plant responses Flashcards
response to abiotic stress?
how can plants respond to drought?
how can plants respond to freezing temperatures?
•freezing
•drought
•increased soil water salinity
•presence of heavy metals eg. lead/
copper
•in ways that reduce water loss= shutting stomata & dropping leaves
•by producing an antifreeze chemical in their cells that decreases the formation of ice crystals (ice crystals can destroy plant cells if they form within them)
responses to herbivory?
what do plants produce to defend against herbivory?
what are the chemicals and what do they do?
•herbivory is a biotic stress factor, plants have adaptations to avoid them being eaten by herbivores
•produce chemicals
•Alkaloids eg. caffeine, nicotine
=bitter tasting or toxic so deters or kills herbivores
•Pheromones
=a signal to nearby plants of the same species that they are under attack from herbivores, triggering other defences
=a signal to attract a herbivorous insect’s natural predators
responses to touch?
•in Mimosa pudica, the leaflets of the touch-sensitive leaves fold rapidly when touched
= could be an adaptation to protect from herbivorous insects and can reduce transpiration rate when leaves are no longer photosynthesising
what is a tropism?
•a growth response of a part of a plant in response to a directional stimulus
-can be towards a stimulus (positive tropism) or away (negative tropism)
types of tropisms:
•Phototropism- in response to light- abiotic stimulus - ensures maximal photosynthesis
•Geotropism- gravity- abiotic- shoots show negative geotropism and roots show positive geotropism= ensures roots/shoots in germinating seeds grow in the right direction regardless of orientation of seed
•hydrotropism- moisture- abiotic- root tips grown towards damper areas in soil = increasing their access to water
•Thigmotropism- touch- abiotic or biotic- important in climbing plants (greater light access), allowing them to detect a living or non-living support and curl around it
•chemotropism- chemicals- abiotic of biotic- tropic response to certain chemicals = pollen tubes grow down the stigma towards the ovules
what experiments were made to investigate phototropism-
Darwin?
-Darwin (1880)
•removed tip of a coleoptile (a sheath that surrounds young grass shoots).
•stopped phototropic response to a unidirectional light source
•he covered the tip with a cap to block out the light, this showed that the tip of the coleoptile was responsible for detecting light
what experiments were made to investigate phototropism-
Boysen-Jensen?
-Boysen-Jensen (1913)
•cut the tip off the coleoptile
•placed a block of agar between the shoot and the tip
•the phototropic response was restored showing that the stimulus for growth was a chemical which was able to diffuse through the agar.
•then replaced the agar with a mica (impermeable to chemicals)
•when the mica barrier was inserted into the lit side = phototropic response
•when inserted into the shaded side = no response
•showed the chemical was travelling down from the root on the side opposite to the stimulus (shaded side) causing growth
what experiments were made to investigate phototropism-
Paál’s experiment (1919)?
•cut off tip of coleoptile then replaced it off-centre in the dark
•the side of the coleoptile that the tip was placed on grew more than the other side causing the coleoptile to curve
•showed that in the light, the phototropic response was caused by a hormone diffusing through the plant tissue and stimulating growth
what experiments were made to investigate phototropism?
went (1926)?
•placed the cut tip of a coleoptile on an agar block, allowing the hormones from the tip to diffuse into the block
•the agar block was then placed on the coleoptile, off-centre and in the dark
•the side of the coleoptile that the agar was on grew more = causing it to curve
•showed that there was a greater concentration of hormone
controlling cell growth by elongation?
•IAA (an auxin) is synthesised in the growing tips of roots and shoots eg meristems
•IAA coordinates phototropism’s to in plants by controlling growth by elongation
•from the meristem, they pass down the stem to stimulate elongation growth
•the IAA molecules activate expansin proteins in the cell wall, these loosen the bonds between cellulose microfibrils
•this makes the cell walls more flexible allowing elongation
what is the phototropic mechanism of IAA on cell elongation?
•the concentration of IAA determines the rate of cell elongation within the region of elongation
•if the concentration of IAA is not uniform on either side of a root or shoots then uneven growth can occur
•when the shoots grow towards the light = positive phototropism
•IAA moves from the side of the shoot with access to light, to the shaded side.
•higher concentrations of IAA on the shaded side results in greater rate of cell elongation
•causing the shoot to bend towards the light
Geotropism in shoots and roots?
•shoots grow away from gravity = negative geotropism
•gravity modifies the distribution on IAA so that it accumulates on the lower side of the shoots
•so IAA increases rate of growth in shoots so grow upwards
•when roots grow towards gravity = positive geotropism
•in roots, higher concentrations of IAA results in a lower rate of cell elongation
•the IAA that accumulates at the lower side of the root inhibits cell elongation
•so the lower side grows at a slower rate than the upper side of the root = root bends downwards
PLANT HORMONES- leaf loss
why do deciduous plants lose their leaves in hot, dry conditions?
when do plants also do this?
in temperate climates, what happens?
-to reduce water loss
-in winter when absorption of water is difficult due to frozen soils, also photosynthesis is limited by low temps and reduced light
-hormones are produced in response to shortening day length in the autumn
describe how hormones cause leaf loss?
