5.16 - Plant responses

Question 1

What is abiotic stress?

Answer 1

A non-living environmental factor that could harm plant e.g. drought, temperature change

Question 2

What are some examples of plant hormones?

Answer 2

• auxins
• gibberellins
• ethene
• ABA (abscisic acid)

Question 3

What are examples of nastic movements in plants?

Answer 3

• Mimosa pudica folds its leaves as a response to touch (limits exposure to abiotic stresses, scares large herbivores and dislodges smaller herbivores)
• Venus fly traps snapping shut to catch prey as a response to touch
• stomatal closure

Question 4

What is the difference between nastic movement and tropisms in plants?

Answer 4

Nastic:
- not directional
- fast and reversible
- caused by a change in cell volume
Tropisms:
- directional
- slow and permanent
- change in growth

Question 5

How does a seed germinate?

Answer 5

• Water absorption is the stimulus that activates the seed
• Gibberellins are produced that stimulate enzyme production (amylase and maltase)
• Enzymes break down food stores in seed so the embryo can produce ATP for growth and metabolic processes

Question 6

How do plants lose their leaves?

Answer 6

• triggered by an abiotic stress such as less light, lower temperature
• reduction in auxin
• ethylene increase at abscission zone
• abscission zone is more sensitive to ethene as parenchyma cells have weaker cell walls
• stimulates enzyme production (cellulase to break down cell walls)
• causes leaf to drop off at abscission zone
• stimulates callus formation to reduce sap loss and protect from pathogens

Question 7

What is the role of hormones in stomatal closure?

Answer 7

• abiotic stress = lack of water
• Abscisic acid (ABA) produced in roots and binds to guard cell plasma membrane on leaf
• Causes ions to be actively pumped out of the cell, increasing water potential in the guard cell
• water diffuses out of the cell
• Guard cell becomes flaccid, so closes
• reduces water loss

Question 8

Roles of auxins in plants

Answer 8

• control cell elongation
• prevent abscission
• maintains apical dominance
• involved in tropisms
• stimulates release of ethene
• involved in fruit ripening

Question 9

Role of gibberellin in plants

Answer 9

• causes stem elongation
• trigger mobilisation of food stores in a seed at germination
• stimulate pollen tube growth in fertilisation

Question 10

Role of ethene/ethylene in plants

Answer 10

• causes fruit ripening
• promotes abscission in deciduous trees

Question 11

Abscisic acid (ABA)

Answer 11

• maintains dormancy in seeds and buds
• stimulates cold protective responses e.g. antifreeze production
• stimulates stomatal closure

Question 12

What are some physical and chemical defences to herbivory?

Answer 12

Physical:
- thorns
- bark
- spikes
- stings
Chemical:
- tannins
- alkaloids
- terpenoids

Question 13

What are tannins?

Answer 13

Part of a group of compounds called phenols. They have a bitter taste which acts as a deterrent to animals and are toxic to insects

Question 14

What are alkaloids?

Answer 14

Bitter tasting nitrogenous compounds. Can affect metabolism and act as drugs e.g. caffeine. This is toxic to insects and fungi, and also can prevent seeds germinating near the plant so there is less competition

Question 15

What are terpenoids?

Answer 15

Form essential oils but also often act as toxins to insects and fungi. Some act as neurotoxins.

Question 16

What is a plant tropism?

Answer 16

a directional growth response of plants

Question 17

What is phototropism?

Answer 17

The growth response to light. Shoots show positive phototropism as they grow towards the stimulus (light)
Roots show negative phototropism as they grow away from light

Question 18

How are auxins involved in phototropism and geotropism?

Answer 18

In shoots: (phototropism)
- Auxins in the meristem tissue in the apex diffuse away from light
- The auxins cause cell elongation on the shaded side of the shoot, so it grows towards the light stimulus.
In roots: (geotropism)
- auxins move towards the lower side of the root and inhibit cell elongation, causing the root to grow downwards (positive geotropism but negative phototropism)

Question 19

What happens as a result of low light intensity?

Answer 19

Etiolation - stems grow long and pale due toa lack of chlorophyll as the plant is using all its energy reserves for cell elongation to find a light source

Question 20

What is apical dominance?

Answer 20

The auxins that are produced at the growing tip at the apex (the top) of a plant stem cause the stem to grow upwards and also stop lateral (side) buds from growing. If the growing tip at the apex of a plant is removed (e.g. due to grazing from herbivores), lateral buds grow from the top of the plant, as the source of auxins has been removed and there is no longer apical dominance

Question 21

What are pheremones?

Answer 21

A chemical made by an organism which affects the social behaviour of other members of the same species. Plants sometimes produce pheromones to alert other plants of insect attacks. These plants then prepare by producing chemicals such as callose to create a physical barrier.

Question 22

What are VOCs?

Answer 22

As plants are not sociable, they usually produce volatile organic chemicals (VOCs) instead of pheromones. These may attract parasitic or predatory insects to control other insects such as aphids.

Question 23

What is geotropism?

Answer 23

A plant growth response to gravity

Question 24

How does ethene ripen plants?

Answer 24

A peak of ethene production triggers chemical reactions such as a greatly increased respiration rate. Climacteric plants such as bananas continue to ripen after being harvested. Non-climacteric fruit such as oranges do not produce large quantities of ethene after harvesting, so do not ripen after harvesting.

Question 25

Why do greengrocers use ethene gas commercially?

Answer 25

Fruit is harvesting before ripening. When fruit is needed for sale it is exposed to ethene gas under controlled conditions. This ensures:
- each batch of fruit ripens at the same rate
- food waste during transport is prevented
- time available for them to be sold is increased

Question 26

How are plant hormones used in rooting powders?

Answer 26

Auxin applied to cuttings propagates them as it stimulates production of roots

Question 27

How are plant hormones used in weed killers?

Answer 27

• weeds interfere with crop plants
• synthetic auxins can be used as a weed killer as crops are often monocots (rice, maize wheat etc.) whereas weeds are often dicots (broad-leaved)
• synthetic dicot auxins are only absorbed by the weeds and affects their metabolism, as the growth rate becomes unsustainable.
• These also have low toxicity to mammals

Question 28

How are plant hormones used in micropropagation?

Answer 28

New plants are grown from a few cells of the original plant.
Plant hormones are used to control production of new cells and the differentiation of the clones into tiny new plants.