Chapter 16 plant hormones and growth

directional growth in response to a stimulus or environmental cues

Tropism definition?

auxins, gibberelllins, ethene and ABA

4 main plant hormones

• seed absorbs water and starts to produce gibberellins

• stimulate the production of enzymes to break down food stores

• Gibberellins switch on genes that code for amylases and proteases to germinate

• ABA acts as an antagonist

How do hormones cause seed germination?

• made in the tips of roots and shoots, in the meristems

• stimulate the growth of the main apical shoot, suppress growth of lateral shoots.

• low conc promote root growth

• presence of auxins means cells stretch more easily

• auxins bind to receptors in the cell membrane causing a fall in pH which is the optimum pH.

What are some of the ways auxins affect growth in plants?

• elongation of plant stems

• affect the length of the internodes (regions between leaves on a stem)

What do gibberellins do?

Synergisms - when hormones work together, giving greater response than they would on their own

Antagonism - opposite effects, promoting and inhibiting

what is synergism and antagonism?

• change in day length

• lack of or excess water

• high winds

• Changes in salinity

examples of abiotic stress?

• leaf loss due to very cold temps and lack of light. The energy needed to keep leaves is more than what is produced by photosynthesis.

• daylight sensitivity - germinating or flowering at certain times of day

• leaf abcission

• making chemicals to stop freezing like solutes and antifreeze

• stomatal control under control of ABA

what happens in abiotic stress?

• light levels fall and so does auxin

• plants therefore make ethene

• base of leaf stalk has an abcission zone which has two layers of cells

• ethene stimulates gene switching which makes enzymes to digest and weaken cell walls in abcission zone

• vascular bundles sealed off and protective layer deposited

leaf abcission process

physical - thorns, barbs, spikes, spiny leaves, folding when touched

chemical - chemicals made to stop animals from eating them

what are the two types of plant responses to herbivory?

tannins - bitter tasting and toxic to insects

alkaloids - bitter, nitrogenous compounds, act as drugs like caffeine, nicotine

terpenoids - form essential oils but can be toxic to insect and fungi, insect repellents

pheromones - affects the social behaviour of the same species

VOCs - made when plant detects attack by insect through saliva, affect the behaviour of predatory insects

chemical defences to herbivory?

• growth of plants in response to light

• result of movement of auxins across the shoot or root

• auxins move towards the shaded side of the plant and result in cell elongation on that side

• this means the plant grows towards the light

• roots grow away from the light

how does phototropism work?

• in shaded conditions plants grow very fast to get to the light

• auxins move away from light and causes cell elongation

• when shoot is growing directly towards the light, the unilateral stimulus is removed and plants grow straight towards the light

effect of unilateral light?

• roots are positively geotropic and roots are negatively geotropic

• Response of plants to gravity

what happens due to geotropisms?

• ethene from ripe fruits can stimulet other unripe fruit to ripen

• when fruits are needed for sale they are exposed to ethene gas so they ripen at the same rate

contro of ripening by hormones?

• application of auxins to cut shoots to stimulate the new production of roots

• A cutting is a small piece of the stem of a plant

• synthetic auxins can cause plants to grow too fast and they die

what are hormone rooting powders and weedkillers?

• auxins can be used in the making seedless fruit

• ethene to promote fruit dropping

• gibberellins used to delay ripening and ageing in fruit

other uses of plant hormones?