ethene
- IAA --> growth stimulant
- made in cells at tips of roots and shoots and meristems
- move in the transport tissues and cell to cell
- high concentrations inhibit gibberellins --> supports apical dominance
- bind to specific receptors in the cell membrane, cause pH to fall to 5 which makes the cell walls flexible and plastic which enables elongation, as the cell matures hormone levels fall and the cell walls become more fixed in shape and inflexible
- low concentrations promote root growth --> produced in the tips and they get some from the shoot when its little --> less auxin means less growth
- Gibberellins
- cause stem elongation of lateral shoots, trigger seed germination, stimulate pollen tube growth in fertilisation
- add water to seed --> embryo produces gibberellins --> gibberellins + water = gibberellic acid -->activates the transcription of amylase --> amylase breaks the stored starch to maltose --> maltase breaks the maltose to glucose --> glucose is respired for ATP to build materials for growth
- they effect the length of the internodes (regions between the leaves the stem) and can make them long and spindly
- Ethene
- causes fruit ripening, promotes abscission
- lengthening of the dark period of the day triggers abscission and a period of dormancy --> falling light = falling auxin to which the leaves respond my making ethene --> the base of the leaf has an abscission layer with two layers of ethene sensitive cells --> these cells produce enzymes that digest and weaken the cell walls of the abscission layer when ethene is present --> vascular bundles are sealed off and fatty material is deposited to form a protective scar when the leaf falls --> hormones made cells retain water and put more pressure on the separation layer --> abiotic factors like the wind make the break
- ABA (abscisic acid)
- maintains dormancy of seeds and buds, stimulates cold protective responses, stimulates stomatal closure
- leaf cells release ABA under abiotic stress --> ABA binds to receptors in the plasma membranes --> this changes the ionic concentrations of the cell so water leaves the vacuoles and the stomata are closed
- Chemical defences to herbivory
- Tannins --> bitter taste and put animals off eating the plant
- Alkaloids --> bitter tasting, nitrogenous compounds which act as drugs affecting the metabolism of the animals that take them in or even poisoning them
- Terpenoids --> form essential oils and act as toxins to insects and fungi
- Pheremones --> communication between groups of the same species ("I'm being attacked, protect yourselves!") or they attract parasites to eat their attackers rather than let their attackers eat them (wasps lay eggs in over larvae so that the baby wasps eat the larvae but leave the plant alone)
- How're they used?
- Ethene used commercially to make fruit perfect ripeness for sale
- Auxin is used in rooting powder
- Synthetic auxins can act as weedkillers that only kill broad-leafed plants (their growth rate increases to a point where its unsustainable and it dies)
- Auxins can be used in the production of seedless fruit
- Ethene is used to promote early abscission
- Cytokinins are used to prevent aging of ripened fruit
- Gibberellins are used to delay ageing and ripening in fruit