5.1.5 Plant responses

Question 1

How do plants defend themselves against herbivory?

Answer 1

By producing chemical defences such as alkaloids and pheromones.

Question 2

What are alkaloids and what do they do?

Answer 2

Bitter-tasting or poisonous chemicals that deter or kill herbivores, e.g., nicotine in tobacco plants.

Question 3

What are pheromones in plants, give an example?

Answer 3

Signalling chemicals that produce a response in other organisms
E.g, cornplants release pheromones which attract parasitic wasps when caterpillars are eating them.
Wasps lay eggs in the caterpillar, killing them

Question 4

How do plants respond to environmental stimuli despite their limitations?

Answer 4

Produce and transport plant hormones that regulate growth and development.

Question 5

What is abiotic stress?

Answer 5

Harmful, non-living environmental conditions

Question 6

What is a tropism?

Answer 6

A directional plant growth response to an external stimulus.

Question 7

What’s the difference between positive and negative tropism?

Answer 7

Poistive tropism = growth towards stimulus
Negative tropism = growth away from stimulus

Question 8

What is phototropism?

Answer 8

Growth in response to light
Shoots = positive, grow towards light
Roots = negative, grow away from light

Question 9

What is geotropism (gravitropism)?

Answer 9

Growth in response to gravity
Shoots = negative, grow upwards (against gravity)
Roots = positive, grow downwards

Question 10

What is hydrotropism?

Answer 10

Growth in response to water (roots grow towards water).

Question 11

What is thigmotropism?

Answer 11

Plant growth in response to contact with an object.

Question 12

What is the main functions of gibberellins in plants?

Answer 12

Stimulate seed germination
Stem elongagtion
Side shoot formation
Flowering

Question 13

How do gibberellins promote stem growth?

Answer 13

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

Question 14

What is seed dormancy?

Answer 14

A period of inactivity in mature seeds that allows survival in adverse conditions.

Question 15

How do gibberellins stimulate seed germination?

Answer 15

Triggering the breaksown of starch into glucose in the seed.
The plant embryo can then use the glucose in respiration to release energy needed for growth.

Question 16

Outline an experiment to investiage the role of gibberellins.

Answer 16

1. Plant 40 plants that are similar age, height & weight in pots
2. Leave 20 plants as they are to grow, with same conditions (controls)
3. Leave the other 20 plants to grow in the same conditions, except water them with a dilute solution of gibberellin.
4. Let the plants grow for about 28 days and measure length of all stems once a week.

Results should show that stem grow more with a dilute solution of gibberellins, suggesting gibberellin stimulates stem elongation.

Question 17

What does it mean when plant hormones are antagonistic?

Answer 17

They have opposing effects
One may promote a response while the other inhibits it.

Question 18

What hormone is antagonistic to gibberellins?

Answer 18

Abscisic acid (ABA)

Question 19

What is the function of ABA as a plant hormones?

Answer 19

Maintains seed dormancy
Inhibits germination.

Question 20

What is the primary auxin in plants, and where is it produced?

Answer 20

IAA
Produced in the meristems of shoot and root tips

Question 21

What are three main roles of auxins like IAA in plants?

Answer 21

• Cell expansion and differentiation
• Supress lateral bud growth (apical dominance)
• Directional growth responses (tropism)
• Inhibition of leaf abscission

Question 22

How do auxin concentrations affect root and shoot growth?

Answer 22

• High auxin = promotes shoot growth, inhibits root growth
• Low auxin = promotes root growth, inhibits shoot growth

Question 23

How do auxins stimulate the growth of shoots?

Answer 23

Cell elongation - cell walls expand and become loose, so cells get longer.

Question 24

Where is the apical bud found?

Answer 24

Found at the shoot tip at the top of a flowering plant

Question 25

What is apical dominance?

Answer 25

The **inhibition** of **lateral bud** growth by auxin (IAA) produced at the shoot tip. Prevents side shoots from growing.

Question 26

What happens when the apical bud is removed?

Answer 26

The plant **won't produce auxin**, so side shoots will start growing by cell division and cell elongation (lateral growth). However, if you replace the tip with a source of auxin, lateral growth is inhibited.

Question 27

What does an **uneven** distribution of auxin cause?

Answer 27

Uneven growth Leading to bending of the plant towards or away from the stimulus.

Question 28

How does auxin cause phototropism in shoots?

Answer 28

Auxin accumulates on the shaded side of the shoot, causing those cells to elongate more, so the shoot bends towards the light.

Question 29

Outline an experiment to investigate the role of auxins in apical dominance.

Answer 29

1. Plant 30 plants that are similar age, height & weight in pots 2. Count & record the no. of side shoots grwoing from main stem of each plants 3. Cut the tip of the shoot of 10 plants and apply a paste containing auxins to the top of stem. 4. Remove the tip of another 10 plants and apply a paste wihtout auxins to the top of stem. 5. Leave the final 10 plants as they are (untreated controls). 6. Let each group grow for 6 days, keeping conditions same, e.g. light intensity, water availability (control variables) 7. After 6 days, count number of side shoots growing from the main stem of each of the plants.

Question 30

Why is a control group neccessary in an experiment?

Answer 30

To provide a **comparison**, so that any observed effect can be attributed to the independent variable, rather than other factors.

Question 31

What hormone stimulates leaf abscission?

Answer 31

Ethene - as leaves get older, more ethene is produced.

Question 32

How does ethene stimulate leaf abscission?

Answer 32

Ethen stimulates the cells in the **abscission layer** to expand, breaking the cell walls and causing the leaf to fall off.

Question 33

Why do plants close their stomata?

Answer 33

To reduce water loss through transpiration.

Question 34

How do guard cells control the opening and closing of stomata?

Answer 34

Guard cell are found either site of a stomatal pore * When guard cells are full of water, they are turgid and the pore is open * When guard cells lose water, they become flaccid, making the pore close.

Question 35

What hormone causes stomatal closure?

Answer 35

Abscisic acid (ABA)

Question 36

What is the role of abscisic acid (ABA) in plants?

Answer 36

ABA helps plants respond to **stress** by triggering **stomatal closure** to reduce water loss. It causes calcium ion channels in guard cells to open, leading to water leaving the cells, making them flaccid and closing the stomata.

Question 37

How is ethene used in the fruit industry?

Answer 37

It stimulates ripening by breaking down cell walls and chlorophyll and converting starch into sugars.

Question 38

What are the commercial uses of auxins?

Answer 38

* In high doses, they can act as **weed killers** (**herbicides**) - cause uncontrolled growth, producing long stems, meaning they can't get enough water or nutrients. * **Rooting powders** - stimulate root growth in stem cuttings, helping new plants grow from one parent plant.