Chapter 16 (5.1.5) Plant responses

state 4 examples of plant hormones

auxins

ethene

gibberellin

abscisic acid (ABA)

function of auxins

causes cell elongation in stems and inhibits growth in roots

prevents leaf fall (abscission) and maintains apical dominance

function of gibberellins

stimulates seed germination, stem elongation, and pollen tube growth in fertilisation

function of ethene

causes fruit ripening, promotes abscission in deciduous trees

function of ABA

stimulates stomatal closing and maintains dormancy of seeds

stimulates antifreeze

describe the role of gibberellins in seed germination

a seed starts to germinate when it absorbs water, activating the production of gibberellins

gibberellins cause enzymes to be released that can breakdown the food stores in the seed so that the embryo plant can use the food to respire and make ATP

gibberellins causes this to happen by switching on genes that code for amylases and proteases

evidence suggests ABA has an antagonistic effect = levels of gibberellins and ABA control when a seed germinates

describe experimental evidence for the role of gibberellins in seed germination

1) mutant plant varieties (do not have the gene that codes for gibberellins) did not germinate = when exposed to an external source of gibberellins, the seeds germinated

2) if gibberellin biosynthesis inhibitors were applied to seeds, these plants were unable to make gibberellins and their seeds did not germinate = when given gibberellin, the seeds then germinated

define tropism

when plants respond via growth to stimuli

tropisms are controlled by specific growth factors (e.g. indoleacetic acid, IAA)

phototropism = light

gravitropism = gravity

water

define IAA

indoleacetic acid is a type of auxin

controls cell elongation in shoots and inhibits growth of cells in the roots

made in the tip/roots/shoots but can diffuse to other cells

describe phototropism in shoots

light is needed for LDS in photosynthesis so plants grow and bend towards light = positive phototropism

1) shoot tip cells produce IAA causing cell elongation

2) IAA diffuses to other cells

3) if there is unilateral light, the IAA will diffuse towards the shaded side of the shoot = higher concentration of IAA there

4) the cells on the shaded side elongate more = results in plant bending towards light source

describe phototropism in roots

roots do not photosynthesise = do not require light

high concentration of IAA inhibits cell elongation = causes root cells to elongate more on the lighter side = root bends away from light

negative phototropism

describe gravitropism in shoots

IAA will diffuse from the upper side to the lower side of a shoot

if the plant is vertical, this causes the plant cells to elongate = plant grows upwards

if the plant is on its side, this will cause the shoot to bend upwards = negative gravitropism

describe gravitropism in roots

IAA moves to the lower side of roots = upper side elongates, root bends down towards gravity and anchors the plant in = positive gavitropism

describe auxins

growth hormones made in cells at the tip of the roots and shoots and in meristems

can move down the stem and up the root in transport tissue and from cell to cell

stimulate growth of the apical shoot

describe the effect of auxins on apical shoot growth

1) auxins synthesised in meristem cells

2) auxins diffuse away from tip

3) auxins bind to receptors in the plasma membrane = pH drops = now at optimum pH for the enzymes needed to keep the walls very flexible and plastic

4) as cells mature, auxin is destroyed by enzymes = pH rises = enzymes maintaining plasticity become inactive

5) wall becomes rigid/ more fixed in shape = cells can no longer expand and grow

why is apical dominance needed

saves energy

prevents side shoots from the same plant competing with the shoot tip for light

allows rapid plant growth

describe the role of auxins in maintaining apical dominance

high concs of auxins suppress the growth of lateral shoots = results in apical dominance (growth of the apical bud)

if the apical bud is removed, the plant will not produce auxins and the side shoots will start growing

auxin becomes less concentrated further down the plant so shoots start to grow near the bottom

high auxin concs inhibit root growth

describe experimental evidence for the role of auxins in apical dominance

if the apical shoot is removed, the auxin-producing cells are removed = lateral shoots grow faster

if auxin is applied artificially to the cut apical shoot, apical dominance is reasserted and lateral shoot growth is suppressed

if the apical shoot is removed, the amount of auxin reaching the roots is reduced = root growth slows = replacing the auxin artificially at the cut apical shoot restores the growth of the roots

state 5 plant responses to abiotic stress

leaf loss

daylength sensitivity

abscission

preventing freezing

stomatal control

describe leaf loss

trees will lose their leaves in countries that have cold winters (temperature climates)

colder = daylight hours decrease = rate of photosynthesis decreases

more energy efficient for plants to lose their leaves

describe daylength sensitivity

photoperiodism = plants being sensitive to a lack of light

plants are sensitive to how long it is dark for

detect dark periods are shorter/daylight hours are longer = causes leaves to bud and flowers to bloom after winter

describe abscission

when light levels decrease (in autumn and winter), ethene switches on genes for enzymes that digest and weaken the cell at the abscission zone (separation layer in a leaf petiole)

causes the leaf to separate from the plant, leaving a waterproof scar behind to protect the rest of the plant

describe preventing freezing

some plants contain chemicals which act as natural antifreeze to prevent the cytoplasm in cells from freezing by lowering the freezing point

describe stomatal control

the evaporation of water out of open stomata provides a cooling effect to a plant

the opening/closing of stomata can be controlled by ABA in response to temperature stress

state physical defences to herbivory

thorns, stings, spikers, barbs, fibrous (inedible) leaves

state 4 chemical defences to herbivory

tannins

alkaloids

terpenoids

pheromones

describe tannins

bitter-tasting chemical compounds

bitter taste discourages animals from eating the plant

toxic to insects

describe alkaloids

nitrogenous, bitter-tasting chemicals

affects the metabolism of the herbivore, sometimes resulting in death

e.g. nicotine, caffeine, cocaine, morphine

describe terpenoids

essential oils that can be toxic to insects and fungi

e.g. citronella (repels insects)

describe pheromones

chemicals produced by an organism which affects the behaviour of other members of that species

animals’ social behaviour are affected by pheromones, but plants use pheromones to communicate about danger

volatile organic compounds (VOCs) act like pheromones for plants = e.g. some trees release pheromones when an insects attacks them, the release of the pheromone can cause neighbouring trees to produce callose to help protect them against the insect attack

describe folding in response to touch

Mimosa pudica is an example of a plant that can move to scare of predators

the leaves fold when they are touched = frightens off larger herbivores and can brush of small insects

state 3 commercial uses of plant hormones

control of ripening

hormone rooting powders and micropropagation

hormonal weedkillers

describe controlling fruit ripening

ethene is used to control ripening (can also be used to promote fruit dropping)

describe rooting powders

used to encourage the growth of new roots from plant cutting

auxins used (can also be used in production of seedless fruit)

describe weedkillers

synthetic auxins are used which act as effective weedkillers