Biology - Plant Responses

regulation of plant growth by external and internal factors

external factors: light, day length, gravity, temperature

internal factors: chemicals called growth regulators

stimulus

anything that causes a reactions in an organism

response

the reaction of an organism to a stimulus

tropism

growth response of a plant to stimulus

positive vs negative tropism

positive - when plant grows towards the stimulus

negative - when plant grows away from the stimulus

types of tropisms

• phototropism - growth response of a plant to light

• geotropism - growth response of a plant to gravity

• hydrotropism - growth response of a plant to water

• thigmotropism - growth response of a plant to touch

• chemotropism - growth response of a plant to a chemical substance

phototropism

• shoot of a plant is positively phototrophic as it grows towards light

• root of a plant is negatively phototrophic as it grows away from light

advantages of phototropism

1. shoot absorbs more light so there is more photosynthesis

2. the roots grow away from the light; helps anchor the plant in the soil

geotropism

• roots of a plant are positively geotrophic as it grows towards gravity

• the shoot of a plant is negatively geotrophic as it grows away from gravity

advantages of geotrophism

1. plant is anchored in the ground

2. allows the roots to absorb water and minerals

hydrotropism

• root of a plant is positively hydrotropic as it grows towards water

• shoot is negatively hydrotropic as it grows away from water

chemotropism

• pollen tubes grow towards chemicals released by the ovule + roots grow towards minerals, Ca and Mg, in the soil (both positive chemotropism)

• roots grow away from harmful acids in the soil (negative)

growth regulator

a chemical that controls the growth of a plant & is produced by the meristem in the root and shoot tip

• produced in small amounts

• transported through vascular tissue

similarities b/n animal hormones and plant growth regulators

• both produced in small amounts at one sit and cause their effect at another

• both

cause and reduction of growth

• growth promotors cause growth

• growth inhibitors reduce growth

growth regulator - auxin (IAA)

site of production - meristem of root and shoot tips

function - cell elongation

effect -

• stimulates root growth @ low concentrations

• inhibits shoot growth @ high concentrations

• causes seedless fruit formation

• causes apical dominance

growth regulator - ethene

site of production - ripe fruit, decaying leaves

function - inhibits growth

effect -

• ripens fruit and as other fruits emit ethene, it ripens other fruit

• cuases leaf fall in autumn

why do gardeners prune plants and cut off the top buds?

because they want to remove apical dominance and allow side branches to grow outwards

mechanism of phototropism

1. auxin produced in meristem of stem tip

2. light is on one side of the shoot/stem

3. auxin diffuses down the shaded side

4. cells on shady site therefore elongate more

5. stem/shoot bends towards the light

commercially prepared plant growth regulators

• rooting powers containing auxins cause roots to form on cuttings from plants

• ethene ripens bananas

• when flowers are sprayed with auxin, seedless fruits form

graph of auxin

• low concs or auxin stimulate root growth and high concs inhibit root growth

• high concs of auxin promote shoot growth and low concs inhibit shoot growth

adverse conditions

harsh conditions which can harm a plant

e.g. high temperatures, lack of water, animal consumption, bacteria

physical adaptations for protection

• spines, thorns or stinging hairs on epidermis deter animals from eating them

• guard cells shrivel when water supply is short, causing stomata to close; reduces water loss

chemical adaptations of plants

• heat-shock proteins created when cells are exposed to high temperatures; proteins surround enzymes and help them keep their shape

• plants promote stress proteins when infected by micro-organisms. they damage the micro-organisms or stimular plant cells to respond to them

to investigate the effect of different concentrations of IAA growth regulator on plant tissue

1. A serial dilution is carried out to produce different concentrations of IAA.

2. Place a circular acetate grid in the lid of each petri dish.

3. Place 5 cress seeds evenly spaced on the same line of the grid

4. Cover the seeds with filter paper and cover with cotton wool, the filter paper prevents the roots and shoots getting caught in cotton wool.

5. Add equal volumes (this makes it a fair test) of a different concentration of IAA solution to each dish.

6. The control contains water only.

7. Tape shut and stand the 8 petri dishes on their side to ensure the root grows down and shoot grows up.

8. Incubate dishes at 25°C for 3-5 days.

results of experiment

The lengths of the roots and shoots are measured. The roots and shoots in the control dish grew due to IAA produced by the seeds. The control results are compared to other results. High concentrations of IAA: Stimulate shoot growth. Inhibit root growth. Low concentrations of IAA: Stimulate root growth. IAA has different effects on the growth of roots and shoots. IAA is concentration dependent.