-a layer of parenchyma cells develops at the base of the leaf stalk, known as the abscission layer
-these parenchyma cells have thin walls, making them weak and easy to break
-the plant hormone ethene stimulates the breakdown of cell walls in the abscission layer, causing the leaf to drop off
-auxins inhibit leaf loss and are produced in YOUNG leaves = making the stalks insensitive to ethene
-the concentration of auxins in leaves DECREASES as they age, until leaf loss can occur in response to ethene
PLANT HORMONES- hormones in stomatal closure
during times of water stress?
-the hormone abscisic acid (ABA) is produced by plants to stimulate the closing of their stomata
describe what happens in stomatal closure?
look at pic
-guard cells have ABA receptors on their cell surface membranes
-ABA binds with these receptors, inhibiting the proton pumps = stopping active transport of H+ ions out of the guard cells
-ABA also causes calcium ions to move into the cytoplasm of the guard cells through the cell surface membranes
-calcium ions act as secondary messengers as they cause channel proteins to open that allow negatively charged ions to leave the guard cells
-this stimulates the opening of more channel proteins that allow potassium ions to leave the guard cells
-the calcium ions also stimulate the closing of channel proteins that allow potassium ions to enter the guard cells
-this loss of ions increases water potential of the guard cells
-water leaves the guard cells by osmosis
-the guard cell becomes flaccid, causing the stomata to close
PLANT HORMONES- seed germination
what happens when a seed is shed from a parent plant?
what does the seed contain?
what hormone is involved in controlling seed germination and stem elongation?
-it is in a state of dormancy as it is metabolically inactive and contains little water = allows seed to survive in harsh conditions until the conditions are right for successful germination
•and embryo = will grow into new plant when seed germinates
•an endosperm = a starch-containing energy store surrounding the embryo
•an aleurone layer = a protein-rich layer on the outer edge of the endosperm
-gibberellins
describe seed germination?
what does ABA do to gibberellins?
look at pic
-when the conditions are right, eg barley seed starts to absorb water to begin germination
-this stimulates the embryo to produce gibberellins
-gibberellin molecules diffuse into the aleurone layer and stimulate the cells there to synthesise amylase
-in barley seeds, gibberellin does this by regulating genes involved in the synthesis of amylase, causing an increase in the transcription of mRNA coding for amylase
-the amylase hydrolyses starch molecules in the endosperm, producing soluble maltose molecules
-the maltose is converted to glucose and transported to the embryo
-this glucose can be respired by the embryo, breaking dormancy and providing the embryo with the energy needed for it to grow
-has the opposite effect to gibberellins, maintaining dormancy by inhibiting amylase production = so the start of germination is determined by the balance of ABA and gibberellins present in the seed
AUXINS AND APICAL DOMINANCE-
the auxins produced at the growing tip at the apex of a plant stem cause what?
what is this called?
why is it best that plants grow upwards?
-the stem to grow upwards and also stop lateral (side) buds from growing
-apical dominance
-towards the light so have more access to energy for photosynthesis
apical dominance experiments-
what happens if the growing tip at the apex is removed (eg due to grazing from herbivores)?
what happens over time to the lateral shoots?
when the plant is decapitated but replaced with an agar block containing auxin? LOOK AT PIC
-lateral buds grow from the top of the plant, as the source of auxins has been removed = no apical dominance
-the lateral shoots curl up towards the light so the plant continues to grow in an upwards direction
-this restores the inhibition of lateral but growth = no lateral buds grow
ROLE OF GIBBERELLINS- stem elongation
what do gibberellins do?
what do dwarf plants have?
what happens when treating dwarf varieties with gibberellins?
-stimulate cell division and elongation in the stem
-very low levels of gibberellins, often due to a mutation in a gene involved in gibberellin synthesis
-results in them growing to the same height as normal varieties = so some farmers apply gibberellin to shorter plants to stimulate growth
ROLE OF GIBBERELLINS- seed germination
what do gibberellins do?
example?
-stimulate germination and the breaking of dormancy
-Arabidopsis plants that do not produce gibberellins can be induced to germinate if gibberellins are applied
-seeds of certain lettuce varieties that require light in order to germinate can be made to germinate in the dark when gibberellins are applied
COMMERCIAL USES OF PLANT HORMONES- selective weedkillers
describe how they work?
-Auxins are growth promoting, in high enough concentrations they can cause such rapid growth in plant tissues eg the roots, to become distorted and damaged = allowing pathogens to enter the plant
-synthetic auxins can be used for this purpose and are applied to the plants in concentrations 100 times greater than the natural hormones found in plants
-effective against weeds that occur in fields of cereal crops or grass lawns as grasses are less sensitive to selective weedkillers than broadleaved weeds so do not die
COMMERCIAL USES OF PLANT HORMONES- rooting powder
describe?
-at low doses, auxins can be used to stimulate cuttings to grow new roots
-they are sold commercially in the form of rooting powders
-the lower end of the cutting is dipped in the powder before being planted in compost and with the correct environmental conditions, roots begin to grow shortly afterwards
